Faisal Qayyum
email: faisal.qayyum@uettaxila.edu.pk

Masood Shah

Osama Shakeel

Fariha Mukhtar

Mehdi Salem

Farhad Rezai-Aria

Numerical simulation of thermal fatigue behavior in a cracked disc of AISI H-11 tool steel

Keywords:

Thermal fatigue is one of the key reasons of material failure in components which are exposed to high temperature cycles. Prediction of crack initiation site and crack propagation speed during thermal cycling helps us predict the life of a component in service. In this research numerical simulation of crack propagation due to thermal cycling on a circular disc has been carried out. The thermal profiles used in the simulation are taken from actual thermal fatigue experiments. The effect of the length of cracks and interaction between adjacent cracks has been investigated. 50 sets of numerical simulation models with different crack numbers and crack lengths have been simulated. The variation in Stress Intensity Factor (SIF), hoop stress and Crack Mouth Opening Displacement (CMOD) has been plotted as a function of primary/secondary crack lengths and number of cracks. Envelopes of possible crack growth have been identified and correspond well to the experimental observations. Results show a significant drop in hoop stress, SIF and CMOD with increase in number of cracks, thus limiting the number of cracks possible in a thermal fatigue crack network.

Introduction

Thermal fatigue is one of the key damage mechanisms in applications involving high temperatures like in power plants, engine valves, gas turbines, steam valves, nuclear reactors, high pressure die casting, and forging operations. Currently no standard procedure for thermal fatigue testing exists. The inherent complexity of thermal fatigue problem, renders standardization of this testing procedure almost impossible.

Different researchers have developed their own specific experimental setups, closely resembling the application being investigated. Some of the setups include a disc heated by induction on the periphery [START_REF] Roux | Image analysis of microscopic crack patterns applied to thermal fatigue heat-checking of high temperature tool steels[END_REF][START_REF] Kamaya | Loading sequence effect on fatigue life of Type 316 stainless steel[END_REF][START_REF] Szmytka | Thermal fatigue analysis of automotive diesel piston: experimental procedure and numerical protocol[END_REF][START_REF] Kuwabara | Thermal-mechanical low-cycle fatigue under creep-fatigue interaction on type 304 stainless steels[END_REF][START_REF] Robertson | Dislocation substructure in 316 L stainless steel under thermal fatigue up to 650 K[END_REF][START_REF] Persson | Simulation and evaluation of thermal fatigue cracking of hot work tool steels[END_REF][START_REF] Sjöström | Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments[END_REF][START_REF] Persson | Thermal fatigue cracking of surface engineered hot work tool steels[END_REF][START_REF] Paffumi | Simulation of thermal fatigue damage in a 316 L model pipe component[END_REF][START_REF] Fissolo | Crack initiation under thermal fatigue: an overview of CEA experience. part I: thermal fatigue appears to be more damaging than uniaxial isothermal fatigue[END_REF][START_REF] Roux | Role of heat-flux density and mechanical loading on the microscopic heat-checking of high temperature tool steels under thermal fatigue experiments[END_REF][START_REF] Medjedoub | Effect of local stress on the heat-checking morphology in high temperature tool steels under thermal fatigue: transition from multi-axiality to uniaxiality[END_REF], triangular blade heated by induction [START_REF] Rezai-Aria | An oxidation fatigue interaction damage model for thermal fatigue crack growth[END_REF], electrical heating of tube followed by water splashing to induce thermal shock [14][START_REF] Haddar | Thermal fatigue crack networks: a computational study[END_REF][START_REF] Maillot | Thermal fatigue crack networks parameters and stability: an experimental study[END_REF][START_REF] Amiable | A comparison of lifetime prediction methods for a thermal fatigue experiment[END_REF][START_REF] Fazarinc | Thermal fatigue properties of differently constructed functionally graded materials aimed for refurbishing of pressure-die-casting dies[END_REF], hot dipping of cylinders in molten aluminum [START_REF] Klobčar | Thermal fatigue of materials for die-casting tooling[END_REF][START_REF] Klobčar | Thermo fatigue cracking of die casting dies[END_REF], laser heating [START_REF] Caliskanoglu | Thermal fatigue and softening behavior of hot work tool steels[END_REF][START_REF] Song | Thermal fatigue on pistons induced by shaped high power laser. part I: experimental study of transient temperature field and temperature oscillation[END_REF], focused halogen lamps heating [START_REF] Mellouli | Thermal fatigue of cast irons for automotive application[END_REF][START_REF] Laird | Crack propagation in high stress fatigue[END_REF], convection/combustion heating [START_REF] Bhattachar | Thermal fatigue behaviour of nickel-base superailoy 263 sheets[END_REF][START_REF] Kerezsi | Using the ASME and BSI codes to predict crack growth due to repeated thermal shock[END_REF], furnace heating with water quenching [START_REF] Ancelet | Development of a test for the analysis of the harmfulness of a 3D thermal fatigue loading in tubes[END_REF][START_REF] Cong | Thermal fatigue resistance of hot work die steel repaired by partial laser surface remelting and alloying process[END_REF] and actual dies [START_REF] Persson | Temperature profiles and conditions for thermal fatigue cracking in brass die casting dies[END_REF][START_REF] Persson | Failure modes in field-tested brass die casting dies[END_REF]. Out of these experimental setups most researchers have worked on the heated disc problem. Analytical models have been developed for heated disc type configuration using Stress Intensity Factor "ΔK" [START_REF] Mowbray | Thermal Fatigue Characterization of Cast Cobalt and Nickel-base Superalloys[END_REF][START_REF] Burlet | Crack growth behaviour in a thermal fatigue test. Experiments and calculations[END_REF][START_REF] Tret'yachenko | Thermal fatigue crack propagation in massive cylinders during their cyclic heating[END_REF][START_REF] Lieurade | Experimental simulation and theoretical modelling of crack initiation and propagation due to thermal cycling[END_REF][START_REF] Sjostrom | Thermal fatigue in hot-working tools[END_REF], for this purpose Mowbray et al. [START_REF] Mowbray | Thermal Fatigue Characterization of Cast Cobalt and Nickel-base Superalloys[END_REF] used ΔK based on the weight function defined by Bueckner to characterize the thermal fatigue crack propagation [START_REF] Bueckner | Field singularities and related integral representations[END_REF]. Others used cyclic J integral [START_REF] Rezai-Aria | Fatigue thermique et fatigue isotherme d'un superalliage à base de cobalt: étude metallurgique de l'endommagement et modelisation[END_REF][START_REF] Gandossi | Crack growth behaviour in 316 L stainless steel components under thermal fatigue loading[END_REF] and strain intensity factor [START_REF] Burlet | Crack growth behaviour in a thermal fatigue test. Experiments and calculations[END_REF][START_REF] Tret'yachenko | Thermal fatigue crack propagation in massive cylinders during their cyclic heating[END_REF][START_REF] Lieurade | Experimental simulation and theoretical modelling of crack initiation and propagation due to thermal cycling[END_REF][START_REF] Sjostrom | Thermal fatigue in hot-working tools[END_REF][START_REF] Skelton | Fatigue at High Temperature[END_REF][START_REF] Koizumi | Crack growth and prediction of endurance in thermal-mechanical fatigue of 12Cr-Mo-V-W steel[END_REF] for characterizing thermal fatigue crack propagation. Most researchers deal with thermal fatigue as a special case of isothermal fatigue. Many researchers have worked on numerical simulation models [START_REF] Kwon | Damage evaluation regarding to contact zones of high-speed train wheel subjected to thermal fatigue[END_REF][START_REF] Qayyum | Comparison of thermomechanical stresses produced in work rolls during hot and cold rolling of cartridge brass 1101[END_REF][START_REF] Paffumi | Experimental and numerical assessment of thermal fatigue in 316 austenitic steel pipes[END_REF] to create better understanding of thermo-mechanical stresses which are produced during the process and are responsible for failure of components.

Thermal fatigue is considered to be composed of two components, initiation phase and the propagation phase. It has been investigated that the initiation might occur due to oxidation sites or due to plasticity, as in the case of stainless steels [unpublished internal results on thermal fatigue of 316 stainless steel, ecole mines albi]. One of the observations made during experimentation was that two diametrically opposed cracks initiate and propagate with almost the same speed through the specimens as represented in Fig. 1 schematics. Secondary cracks are created later and their propagation speed is lower than the primary cracks. The total number of cracks (primary and secondary) almost never exceeds 8 in number in oxidizing steel. This material was chosen because due to oxidation large number of cracks initiate, while the cracks having sufficient SIF propagate during thermal cycling. This phenomenon has been presented in Fig. 1.

This research aims to determine the interaction between primary and secondary cracks. It also aims to determine the threshold value of number of secondary cracks that can propagate. The threshold value is found to be dependent on the stress relaxation produced due to addition of secondary cracks around the periphery of the specimen.

All in all 50 simulation models with different crack number and crack lengths were run to determine the threshold of crack number. The drop in SIF, hoop stress and CMOD is plotted as a function of number of cracks and primary/secondary crack lengths. These plots give an envelope of propagation, clearly showing the parameters necessary for the crack propagation to occur. The results obtained were observed to be in agreement with the experimental observations.

