Anni Määttänen
email: anni.maattanen@latmos.ipsl.fr

Constantino Listowski

LATMOS Franck Montmessin

L Maltagliati

L Joly

Aurélie Reberac

Jean-Loup Bertaux

J.-L Bertaux

SPICAM Climatology of Aerosol Vertical Distribution through UV Occultations

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Introduction

We will present observations of aerosol profiles derived from MEx/SPICAM solar occultations in the ultraviolet ( [START_REF] Määttänen | A complete climatology of the aerosol vertical distribution on Mars from MEx/SPICAM UV solar occultations[END_REF]) completed with UV stellar occultations. This dataset allows, as described by [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF] for stellar occultations, the retrieval of vertical profiles of the abundance of gaseous species (CO2, O3), aerosol optical depth in the UV, and the Ångström coefficient, which can be, through some hypotheses, related to particle size. The first solar occultation results were presented by [START_REF] Listowski | Solar Occultation with SPICAM/UV onboard Mars Express: Retrieving Aerosol and Ozone Profiles[END_REF]. Since then, the SPICAM technical team has provided new corrections for the data, giving access to previously unused orbits, and new geometry files. A data analysis code, including a correction for mechanical oscillations induced by the MARSIS radar antennae, has been developed and used in the analysis. We have analysed all solar occultations of adequate quality between orbit numbers 0-10000 (Mars Years 27-30) spanning 4 MY with a good seasonal and spatial coverage. The results reveal the dust haze near the surface, and detached layers (possibly clouds) at high altitudes. We will present a global overview of the results and some specific cases of interest from the several hundreds of occultations analysed, and complete the solar occultation dataset with a comparison to the UV stellar occultations.

The abstract is based on [START_REF] Määttänen | A complete climatology of the aerosol vertical distribution on Mars from MEx/SPICAM UV solar occultations[END_REF] where the solar occultation results have been published.

Dataset

SPICAM (SPectroscopie pour l'Investigation des Caractéristiques Atmosphériques de Mars) is observing Mars in the ultraviolet (118-320 nm) and in the near infrared (1-1.7 µm) ranges. [START_REF] Bertaux | SPICAM on Mars Express: Observing modes and overview of UV spectrometer data and scientific results[END_REF] describes the instrument and the observation techniques in detail. In the occultation mode, the UV channel allows to infer the vertical distribution of CO2, O3, and aerosols. We are focusing here on the UV solar occultation dataset of about 650 profiles, and we are also including over 800 UV stellar occultations mainly from MY27-28 (with some observations in MY29-30) to achieve a better overall coverage in latitude and Ls. We also wish to compare the results of the two datasets that mainly differ only by the local time of observation acquisition (solar occultations at the terminator, stellar occultations during the night). The calculated transmission spectra are fitted with the Beer-Lambert law, which takes into account extinction by gaseous species and aerosols. Aerosol extinction is modeled with the so-called alpha-model, providing access to the Ångström coefficient, which depends on the size of the aerosols. However, we can not determine the composition of the particles through the observations, which can make the size determination dependent on the assumed composition. Inversely, for very large Ångström coefficients only ice composition is possible, but again there is no way to discriminate between H2O and CO2 ices.

Hazetops

The so-called hazetop (the altitude where the slant opacity drops to τ = 1 from the higher near-surface values) gives information about the vertical extent of the lowlying, well-mixed continuum of aerosols (haze) that prevails near the surface of Mars. This haze should be composed of dust outside the polar regions: the polar hoods are formed of fairly low-lying clouds. The source of the dust haze is lifting from the surface and mixing through atmospheric convection. The hazetop can thus give some information on the atmospheric activity and its variations.

Figure 1 shows the variations of the hazetop as a function of latitude and season. In this figure all the observations are mixed regardless of the year of acquisition. The stellar occultations included in the figure cover a larger (unpublished) dataset than just those analyzed by [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF].

In general, higher hazetop values are observed at warmer seasons and regions, i.e., near the equator, and in the southern hemisphere during southern spring and summer (Ls = 180-360 • ). Here convection acts more efficiently and the general circulation is stronger: aerosols can be lifted to and maintained at higher altitudes. This happens in particular around the perihelion (Ls = 220-300 • ) where dust storms occur more often ( [START_REF] Smith | Interannual variability in TES atmospheric observations in Mars during 1999-2003[END_REF]). Seasonal temperature changes modulate the cloud formation level as well. Consequently cloud formation regulates the dust haze extent through scavenging of dust particles and sedimentation to the lower atmosphere.

Detached layers

In almost half (48%) of the analyzed solar occultation profiles, and in more than two thirds (70%) of the stellar occultation profiles, the slant optical depth has a clear local maximum that we can identify as an aerosol layer (including multiple layers, 13% of observed layers in solar occultations). These layers are observed at all latitudes and seasons throughout the analysed dataset (Figure 2). In most cases, these maxima are found at high altitudes above the near-surface haze, but clear layer structures are as well found within the near-surface haze. It should be kept in mind that the slant optical depth includes information from several altitudes along the line of sight. Thus certain effects may be due to the geometry of the observations. For example, a layer observed within the haze, may be a result of the projection of a layer in the fore or background of the local vertical. Our discussion here concentrates mainly on the more frequent cases of layers above the haze (i.e., high-altitude detached layers). A systematic investigation was performed to detect these clearly-defined atmospheric structures. A detached layer may consist of ice condensates or it might be a dust plume. However, in the occultation mode, we cannot spectrally distinguish between these cases. In contrary to what [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF] found, such structures are not limited to mid-southern latitudes in our data.

Seasonal behavior of hazetop and detached layers

Some interesting phenomena were observed when following the seasonal evolution of the hazetop and the detached layers. Around the northern autumn equinox in MY27 the hazetop altitudes change nearly simultaneously, but in the opposite directions. In the north, the haze remains confined below 40 km after the change, whereas in the south the hazetop altitude increases and high-altitude detached layers are observed very frequently. This might be the first indications of the starting dust storm season in the south. The northern occultations scan the polar regions, showing most probably the development of the polar hood. Here the slant opacities are quite high in the lower atmosphere, and the transition from aerosol-loaded to aerosol-free atmosphere is very sharp.

Observations at the southern spring/summer (Ls = 200-290 • ) were acquired during two campaigns, at MY 28 and MY29. They include the global dust storm of MY28, and we also should be seeing the early ("preseason") dust activity in MY29 pointed out by [START_REF] Smith | THEMIS observations of Mars aerosol optical depth from 2002-2008[END_REF]. We can observe seasonal variations of the hazetop in the southern summer through the southern hemisphere parts of the campaigns (Ls = 170-270 • ) that reveal an increase in hazetop altitudes in the southern midlatitudes as the local spring advances and the circulation becomes more vigorous on average. There is an abrupt increase in the hazetop altitude at around Ls = 240 • . This can also be seen in the detached layers. High-altitude layers are clearly more abundant at this season, with the highest layers of our whole dataset observed during the dust storm.

In the early winter in the southern hemisphere, we have observed a distinct, quite constant, but thin aerosol layer at around 40 km with τaer <0.1 in about 10 consequent profiles (Ls = 43-53 • ). This layer is detected only in occultations made directly south of the Hellas basin (70-72 • S). This layer stands out from the data since the majority of the detached layers in this season are found in the northern leg of the campaign, and since the shape of the profile and its altitude are very constant. In all the profiles displaying this persistent layer, the Ångström coefficients in the layers are greater than 2.6, indicating that only ice crystals can explain the wavelength dependence of the opacity. The effective radii of the crystals in the layer are very small, of the order of 100 nm or slightly less, and the smallest crystals are found in range of 50nm. We believe this cloud is related to the forming south polar hood.

