Christophe Lécuyer
email: christophe.lecuyer@univ-lyon1.fr

Ana-Voica Bojar

Valérie Daux

Serge Legendre

Geographic variations in the slope of the δ2H-δ18O meteoric water line over Europe: A record of increasing continentality

Keywords: meteoric water line, stable isotopes, continentality, water cycle, Europe

come

O) during phase changes and distillation processes in the atmosphere [START_REF] Craig | Isotopic variations in meteoric waters[END_REF][START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]Rozanski et al. 1993). Therefore, they have been recognized for decades as powerful natural tracers of the water cycle [START_REF] Craig | Isotopic variations in meteoric waters[END_REF][START_REF] Craig | Deuterium and oxygen-18 variations in the ocean and the marine atmosphere[END_REF][START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]Rozanski et al. 1993;[START_REF] Gat | Oxygen and hydrogen isotopes in the hydrologic cycle[END_REF][START_REF] Gat | Environmental isotopes in the hydrological cycle[END_REF]Rozanski 2005).  2 H and  18 O of meteoric waters have been extensively used to identify the main sources of water evaporation [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF]Uemura et al. 2008), migration pathways of humid air masses [START_REF] Masson-Delmotte | GRIP deuterium excess reveals rapid and orbital-scale changes in Greenland moisture origin[END_REF]Vimeux et al. 2005;[START_REF] Guan | Deuterium excess variations of rainfall events in a coastal area of South Australia and its relationship with synoptic weather systems and atmospheric moisture sources[END_REF], temperature effects under temperate climates [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]Yurtsever 1975;Rindsberger et al. 1983;Rozanski et al. 1993;[START_REF] Gat | Oxygen and hydrogen isotopes in the hydrologic cycle[END_REF], amount effects under humid tropical and equatorial areas [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]Rozanski et al. 1993), and the recycling of continental moisture released during the evaporation of soils, rivers, lakes and closed marine basins [START_REF] Gat | Evolution of the isotopic composition of atmospheric waters in the Mediterranean Sea area[END_REF][START_REF] Froehlich | Deuterium excess in precipitation of Alpine regions-moisture recycling[END_REF]Wang & Yakir 2000;[START_REF] Gibson | Regional water balance trends and evaporationtranspiration partitioning from a stable isotope survey of lakes in northern Canada[END_REF][START_REF] Aemisegger | Deuterium excess as a proxy for continental moisture recycling and plant transpiration[END_REF][START_REF] Langman | Spatial distribution of δ 2 H and δ 18 O values in the hydrologic cycle of the Nile Basin[END_REF]Wang et al. 2016;[START_REF] Juhlke | Assessing moisture sources of precipitation in the Western Pamir Mountains (Tajikistan, Central Asia) using deuterium excess[END_REF]. [START_REF] Craig | Isotopic variations in meteoric waters[END_REF] and refined later by [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF] who considered mean isotopic compositions weighted by the amount of precipitation. Basically, the isotopic compositions obey a Rayleigh's distillation law in both evolving reservoirs of vapour water and condensed water.

Moreover, condensation in clouds globally proceeds from warm low latitudes towards cold high latitudes. Air temperature in the residual cloud tends to decrease resulting in an increasing isotopic difference between precipitation and water vapour. The GMWL relationship is very useful in hydrology to correlate experimental data and to understand how the atmospheric water vapour forms the various sources of freshwater on the continents. O composition of its source (SMOW), meaning that atmospheric vapour is not in equilibrium with seawater. Such pattern is mainly the consequence of the evaporation of the sea surface under an atmosphere with a relative humidity (RH) lower than 100%. Indeed, evaporation of water involves both equilibrium and kinetic effects whose respective magnitudes depend on temperature, relative humidity and wind velocity [START_REF] Craig | Deuterium and oxygen-18 variations in the ocean and the marine atmosphere[END_REF]. Consequently, the slope and intercept values of the GMWL record the evaporative conditions (T, RH) that prevail at the source of the humid air masses.