Experimentation

The experimentation was done by Salem M. [START_REF] Salem | Etude d'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium: effet de l'interaction avec l'aluminisation et l'oxydation[END_REF] in his PhD thesis, and detailed material model for AISI H11 tool steel which have been incorporated in the numerical simulation model were developed by Ahmer [START_REF] Ahmer | An Investigation on Thermomechanical Behaviour of a Tool Steel X38CrMoV[END_REF]. The experimental setup and geometry of the used specimen is shown in Fig. 2. Heating was done by induction coils while cooling was done by cold water flowing through internal pipe at 25 °C. 6 thermocouples were spot welded on the disc at positions specified in Fig. 3. This specimen was heated using an induction heating system with 1.3 s heating cycle and 7.2 s cooling cycle. The resulting thermal profile is shown in Fig. 3 and is compared with the thermal profile obtained from simulation. The cracks were observed and measured using binocular microscope during testing, which helped in determining fatigue crack propagation rate and crack locations.

Numerical simulation

Numerical simulation was carried out on ABAQUS Standard™ 6.12. Decoupled thermo-mechanical approach was adopted during simulation. Simulations were carried out for a disc type specimen on a pipe configuration. Sectioned model of the assembly is shown in Fig. 2 (b). Thermal analysis was carried out for 10 thermal cycles to obtain steady state results which will be discussed later. Temperature change of each node was recorded after specific time intervals. Saved temperature changes at each node at specific time intervals were fed to the stress analysis file as an input. Results of stress development and crack opening during thermal cycling were observed.

Due to thermal cycling hundreds of microcracks develop on the surface of the specimen (upper left of Fig. 1). These microcracks are shown to be formed due to the presence of oxidation at the surface of the specimens. Some of these microcracks later develop into macro cracks (lower right of Fig. 1) and propagate through the section of the specimen. The primary cracks (minimal of 0.5 mm length), as they are called are the diametrically opposed cracks that appear initially and are always longer in length than the other cracks on the periphery of the specimen.

Material data

Specimen was made of AISI H-11 hot work tool steel with chemical composition as shown in Table 1. Density of 7840 kg/m 3 and a Poisson's ratio of 0.3 were used. The temperature dependent properties used during simulation are shown in Fig. 4.

Thermal analysis

The disc part of the specimen in Fig. 2 (b) was modeled as a 2D deformable shell of 30 mm outer diameter and 18 mm internal diameter with a thickness of 1 mm. The hollow shaft inside was modeled as a 2D deformable shell of 18 mm outer diameter and 12 mm internal diameter having length 40 mm as shown in Fig. 5. Temperature dependent material properties of H11 tool steel (Fig. 4) were assigned to the material model. Both instances were assembled together in the assembly module by using coaxial constraint command. Transient heat transfer model was selected in step module for 10 thermal cycles giving a total time period of 85 s for the analysis. Heat sink of 25 °C was applied inside shaft. While defining boundary conditions, external surface of shaft and internal surface of disc were constrained with "tie interaction" of continuous heat transfer condition. Surface heat flux of 5.15 W/mm 2 was applied on the outer rim of flange with amplitude shown in Fig. 3. In Fig. 3, 1 means maximum flux and 0 means no flux. The amplitude profile values were adjusted so that the thermal profile obtained by simulation at different points along the radius of the disc matches the thermal profile in the experimentation. This indirectly takes into account the heating due to the induction coil, radiative losses, natural convection losses as well as the conduction loss due to the cooling channel. The comparison between experimental and numerical values is shown in Fig. 3. Close correlation exists up to a depth of 2 mm. After that the experimentation shows almost 20% higher temperatures than the numerical simulation. The amplitude follows the response of the induction heating system.

The meshed assembly is shown in Fig. 5. It was assured that aspect ratio of all elements is almost equal to 1. The model was also checked for mesh dependency. The simulation model schematic is shown in Fig. 5. The thermocouple positions in Figs. 3 and5 were used to calibrate the thermal model.

Stress and fracture analysis

For mechanical analysis the disc and shaft again modeled as 2D deformable shells with all properties as discussed in the Material data section. Temperature dependent material properties of H11 tool steel were assigned to the disc and shaft. An incremental plasticity model was used for the plastic deformation of the specimen.

The primary and secondary cracks of different lengths and numbers were modeled on disc using surface partition command. The choice of crack lengths follows the experimental observations. Initially the experiments were conducted on H11 steel. This steel develops an oxide scale due to thermal cycling and the detection of a crack under this scale was difficult. Also the crack initiation point is on the curve of the specimen. Both these factors affect the minimum resolution of the crack observations which is 0.5 mm using a binocular microscope. (Observations with higher powered microscopes are difficult due to low depth of field of the microscopes and heavily instrumented specimen, which has 6 thermocouples of 0.1 mm thickness spot welded on the surface).

Once the resolution is identified a complete range of possibilities of crack lengths is simulated. The only constraint was that the primary crack has to be longer than the secondary crack length. A 16 cracks numerical simulation set containing 1 primary crack of 2 mm length and 15 secondary cracks of 0.5 mm length is shown in Fig. 6. Cracks were defined in assembly module using contour integral command. Collapsed element side seam with mid side node parameter of 0.24 was defined for all cracks. The set of cracks defined during simulation are shown in Table 2. Static general model was selected in step module for time period of 85 s which simulates 10 thermal fatigue cycles. It was observed that although the thermal profile stabilizes at the third cycle, the stress-strain response and the J-integral does not stabilize up to the 7th cycle. It was thus decided to run the thermal fatigue experiments up to 10 cycles so that stability in mechanical response can be assured. Due to cyclic simulation the crack faces come into contact during the simulation. The crack faces were defined as contact pairs with frictionless contact to avoid any elemental penetration during the simulation. Coefficient of friction is considered as frictionless because in mode I crack no sliding occurs. Since the simulation model is based on mode I crack opening configuration, keeping in view the simplicity of the model, the effect of sliding was not considered and hence frictionless contacts were defined.

The detailed meshing for crack tip is given elsewhere [START_REF] Shah | An estimation of stress intensity factor in a clamped SE (T) specimen through numerical simulation and experimental verification: case of FCGR of AISI H11 tool steel[END_REF]. The meshed assembly is shown in Fig. 6. In total, 1 thermal and 49 mechanical simulation models were run to achieve the desired results.

Results and discussion

Thermal fatigue is caused by thermal gradients in a component which results in material expansion and strains eventually resulting in production of thermal stresses in the component due to the constraint of the material itself. The phenomenon is majorly strain controlled deformation. As a consequence the largest strains and stresses occur at the surface with thermal cycling, because of which a large number of cracks initiate at the surface. When the components (in this case a disc) develop cracks, its compliance increases, which leads to reduction of the stresses and strains. When this happens the formation of new cracks becomes less likely and the propagation of individual macro-cracks is reduced. The reduction in stresses/strains is directly linked to the circumferential distance between cracks and the crack depth, which results in a typical number of cracks. Due to the thermal gradients the stresses also decay with the distance from the heated surface. As a consequence crack propagation rate slows down and crack often arrests at a specific depth.

Numerical simulation gives us an insight of how heat transfer takes place during each cycle, and in response to cyclic thermal fatigue how mechanical stresses are produced in the disc. It is also observed how number of cracks (n) and crack lengths (a s and a p ) affect the stresses produced in the disc. The thermal cycle stabilizes typically at the third cycle in simulation and experimentation alike. Fig. 3 also shows the comparison of the experimental and numerical thermal response and a good correlation is observed. In this section the stress-strain response is discussed followed by the effects of multi-cracking on the stress distribution and J-integral ahead of the crack tip.

Stress-strain response

The stress-strain response is studied initially on the un-cracked disc. This response is important in determining the crack initiation sites. Isotropic hardening temperature dependent material model was used. The first cycle shows a large amount of plastic deformation in compression. The initial heating results in expansion of the material which results in axial compression of flange, and the compressive stress increases till the temperature reaches 525 °C. Beyond this temperature, although the expansion continues, the yield strength of the material starts to drop. 550 °C presents the first tempering temperature of this material and is also considered to be the limit of hot hard behavior. This softening results in a large amount of plastic deformation and reduction in compressive stress, visible is hoop stress vs temperature plot Fig. 7. The drop in stress is substantial, from 820 MPa to 530 MPa compressive.

As the disc cools, a tensile residual stress is observed due to yielding in the first cycle. It is believed that this tensile residual stress is responsible for the thermal fatigue cracking in metals. After the first few cycles, the plastic deformation is greatly reduced and the stress-strain curve stabilizes. However, there is some amount of plastic deformation in each cycle. This plastic deformation eventually causes enough damage to initiate thermal fatigue cracks.

Drop of tensile stress in flange due to multiple cracks

The presence of cracks affects the hoop stresses produced at the edge of the disc. Fig. 8 (a,b,c,d) represents the smallest primary crack length and secondary crack length of the simulation. Minimal (almost non-existent) effect of increase in number of cracks (n) on the periphery of the disc during compression cycle is observed. However, the effect of the number of cracks (n) on the tension part of the stress-strain response is significant. In Fig. 8 (a) it can be observed that there is a drop in tensile stress where the cracks are present. However, between these points there is a large "plateau" of constant tensile stress. For 4 cracks (Fig. 8 (b)) the "plateau" still exists, however the maximum value of the tensile stress slightly drops. It is seen that for 8 cracks (Fig. 8 (c)) there is almost no constant stress zone left on the periphery of the crack. Also, the value of the maximum tensile stress drops from 340 MPa to 320 MPa. This shows that the tensile stress peaks (that would exist in case of the uncracked specimen) are now completely relieved by the 8 cracks around the periphery. A further increase in the number of cracks to 16 (Fig. 8 (d)) reduces the tensile stress peaks to 280 MPa and there is no "plateau" showing that the tensile stress that initially caused the crack initiation cannot be achieved now. This is in agreement with experimental results [START_REF] Salem | Etude d'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium: effet de l'interaction avec l'aluminisation et l'oxydation[END_REF] where the maximum number of cracks ever achieved never cross 8.