Interannual variations

The seasonal variation of hazetop altitude (for all occultations regardless of latitude, longitude, or Mars Year) can be seen in Fig. 3. The figure reproduces a similar figure of [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF], and includes their data and the additional stellar occultation data analyzed later. Fig. 3 the seasonality in the hazetop altitude, shown already by [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF], but with our more extensive dataset the increase in hazetops during the second half of the year is less clear than in [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF]: Here we see much larger variability during the southern spring/summer. The most prominent feature of the plot is the minimum at around Ls=90 • . Our observations include the dust storm of MY28 providing the large hazetop altitude variations and large hazetop values in the southern summer. Compared to stellar occultations, the solar occultations span a wider range of latitudes during a given season, inducing more scatter for a given Ls.

We can now discuss further the inter-annual variability with the help of Figure 1. Two solar occultation observation campaigns were acquired in the southern hemisphere near 65 • S during the same season (Ls = 34-110 • ), but they account respectively for MY27 and MY29. They display similar average hazetop values in this region and season. Alrso the MY27 stellar occultations show similar values. It seems that the hazetop follows a quite undisturbed seasonal cycle at this period at mid-to-high southern latitudes since we discover almost the same hazetop altitude two MY apart, and with a global dust storm in between. For these same solar occultation campaigns, the mean layer altitude of the detached layers in the southern hemisphere shows a very small 4 km difference. However, the layers in MY29 are almost twice as thick (in vertical extent, 11 km) as those of MY27 (6 km), and show higher opacities with respect to the background. This might be the effect of the global dust storm of MY28. The storm has charged the atmosphere full of condensation nuclei, which have had a long-term effect on the microphysical properties of clouds without changing their formation altitude.

The most significant source of inter-annual variability in the solar occultation data is the global dust storm of MY28. We have observations on two campaigns for the southern spring/summer season. If we investigate separately MY28 and MY27, we can mention that the mean hazetop altitude observed only slightly later in season in MY27 is 23 km lower in the southern and 14 km lower in the northern hemisphere than during the dust storm of MY28. This reveals the remarkable vertical extent of the dust advected by the storm. Also the detached layer average altitudes of MY27 are clearly lower (59-61 km), compared to MY28 (77-81 km). Occultations can not be used to define the composition of these layers, but probably the atmosphere is heated strongly by the suspended dust causing the condensation level of water (and consequently the cloud formation level) to be found at higher altitudes.

Ångström coefficients

The alpha-model used to fit the wavelength dependence of the aerosol extinction gives access to the particle size. However, several assumptions are required before the conversion can be made, and thus unambiguous conclusions are difficult to draw. Thus, we have mainly investigated the vertical variation of the coefficient. We have looked for vertical trends of the coefficient and the particle effective radii in correlation with the opacity profiles in the solar occultation data. It might be expected that in the dust haze the particle radii decrease ( Ångström coefficient increases) with increasing altitude simultaneously with a similar trend in opacity. In addition, if the detached layers are ice clouds, one might expect the radii to be larger (smaller Ångström coefficients) than in the surroundings, since the ice crystals form on dust particles. Contrary to the conclusions of [START_REF] Montmessin | Stellar occultations at UV wavelengths by the SPI-CAM instrument: Retrieval and analysis of Martian haze profiles[END_REF], in the solar occultation data, we have not always seen this "expected" behavior of increasing coefficient (decreasing particle size) with increasing altitude, nor do we have a systematic decrease (increase in particle size) within the detached layers. Thus it is very difficult to draw conclusions on the behavior of the effective radii in the haze and on the composition of the detached layers. In one exceptional case of the detached layer near Hellas mentioned above, the particles were identified as ice crystals thanks to their large Ångström coefficients.

Conclusion

The four Martian years of data of SPICAM occultations show that the seasonal behavior of the hazetop follows closely the seasonal variations of the temperature. The highest hazetops are observed at the warm season around the southern summer solstice. Observations during the MY28 dust storm reveal very high hazetops and detached layers. The high northern latitudes in the autumn give the opposite example of very calm vertical distribution with a low hazetop. These data show that the assumption of a well-mixed dust in the lowest atmosphere is not the only option, but that the martian dust vertical distribution is very variable. We observed detached layers in more than half of the analyzed orbits (accounting for both solar and stellar occultations). The highest detached layers are observed during the global dust storm of MY28, but evident single and multiple layers are observed also during other seasons.

The solar occultation dataset can be accessed through the Supplementary Material of the article [START_REF] Määttänen | A complete climatology of the aerosol vertical distribution on Mars from MEx/SPICAM UV solar occultations[END_REF] and the Europlanet IDIS atmospheres node that can be found at http://www.bdap.ipsl.fr/idis/.

Figure 1 :

 1 Figure 1: The whole hazetop dataset as a function of latitude and season. The circles and stars represent solar and stellar occultations, respectively. The color scale gives the hazetop altitude.

Figure 2 :

 2 Figure 2: The detached layer dataset as a function of latitude and season. The circles and stars represent solar and stellar occultations, respectively. The color scale gives the detached layer altitude.

Figure 3 :

 3 Figure 3: The hazetop altitude as a function of season.


A Fedorova
email: fedorova@iki.rssi.ru

A Rodin

IKI O Korablev

F Montmessin

A Määttänen

J-L Bertaux

OBSERVATIONS OF A BIMODAL SIZE DISTRIBUTION FOR THE AEROSOL PARTICLES ON MARS BY SPICAM/MEX

Introduction:

Dust aerosols play an important role in the Martian climate system. The spatial and temporal variability and optical properties of atmospheric aerosols has been a target of almost every major spacecraft mission to Mars. Airborne dust is composed mostly of mineral particles lifted from the surface by nearsurface winds and presumably by "dust devils", small-scale convective vortex. Dust participates in heating and cooling of different atmospheric layers absorbing, scattering and reemitting in thermal IR solar radiation [START_REF] Medvedev | Influence of dust on the dynamics of the martian atmosphere above the first scale height[END_REF][START_REF] Forget | Revisiting the radiative impact of dust on Mars using the LMD Global Climate Model[END_REF]. Aerosol particles serve also as cloud condensation nuclei (CCN) in the Martian atmosphere and thus help regulate the transfer of water between the hemispheres [START_REF] Montmessin | Origin and role of water ice clouds in the Martian water cycle as inferred from a general circulation model[END_REF].

Since condensate clouds were observed in Martian atmosphere, besides the main micron-size mode extended microphysical modeling requires a small dust particle population in the higher region of the atmosphere in order to form a clouds agreeing with data. Some observations supported the bimodal distribution against the mono-modal mostly used to characterize the Martian dust. Based on Viking limb radiance observations and microphysical modeling, [START_REF] Montmessin | New insights into Martian dust distribution and water-ice cloud microphysics[END_REF] [START_REF] Montmessin | New insights into Martian dust distribution and water-ice cloud microphysics[END_REF] have derived a bimodal distribution at L s =176° / 15°S (shortly before the dust storm), featuring two maxima with r eff =1.8 μm and v eff =0.5 μm for the large mode and <0.2 μm for the small mode, with a ratio of populations (small to large) around 25. [START_REF] Markiewicz | Optical properties of the Martian aerosols as derived from Imager for Mars Pathfinder midday sky brightness data[END_REF] have reported a possible presence of bimodal distribution based on the Imager for Mars Pathfinder (IMP) data on the midday sky brightness in visible-near-IR filters at L s ~156° [START_REF] Markiewicz | Optical properties of the Martian aerosols as derived from Imager for Mars Pathfinder midday sky brightness data[END_REF].

Data analysis:

The spectral range of SPICAM IR (1-1.7 μm) includes three relatively strong CO 2 absorption bands (1.43, 1.57 and 1.6 μm) and the 1.37μm H 2 O band [START_REF] Korablev | SPICAM IR acousto-optic spectrometer experiment on Mars Express[END_REF]. To reduce the number of spectral sampling points in solar occultation, the IR spectrum consists of three "windows" and a set of several continuum points [see 6-7 for details]. We used the set of 10 continuum wavelengths outside gaseous absorption bands: 996.4, 1093.7, 1158.2, 1197.0, 1241.4, 1272.9, 1304.4, 1321.9, 1514.6 and 1552.2 nm.