However, it is of prime concern that large variations in the slope and intercept values are observed at the regional or local scale [START_REF] Kendall | Distribution of oxygen-18 and deuterium in river waters across the United States[END_REF]Sharp 2017). For example, the case of tropical islands with slopes close to 5 or 6 was identified in the pioneering study of [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]. Indeed, the geographic distribution of  18 O and  2 H values of precipitations also partly reflects the partition of climate modes at a regional scale. As underlined by [START_REF] Kendall | Distribution of oxygen-18 and deuterium in river waters across the United States[END_REF], LMWL from the western and southern areas of USA have S lower than 6. Sharp (2007) concluded that the GMWL represents a weighted average of all LMWL which slopes are commonly lower than 8 with a large range of intercept values that may be either negative (down to -2) or positive (up to 15). Variations of slope and intercept values constitute a critical aspect when considering the objectives to reconstruct past climates on the basis of continental proxies. Consequently, the identification of regional processes needs to be integrated into models simulating past or future climates of the Earth, especially for glacial stages or super-greenhouse periods. We have chosen the example of the European continent well documented by the isotopic data collected through time by the numerous IAEA sites. Europe is also characterized by one main source of precipitation that formed from the prevailing moisture-bearing westerly winds, except in the Mediterranean Basin. Indeed, the Mediterranean basin is characterized by a specific  18 O-T relationship compared to the rest of Europe as shown, for example, by [START_REF] Gat | Evolution of the isotopic composition of atmospheric waters in the Mediterranean Sea area[END_REF] and [START_REF] Lécuyer | Tsunami sedimentary deposits of Crete records climate during the 'Minoan Warming Period' (≈3350 yr BP)[END_REF]. This area is characterized by specific patterns of atmospheric circulation with Increasing slopes S record the increasing degree of continentality that are defined here by indices quantifying the intra-annual temperature and precipitation amplitudes. We also show the important role of the dew point of a humid air mass that partly controls the slope S of meteoric water lines under mid latitudes. The influence of neighbouring water masses (oceans, close marine basins and lakes) on the amount of vapour water potentially recycled into local precipitations is also discussed.

Basic principles

Continentality

IAEA stations are geo-referenced in terms of latitude () and longitude () coordinates. Since the beginning of the XX th century, indices of continentality have been proposed. [START_REF] Conrad | Usual formulas of continentality and their limits of validity[END_REF]'s K index, derived from [START_REF] Gorczyński | Sur le calcul du degré du continentalisme et son application dans la climatologie[END_REF] and [START_REF] Johansson | Über die Asymmetrie der meteorologischen Schwankungen[END_REF]'s ones, depends on A T , the temperature difference between the warmest and the coldest month of the year, and is defined as follows:

𝐾 = 1.7𝐴 𝑇 sin(𝜆+10) -14
(1)

This continentality index K has been revised by [START_REF] Hela | Regional distribution of the continentality in the climate of the oceans[END_REF] as follows:

𝐾 = 1.97𝐴 𝑇 0.92+sin(𝜆-53) -13.3 (2) A C C E P T E D M A N U S C R I P T
These equations are all based on two boundary conditions that are an index of 0 for a theoretical oceanic climate without any influence of emerged lands and an index of 100 for a continental climate without any influence of a water mass.

Relative Humidity and Dew Point

The relative humidity RH (in %) is basically the ratio between the actual water vapour pressure P w and the saturation water vapour pressure (P s ):

𝑅𝐻 = 𝑃 𝑤 𝑃 𝑆 100 (3) with 𝑃 𝑆 = 𝐶. 𝑒𝑥𝑝 (𝐴.𝑇) (𝐵+𝑇) (4)
and

𝑃 𝑤 = 𝑛𝑅𝑇 𝑉 (5)
T and V are respectively the temperature and the volume of the gas mixture, R is the ideal gas Here, we propose to refer to a review of these equations and parameters according to [START_REF] Gibbins | A survey and comparison of relationships for the determination of the saturation vapour pressure over plane surfaces of pure water and of pure ice[END_REF], [START_REF] Alduchov | Improved Magnus form approximation of saturation vapor pressure[END_REF] and [START_REF] Lawrence | The relationship between relative humidity and the dewpoint temperature in moist air: A simple conversion and applications[END_REF]. In practice, the dew point T d is usually calculated from RH:

A C C E P T E D M A N U S C R I P T 𝑇 𝑑 = 𝐵[𝑙𝑛( 𝑅𝐻 100 )+ 𝐴𝑇 𝐵+𝑇 ] 𝐴-𝑙𝑛( 𝑅𝐻 100 )- 𝐴𝑇 𝐵+𝑇 (6)
where T is the air temperature ("dry-bulb temperature") in °C. This equation applies for RH > 0 over the temperature range -40°C ≤ T ≤ +50°C. For a RH of 100, T d equals T while at RH < 100% T d is always lower than T. At the surface of the Earth, T d varies from +30°C under equatorial and humid tropical belts down to -40°C at the poles and under high-latitude continental areas such as Canada and Siberia [START_REF] Mourshed | Climatic parameters for building energy applications: A temporal-geospatial assessment of temperature indicators[END_REF].