Fig. 9 shows the comparison for long cracks as well. It is seen that for maximum crack length the tensile stress values drop to 120 MPa in the case of 16 cracks, thus eliminating any possibility of having more than 8 cracks to propagate around the periphery. The difference in compressive stresses between a p = 1 mm (lower compressive stress) and a p = 4 mm (higher compressive stress) is due to the numerical model that has been used. The contact of the crack faces is necessary to accurately simulate the compressive stresses. In the numerical model, contact of the crack faces is made except for the last element of the crack tip.

Here the contact cannot be defined because the upper and lower face has an intersecting point (crack tip). During simulation, the crack tip elements cross each other. The simulation generally gives satisfactory results because the area represented by the crack tip is much lesser than the total crack faces. But in the case of very short cracks, the effect of this (non-contact definition) is slightly more pronounced. The effect is manifested as a lower compressive stress simulated. In the extreme cases as is shown in Fig. 9 this error is almost 8%.

The J-integral calculations are important during the tensile cycle of the TF experiment. This difference of compressive stress might not greatly affect the J-integral values.

J-integral as a function of crack length

Once the cracks initiate, they will continue to propagate under the effects of the tensile stress. The crack propagation speed has been found to be fast up to 1 mm crack length and slows down considerably afterwards. The primary crack propagation arrests at a length of 3.5 mm maximum while the secondary cracks rarely exceed 2.5 mm in length. Fig. 10 [44] shows some of the results obtained experimentally (in this case only five cracks were tracked while others do exist, but they were not tracked).

Development of crack tip stress field

When a crack is introduced into the periphery, it initially acts as an un-cracked ligament during the first heating cycle. Following this during the cooling cycle the initial yielding at the highest temperature causes tensile stresses which open the crack as shown in the Fig. 11. The second cycle causes crack closure (forward yielding) and then during subsequent cooling (second cycle) a more severe crack opening is observed.

The contour integral plot in Fig. 12 represents the development of crack tip stress field. Sharp rise in the second cycle as compared to the first cycle is indicative of the residual crack opening created due to first yielding of the cracked specimen (Fig. 12). The numerical simulation of the contour integral shows that the J-integral remains constant, (and high) up to 1 mm crack length (Fig. 12). Beyond this point the value sharply reduces and is minimal at 3 mm crack length. At 4 mm crack length the value is almost zero, indicating that there is not much possibility of crack growth at this crack length. This fact is experimentally verified with experimentally obtained crack lengths [START_REF] Salem | Etude d'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium: effet de l'interaction avec l'aluminisation et l'oxydation[END_REF].

Envelopes of crack propagation

The effect of the number of secondary cracks of 0.5 mm length on the J-integral of the primary crack is shown in the surface plot in Fig. 13 (a). The red and light blue shades within 0 ≤ n ≤ 7 (where n = number of secondary cracks) shows the envelope of propagation. Beyond n = 7 there is a sharp drop in the J-integral value that would arrest any crack. Also beyond a p = 3 mm the J-integral value drops to zero, indicating that no propagation is possible. Parallel surfaces can be obtained for different values of a s . In the same manner the J-integral surface plot for the other extreme condition with a p = 4 mm and different a s is shown in Fig. 13 (b). It is clear here that the blue part of the surface indicates the zone of non-propagation. This surface shows that the maximum secondary crack length could not exceed 2.5 mm. Practically this observation seems to be true by experimental verification [START_REF] Salem | Etude d'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium: effet de l'interaction avec l'aluminisation et l'oxydation[END_REF].

CMOD as a function of a p and a s

Numerous researchers have worked on relating the fatigue crack growth rate to cyclic J-integral [START_REF] Dowling | Geometry Effects and J Integral Approach to Elastic-plastic Fatigue Crack Growth[END_REF][START_REF] Dowling | Fatigue crack growth during gross plasticity and the J-integral[END_REF][START_REF] Chow | Cyclic J-integral in relation to fatigue crack initiation and propagation[END_REF] [50] and to the Crack Tip Opening Displacement (CTOD) [START_REF] Laird | Crack propagation in high stress fatigue[END_REF][START_REF] Pelloux | Crack extension by alternating shear[END_REF][START_REF] Neumann | New experiments concerning the slip processes at propagating fatigue cracks[END_REF][START_REF] Hamam | Simulation de la propagation des fissures par fatigue dans les toiles des roues de train sous chargement à amplitude variable[END_REF][START_REF] Schweizer | Mechanisms and modelling of fatigue crack growth under combined low and high cycle fatigue loading[END_REF]. The ASTM standard E399 [START_REF] Standard | Standard test method for plane strain fracture toughness of metallic materials[END_REF] also gives an indirect crack length measurement method using the CMOD. The CMOD values were thus extracted for the different thermal cycles to see if they represent the Jintegral values at the crack tip as shown in Fig. 14 It is seen that the CMOD values of the primary or secondary crack do not represent at all whatever is happening at the crack tip. The values of the CMOD increase continuously while the crack propagation rate and J-integral drop continuously during the propagation beyond 1 mm crack length. The CMOD values do not change a lot even for 16 cracks when the secondary cracks have small length (Fig. 14 (a)). CMOD values for 16 cracks shows a sharp decline, which is representative of stress relaxation around the periphery for longer crack lengths.

Conclusion

The study to verify the limiting number of cracks produced on the rim of an H11 tool steel disc exposed to thermal fatigue was carried out using numerical simulation technique. Decoupled thermal mechanical numerical simulation technique was successfully used to get an insight of a thermal fatigue phenomenon. Thermal profiles were taken from actual thermal fatigue experiments. Isotropic hardening temperature dependent material model was used. 50 sets of numerical simulation models with different crack numbers and crack lengths were simulated to determine the interaction between the primary and secondary cracks. Significant drop in hoop stress and SIF with increase in number of cracks was observed. Envelopes of crack propagation were determined by plotting surface plots of J-integral as a function of n, a p and a s .

1. It was observed that the initial heating causes expansion of the material which results in axial compression of flange, and the compressive stress increases till the temperature reaches 525 °C. Beyond this temperature yield strength starts to drop which softens the material and results in drop of compressive stress. 35% drop in compressive stress was observed for first cycle in Fig. 7 (from 820 MPa to 530 MPa compressive) which is considered to be the most damaging. 2. It is obvious that crack initiation and propagation in the disc occurs due to tensile stresses. It was observed that increases in number of cracks produced in the disk substantially decreases maximum tensile stresses on the rim which limits the number of cracks produced due to the phenomena of stress relieving. In the current scenario it was observed that the number of cracks propagating will never exceed 8. 3. It was observed that crack tip stress field fully develops after multiple fatigue cycles. It was also observed that value of Jintegral on crack tip decreases with increase in crack length. To elaborate this phenomenon of crack arrest surface plots of J-integral as a function of n, a p and a s were developed to find the crack propagation envelopes for primary and secondary crack lengths. As the maximum number of cracks never cross the value of 8 the primary crack length never exceeds 3 mm and secondary crack lengths never cross 2.5 mm. n ≤ 8, a p ≤ 3 mm and a s ≤ 2.5 mm, these are considered to be the threshold values. 4. It was observed that although the value of J-integral drops as the crack length increases, the CMOD keeps on increasing. It is concluded that in the current scenario CMOD does not represent a good measure of the crack tip stresses.

Fig. 1 .

 1 Fig. 1. Crack initiation and propagation in an oxidizing steel.

Fig. 2 .

 2 Fig. 2. (a) Experimental setup. (b) Specimen Shape (all dimensions are in mm) [44].

Fig. 3 .

 3 Fig. 3. Comparison of experimental and simulated thermal profile, and (inset figure) amplitude factor of maximum energy.

Fig. 4 .

 4 Fig. 4. Temperature dependent material data of H-11 tool steel [44,45].

Fig. 5 .

 5 Fig. 5. Meshed assembly of disk and shaft.

Fig. 6 .

 6 Fig. 6. Meshed assembly of simulation set containing 1 primary crack of 2 mm length and 15 secondary cracks of 0.5 mm length.

Fig. 7 .

 7 Fig. 7. Hoop stress vs temperature curve.

Fig. 8 .

 8 Fig. 8. Distribution of hoop stress along periphery of the flange.

Fig. 9 .

 9 Fig. 9. Comparison of σ Hmax at periphery of disk as a function of n, a s and a p .

Fig. 10 .

 10 Fig. 10. Crack growth in disc specimen. Primary cracks are longer than the secondary cracks. a pmax = 3.5 mm, a smax = 2.5 mm. Cracks accelerate up to 1 mm length [44].

Fig. 11 .

 11 Fig. 11. Complete development of a crack tip stress field in two consecutive thermal cycles.

  (a) and (b).

Fig. 12 .

 12 Fig. 12. J-integral stabilization graph when primary crack length a p 0.5, 1.0, 2.0 and 3.0 mm.

Fig. 13 .