The UV spectrometer measures a spectrum from 118 to 310 nm and includes the absorption bands of CO 2 , O 3 and continuum extinction by aerosols. The retrieval method separates the various species in a way described in [START_REF] Montmessin | Stellar Occultations at UV wavelengths by the SPICAM instrument: retrieval and analysis of Martian haze profiles[END_REF][START_REF] Määttänen | A complete climatology of the aerosol vertical distribution on Mars from MEx/SPICAM UV solar occultations[END_REF]. For the present study, aerosol extinctions at wavelengths of 200, 250 and 300 nm have been used. The hypothesis of a bimodal size distribution for Martian dust, the main motivation of this work, has been tested against observations chosen at the beginning of summer in the northern hemisphere in MY29 (L s =56-97°). This season is especially interesting due to the recent detection of supersaturation of water vapour that was previously discussed for its link to the size and nature of dust condensation nuclei [START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF]. The series of observations consists of 9 profiles in the northern hemisphere (fig. 1) and 11 at the southern hemisphere (fig. 2) at latitudes of 40-65°N and 30-62°S respectively. The latitudes cover cold dust-clear area near the polar night in the southern hemisphere and the middle and high northern latitudes corresponding to the edge of aphelion cloud belt and the range free of clouds at 60°N. The interpretation of the profiles using Mie scattering theory with adequate refractive indices of Martian dust and H 2 O ice particles allows to retrieve a particle size distribution and number density [START_REF] Fedorova | Solar Infrared Occultations by the Spicam Experiment on Mars-Express: Simultaneous Observations of H 2 O, CO 2 and Aerosol Vertical Distribution[END_REF]. Figures 3 and4 present vertical distributions of particle size for dust, ice and small mode and number density profiles for 5 orbits in the Northern hemisphere for latitudes >60N, L s =65-85° and for 3 orbits in the Southern hemisphere for latitudes >50S, L s =61-81° as examples.

Summary:

For the first time, a combined analysis of the UV and IR channels in the spectral range from 200 to 1550 nm has allowed to derive unambiguously a bimodal size distribution for Martian aerosols in an altitude range of 10-70 km and to track its vertical and temporal evolution:

1) The main mode of the distribution has been determined both for H 2 O and dust particles with a retrieved average radius of 0.7 and 1.3 μm, respectively. We are unable to separate ice and dust from SPICAM observations, so we based our interpretation on MCS observations for the same season and MY29 [START_REF] Mccleese | Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols[END_REF] 2) A small mode of submicron particles has been detected for both hemispheres. The average radius is 0.044 μm and number density varies from 2 cm -3 at 60 km to 10 4 cm -3 at 20 km for the Northern hemi-sphere. The average radius is 0.066 μm and the number density ranges from 1 cm -3 at 60 km to 10 3 cm -3 at 20 km for the southern hemisphere The small mode extends vertically up to 70 km in the Southern hemisphere whereas it remains around 30-40 km in the Northern hemisphere.

Figure 3. Retrieved profiles of particle size and number density for the two modes of the bimodal distribution, assuming dust or ice refraction index.

The results for five orbits in the Northern hemisphere for latitudes >60°N are presented. 3) Low-altitude clouds have been observed at 20-30 km. Such clouds correspond to the poleward edge of the aphelion cloud belt both in the Southern and the Northern hemisphere. Their opacity in UV and IR is below 0.03. 4) Clouds in the Southern hemisphere have been detected at 40-50 km in the UV only with a total opacity below 0.05, too small to be detected in nadir (sub-visible clouds). The average particle size is ~0.06 μm and the number density in the clouds varies from 100 to 5000 cm -3 .

5) The highest level of supersaturation at the Northern hemisphere at altitudes of 30-50 km reported in [START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF] for latitudes > 60°N is consistent with the observed absence of any particles (small or large) above 30 km in our observations. For latitudes below 60°N it is more difficult to explain the supersaturation since at least 10-100 cm -3 particles has been observed at 30-50 km with r eff ≈ 0.05 μm on average. Our estimation of the critical saturation ratio based on the nucleation rate calculation gives S crit ~2 for particles from 0.01 to 0.1 μm. The high supersaturation may be explained by high values of S crit at low temperatures reported by several authors [START_REF] Ladino | Laboratory investigation of Martian water ice cloud formation using dust aerosol stimulants[END_REF]. In the Southern hemisphere the small particles with r~0.07 μm are always present but S crit <4 derived by [START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF] [START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF] is within the uncertainties of nucleation rate and temperature dependence.

6) Brownian coagulation quickly removes particles with r<0.1 μm and number density ≥1000 cm -3 . For the small mode with r eff =0.04-0.05 μm and N~10 3 cm -3 in the Northern hemisphere a source of particles is required to balance the loss by coagulation. Micrometeorite mass flow estimations are too low, whereas a surface source (wind stress dust lifting and dust devils) appears more plausible. In the Southern hemisphere the small mode with r eff =0.07 μm and N<10 2 cm -3 is more stable and can survive 50-100 days or more. Similar to the Earth aerosol distribution, coagulation of small particles could explain the bimodal type of distribution.

The Mars Express occultation database is large and is being continuously populated [START_REF] Määttänen | A complete climatology of the aerosol vertical distribution on Mars from MEx/SPICAM UV solar occultations[END_REF][START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF]. Further analysis of the IR and UV solar occultations would help to constrain the seasonal and geographical variations of the small and large modes of aerosols. Considering the impact of aerosols on present-day Mars climate, the present study opens a new path of exploration for modelers in their research.

Figure 1 .

 1 Figure 1. The IR and UV extinctions for the Northern hemisphere. Black lines are the IR extinctions for 12 wavelengths, and red lines are the UV extinctions for three wavelengths.

Figure 2 .

 2 Figure 2. The IR and UV extinctions for the Southern hemisphere.

Figure 4 .

 4 Figure 4. Same as in Fig. 3 for three orbits in the Southern hemisphere for latitudes >50°S.3) Low-altitude clouds have been observed at 20-30 km. Such clouds correspond to the poleward edge of the aphelion cloud belt both in the Southern

Acknowledge: We thank our collaborators at the three institutes for the design and the fabrication of the instruments (Service d'Aeronomie/France, BIRA/Belgium and IKI/Moscow). AF, OK and AR acknowledge the program 22 of RAS and the Russian Government grant to the Moscow Institute of Physics and Technology № 11.G34.31.0074 for finance support.


D P Mulholland
email: d.p.mulholland@reading.ac.uk

A Spiga

SUB-GRID SCALE PROCESSES: TOWARDS A NEXT-GENERATION DUST LIFTING SCHEME FOR MARTIAN GCMS

Introduction:

A realistic spatio-temporal variability in atmospheric dust loading is essential to accurately simulate the Martian climate, yet dust lifting in Mars Global Climate Models (MGCMs) is still in relative infancy. We detail several improvements can be made to lifting schemes, following recent developments in observational data and MGCM physics, and taking inspiration from terrestrial dust emission work. The impacts of these additions are found to be significant, though they act in somewhat opposite directions -which perhaps allowed earlier dust-lifting MGCMs to perform as well as they have.

Macroscale lifting rate calculation:

MGCMs are typically run at a horizontal resolution of 5°, and where dust lifting schemes are utilised, lifting rates are calculated at the same resolution. Besides the neglect of sub-grid scale variability (which is dealt with in the next section), several other factors have been absent from previous dust-lifting MGCMs [1,2,3].

Heterogeneous surface roughness. In the absence of more detailed information, previous MGCMs have assumed a uniform surface aerodynamic roughness length (z 0 ), usually 1cm, following measurements made by the Viking landers [4]. Recently a global z 0 map has been derived from TES rock abundances [5], and is suitable for use by MGCMs. The map, shown in Figure 1, features z 0 variation of around two orders of magnitude, and reveals that the northern hemisphere is, broadly speaking, considerably smoother than the southern hemisphere. Despite this, dust storms are frequently observed in the southern hemisphere [6].