Slopes of Meteoric Water Lines (MWL) and deuterium-excess

Starting from a generic formulation of the linear equation describing the Global Meteoric Water Line (GMWL) expressed as follows:

 2 H = S. 18 O + b (7)
with 'S' being the slope and 'b' the intercept of the linear equation. For a slope S = 8, the intercept 'b' is called the "deuterium excess" (d or d-excess) defined for the first time by [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF].

Indeed, during evaporation of a water mass, the water vapour is less depleted in deuterium than in 18 O as a consequence of the differential rates of diffusivity of water isotopologues at the interface where the transition between the two states of water takes place. This parameter is especially sensitive to the physical conditions (temperature and moisture content of the air) prevailing during the evaporation of the sea surface [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF][START_REF] Johnsen | The origin of Arctic precipitation under present and glacial conditions[END_REF][START_REF] Pfahl | Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean[END_REF]. [START_REF] Craig | Isotopic variations in meteoric waters[END_REF] found that the global mean meteoric water has a d-excess of 10‰, reflecting sea surface waters out of equilibrium with the atmosphere. This deuterium excess corresponds to a temperature close to 25°C and a mean relative humidity lower than 100% at the seawater-atmosphere interface. More precisely, [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF] computed a RH value of 81% and a temperature of evaporation of 26°C for the GMWL equation determined by [START_REF] Craig | Isotopic variations in meteoric waters[END_REF] while comparable values were obtained for [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF] equation (RH = 79.5% and T = 25.4°C). It is worthy to note here that a higher value of 85% for RH was proposed by [START_REF] Clark | The environmental isotopes[END_REF]. Slope S is an interesting second-order

A C C E P T E D M A N U S C R I P T
at University College London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from parameter as its relative variation can be analysed independently of a reference linear equation such as the GMWL (S = 8; d = 10):

S = ( 2 H -b)/ 18 O (8)
This slope S corresponds to the ratio between the isotopic fractionation factors  defined for the two isotopic systems at equilibrium between the two co-existing water phases, which means liquid water and water vapour or water ice and water vapour. In the case of the liquid water  water vapour state change, the mean slope S is expressed as follows:

𝑆 = ( 𝐻 2 𝐻 1 ⁄ ) 𝑙𝑖𝑞𝑢𝑖𝑑 ( 𝐻 2 𝐻 1 ⁄ ) 𝑣𝑎𝑝𝑜𝑢𝑟 -1 ( 𝑂 18 𝑂 16 ⁄ ) 𝑙𝑖𝑞𝑢𝑖𝑑 ( 𝑂 18 𝑂 16 ⁄ ) 𝑣𝑎𝑝𝑜𝑢𝑟 -1 = 𝛼 2 (𝑙𝑖𝑞𝑢𝑖𝑑-𝑣𝑎𝑝𝑜𝑢𝑟) -1 𝛼 18 (𝑙𝑖𝑞𝑢𝑖𝑑-𝑣𝑎𝑝𝑜𝑢𝑟) -1 (9)
The rainfall is usually considered at isotopic equilibrium with vapour because the formation of water droplets in clouds takes place when saturation or oversaturation in water vapour is reached. -Stations that are under the regional climatic influence of the Mediterranean basin were excluded. Indeed, The Mediterranean basin is characterized by a specific  18 O-T relationship compared to the rest of Europe as shown, for example, by [START_REF] Gat | Evolution of the isotopic composition of atmospheric waters in the Mediterranean Sea area[END_REF] and [START_REF] Lécuyer | Tsunami sedimentary deposits of Crete records climate during the 'Minoan Warming Period' (≈3350 yr BP)[END_REF]. This area is characterized by specific patterns of atmospheric circulation with dry and cold continental air masses that interact with a marine basin characterized by high evaporation rates and relative high sea surface temperatures (≈ 20°C). Such air-sea interactions favor the genesis of cyclones, especially in the eastern part of the Mediterranean Sea. The Mediterranean climate is characterized by warm to hot dry summers along with mild and wet winters (see [START_REF] Lionello | The Mediterranean climate: An overview of the main characteristics and issues[END_REF] for a detailed definition). It corresponds to a geographic area located on the west side of continents between 30° and 40° of latitude in the vicinity of a large mass of water (Figure 1B).

-Stations located at elevations equal to or higher than 1000 m were discarded to avoid isotopic effects related to the orographic lifting of humid air masses.