 13 Fig. 13. Surface plot of J-integral as a function of, (a) a p , n and (b) a s , n.

Table 1

 1 Chemical composition of AISI H-11 tool steel.

	Elements	Cr	Mo	Si	V	C	Ni	Cu	Mn	P	S
	wt.%	4.75-5.50	1.10-1.75	0.80-1.20	0.80-1.20	0.32-0.45	0.3	0.25	0.20-0.50	0.03	0.03

Table 2

 2 Set of cracks defined during simulation.

	Number of cracks	Length of primary crack	Length of secondary cracks	Total number of simulations
	(n)	(a p )	(a s )
	1	0.5, 1, 2, 3, 4	0	5
	2	0.5, 1, 2, 3, 4	0.5	11
		2, 3, 4	1
		3, 4	2
		4	3
	4	0.5, 1, 2, 3, 4	0.5	11
		2, 3, 4	1
		3, 4	2
		4	3
	8	0.5, 1, 2, 3, 4	0.5	11
		2, 3, 4	1
		3, 4	2
		4	3
	16	0.5, 1, 2, 3, 4	0.5	11
		2, 3, 4	1
		3, 4	2
		4	3

Acknowledgment

Experimentation of this research was conducted at the Université de Toulouse; INSA, UPS, Mines Albi, ISAE; ICA (Institut Clément Ader) Route de Tiellet, Campus Jarlard, Albi, France. The simulation was done at the Fracture Mechanics and Fatigue Lab., Department of Mechanical Engineering, University of Engineering and Technology Taxila (UET/ASR&TD/RG-1250 and UET/ASR&TD/ RG-348), Pakistan. The study was funded and supported by both institutes which are gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.engfailanal.2016.01.015.


J.-P Vernier

T D Fairlie

T Deshler

M Venkat Ratnam

H Gadhavi

M Natarajan

A K Pandit

A Raj

A H Kumar

A Jayaraman

J Vernier

S Kumar

H Kumar

A K Singh

N Rastogi

P R Sinha

S Tiwari

T Wegner

N Baker

D Vignelles

G Stenchikov

I Shevchenko

J Smith

K Bedka

A Kesarkar

V Singh

J Bhate

V Ravikiran

J.-P Vernier

Raj S T Akhil

M.D Rao

S Ravindrababu

A Patel

H Vernier

F G Wienhold

H Liu

T Knepp

L Thomason

J Crawford

L Ziemba

J Moore

S Crumeyrolle

M Williamson

G Berthet

F Jégou

J B Renard

Jean-Paul Vernier
email: jeanpaul.vernier@nasa.gov

BATAL: The Balloon measurement campaigns of the Asian Tropopause Aerosol Layer

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I.

Introduction.

Asian pollution sources and their strength have changed over the last few decades.

Recent satellite observations have shown that the highest NO2 and SO2 emissions have shifted from China to India [START_REF] Krotkov | Aura OMI observations of regional SO2 and NO2 pollution changes from 2005 to 2015[END_REF]). China's SO2 emissions have decreased by 50% after the installation of desulfurization system in their power plants, while SO2 emissions in India have doubled between 2005 and 2015 [START_REF] Krotkov | Aura OMI observations of regional SO2 and NO2 pollution changes from 2005 to 2015[END_REF]. The shift in the highest SO2 emissions toward the tropics could have serious implications for their redistribution in the atmosphere. During the Asian Summer Monsoon (ASM), tropical convective systems provide a vertical transport pathway for boundary layer pollutants to reach the Tropical Tropopause Layer (TTL), which is a gateway to the global stratosphere.

SO2 and other gas-phase precursors transported into the TTL can be converted into sulfuric acid droplets and other aerosols with consequences for the radiative balance, stratospheric ozone chemistry, and the properties of cirrus clouds near the tropopause [START_REF] Kremser | Stratospheric aerosol-Observations, processes, and impact on climate[END_REF].

Satellite observations have revealed the presence of pollution markers (e.g., CO, HCN, CH4) in the UTLS region during the ASM [START_REF] Park | Transport above the Asian summer monsoon anticyclone inferred from Aura MLS tracers[END_REF][START_REF] Randel | Deep convective influence on the Asian summer monsoon anticyclone and associated tracer variability observed with AIRS[END_REF][START_REF] Randel | Asian Monsoon Transport of Pollution to the Stratosphere[END_REF]. Together with model simulations (e.g., [START_REF] Lawrence | Atmospheric pollutant outflow from southern Asia: a review[END_REF], these studies suggest that deep convection can serve as a conduit for the transport of gas-phase boundary-layer pollutants to the UTLS. Pollutants can accumulate in the UTLS under anticyclonic flow conditions (the Asian anticyclone) which characterize the UTLS between the Eastern Mediterranean Sea and China during the ASM, and which encompasses Northern India.

Figure 1 shows a sketch of the transport mechanisms associated with the ASM. An overpass of the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) lidar for August, 12 th 2017 shows the vertical extension of convective storms reaching the UTLS over India. Air transported via the ASM anticyclone to the tropical upper troposphere can reach the lower stratosphere via relatively slow diabatic ascent [START_REF] Garny | Transport pathways from the Asian monsoon anticyclone to the stratosphere[END_REF]. In addition, strong Mesoscale Convective Systems (MCSs) in India can directly penetrate the lower stratosphere. Detrainement of air from the anticyclone due to transience in the flanking jet streams, in particular Rossby-wave breaking along the subtropical jet can lead to irreversible exchange of air with the extratropical lower stratosphere [START_REF] Vogel | Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012[END_REF], with implications for the lower stratospheric composition throughout the hemisphere.

In addition to trace gases, aerosol concentrations are enhanced in the UTLS during the ASM. CALIPSO satellite observations have shown a recurrent aerosol layer in clear-sky summertime backscatter profiles near 13-18 km from the Eastern Mediterranean Sea to Western China and India [START_REF] Vernier | CALIPSO detection of an Asian tropopause aerosol layer[END_REF][START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF], called the Asian Tropopause Aerosol Layer (ATAL). The ATAL was confirmed through solar occultation observations by Stratospheric Aerosol and Gas Experiment (SAGE) II [START_REF] Thomason | Improved SAGE II cloud/aerosol categorization and observations of the Asian tropopause aerosol layer: 1989-2005[END_REF] after improving the cloud-aerosol separation approach using the ratio between aerosol extinction coefficients retrieved at two wavelengths. Analysis of long-term satellite measurements of UTLS aerosols suggests that ATAL's aerosol optical depth has increased by 2-3 times since the late 1990's pointing out its possible connection with Asian pollution growth [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF]. Satellite observations of ATAL are consistent with early measurements carried out from the Tibetan plateau during the late 90's. Ground based lidar in Lhasa (Tibet, China),

showed the presence of an aerosol layer between 14-19 km with SR = 1.1-1.2 and δ < 5% [START_REF] Kim | Enhancements of aerosols near the cold tropopause in summer over Tibetan Plateau: Lidar and balloon borne measurements in 1999 at Lhasa, Tibet, China[END_REF]. In addition, balloon measurements at the same location in August 1999

have shown an enhancement of small particles of effective radius <0.6μm near the tropopause [START_REF] Tobo | Balloon-borne observations of high aerosol concentrations near the summertime tropopause over the Tibetan Plateau[END_REF]. More recently, in situ backscatter measurements using the Compact Optical Backscatter AerosoL Detector (COBALD) and aerosol size distribution from the Printed Optical Particle Spectrometer (POPS) have confirmed the presence of the ATAL [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF][START_REF] Pengfei | Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone[END_REF].

The presence of aerosol near the tropopause could impact the Earth's radiative balance, and the properties of cirrus clouds. The direct summertime radiative forcing resulting from the AOD increase of the ATAL, since the late 1990s, was estimated near -0.1 W/m 2 using radiative parameters corresponding to sulfate aerosol. This represents 1/3 of the total radiative forcing due to increased CO2 over the same period [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF]. Thus, the ATAL appears to be a significant player in the Earth's radiative budget of the Northern Hemisphere at decadal scale. Aerosol near the tropopause can potentially affect stratospheric ozone. [START_REF] Solomon | Monsoon circulations and tropical heterogeneous chlorine chemistry in the stratosphere[END_REF] suggested that heterogeneous chlorine chemistry occurring on aerosol in the lower stratosphere during the ASM could impact the local budget and longterm trend of ozone in the UTLS. Finally, the presence of aerosol near the tropopause can potentially alter the properties of TTL cirrus clouds. [START_REF] Pandit | Long-term trend analysis and climatology of tropical cirrus clouds using 16 years of lidar data set over Southern India[END_REF] showed that the frequency of sub-visible cirrus (SVC) in South India has increased by 9% from 1998-2013, with possible connections with the positive trend in ATAL's AOD since the late 90's [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF].

Since 2014, we have organized a series of balloon field campaigns in India and Saudi-Arabia during the ASM to better understand the nature, formation, and impacts of the ATAL.

We chose 3 balloon sites in India to sample air masses directly influenced by convection and to capture the horizontal gradient of aerosol in the UTLS region from the edge to the center of the ASM Anticyclone. The location in Saudi Arabia was selected for measurements downwind from the monsoon region and study the evolution of air masses along their transport.

The BATAL campaigns specifically address the following science questions: We used different balloon platforms (Table 1, Figure 3) over the 4-year BATAL effort to address the science questions described in section 1:

-

ZF : A New concept for TTL measurements

In order to achieve extended flotation in the UTLS, the Zero-pressure Flights (ZF), included a ballast module to compensate for altitude loss due to extremely cold temperature near the tropopause. Figure 4 -LF/MF: The optical properties of the ATAL were studied using a backscatter sonde, called COBALD (Compact Optical Backscatter AerosoL Detector) flown under a meteorological sonde for Light Flights (LF) with 1200 g latex balloons and a total payload weight of less than 1.3 kg. Medium Flights (MF) included, in addition to LF payloads, ozone and water vapor sensors, and sometimes a light-weight aerosol counter to study the size distribution of the ATAL and the chemical composition of the UTLS.