Figure 1: The surface roughness length (z 0 ) map from [5] (shading; dark areas feature large z 0 values), with observed dust storm locations from [6] overlaid.

Surface roughness length impacts the calculation of both the drag velocity, , and the threshold drag velocity for wind stress lifting, ; thus, it should strongly control dust lifting rates [7]. Roughness length affects the distribution of momentum between the underlying surface (from which dust is lifted) and roughness elements (rocks and boulders, and vegetation on Earth) that impede the near-surface flow. This is encapsulated by the drag partition function, f eff , defined as the ratio of the threshold drag velocity over a smooth surface ( ) to the threshold drag velocity for the rough surface in question. The required threshold is therefore Several functional forms for f eff have been proposed [8,9], all of which show a decrease in f eff with increasing z 0 , leading to increased thresholds for rough areas. At z 0 = 1cm, threshold is predicted to be 2-4 times the 'smooth' threshold, : the latter has typically been used as the guideline level for calculated or prescribed thresholds in dustlifting MGCMs to date.

Increases to the threshold drag velocity over rough surfaces are offset by increases to , such that the spatial distribution of the impact on dust lifting 'difficulty' depends on the precise form of f eff used.

Figure 2 shows that if the form of Raupach et al. [8] -which appears to be supported by recent work [10,11] -is used, dust lifting becomes relatively more difficult in rough areas, such as the southern midlatitudes, than it does in aerodynamically smooth areas.

Figure 2: Increase in the 5m windspeed required to meet the lifting threshold for lifting, relative to the windspeed needed when using a uniform z 0 = 1cm, for various f eff functions.

Lifting threshold. It is the threshold over smooth surfaces, , then, that should be calculated according to theoretical formulae, and later modified according to local roughness length. If the conventional view of dust emission -that it occurs whenever sand-sized particles are mobilized in saltation -is retained, the threshold can be calculated with relatively high confidence using a formula such as that of [12]. An alternative theory for Martian wind stress lifting [13] invokes direct detachment of low-density dust particle aggregates rather than saltation as the relevant process. This results in a that is perhaps 20% lower; however, this does not significantly alter the results shown below.

Boundary layer instability. Lifting rates are also dependent, for a particular near-surface wind velocity, on atmospheric stability in the planetary boundary layer (PBL). MGCMs have, until now, assumed a logarithmic wind profile in order to derive the drag velocity from near-surface windspeed, but such an assumption is valid only in conditions of neutral stability. The new version of the Laboratoire de Météorologie Dynamique (LMD) MGCM includes a boundary layer scheme that accounts for variations in stability [14]. The strong diurnal cycle in PBL stability results in more pronounced diurnal variation in than has previously been simulated by MGCMs.

The result of these combined effects is that MGCM surface winds, taken from the Mars Climate Database [15], apparently never reach the magnitudes required to initiate dust lifting almost anywhere on the planet (Figure 3).

Sub-grid scale variability:

In order to explain the discrepancy between MGCM predictions and the observational record for southern hemisphere dust storms, we investigate the potential for sub-grid scale variability in both z 0 and to increase MGCM dust lifting rates. In both cases, net emission from a surface gridbox increases due to the resolution of areas within the gridbox with conditions (the combination of and ) that are more favourable for dust lifting than are the gridboxmean conditions.

Roughness length. Lifted fluxes were calculated using the full, 1 / 8 ° z 0 map, and compared to those from a map smoothed to 5° resolution. Due to the presence of lower minimum roughness lengths at full resolution, lifting begins at lower values of , which implies that it is appropriate to reduce thresholds in an MGCM, in order to capture these earlier 'switchon' points. The reductions required are particularly large in high-z 0 areas, as a consequence of the (f eff ) -1 dependence of the threshold. With the Raupach et al. drag partition function, reductions of as much as 50% are required to reproduce 1 / 8 ° lifting rates at MGCM resolution.

Surface windspeed gustiness. Windspeed variability over the spatial (~100 km) and temporal (~30 minutes) resolution of an MGCM must be parameterised in order to account for the formation of hotspots in [16], where the lifting threshold may be met, even whilst the gridbox-mean is below the threshold value. Results from the LMD Large Eddy Simulation model [17] show that, at a ~km scale, gusts of 2-3 times the mean value are common (Figure 4), and therefore that the hotspot theory is indeed relevant to Martian dust lifting, with respect to windspeed variability. Using convective updraft velocity as a proxy for horizontal gustiness, Mars Climate Database data show that gustiness peaks in the subtropics in local spring and summer, and that gustiness is, on average, stronger in the southern hemisphere than in the northern hemisphere. Using a Weibull distribution to represent variability in , activation points for dust lifting are lowered, resulting in equivalent reductions to MGCM-resolution thresholds of 20-60%, depending on the shape parameter used in the Weibull distribution, and the minimum probability for lifting activation imposed on .

Taken together, sub-grid scale variability in and in z 0 appears to be sufficient to allow MGCM dust lifting in the rough southern midlatitudes, where it was previously out of reach of model winds. Results will be presented, showing how parameterisation of these effects both increases mean model lifting rates and modifies the spatial and temporal distribution of dust emission.

Conclusions:

We have found that several dust lifting scheme components, thus far neglected from MGCM schemes, exert a strong influence on lifting rates, altering both magnitude and spatial distribution. To maintain realistic dust storm frequency when including heterogeneous surface roughness lengths in lifting threshold calculations, parameterisation of sub-grid scale variability, in both and z 0 , appears to be essential. Both of these considerations boost lifting preferentially in the high-z 0 regions of the southern subtropics and midlatitudes, and it is therefore expected that both will have an impact on the representation by MGCMs of major dust storms, many of which originate in these regions. It remains for the additions described here to be evaluated fully, through incorporation into a dust-lifting MGCM.

Figure 3 :

 3 Figure 3: Mars Climate Database output for a typical no-dust-storm year, showing the maximum drag velocity as a fraction of the local lifting threshold.

Figure 4 :

 4 Figure 4: Daytime maximum , from an LMD LES model simulation using a background wind equivalent to 1 ms -1 .


T Navarro
email: navarro@lmd.jussieu.fr

J-B Madeleine

F Forget

A Spiga

E Millour

MODELING OF THE MARTIAN WATER CYCLE WITH AN IMPROVED REPRESENTATION OF WATER ICE CLOUDS

Introduction:

The Martian water cycle has been studied for a long time thanks to remote observations and the use of GCMs (Global Climate Model). Thanks to GCMs, the comprehension of the water cycle has been improved, revealing for instance the importance of clouds in the global water transport [START_REF] Montmessin | Origins and role of water ice clouds in the Martian water cycle as inferred from a general circulation model[END_REF] with implications for atmospheric dynamics, paleoclimates. From that perspective, the modeling of the Martian water cycle by the LMD (Laboratoire de Météorologie Dynamique) Mars GCM has been improved by implementing significant processes, such as radiatively active water ice clouds and a microphysical scheme described in [START_REF] Montmessin | New insights into Martian dust distribution and water-ice clouds microphysics[END_REF], including processes such as nucleation, growth of ice particles, sedimentation, coalescence and scavenging of dust nuclei.

Model description:

Various recent improvements in LMD Mars GCM impact on the water cycle. Dust is represented via a twomoment scheme of lognormal distributions, allowing the independent transport of both dust mass and particles number, enabling dust particles radius prediction. Additionally, a new microphysical scheme includes the nucleation and growth of water ice particles onto dust particles, and thus enables supersaturation. Interaction with airborne dust is thus represented through scavenging, that is to say the redistribution of ice-trapped dust below the clouds after sedimentation.

Moreover, water ice clouds are now radiatively active and interact with radiative transfer processes in both visible and infrared bands [START_REF] Madeleine | The influence of radiatively active water ice clouds on the Martian climate[END_REF]. As for dust, ice particles are accounted for using a lognormal distribution, i.e. their radius is not fixed and can vary in both time and location.