-LMWL have only been calculated for stations for which at least 5 years of record are available with at least 9 months recorded in the year in order to increase the robustness of the database.

-Only LMWL characterized by robust coefficients of determination (R 2 ≥ 0.75) have been kept in the final database. Most IAEA stations have R 2 ≥ 0.9 and the median value is 0.98 (Supplementary Figure 2).

Finally, filtered data allowed the calculation of the slopes of the corresponding

 2 H- 18 O LMWM
determined for each station (see Supplementary Table 1). One example is provided with the IAEA station in Berlin, Germany (Figure 2). Slopes S of LMWL have been ultimately correlated to the longitude of IAEA stations, the annual amplitude of air temperatures and precipitation, the Hela index K of continentality and the mean annual dew point.

A C C E P T E D M A N U S C R I P T

Results

Continentality

Studied European IAEA stations range from -10°W to 57°E in longitude and from 42°N to 69°N

(Figure 1A). The index K of continentality [START_REF] Hela | Regional distribution of the continentality in the climate of the oceans[END_REF]) increases significantly eastward (Figure 3A) according to equation ( 10 It is worthy to note that the temperature amplitude in our data set is strongly correlated with the longitude but not the latitude. This last observation is not consistent with the well-known increase in temperature amplitude with increasing latitude at the global scale, as mentioned by [START_REF] Hela | Regional distribution of the continentality in the climate of the oceans[END_REF]. The apparent paradox between our observations and this basic principle is mostly solved by considering the Southwest -Northeast distribution of the IAEA stations, which results in a positive relation between latitude and longitude (Figure 1A). We can also consider that the northwestern part of Europe receives a sizable amount of heat from the Gulf Stream, which contributes to reduce the latitudinal temperature gradient. Consequently, most IAEA stations have longitudes that increase with latitudes (Figure 3C).

Relative Humidity and Dew Point

According to maps computed by [START_REF] Mourshed | Climatic parameters for building energy applications: A temporal-geospatial assessment of temperature indicators[END_REF]

The slope S

Slope S of LMWL recorded at European IAEA stations are reported versus  (Figure 4A). These The value of 15 added to  was arbitrary chosen to make all the longitude values positive.

Interpretation of data and discussion

Slope of the LWML as a function of equilibrium temperature

Genesis of rain or snow during the condensation of cloud water vapour is considered as an isotopic equilibrium process, hence only related to air temperature [START_REF] Juhlke | Assessing moisture sources of precipitation in the Western Pamir Mountains (Tajikistan, Central Asia) using deuterium excess[END_REF]. According to this model, the hydrogen and oxygen isotope ratios of precipitation are highly sensitive to the latitude and altitude of the site as well as air temperature (Rozanski et al. 1993). The stable isotope compositions also correlate with increasing continentality because of the successive precipitation events that obey the Rayleigh distillation process [START_REF] Clark | The environmental isotopes[END_REF][START_REF] Gat | Environmental isotopes in the hydrological cycle[END_REF].

Several temperature-dependent isotope fractionation equations have been published so far in the scientific literature for both Of great importance for the understanding of the isotopic composition of precipitation, which takes place in the atmosphere below the "freezing level" or 0°C isotherm, is the knowledge of the isotopic fractionations between water ice and water vapour. For example, [START_REF] Majoube | Fractionation factor of 18 O between water vapour and ice[END_REF] proposed the following equation for the oxygen isotopes in the temperature range -30°C to 0°C: 15) to (18), we define S = ( 2 H-1)/( 18 O-1) for the liquid water-water vapour and water ice-water vapour systems as functions of temperature (Figure 5). We also emphasize that T d values over Europe are compatible with S increasing eastward from ≈ 6 to ≈ 9). Supercooled water droplets may form in high-altitude clouds at temperatures as low as -40°C. In this case, S would approach a value close to 9 (Figure 4B).

Sources of air moisture over Europe

Moisture over Europe is mainly transported by the westerlies that carry the vapour generated by the evaporation of the Central and North Atlantic surface waters. Outside the area under the Mediterranean influence, moisture over Europe mainly comes from the Central and North Atlantic surface waters, and is transported eastwards by the westerlies. Arctic winds, bringing moisture from the Norwegian Sea, can reach Europe during winter.