-BF: In order to study the influence of convection on the ATAL, we used a controlled balloon system, developed by Smith and Williamson, Inc., called Boomerang. The rational was to have a flexible and simple system to launch payloads of less than 10 kg in weather conditions which would not be possible for ZF or HF flights. The goal was to extend the measurement duration in the UTLS region and attempt to fly in convective storms. Boomerang was used with a 3000g balloon and a secondary balloon (1200g) to which was attached with a pressure sensitive cut down device (Doongara). The secondary balloon was cut at a given altitude to rapidly slow down the ascent rate. The two Boomerang flights conducted from Varanasi in August 2016 included an aerosol impactor and COBALD.

Boomerang

Boomerang is a radio-controlled balloon system which includes a valve and a ballast module. The valve allows venting of helium gas from inside the balloon, controlling the internal pressure, reducing the buoyancy force and thus the ascent rate. A ballast module comprises sand which can be released to make the system lighter, increasing the balloon ascent rate.

b. Payloads.

In order to characterize the optical, physical and chemical properties of the COBALD is a light-weight (540 g) instrument which consists of two high-power

LEDs that emit about 250 mW optical power each at wavelengths of 455 and 940 nm, respectively. The backscattered light from the molecules, aerosols, or ice particles is recorded by a silicon photodiode using phase-sensitive detection. The backscatter ratio between aerosols and molecules is inferred from the signal using the radiosonde air pressure and temperature to calculate the molecular backscatter. This ratio is not quantified absolutely, but the analysis of the entire sounding profile imposes physical constraints such that it is confined to an absolute error interval of 5%, while precision along the profile is better than 1% in the UTLS region [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF]. We use the Color Index (CI), the ratio between SR-1 at 940 nm and 455 nm to eliminate ice clouds in the COBALD data. The COBALD data are usually calibrated in the upper stratosphere above 30 km in the quasi-absence of aerosols.

University of Wyoming OPC (Wyo)

The Optical Particle Counter (OPC) flown by the University of Wyoming [START_REF] Deshler | Thirty years of in situ stratospheric aerosol size distribution measurements from Laramie, Wyoming (41N), using balloon-borne instruments[END_REF] provides vertical profiles of size-resolved aerosol concentration at 8 radii between 0.075 and 15 μm. A second OPC with an inlet heated to 180ºC was included to determine the non-volatile fraction of aerosol concentration. In addition, a Condensation Nuclei (CN)

counter was added to the OPCs to measure the total aerosol number concentration [START_REF] Campbell | Condensation nuclei measurements in the midlatitude (1982-2012) and Antarctic (1986-2010) stratosphere between 20 and 35 km[END_REF].

Langley OPC (Lan)

A small balloon-borne OPC payload was developed based upon the adaptation of the MetOne OPC profiler to make aerosol concentration measurement in 8 radii between 0.15 to 5 micron up to the middle stratosphere. Flow and electronic systems are adapted for balloon flight application and data are transmitted through an iMet radiosonde (Vernier et al., 2017, in preparation).

Balloon-borne Aerosol Impactor (BAI)

To assess the composition of UTLS aerosols, a Balloon-borne Aerosol Impactor (BAI) was developed. A ground-based impactor was attached to a scroll pump and controlled by an electronic board based upon meteorological and GPS data from an iMet radiosonde. The pump was switched on and off at the desired pressure to collect aerosols in a given altitude range. We used a 4-stage aerosol impactor from California Measurement Inc. with cutoff radius of 0.025, 0.075, 0.25 and 1 µm at a flow of 7 lpm (liter per minute).

LOAC

LOAC (Light Optical Aerosol Counter) is a miniature OPC [START_REF] Renard | LOAC: a small aerosol optical counter/sizer for ground-based and balloon measurements of the size distribution and nature of atmospheric particles -Part 1: Principle of measurements and instrument evaluation[END_REF]. It contains a laser (650 nm) and measures the intensity of the light scattered at two angles 12° and 60° [START_REF] Lurton | Light scattering at small angles by atmospheric irregular particles: modelling and laboratory measurements[END_REF]. The particle number concentration is provided over 19 size classes from 0.1 to 50 µm in radius at sampling interval of 1 min. Uncertainties are estimated to be of ± 30 % for smaller size bins [START_REF] Renard | LOAC: a small aerosol optical counter/sizer for ground-based and balloon measurements of the size distribution and nature of atmospheric particles -Part 1: Principle of measurements and instrument evaluation[END_REF]. In addition to number concentration, the LOAC gives a specific index relative to the main optical nature of the aerosol sampled, called typology, which is obtained by combining the intensities of light at two scattering angles [START_REF] Renard | LOAC: a small aerosol optical counter/sizer for ground-based and balloon measurements of the size distribution and nature of atmospheric particles -Part 1: Principle of measurements and instrument evaluation[END_REF]. Typology is calibrated in laboratory with response given by known samples. Three LOAC OPCs were launched from Varanasi during BATAL 2015. The first two LOAC instruments flown using MF balloons together with a CFH, a Lan OPC, and an ozonesonde did not reached ATAL altitudes. The third LOAC instrument launched in a LF configuration reached the lower stratosphere. No significant enhancement in the aerosol content was detected in the tropopause region possibly due to the regular losses of telemetry encountered at these altitude levels.

ii) Temperature, pressure, ozone and water vaport measurements.

Ozone and water vapor concentration measurements accompanied aerosol observations on a number of occasions to study the behavior of the ATAL and UTLS chemistry, using an ozonesonde and a Cryogenic Frost Point Hygrometer (CFH), respectively. We used iMet radiosondes onboard most of the balloon flights, and occasionally a Meisei (RS-11G) radiosonde, to transmit data, follow the balloon trajectory, and obtain the landing location. We included SPOT and GSM based locators on most of the flights to determine the exact location of the landing and improve the chance of recovery.

Radiosondes

We used Meisei (RS-11G) and iMet radiosondes. These instruments measure

Ozonesondes

We launched Electrochemical Concentration Cell (ECC) Ozonesondes [START_REF] Komhyr | Electrochemical concentration cell ozonesonde performance evaluation during STOIC 1989[END_REF] to measure the vertical distribution of ozone from the surface to balloon burst altitude.

The measurement is based on electrochemical oxidation of potassium iodide by ozone in aqueous solution. The accuracy of the ECC Ozonesonde is ~5 -10%. More details of the ozonesondes being used at NARL in Gadanki is provided by Akhil Raj et al. (2015).

Cryogenic Frost point Hygrometer (CFH)

The CFH measures water vapor concentration through the chilled-mirror principle using a cryogenic liquid as a coolant. A small mirror attached at the end of the cold finger piece is electrically heated and cryogenically cooled to maintain a constant thin layer of frost that is optically detected. The mirror temperature is then equal to the ambient dew point or frost point temperature and is measured by a small thermistor embedded in the surface of the mirror. The frost-point temperature is used to calculate the partial pressure of water vapor in air (Goff and Gratch, 1946) and determine the water vapour mixing ratio (WVMR) with an uncertainty of about 4% in the lower tropical troposphere to about 10% in the middle stratosphere and tropical tropopause [START_REF] Vömel | Accuracy of tropospheric and stratospheric water vapor measurements by the cryogenic frost point hygrometer: Instrumental details and observations[END_REF]. We used the version 4 of the CALIOP level 1 data which are based upon a new calibration scheme of the nighttime profiles between 36-39 km [START_REF] Vernier | Tropical stratospheric aerosol layer from CALIPSO lidar observations[END_REF]. The SR is calculated from the averaged total attenuated backscatter at 532 nm every 1 degree along each orbit track and corrected from ozone absorption and molecular scattering using air density from the NASA GEOS-5 assimilated dataset. The data have been cleared from clouds when at least 3 of 5 consecutive vertical bins have depolarization ratio values greater than 5 % below 20 km. All data below clouds are removed to avoid cloud attenuation effects.

Rogers et al. (2011) showed that the error at mid-latitudes of the CALIOP data is near 5% compared to aircraft high spectral resolution lidar measurements. We anticipate that the residual error from aerosol contamination in the calibration region of 36-39 km is near 2% in the tropics. Thus, the total relative error on the CALIOP SR is expected to be less than 6%.

The ATAL is seen in Fig. 7 through elevated mean SR near 1.13 +/-0.07 from the Eastern Mediterranean Sea to Western China between 360-440 K potential temperature surfaces. The aerosol layer is thinner near the equator and about 3-4 km thick between 30-40N.

The CALIOP and corresponding COBALD profiles obtained during the BATAL balloon flights are shown in Fig. 8. We use the Color Index (CI), i.e., the ratio between SR-1 at 940 nm and 455 nm, to eliminate ice clouds in the COBALD data. We relax the threshold on the CI to 10 since the value of 7 proposed by [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF] seems to remove clear sky data. After visual inspection, we find that the new filter does not retain sub-visible cirrus clouds.

The satellite and in situ measurements agree within ~10% from 14 and 24 km for the flights at Gadanki and Hyderabad (Fig. 8, left). The averaged profile from Thuwal (Fig. 8 Nevertheless, the ATAL is seen in both balloon and satellite profiles within the coldest region of the atmosphere between 14-19 km. CALIOP observations show the absence of an ATAL during the winter season (Figure 2; [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF]. Throughout the BATAL campaigns in India, SR maxima were often located near the cold point tropopause, suggesting the influence of temperature on the processes involved in aerosol formation and/or growth.

b. Adavanced microphysical properties of the ATAL. During BATAL 2015, we conducted two heavy balloon flights, carrying ~50 kg of instrumentation, telecommunication, and recovery systems from the TIFR balloon facility, Hyderabad (India). These balloons carried two OPCs of the University of Wyoming, one with an ambient inlet and another with an inlet heated to temperature > 180ºC to derive the aerosol volatility fraction. In addition, a Condensation Nuclei (CN) counter from the University of Wyoming was included to derive the concentration of particles with radii greater than 10 nm.