The drier water cycle issue in GCMs.

Implementation of clouds radiative effects generates a thick polar cloud cover over the Northern hemisphere during Northern summer, decreasing ground temperature by reducing incoming solar flux [1]. As the North pole is the main source of atmospheric water vapor, the resulting water cycle is then very dry in comparison to observations and simulations without radiatively active clouds (RAC). The reason of the formation of such thick polar clouds during polar day has been identified as the consequence of retroactive processes between local temperature and both nucleation and growth of ice particles, driven by the use of an explicit scheme for nucleation and growth.

Simulation parameters:

This issue has been solved by the use of a smaller integration timestep for microphysical processes, in the integration of nucleation and ice growth explicit schemes.nce a North pole free of clouds during summer solstice obtained in GCM simulations, further unknown parameters have to be tuned in order to adequately model the Martian water cycle. These are the properties of permanent ice reservoirs in the GCM grid, (i.e. their position, albedo and thermal inertia), but also the effective variance of ice particles distribution for sedimentation flux and the contact angle parameter for heterogeneous nucleation.

Simulation results:

The water cycle is modeled with the use of RAC. Zonal mean of aphelion cloud belt opacities are well matched (Figure 2). However, the transport of water vapor from the North pole to tropics is underestimated while the sublimation peak during Northern solstice is overestimated (Figure 1). The reason is that the trap of water in the ascending branch of the Hadley cell is more significant with RAC given the set of parameters used to match observed cloud opacities (Figure 3). Including RAC improves modeled temperatures, by warming the atmosphere above the clouds, in better agreement with observations [START_REF] Madeleine | The influence of radiatively active water ice clouds on the Martian climate[END_REF]. This affects the atmosphere not only on a local scale, but on the whole planet, as the global circulation is modified. Supersaturation is modeled and compared to observations [START_REF] Maltagliati | Evidence of water vapor in excess of saturation in the atmosphere of Mars[END_REF] with predictions of ratios up to 1000 above 40 km, that cannot be observed due to the tiny quantities of water vapor. Dust scavenging by clouds enhances the formation of dust detached layers in rare and very localized areas and periods, but generally fails to explain the global distribution of detached layers observed from orbit [START_REF] Mccleese | Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols[END_REF].

Conclusion:

The use of GCMs with RAC has changed the way we model the Martian water cycle and thus we have to address the issue of how ice clouds interact with the whole atmosphere. With the use of microphysics with a smaller integration timestep we could fix the issue of the dry water cycle, and then simulate water vapor quantities and water ice opacities in better agreement with observations. Atmospheric temperatures are better represented thanks to the interactions between clouds and heating rates they yield. Many issues remain to be addressed, such as the cross-equatorial transport of water and the role of supersaturation that allow vapor to pass the trap of the aphelion cloud belt. It is noticeable that dust scavenging by water ice clouds cannot explain the observed detached dust layers. Also, it would be important to have a fine spatial modeling of Northern pole permanent ice reservoirs, and what it can tell us about their stability and the possible need for additional sources of atmospheric vapor, such as the regolith.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1: Zonal mean of atmospheric vapor as modeled with the GCM with RAC and microphysics (top) and observed by TES (bottom)


André Sobczak
email: asobczak@audencia.com

Ensuring the effective implementation of transnational company agreements

Keywords: transnational collective agreements, transnational social dialogue, European social dialogue, European Works Councils

Over the past decade, a number of European companies have negotiated transnational company agreements. These agreements may help improve labour standards in the subsidiaries, and even more for those employed by subcontractors or suppliers. They may also encourage social dialogue at these levels. However, given their uncertain legal status, how it is possible to ensure their effective implementation throughout the company's subsidiaries? What matters is less the legal status of the transnational collective agreement than the active involvement of the various actors involved in the process. The role of the signatories to the agreement is of course crucial, but its effective implementation also requires the commitment of local managements and workers' representatives. This article draws on discussions and interviews with managers and worker representatives directly involved in the implementation of transnational company agreements, and identifies ways to encourage this kind of commitment.

Introduction

During the last two decades, transnational social dialogue within the EU has developed at crossindustry [START_REF] Clauwaert | 2011: 20 years of European interprofessional social dialogue: achievements and prospects[END_REF][START_REF] De Boer | Broadening without Intensification: The Added Value of the European Social and Sectoral Dialogue[END_REF], sectoral [START_REF] Léonard | European Sectoral Social Dialogue: An Analytical Framework[END_REF][START_REF] Perrin | European sectoral social dialogue and national social partners[END_REF] and company levels [START_REF] Schoemann | Codes of conduct and international framework agreements: New forms of governance at company level[END_REF][START_REF] Telljohann | European and international framework agreements: new tools of transnational industrial relations[END_REF]. This development is linked to increasing internationalization of business and the consequent need for social regulation at international level (Papadakis, 2008). Even though links increasingly exist between the various levels of transnational social dialogue [START_REF] Léonard | Accords transnationaux d'entreprises et dialogue social sectoriel européen : quelles interactions ?[END_REF], we are still far from the sort of systematic, formalized structure linking the three levels which exists in some national contexts. In particular, whereas Articles 154 and 155 of the EU Treaty define precise rules for the negotiation and implementation of transnational agreements at cross-industry and sectoral levels, there is currently no specific legal framework for transnational company agreements [START_REF] Ales | Transnational Collective Bargaining: Past, Present and Future[END_REF][START_REF] Sobczak | Legal Dimensions of International Framework Agreements in the Field of Corporate Social Responsibility[END_REF].

However, the lack of a specific legal framework has not hindered the development of transnational company agreements; indeed some argue that certain employers would not have signed such agreements if these had a clear legally binding character.

Whereas transnational social dialogue at the cross-industry and sectoral levels remains almost exclusively limited to the EU, almost half of the existing transnational company agreements have a worldwide scope of application, covering workers in all subsidiaries (European Commission, 2008).

The majority of the companies that have signed these agreements have their head office in one of the EU member states, mainly Germany or France, even if the last five years have seen an increasing development of transnational company agreements in countries outside the EU. The content of agreements of worldwide scope is quite different from those applying only within Europe, usually covering various aspects of corporate social responsibility [START_REF] Hammer | International Framework Agreements: global industrial relations between rights and bargaining[END_REF]. First, they refer, almost as a matter of course, to the fundamental social rights defined by the International Labour Organisation (freedom of association, equal opportunities and prohibition of forced and child labour). Second, they often cover working conditions issues that are also discussed at national level, such as health and safety, training, responsible restructuring or respect for national and international minimum standards regarding remuneration and working time. Third, they often also include broader issues concerning relations with other stakeholder groups, such as environmental protection, involvement in local communities or the fight against AIDS. Agreements with a purely European scope usually focus on one specific subject (European Commission, 2008). Most such texts are related to restructuring, either setting out social guarantees for a specific plant or defining general principles for the future, such as rights on information and consultation of workers' representatives or lifelong training (Moreau, 2009;[START_REF] Scarponi | Gli accordi transnazionali sulle ristruturazioni d'imprese[END_REF]. Other European agreements concern equal opportunities or diversity management. Finally, some texts at this level cover technical matters such as data protection or financial participation.

The negotiations involve a variety of actors. Those with a worldwide scope always involve one or more Global Union Federations (GUFs). However, almost half of these texts are co-signed by the national union(s) of the country where the company has its head office, and a quarter by the company's European Works Council (EWC). Two-thirds of the agreements with a European scope of application are signed by the EWC, and the rest are negotiated by European Trade Union Federations (ETUFs) or GUFs. National unions are only very rarely signatories of agreements with a purely European scope, indicating that the European organizations have achieved sufficient legitimacy to replace national ones. This does not yet seem to be the case at international level.

None of these mechanisms helps to clarify the legal status of the negotiated agreements [START_REF] Daugareilh | La négociation collective internationale[END_REF][START_REF] Drouin | International Framework Agreements: A Study in Transnational Labour Regulation[END_REF][START_REF] Sobczak | Legal Dimensions of International Framework Agreements in the Field of Corporate Social Responsibility[END_REF]. GUFs and ETUFs are organized at sectoral level and can negotiate agreements at this level with employers' organizations, if these exist. However, there is no legal authority for them to negotiate agreements with specific companies. EWCs, by contrast, represent employees at company level, but the directive establishing them does not give them any right to negotiate collective agreements. Finally, national unions that represent the employees at the company's head office have no legitimacy to negotiate agreements that apply to the workers in subsidiaries in other countries.

Given this uncertain legal status, how it is possible to ensure the effective implementation of transnational company agreements throughout a company's subsidiaries in order to strengthen workers' rights [START_REF] Fichter | Kann transnationale Solidarität auf Konzernebene organisiert werden? Der Ansatz der internationalen Rahmenabkommen[END_REF][START_REF] Scarponi | Gli accordi transnazionali a livello di impresa: uno strumento per contrastare il social dumping[END_REF]? Effective implementation probably depends less on the legal status of the transnational collective agreement than on the power and active involvement of all relevant actors in the process. The role of the signatories is of course crucial, but its effective implementation also requires the commitment of local managements and workers' representatives.

This article draws on discussions and interviews with managers and worker representatives directly involved in the implementation of transnational company agreements, and identifies several means to create or strengthen such commitment. I develop my argument in three sections. First, I discuss the impact of legal status on the effective implementation of transnational company agreements, in an attempt to go beyond the traditional distinction drawn between hard and soft law.

Second, I analyze various procedures which may be developed at corporate level to ensure effective implementation, highlighting the fact that current solutions are often insufficient. Third, I explore means of enhancing the active involvement of local stakeholders, both managers and worker representatives. In conclusion, I discuss possible future research in this area.

Distinguishing between legal status and effective implementation

The lack of a precise legal status for transnational company agreements does not mean that these texts have no legal effects (Lo Faro, this issue). Nor does it mean that they have no impact on working conditions or the quality of social dialogue within the company [START_REF] Mund | Soft law: second best solution or a privatisation for social rights? Some pointers for a future discussion[END_REF]. In several national industrial relations systems, collective agreements are not legally binding and are considered 'gentlemen's agreements', yet this does not mean that employers do not respect them (Rehbahn, 2003;[START_REF] Rebhahn | Collective labour law in Europe in a comparative perspective (Part 2)[END_REF]. Clearly, then, the legally binding nature of a collective agreement is neither a sufficient, nor a necessary condition for its effective implementation.

There are many examples of legally binding labour standards which are not effectively implemented in the companies to which they in theory apply, as is particularly evident in the area of equal opportunities and anti-discrimination. Over recent decades, many legally binding texts have been adopted at EU and national level to strengthen the rights of workers and job applicants in this area (Jacobs, 2005); but unequal treatment still occurs. Similarly, despite EU and national legislation requiring that workers' representatives be informed and consulted before decisions are taken that may lead to the loss of jobs, this principle is not effectively respected, since consultation usually occurs only after the decision is already taken. To a certain extent, the development of transnational company agreements is itself evidence that the legally binding character of a labour standard is insufficient to guarantee its effective implementation. Indeed, many of the provisions in these agreements, in particular those with a worldwide scope, refer to respect for ILO conventions or even national labour laws that are already legally binding. They may, of course, extend these rights to new categories of workers, in particular employees in subsidiaries with lower labour standards and those working for suppliers and subcontractors who are often less protected in law and in fact [START_REF] Sobczak | Codes of Conduct in Subcontracting Networks: A Labour Law Perspective[END_REF]. Their main added value, however, is not in the creation of new rights, but in the commitment of all the signatories to contribute proactively to the effective implementation of existing standards. GUFs and ETUFs therefore pay increasing attention to the definition of precise measures for the implementation and monitoring of transnational company agreements. In other words, by concluding such agreements, companies and their union counterparts aim to complement the public authorities, in particular those carrying out labour inspections, in monitoring compliance with labour standards. This may raise questions about the legitimacy of these private actors, but it also offers new opportunities, as long as the new private monitoring procedures do not replace the public ones.

On the other hand, as Trubek and Trubek (2005) put it, 'soft law may be harder than you think'. Soft law is defined by a lack of legal enforceability, which is not the same as effectiveness [START_REF] Lascoumes | Théories et pratiques de l'effectivité du droit[END_REF]. Several examples show that practices which are in no way legally binding may be effectively implemented, if the parties involved understand and accept the aims and either feel pressure from external stakeholders or have an interest in creating a long-term interorganizational learning process (Berthoin [START_REF] Antal | Beyond CSR: Organisational Learning for Global Responsibility[END_REF]. Civil society and consumers are, indeed, increasingly interested in the social practices of companies. They expect them to respect their commitments in this area and even to resist social dumping by refusing to exploit national jurisdictions offering a lower level of worker protection. Companies which do not respect their 'voluntary commitments' in the area of corporate social responsibility may be subject to boycotts or even legal proceedings, charged with misleading advertising [START_REF] Sobczak | Are Codes of Conduct in Global Supply Chains Really Voluntary? From Soft Law Regulation of Labour Relations to Consumer Law[END_REF]. Even in the area of social dialogue, there are several European countries where collective agreements are respected not because they are legally enforceable, but because of the need of the social partners to create mutual trust in order to establish long-lasting relationships.

Consequently, the difference between hard and soft law should not be overstated. They are not totally different in nature; rather, there are variations in effectiveness within both categories. In theory, legally binding labour standards are more likely to be implemented effectively than non-binding ones; but this presupposes that the public authorities have the will and capacity to monitor the application of the relevant legislation and to sanction any breach. This is not always the case [START_REF] Hepple | Enforcement: the law and politics of cooperation and compliance[END_REF]. In many countries, legally binding standards are observed only if the local actors are aware of the legislation, consider it relevant and legitimate and have the power to enforce it, either through lawsuits or through such means as organizing collective action or alerting the media and other stakeholders, including those in other countries. These are the same measures that may be used to press managers to respect non-binding texts.

In order to ensure the effective implementation of a labour standard, it is therefore less important to achieve legally binding provisions than to set up appropriate internal monitoring processes and to provide local stakeholders with knowledge and understanding of the standard. The considers that monitoring mechanisms are a key ingredient of transnational company agreements, since they make them into practical, living documents. This better enables monitoring of a company's performance and means that agreements can be amended or improved in the light of experience.

Consequently, the GUFs do not consider the adoption of a legal framework for transnational company agreements a priority. Rather, they try to integrate more detailed provisions on implementation and monitoring into the agreements which they negotiate [START_REF] Schoemann | Codes of conduct and international framework agreements: New forms of governance at company level[END_REF][START_REF] Telljohann | European and international framework agreements: new tools of transnational industrial relations[END_REF]. This strategy is compatible with the strong opposition of employers' associations towards any kind of legal framework, even on an optional basis, for transnational collective bargaining, and the resulting difficulties facing the European Commission in moving forward in this field (Dufresne, this issue). In this context, in order to guarantee the impact of transnational company agreements, it may be more efficient to concentrate efforts on involving all relevant actors in the implementation process, both at head offices and in subsidiaries, rather than fighting for a legal framework that is unlikely to be adopted.