Figure 9 shows data from the balloon flight on August 13 rd . We found a relatively high concentration of aerosol (up to ~25 part/cm 3 ) near the tropopause (16-19 km) with r > 0.075 μm, comparable to those observed in the boundary layer. Particle number concentration for r > 0.15 and r > 0.25 μm is smaller by a factor of 30 and 300, respectively. The volatility fraction, defined for a particular size as the difference between the unheated and heated number concentration devided by the unheated concentration, shows that 80-95 % of these aerosols have a volatile component, thus likely partially or totally liquid in the 16-18 km altitude range. The volatile fraction of aerosol at 0.075 μm is shown in Figure 9B. The shapes of the total/non-volatile aerosol concentration profiles are very similar with a peak near the cold point tropopause (Fig. 9B), suggesting that aerosols with a non-evaporating, and thus likely solid, core had been transported at those levels. The CN measurements were saturated throughout the troposphere and decreased sharply near the tropopause (Fig. 9A). High aerosol concentrations near the tropopause coincide with a peak in SR (~17 km) of 1.9 associated with low Color Index (CI<10) values measured by COBALD in contrast with ice clouds showing CI near 20 (15-16.5km) (Fig. 9D). c. Ozone and water vapor behavior near the ATAL.

In addition to the aerosol measurements described above, we also flew an ozonesonde and a CFH for ozone and water vapor measurements, respectively. Interestingly, enhanced water vapor (up to 7-8ppmv) near 17-18 km were observed just above the cold point tropopause and the peak in aerosol concentration (Fig. 9C and9A). The ozone partial pressure profile shown in Fig. 9C displays a sharp increase from 0 to 2 mPa between 17 and 18 km.

The presence of a moist layer near the tropopause strongly suggests the influence of convection up to these levels. Deep convection is also known to inject tropospheric air with low ozone concentrations which could explain the very low ozone values near the cold point tropopause. In addition, heterogenous ozone chemistry in the lower stratosphere could also result in ozone depletion through chlorine activation on aerosol [START_REF] Solomon | Monsoon circulations and tropical heterogeneous chlorine chemistry in the stratosphere[END_REF]. Thus, the behavior of ozone, water vapor and aerosol can be strongly inter-related via the influence of deep convection.

d. Composition of the ATAL

In order to study the composition of the ATAL, we developed a Balloon-borne Aerosol Impactor (BAI, section IIb) to collect size-segregated aerosols on filters near the tropopause and analyze their composition through Ion Chromatography (IC). The BAI flew on 3 ZF flights on August 8 th ,15 th and 21 st at TIFR. The pump, connected to a 4-stage impactor, was switched on automatically below 150 hPa (~14 km) and above 70 hpa (~20 km). In order to collect sufficient particles on filters, we maximized the floating duration of the balloons near the tropopause using the ZF concept (see section II). During those flights, the pump was on for 1h 20min on ZF1, 1h 47min on ZF2 and 2h 47min on ZF3. The volume of air passing through the impactor was controlled and calculated with a mass flow meter. A total volume of 566 L and 917 L of air was sampled during ZF2 and ZF3, respectively. In addition, the BAI was run on the roof top of TIFR balloon facility on August 23 rd for 45 min to provide a comparison basis for the flight measurements. After 3 succesfull ZF flights, the BAI was recovered in good condition and brought back to TIFR. The filters were loaded and unloaded from the BAI in a clean room of class 7 and refrigerated at -24 o C. The filters were transported from TIFR to the Physical Research Laboratory in Ahmedabad by flight using a cooler with dry ice to maintain temperature at freezing levels. Figure 10 shows the concentration of ions measured through IC at the Physical Research Laboratory in Ahmedabad. We note the presence of nitrate ions with a concentration about 100 ng/m 3 on ZF2 and ZF3, nearly 6 times smaller than ground levels. Traceable amount of Calcium ions were also observed on both ZF2 and ZF3 filters. We note the absence of sulfate anions on both ZFs as opposed to the high concentration of 950 ng/m 3 observed on the ground. These preliminary results are consistent with nitrate being an important component of ATAL, as suggested by the model results of [START_REF] Gu | Summertime nitrate aerosol in the upper troposphere and lower stratosphere over the Tibetan Plateau and the South Asian summer monsoon region[END_REF], with the additional presence of some mineral dust aerosol.

How does convection affect the ATAL ?

a. Convective influence on the measurements. We assess the influence of convection on the BATAL measurements using a combined approach with trajectory calculations and cloud top brightness temperature observations from the HIMAWARI-8 geostationary satellite used as a proxy for deep convection. We used an overshooting cloud and anvil detection algorithm described by [START_REF] Bedka | A Probabilistic Multispectral Pattern Recognition Method for Detection of Overshooting Cloud Tops Using Passive Satellite Imager Observations[END_REF] to find when and where measurements were likely affected by convection based on backtrajectories initialized at the location of balloon measurements (Figure 11). Figure 12 (bottom) shows back-trajectories colored according to their respective altitudes and initialized from the two Heavy balloon Flights (HF) from Hyderabad (08/08/15 and 08/13/15) between 14-18 km (Fig. 11). We show only the trajectories influenced by deep convection during the 72 hours preceeding the balloon measurements. The position of the convective systems are shown with filled red circles. Air masses from the upper troposphere (~14km) tend to follow trajectories within the inner core of the Asian Anticyclone, while those at higher altitudes follow the edge. The flight on August 8 th was likely influenced by regional convection (within 10h of the flights) over the Bay of Bengal and along the Indian central and east shore.

Brightness temperatures indicate cloud tops reaching the upper troposphere but probably not penetrating the tropopause. Aerosol concentration (Fig. 11), backscatter and ozone concentration (not shown) increases near 14-16 km may indicate the convective transport of regional Indian pollution and its influence on these measurements. The peak in ozone (not shown) could have been produced via photochemical production due to anthropogenic or lightning-induced NOx emissions.

The peak in aerosol concentration and water vapor (Fig. 9C) near the cold point tropopause observed on August 13 th could have originated from earlier convection over the Bay of Bengal, Thailand, Laos and South China where trajectories ~17 km appear to intersect deep convective systems at these altitudes. However, identifying a specific convective system influencing the measurements is complicated since convection frequently reaches the tropopause during the ASM. It is easier to identify recent and regional MCSs affecting the measurements due to the better accuracy of shorter trajectory calculations. During the 2016 BATAL campaign, we had a balloon flight in the vicinity of a deep convective system, which will be discussed in the next section. b. Overshoot of ice particles in the lower stratosphere.

We conducted a balloon flight on August, 12 th 2016 during an active monsoon period using the Boomerang controlled system (see section II). The time series of temperature, altitude, ascent rate (colored) and COBALD scattering ratios at 455 nm and 940 nm are shown in Fig 13 . An abrupt decrease in the ascent rate from 6 m/s to 3 m/s near 14 km, 23 min after the launch, corresponds to the release of the secondary balloon. The valve of Boomerang system was activated several times to reduce the ascent rate near 1-2 m/s and to maintain a very slow ascent through the UTLS region. After crossing the cold point tropopause (at 52 min) near 16.5 km (-80°C), the system reached the lower stratosphere and sampled layers of elevated SR near 18.5-19 km. COBALD color index values above 10 associated with these stratospheric features indicate the likely presence of ice crystals. We computed backtrajectories from the location of the balloon flight to determine if convection could be associated with these layers. Figure 14a (left) shows a Himawari-8 10.4 micron cloud top brightness temperature (BT) image across southeast Asia overlaid with colored squares corresponding to the position and altitude of back-trajectory air parcels sampled by the balloon ~36 hours later. Air parcels above 17.5 km intersected a Mesoscale Convective System (MCS) over southeastern China at that time. A zoomed image of the MCS (Fig. 14b)

shows the presence of Overshooting cloud Tops (OT, black arrows) and Above-Anvil Cirrus Plumes (AACP, white outlines). OTs were persistently generated by this MCS throughout its lifetime, reaching 189 K BT, 4 K colder than the cold-point tropopause observed by the nearby 12 Z sounding from Kunming, China. At the time of this image, AACPs, indicative of injection of ice into the lower stratosphere [START_REF] Setvak | Satellite-observed cold-ring-shaped features atop deep convective clouds[END_REF]Homeyer et al. 2017), were being generated by OTs that intersected with the back-trajectories. The AACPs radiate at the lower stratospheric temperature (~200 K, 18.5-19 km altitude according to the Kunming sounding), which is 10+ K warmer than the OT BT. OT regions to the south had also generated AACP earlier in the MCS lifetime. Though a 36-hour back trajectory has some error, given that this MCS had repeatedly generated AACP throughout its north-south extent, this MCS is a strong candidate to explain the presence of ice layers in the lower stratosphere observed by the balloon. Other convective storm systems over India during the balloon campaigns have also shown the signature of convection overshooting the cold point tropopause and reaching the lower stratosphere, providing strong evidence that deep convection during the Summer Asian Monsoon can influence the lower stratospheric composition.

IV.

Discussion.

a. ATAL and New Particle Formation

Balloon measurements made from the TIFR balloon facility,Hyderabad during BATAL strongly suggest that the ATAL is composed primarily of small (r < 0.25 μm), liquid (mostly volatile, ~80-95%) aerosol with or without solid cores, correlated with enhanced water vapor and virtually no ozone. Moist layers observed near and above the cold point tropopause suggest the influence of deep convection, which could also be responsible for transporting aerosol and/or their gas-phase precursors. Up to 100 pptv of SO2 were measured in the Eastern part of the Asian Anticyclone during the Oxidation Mechanism Observation campaign (H. Schlager, personal communication with D. Fairlie), suggesting that SO2, as a precursor of sulfate aerosol, can survive convective storms and reach the upper troposphere.