Coordinating implementation at head office

Most transnational company agreements, in particular the most recent ones, contain precise provisions as to how the signatories at the head office should monitor implementation within the different subsidiaries. Unlike unilateral corporate social responsibility initiatives such as codes of conduct, which are increasingly monitored through external audits [START_REF] Schoemann | Codes of conduct and international framework agreements: New forms of governance at company level[END_REF], such processes are usually internal and based on social dialogue, since they involve workers' representatives at different levels in the company. This choice has advantages for both management and trade unions. Managers at the headquarters establish an internal warning system which can give them information on social problems in the subsidiaries, which may be hidden from them by local management, and can allow them to implement corrective actions before the situation reaches the media, social rating agencies and the general public [START_REF] Sobczak | La responsabilité globale, moteur du dialogue social[END_REF]. The procedures developed to monitor transnational company agreements may thus be seen as a way of managing reputational risks [START_REF] Sobczak | Syndicats et responsabilité sociale des multinationales[END_REF]. For trade unions, developing such internal control mechanisms not only improves compliance with labour standards throughout the subsidiaries, but also helps to strengthen local unions and to promote transnational cooperation. This in itself is one of the reasons for negotiating such agreements [START_REF] Müller | Internationale Rahmenvereinbarungen. Chancen und Grenzen eines neuen Instruments globaler Gewerkschaftspolitik[END_REF].

To coordinate implementation at the company's head office, almost all transnational company agreements provide for at least an annual meeting between the signatories to discuss the initiatives taken, the challenges met and the results obtained. For example, the 2005 agreement on corporate social responsibility and international industrial relations between the French building materials company Lafarge and the BWI, ICEM and World Federation of Building and Woodworkers' Unions (WFBW) established 'a reference group consisting of representatives of the Lafarge management and the signatory international federations [to] meet at least once a year, or whenever necessary, to follow up and review the implementation of this agreement'.

Some transnational agreements open up these meetings to workers' representatives other than those who have signed the text. This is, for example, the case of texts with worldwide scope which have not been signed by the EWC: the ETUF or GUF may have refused to allow it to sign the agreement since its members are not necessarily trade unionists, but may still admit its legitimacy in the monitoring process (Dufresne, this issue). Thus the 2008 international framework agreement between Italcementi and BWI to promote and protect workers' rights provided for 'a reference group... composed of representatives of Italcementi and the concerned BWI affiliated union(s) in the home country of the company, the EWC Coordinator and a BWI Coordinator. It will meet at least once a year, or when necessary, to evaluate reports on compliance and to review the implementation of the agreement.' Some agreements contain very detailed provisions concerning the annual meeting, such as the documents to be sent to the workers' representatives beforehand, the time to be spent on discussions and the budget to be provided by the company for translations and travelling [START_REF] Sobczak | La responsabilité globale, moteur du dialogue social[END_REF]. The Corporate Social Responsibility' (CCSR) to meet annually, adding that 'in the event of a request from the majority of the members, and with the agreement of the Chairman, a special session may be held'.

'A preparatory meeting and a follow-up meeting' is to take place on each occasion. Participants 'shall be given time in the company that employs them to prepare and participate in the annual CCSR meeting': 4 days a year for ordinary members, with extra allowances for members of the CCSR Bureau, and a special provision for the Secretary. In addition, EDF 'covers the costs directly related to the functioning of the follow-up on the CSR Agreement (CCSR meetings, Bureau, interpretation and translations). The transportation costs are covered by the companies of the Group'. Management is to arrange language training in French or in English, where necessary.

The requirement to hold these regular meetings is an interesting initiative that does not exist for many hard law standards. The legislator tends to consider that the legally binding character of the latter is sufficient to guarantee effective implementation without any further systematic monitoring procedures. The non-legally binding character of transnational company agreements, in contrast, encourages their signatories, in particular the workers' representatives, to develop specific processes to ensure that the provisions have a tangible effect on the working conditions and the quality of social dialogue in the various subsidiaries. Furthermore, these regular meetings contribute to the recognition of the transnational group as a new and legitimate level of social dialogue [START_REF] Müller | Internationale Rahmenabkommen: Neue Räume für den gewerkschaftlichen Internationalismus?[END_REF][START_REF] Schoemann | Codes of conduct and international framework agreements: New forms of governance at company level[END_REF].

Analysis of the implementation processes within different transnational company agreements

shows, however, that organizing annual meetings is not enough. To promote effective implementation it may be necessary to prepare detailed reports on actions carried out in the different subsidiaries to conform to the principles defined in the text, and to develop performance indicators. The global agreement on fundamental rights, social dialogue and sustainable development between GDF SUEZ, BWI, ICEM and PSI in 2010 stated that 'each company covered by this agreement will propose its methods of application within the framework of joint action plans. The application is intensely monitored with the trade union partners based on accessible and objective indicators. Where trade union partners cannot be involved, other methods will be examined and jointly agreed.' Defining such performance indicators, which must be consistent with other indicators used by the company in the area of corporate social responsibility and more generally, might risk creating a shift from a bilateral to a unilateral approach. Management is unlikely to agree to discuss these performance indicators with workers' representatives, since they are a key tool for the management of human resources. Given the often rather broad scope of transnational company agreements, in particular those which apply globally, it is also important to provide worker representatives at the headquarters with information or even training on the issues covered by the agreement. This is true, for example, if the agreement covers matters such as environmental protection or the fight against AIDS, which are not the standard subjects dealt with by workers' representatives and may thus require additional expertise. Such an approach may help to stimulate an organizational learning process aimed at improving working conditions and social dialogue in all parts of the company through cooperation with workers' representatives (Berthoin [START_REF] Antal | Beyond CSR: Organisational Learning for Global Responsibility[END_REF]. In some cases, the existence of a transnational company agreement allows headquarters representatives to organize visits to subsidiaries in different parts of the world, to give them a better knowledge of local priorities and expectations.

Although such visits may be difficult to organize and costly, they are more likely to produce the expected effect on working conditions and social dialogue, but also in terms of risk management, than just the signature of the agreement and minimal involvement in its monitoring by means of an annual meeting that is not sufficiently prepared.

As well as developing such a proactive monitoring procedure, the signatories to transnational company agreements may define complaint mechanisms to allow workers in the subsidiaries to report violations of the principles set out in the text. Some transnational agreements contain such provisions. The 2005 framework agreement between the aeronautics company EADS and its EWC specified that 'trade union or works council representatives may inform central management orally or in writing of any breach' of one or more terms of the agreement. In such cases, 'EADS central management shall take appropriate measures to eliminate any breach of the aforesaid principles, reporting on any measures taken to a plenary session of the EWC' and 'the EWC may submit proposals for corrective measures negotiations take only a few months, and both parties rarely mention major difficulties in the process [START_REF] Schoemann | Codes of conduct and international framework agreements: New forms of governance at company level[END_REF]. What seems much more difficult is to convince local stakeholders of the benefits of a transnational agreement, which may contain principles that are not in line with local legislation, traditions and practices. Whereas local workers' representatives may see the transnational agreement as an opportunity to improve working conditions and to strengthen their position in social dialogue, local managers often perceive the principles it contains as constraints that are not always in line with their own management culture. This is particularly true for the provisions relating to social dialogue, since the added value of workers' rights on information and consultation is far from accepted in all parts of the world, and may provoke open hostility in Asia, and also in North America.

To help local managers and worker representatives understand the aims of the transnational agreement and to reflect on the way it may enrich their own practices, it seems important to involve them at an early stage of the process. Unfortunately, such an approach is far from widespread among existing agreements. For the employer, managers at head office conduct the transnational negotiations and at best consult informally certain local managers as to the process or particular critical issues; it is very unusual for local managers to be included in the negotiating team. This may be explained by a desire to facilitate and accelerate negotiations, as well as by the difficulties involved in selecting the right local managers, yet the experience of those companies which have involved local managers in the negotiation process shows that this procedure favours faster and more effective implementation.

First, local managers feel a closer connection with the agreement if they have been involved at an early stage rather than learning about it once it has been signed. Second, their involvement may make it possible to avoid potential tensions with local legislation or cultures.

The same reasoning applies to the involvement of local workers' representatives. It may be tempting to negotiate transnational company agreements with transnational representative bodies such as GUFs, ETUFs or EWCs. This may avoid long discussions as to the representativeness of the various local actors, and accelerate the negotiation process. The risk is, however, that local representatives then learn of the transnational agreement without understanding the intentions of the signatories, nor how it can be adapted to their local context. The transnational agreement is thus likely to remain a virtual prospect without real impact on the working conditions and on local social dialogue national unions in 2006 provided that 'in each of the major countries local social observatories will be set up. These will be made up of human resources divisions and labour unions. The social observatories will monitor the application of the Global Framework Agreement on an annual basis using a common monitoring document to be created jointly by the parties to this agreement.'