We performed numerical simulations using the microphysical model M7 [START_REF] Vignati | M7: An efficient size-resolved aerosol microphysics module for large-scale aerosol transport models[END_REF]. The aerosol dynamic processes in M7 include nucleation, coagulation, and condensation of sulfuric acid. Aerosol physical properties (size distributions and concentrations) were calculated along back-trajectories intialized at the balloon locations and using GEOS-5 meteorological fields. ~100 ppt of SO2 was injected at the beginning and along the trajectories when air parcels were influenced by deep convection (see section III.d).

Consistent with previous studies [START_REF] Weigel | In situ observations of new particle formation in the tropical upper troposphere: the role of clouds and the nucleation mechanism[END_REF][START_REF] Crumeyrolle | New particle formation events measured on board the ATR-42 aircraft during the EUCAARI campaign[END_REF], sunlight, low temperatures as well as low pressure are favorable to new particle formation. The fast vertical transport, in deep convective storms, of gas-phase aerosol precursors from the surface up to 17 km is thus highly favorable to nucleation events. Within a few days (1.5 days on average), aerosols grow by condensation and coagulation processes from a few nm to 100 nm. The extinction coefficients, calculated using a Mie code with a refrative index of 1.521 (representative of ammonium sulfate ; Seinfeld and Pandis, 2006), ranges from 0.2.10 -3 to 2.4.10 -3 km -1 consistent with CALIOP observations. Moreover, SO2 injected in the model along the transport to simulate the influence of deep convection is found to condensate on existing particles, enhancing the extinction coefficients by a factor of 1.5.

b. Cloud-aerosol relation in the UTLS.

Thin cirrus clouds have been observed very frequently near the cold point tropopause during the BATAL campaigns. Coincident OPC/ COBALD measurements can be used to investigate ATAL's interaction with clouds. Figure 11 shows the aerosol concentration profiles for particles of radius between 0.075 µm and 0.15 µm measured during the two heavy flights carried out from Hyderabad in 2015. Part of the profiles are colored in red when the CI from COBALD is greater than 10 and indicate the presence of large ice particles also observed through the larger channels of the Wyoming OPC (not shown). The concentration of aerosol is very similar on both flights at altitudes above ~18 km, 1 km above the cold point tropopause (see arrows). We observe a sharp decrease of aerosol concentration from 10 part/cm 3 to 2 part/cm 3 between 18 and 20 km. We observe a strong variability within the two profiles below 18 km with a factor of ~25 in aerosol concentration between the peaks near 17 km (08/13) and 15 km (08/08) compared to the minimum values near 16 km on both flights.

CN measurements during those two flights (not shown) were saturated throughout the troposphere. However, these profiles converge (toward ~20 part/cm 3 ) near 18-18.2 km like those shown in Fig. 10.

We also note that the aerosol minimum observed on 08/13 between 15-16 km is associated with the presence of an ice cloud (red) layer as observed by COBALD. In addition, the apparent drop in aerosol concentration near 17 km on 08/08 also coincides with a thin cirrus cloud near the cold point tropopause. The presence of ice clouds associated with reduction or minimum in aerosol concentration could be consistent with aerosol removal processes such as in-cloud scavenging of aerosol droplets. In contrast, aerosol concentration peaks suggest the presence of a source mechanism. Figure 9C shows an enhancement of water vapor mixing ratio near 17-18 km coincident with the peak in aerosol concentration on August 13 th , also implying that the air masses at these levels were likely influenced by deep convection. Thus, convectively influenced air masses appear to be a source of aerosols up to 1km above the cold point tropopause. Overall, these observations suggest that the aerosol budget is balanced by source and removal mechanisms associated with convection up to 18 km, ~1km above the cold point tropopause in the monsoon region.

Conclusion and perspective.

We provide an overview of the balloon-borne activities from India and Saudi-Arabia The first offline chemical analysis of the ATAL reveals interesting but puzzling results. The presence of nitrate aerosol can be possibly explained by the gas-to-aerosol conversion of HNO3 from NOx produced through lightning or transported by convection from the boundary Layer [START_REF] Gu | Summertime nitrate aerosol in the upper troposphere and lower stratosphere over the Tibetan Plateau and the South Asian summer monsoon region[END_REF]. However, the absence of sulfate aerosol seems to be contradictory with the expectation of increasing influence of sulfur emissions in Asia over the past few decades on aerosol trends in the UTLS [START_REF] Vernier | Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution[END_REF].

Additional observations are needed to address these puzzling questions. NOx/NOy measurements would help constrain the role of gas-to-aerosol conversion in the formation of nitrate aerosol. Additional pollution tracers, such as CO, would help to determine the relative importance of in situ lightning production of NOx versus convective transport from polluted regions to the abundance of nitrate aerosol. In addition, online aerosol mass spectroscopy would help understanding whether volatile aerosols (e.g. NH4), which would disappear during offline analysis, could be present in the ATAL. A suite of aerosol and ice measurements would contribute to an improved understanding of aerosol-cloud interactions in the UTLS region. In order to study the impact of the ATAL on heterogeneous ozone chemistry, ClO and ClONO2 would also need to be measured. The BATAL project has been approved to continue for another 3-4 years, and NASA and ISRO aim to augment their level of collaboration on this program. Additional balloon payloads will be considered to help address the remaining science questions regarding the composition of the ATAL and its interaction with convection and cirrus clouds. Additionally, the results of the BATAL campaigns will be used to formulate a future NASA-ISRO airborne campaign with NASA high altitude aircraft. red lines on the cross-section. [1.Gadanki,2. Hyderabad,3. Thuwal,4. Varanasi].

I.

  What are the optical properties, size distribution and composition of the ATAL ? II. How do ozone and water vapor behave near the ATAL? III. How does convection affect the ATAL II. Description of the BATAL campaigns. The BATAL campaigns have comprised a total of ~68 balloon flights between 2014-2017 from India and Saudi Arabia, with payload weights ranging from 1.5 kg to 50 kg. The effort has involved a number of institutes from the United States (Science Systems and Applications, Inc., NASA Langley Research Center, and University of Wyoming), France (LPC2E/CNRS/University of Orléans), Switzerland (Swiss Federal Institute of Tech.), Saudi-Arabia (King Abdullah University of Science and Technology: KAUST), and India (National Atmospheric Research Laboratory: NARL, Tata Institute of Fundamental Research: TIFR, Banaras Hindu University: BHU, Physical Research Laboratory: PRL). Figure 2 shows the position of the launch sites (Gadanki, Hyderabad, Varanasi and Thuwal) and the trajectories of the balloon flights conducted during the campaigns. Most of the time, the balloons drifted westward in strong easterly winds of up to 40 m/s near the cold point tropopause. Most of the payloads launched from India were recovered after landing within 300 km from the launch site while most of the balloon flights from Thuwal (Saudi Arabia) landed in the Red Sea. a. Ballooning during BATAL.

  HF: The characterization of the optical and physical properties of the ATAL is addressed with aerosol payloads onboard Heavy balloon Flights (HFs) with total payload weight greater than 30 kg using 3000 m 3 polyethylene balloons. HFs took place from the TIFR balloon facility,Hyderabad (India), in August 2015 and 2017. The balloon facility manufactured the balloons and provided the logistical support for the launches and the recovery of the payloads. During those flights, we also studied the behavior of the ozone and water vapor near the ATAL. -ZF: The chemical composition of the ATAL has been studied using an aerosol impactor onboard balloon flights floating near the tropopause for several hours, called Zeropressure Flights (ZF). The float duration was critical to collect a sufficient concentration of aerosols on the filters mounted on the aerosol impactor. Filters were sent post-flight for Ion Chromatrography (IC) analysis within 12 hours after launch. 4 ZF flights were conducted from TIFR since 2015. The time evolution of the GPS altitude of those flights is shown in Fig. 5. The first flight (ZF0) in 2016 did not have a ballast module and the floating duration between 14 and 18 km was only 50 min. In 2017, we conducted three additional balloon flights and each one of them included ballast module. The maximum floating duration above 14 km of 2h 50 min was achieved during ZF3.

  shows a sketch of the Static Launch Method (SLM) used for the ZFs at the balloon facility of TIFR. During the launch preparation, the plastic balloon is fully deployed and the top portion of it is blocked under a small aluminum roller mounted on the of top a trolley for initial balloon inflation. The bottom part of the balloon is connected to the flight train with the parachute, the communication device and the science payloads. The ballast module, the last element of the flight train, is hooked to a vehicle. Tow balloons (launch balloons of 20 kg lift capacity) are tied to the communication payload to lift the load line into the air. During the inflation of balloon, adequate tension is taken by moving the vehicle forward or backward. During the launch, the balloon is released first from the roller until the load line becomes vertical. At that time, the anchor nylon rope between the ballast module and vehicle is cut. Immediately after releasing the payload, a telecommand is given to separate the tow balloons.

  temperature from -90 o C to 50 o C with resolution of 0.1 o C and accuracy of 0.5 o C. Wind speed resolution is 0.1m/s with an accuracy of 0.2 m/s. Further details of this radiosonde system, data base and quality checks applied to the data are reported by Venkat Ratnam et al. (2014).

1 .

 1 Figure6shows maps of carbon monoxide (CO), sulfate, nitrate, and black carbon (BC) concentrations at 100 hPa for 21 UTC, August 3, 2017, as predicted by NASA GEOS-5 on July 30, 2017 at 00 UTC. High levels of CO in the upper troposphere forecasted over Gadanki during BATAL 2017 likely resulted from the convective transport of polluted air from eastern China followed by westward transport along the southern edge of the Asian Anticyclone. Sulfate and black carbon aerosol layers were also approaching South India at that time. We used forecasts to make the necessary logistical arrangements and plan balloon flights ahead of time. Characterization of the optical, physical and chemical properties of the ATAL. a. Backscatter measurements from CALIOP and COBALD.One of the main science objectives of the BATAL campaigns was to characterize the optical properties of the ATAL at multiple locations within the Asian Anticyclone through backscatter measurements using the COBALD backscatter sonde and to compare those measurements with the Cloud-Aerosol LIdar and Orthogonal Polarization (CALIOP) onboard CALIPSO. Three locations were selected in India to study the aerosol gradient from the southern edge (Gadanki) to the interior (Varanasi) of the Asian Anticyclone. In addition, we also chose one site in Saudi Arabia (Thuwal) to sample the outflow from the Indian monsoon through the westward circulation prevailing in the UTLS. Figure7shows a map of Scattering Ratio (SR) at 532 nm, optically equivalent to an aerosol mixing ratio, averaged from CALIOP observations acquired between July and August 2015. Corresponding cross-sections averaged between 15-45N latitudes (bottom) and 5-105E longitudes (right) illustrate the 3-D distribution of the UTLS aerosols.

  , right) exhibits a larger difference compared to CALIOP with an apparent offset of 0.3-0.4. The retrievals of COBALD and CALIOP SR profiles are based upon independent calibration procedures which are usually applied in the upper stratosphere. However, since the bursting altitude of a balloon flight is usually below 34 km, COBALD profiles are referenced below this level while CALIOP backscatter profiles are calibrated between 36 and 39 km. The COBALD profiles are artificially compensated for the presence of aerosol in the calibration region by multiplying by a correction factor. Differences in the calibration procedure are therefore the likely cause of the differences observed at Thuwal between COBALD and CALIOP.

  between 2014-2017 during the BATAL campaigns. With 68 balloon flights and payloads dedicated to study UTLS aerosols, together with water vapor and ozone sensors, we show the first set of advanced in-situ measurements of the ATAL with COBALD backscatter sonde, OPC size distributions and offline chemical analysis. We found that the ATAL is comprised of mostly small (r < 0.25 μm), liquid (~ 80-95%) aerosols with relatively low SR at 532nm (SR < 1.2), consistent with CALIOP satellite observations. Chemical analysis of particles collected near the tropopause indicates the dominant presence of nitrate aerosol with concentration of about 100 ng/m 3 . Balloon and satellite observations consistently showed a drop in aerosol concentration (CN, r > 75 nm (total), r > 75nm (non-volatile aerosol)), and backscatter near 18 km (~400-420 K), ~1km above the cold point tropopause, and 1-1.5 km above the 380 K isentropic level. In addition, enhanced water vapor and ice particles have been observed during the BATAL campaigns at and above the cold point tropopause. These indicate that convection can potentially influence aerosol loadings well above the tropopause level.

  Figure captions.

Figure 1 . ( 1 )

 11 Figure 1. (1) Deep convection from the Asian Summer Monsoon transports air masses from

Figure 3 .

 3 Figure 3. Ballooning during BATAL. Preparation of: (1) Heavy balloon Flights (HF) from

Figure 4 .Figure 5 .

 45 Figure 4. Sketch of the Static Launch Method used during the zero-pressure flights at the

Figure 6 .

 6 Figure 6. Concentrations of carbon monoxide (CO), sulfate, nitrate, and black carbon (BC)

Figure 7 .

 7 Figure 7. Map of Scattering Ratio (SR) at 532 nm after averaging the CALIPSO cloud-

Figure 8 .

 8 Figure 8. SR profiles (black stars) at 532 nm derived from the COBALD balloon flights in

Figure 9 .

 9 Figure 9. Heavy balloon flight ascent profile of data collected on August 13 th , 2015 from

Figure 10 .

 10 Figure 10. Cations and anions concentration obtained from Ion Chromatography analysis to

Figure 11 .

 11 Figure 11. Aerosol concentration profiles for particles within 0.075 µm < r < 0.15 µm from

Figure 12 .

 12 Figure 12. Back trajectories initialized from the balloon flights on 08/13/2015 (top) and

Figure 13 .

 13 Figure 13. (Top) Altitude, temperature, scattering ratios at 455nm and 940 nm and Color

Figure 14 . 1 .

 141 Figure 14. (a) A Himawari-8 10.4 micron brightness temperature image at 1400 UTC on 11

Figure 1 . ( 1 )

 11 Figure 1. (1) Deep convection from the Asian Summer Monsoon transports air masses from

Figure 2 .

 2 Figure 2. Balloon trajectories from Gadanki [13.46 o N, 79.17 o E], Hyderabad [17.47 o N, 78.58 o E], Varanasi [25.27 o N, 82.99 o E] and Thuwal [22.32 o N, 39.10 o E] during the BATAL

Figure 4 .Figure 5 .

 45 Figure 4. Sketch of the Static Launch Method used during the zero-pressure flights at the

Figure 8 .

 8 Figure 8. SR profiles (black stars) at 532 nm derived from the COBALD balloon flights in

  the influence of convection on the balloon measurements. All ground-based support during the BATAL campaigns provided quasi-collocated observations with the balloon-borne measurements to better characterize the vertical structure and properties of aerosols and clouds. //gmao.gsfc.nasa.gov/weather_prediction/). The forecasted fields used for the BATAL campaign included U and V winds, water vapor, cloud fraction, ozone, carbon monoxide (CO), sulfur dioxide (SO2), column aerosol optical depths, and aerosol coefficients.

	(https:
	ii. Modelling support.
	Throughout the BATAL campaign in India weather forecasts
	(http://forecast.narl.gov.in/weather) provided by NARL and near-real time Doppler weather
	radar images obtained from Indian Meteorological Department
	simulations have been carried out with the MOZCART gas-phase chemistry (Emmons et al.,
	2010) and GOCART aerosols (Chin, 2002).
	The GEOS "Forward Processing" (FP) system generate weather analysis, assimilation
	products,	and	10-day weather	and composition	forecasts

d. Ground-based and modelling support. i. Ground-based support Throughout the campaign at the National Atmospheric Research Laboratory (Gadanki) and at the King Abdulah University for Science and Technology (Thuwal), observations were available from additional instrumentations, listed in Table 4. These include ground-based lidar observations which can be compared with COBALD in-situ backscatter measurements from the boundary-layer to the middle stratosphere and can be used to validate CALIPSO satellite aerosol retrievals. A current project at KAUST uses COBALD/AERONET and MPL measurements to help characterize the vertical and temporal structures of dust in the Boundary Layer. Ground-based lidar measurements from Gadanki are used to characterize the ATAL and sub-visible cirrus clouds near the tropopause. In addition, the MST radar from Gadanki can help assess (http://www.imd.gov.in/section/dwr/dynamic/dwr.htm) supported flight planning for balloon launches. Starting with the 2017 campaign, regional and global chemical transport simulations have been carried out using Weather Research and Forecasting (WRF) model coupled with chemistry (WRF-Chem), version 3.6.1, and the NASA Goddard Earth Observing System (GEOS), version 5, (https://gmao.gsfc.nasa.gov/forecasts/) with chemical and aerosol forecasts over South Asia.

WRF-Chem was run with a horizontal resolution of 27 km and 30 vertical levels from the ground to 50 hPa. Meteorological initialization and boundary conditions were derived from the NCEP-GFS data available every six hours. Anthropogenic emissions have been obtained from HTAP-EDGAR

(Janssens-Maenhout et al., 2015)

that contains the annual anthropogenic emissions of greenhouse gases and air pollutants at 0.1x0.1 degree spatial resolution. The

Acknowledgement

We thank Drs. J. Kaye and K. Jucks at NASA HQ for supporting the BATAL campaigns since 2014 through the UARP program. We acknowledge technicians and engineers from NARL, TIFR, BHU and KAUST for their help during BATAL. We also thank the Indian Meteorological Department for providing Doppler weather radar images and MOSDAC for providing INSAT-3D satellite images. We acknowledge Timothy Marvel for his help to make Fig.1. We also thank Jacquelyn Charity-Hodnett and Kishore Kumar for their logistical supports. LABEX VOLTAIRE "Étude des géofluides et des VOLatils-Terre, Atmosphère et Interfaces-Ressources et Environnement" (VOLTAIRE) (ANR-10-LABX-100-01) managed by the University of Orléans. The GEOS-5 data used in this study have been provided by the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center through the online data portal in the NASA Center for Climate Simulation.

Figure 14. (a) A Himawari-8 10.4 micron brightness temperature image at 1400 UTC on 11 August 2016 over Southeast Asia. Back-trajectory air parcels at the time of the image, initialized from balloon flight position launched from Varanasi (India) 32 hours after, colored by their altitude, are overlaid. (b) A zoomed image at 1410 UTC over the region outlined by the red box in panel (a), contrast enhanced to more clearly show overshooting cloud tops (black arrows) and above-anvil cirrus plumes (white outline), embedded within the anvil of this mesoscale convective system. The locations of back-trajectory parcels above 17.5 km from 11-17 UTC are overlaid with white circles, illustrating the uncertainty in parcel position at the time of the satellite image.

red lines on the cross-section. [1.Gadanki,2. Hyderabad,3. Thuwal,4. Varanasi].