There is of course a risk of reintroducing national differences, but the aim of the transnational agreement is usually less to harmonize working conditions throughout the company than to set up minimum standards that may always be improved at subsidiary level. In other words, the idea is to create a multi-level system of social regulation combining transnational and national norms rather than setting the two against each other [START_REF] Marginson | European Integration and Industrial Relations: Multi-Level Governance in the Making[END_REF][START_REF] Sciarra | Collective Exit Strategies: New Ideas in Transnational Labour Law[END_REF].

It is important to ask local management to report on the specific progress made with reference to the provisions of the agreement. Some companies organize annual meetings with the top management of all subsidiaries to prepare for the meeting of head office signatories. In a few cases, performance indicators are developed to measure the results obtained. For local management, this underlines the importance attached by head office to the effective implementation of the agreement. It also allows headquarters managers to communicate more effectively on the actions developed at local level, the results obtained and the remaining challenges. Some companies have used this knowledge to create databases of innovative practices to inspire those in other subsidiaries.

It may also be desirable to train local workers' representatives, to help them develop the right initiatives to achieve the aims set out in the transnational agreement. Some companies, for example, organize training on issues frequently included in these agreements but which do not necessarily correspond to the traditional activities of local workers' representatives, such as the integration of disabled workers or the in-depth analysis of the company's accounts in order to make appropriate proposals in the case of restructuring. Such training programmes, which can be jointly organized by management and workers' representatives, seem necessary for the effective implementation of these agreements, in particular if they cover issues which are new to the local workers' representatives.

To promote effective implementation of transnational company agreements, therefore, the signatories at head office level must thus develop innovative processes aimed at involving local management and workers' representatives and encouraging them to integrate provisions of the agreement into their strategies and activities. In other words, the signing of a transnational company agreement does not mark the end of the negotiation process, but rather the starting point of an organizational learning process throughout the subsidiaries. What counts is less the formal content of the agreement than the procedures put in place to encourage and maintain this learning process. It does not seem necessary to include all details concerning the implementation process in the text of the agreement. Such an approach may even risk discouraging decentralized initiatives by local players if they have not been included in the transnational agreement. The transnational agreement should rather be seen as offering a framework for organizational learning at all levels of the company rather than as a text designed to harmonize working conditions and social dialogue throughout the company.

Conclusions

This article is based on interviews with managers and workers' representatives who have been involved in the negotiation of transnational agreements. These interviews have been mainly limited to companies with their headquarters in France, where most such agreements have been negotiated.

Future research should cover a broader range of companies, in order to analyze whether those in other countries have developed similar procedures to implement these agreements effectively, and whether they have had the same positive experience. Interviews should of course also be conducted in at least some of the subsidiaries, to find out how local managers and workers' representatives perceive these agreements and their involvement in implementation. It would also be interesting to broaden this research to the impact on working conditions and social dialogue for suppliers and subcontractors, since many transnational company agreements contain provisions that cover these. Such an ambitious research project would require global networks of researchers, in order to organize interviews in different countries with the local actors. It also presupposes support from the head office signatories, to convince local actors to take part in such research. For all these reasons, the launching of this type of research project is a difficult task. Nevertheless, it seems to be a necessary next step in order to

  GUFs and ETUFs seem to recognize this. For example, the International Metalworkers' Federation (IMF) considers that the enforcement of transnational company agreements depends almost entirely on union strength. It therefore recommends and encourages its affiliates to report any breaches of such agreements by local managers. It considers that workers' representatives should plan how to implement the agreement before signing, and should develop education and training to ensure that affiliates are equipped to ensure genuine implementation. Similarly, the Building and Wood Workers' International (BWI) sees the success of any global company agreement as depending on the strength of unions at national level: proper implementation is only possible if workers are organized in free trade unions and are able to bargain collectively at the national and enterprise levels. Along the same lines, the International Federation of Chemical, Energy, Mine and General Workers' Unions (ICEM)

  corporate social responsibility between the French electricity company EDF and ICEM, Public Services International (PSI), International Federation of Mining and Energy (IFME) and several national unions, provided for the creation of a 'Consultation Committee on EDF Group

to ensure compliance with this agreement to the management of EADS or the relevant Business Unit.' Finally, the agreement states that 'in a situation of conflict, the arbitration process will be mutually agreed by the EADS head of Human Resources and the EWC.' Usually, the workers or their local representatives must initially discuss the problem with local management. If the problem cannot be resolved at this level, the worker or the local union may contact the national union, which will discuss the issue with the national company headquarters. If the problem can still not be resolved, it becomes the responsibility of the signatories of the transnational agreement to deal with the conflict. This approach, respecting the principle of subsidiarity, has the advantage of involving managers and workers' representatives at all levels, while giving priority to conflict resolution at local level. It would be dangerous to give workers in the subsidiaries the impression that head office could be brought in to challenge any decision by local managers; and local actors would not be encouraged to take initiatives to adapt the transnational agreement to local circumstances, if all decisions were to be taken by the head office. Hence while the signatories at head office are responsible for coordinating implementation of the transnational company agreement, the main actors are those in the subsidiaries, since they need to find ways to achieve the aims defined in the text while taking account of the local culture and context.

Involving local stakeholders in implementation

Involving local managers and workers' representatives in the implementation of transnational company agreements is both one of the most difficult and one of the most important challenges for the signatories at headquarters level. Agreeing on the common values and principles that should govern working conditions and social dialogue in all parts of the company should not be too complicated a task for managers and workers' representatives at central level. Many transnational company agreements have similar contents (da Costa et al., this issue), which shows that once the two parties have decided to open negotiations, they have a fairly clear idea of the outcome. In most cases, [START_REF] Marginson | European Collective Bargaining: A Virtual Prospect?[END_REF]. Conversely, by involving local representatives at an early stage, the signatories at head office level can encourage effective implementation of their transnational agreement.

Such involvement, however, remains the exception. Only half of those transnational agreements with a worldwide scope of application, and very few of those with a European scope, have been co-signed by national unions. Furthermore, in almost all cases, the only national unions involved in the negotiation process are those of the country of the head office. EDF seems to be the only company that has involved national unions from all countries in which it has subsidiaries [START_REF] Sobczak | La responsabilité globale, moteur du dialogue social[END_REF], a choice that was ambitious but possible, given the moderate number of its subsidiaries. The French car-producer Renault negotiated its transnational agreement with the IMF, but informed the various national unions of the process and invited them to co-sign the agreement to give it additional legitimacy. This option seems easier to replicate than the EDF model. It is close to the internal procedure set out by the European Metalworkers' Federation (EMF), which has decided not to open any negotiation on a transnational company agreement before consulting the various national unions and getting a mandate to negotiate (Dufresne, this issue). Moreover, EMF will not sign the final agreement until the national unions have approved the result.

Once a transnational company agreement has been signed, the signatories at head office level must encourage local management and workers' representatives to implement effectively the principles defined in the text. According to the 'basic principles of social responsibility' agreed between Bosch, its EWC and the IMF in 2004, the aim of the signatory parties is to implement these principles throughout the whole group. These principles will be incorporated into the 'Management System Manual for Quality, Environment, Safety, and Security in the Bosch Group'. The agreement states that the 'responsibility for their implementation lies with the senior management of the respective divisions, regional subsidiaries and company locations.' Some transnational agreements refer explicitly to implementation through decentralized social dialogue, which is an excellent way to involve the local actors. For example, the Global Framework Agreement on Social Responsibility between PSA Peugeot Citroën and the IMF, EMF and several continue to learn more about the effective implementation of transnational company agreements and thus guarantee their future development.

Author biography

André Sobczak is Associate Professor and Research Director in Management at Audencia Nantes School of Management.