Westerlies bring to western Europe warm and humid air masses responsible for large amounts of precipitation during winter and the intermediate seasons. During summer, westerlies tend to bring cooler air than that present over the continent [START_REF] Barriopedro | Witnessing North Atlantic westerlies variability from ships' logbooks (1685-2008)[END_REF]. At a synoptic scale and assuming a closed water cycle (e.g. [START_REF] Jouzel | Deuterium and oxygen-18 in present-day precipitation: data and modelling[END_REF], deuterium excess content of average oceanic water vapour at the site of evaporation is mainly controlled by relative humidity (RH), sea surface temperature (SST) and, to a lesser extent, wind velocity, [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF]Aemsigger et al. 2014). Precipitation along the GMWL is generated from a vapour formed at RH and SST values of 85% and 25°C, respectively [START_REF] Clark | The environmental isotopes[END_REF]. Several linear relationships between RH and SST have been established yet [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF]Jouzel et al. 1996;[START_REF] Pfahl | Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean[END_REF][START_REF] Aemisegger | Deuterium excess as a proxy for continental moisture recycling and plant transpiration[END_REF][START_REF] Pfahl | What controls deuterium excess in global precipitation?[END_REF]. All of them indicate that the deuterium excess increases with temperature and decreases with RH (d = 0 for RH = 100%). In other words, it means that for temperatures higher than 25°C and RH lower than 85% at the source of evaporation, the first generated LMWL will have a slope lower than 8. Indeed, LMWL with low slopes, as low as 5, have been observed and discussed by [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF] in the case of several tropical islands such as Hawaii, USA. [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF] considered that such low slopes close to 5 cannot be explained by a "simple Rayleigh condensation process under equilibrium conditions … since the calculation on such processes gives 7.5". Indeed, a cloud can receive additional vapour from local oceanic sources along its travel path and this vapour might not be strictly produced under equilibrium conditions but to some extent by kinetic/non-equilibrium isotopic fractionation because of the fast evaporation process. In the case of the Atlantic Ocean and the western European coast, we can suspect that humid air masses are fed by moisture derived from a nearby evaporating oceanic surface. Such process may explain why LMWL recorded at stations located along the western Atlantic coast (La Coruna, Spain; Valentia, Ireland; Wallingford and Keyworth, UK) have slopes close to 6 instead of 8.

Slopes of MWL over Europe tend to slightly increase eastward even if we do not take into account the large potential reservoir of moisture represented by the Atlantic Ocean bathing the western coast of Europe (called here "Atlantic Side Effect"). It is worthy to note that the origin of precipitation at a regional scale can generally be ascribed to three main sources that are local soil evaporation, plant transpiration and externally advected moisture [START_REF] Bisselink | Precipitation recycling: Moisture sources over Europe using ERA-40 data[END_REF].

According to [START_REF] Gibson | Regional water balance trends and evaporationtranspiration partitioning from a stable isotope survey of lakes in northern Canada[END_REF], soil-derived moisture is depleted in 18 O and 2 H relative to its source while transpiration does not produce isotopic fractionation between moisture and soil water because "the transfer of moisture to the atmosphere by plants is essentially quantitative, and heavyisotope enrichment is restricted to internal plant waters". About 60% of the total evapotranspiration flux over land enters the atmosphere via transpiration (Schlesinger & Jasechko 2014), which means that local soil evaporation represents about 40% of the total flux of recycled moisture (Trenberth 1999;[START_REF] Bisselink | Precipitation recycling: Moisture sources over Europe using ERA-40 data[END_REF][START_REF] Froehlich | Deuterium excess in precipitation of Alpine regions-moisture recycling[END_REF][START_REF] Kong | Quantifying recycled moisture fraction in precipitation of an arid region using deuterium excess[END_REF]Wang et al. 2016;Wypych et al. 2018). In Europe, the atmospheric water vapour

A C C E P T E D M A N U S C R I P T
at University College London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from content, which depends on air temperature, decreases along a southwest-northeast gradient except during the summer season (Wypych et al. 2018). Indeed, water vapour pressure decreases from about 15 to 5 kg.m -2 in January, and from about 20 to 10 kg.m -2 in April and October, and increases from about 20 to 30 kg.m -2 in July (Wypych et al. 2018). Recycling of soil moisture mixed with the externally advected moisture carried out by the westerlies may contribute to decrease the slope of LMWL at the westernmost stations.

Rain drops are subject to evaporation during their fall, in particular when they travel through a dry and warm column of air (Stewart 1975). Evaporation of rain drops is known to change dramatically the original composition of rainfall in the warmest and driest regions of the world such as Central-Eastern Africa [START_REF] Balagizi | Influence of moisture source dynamics and weather patterns on stable isotopes ratios of precipitation in Central-Eastern Africa[END_REF][START_REF] Balagizi | Key factors of precipitation stable isotope fractionation in Central-Esatern Africa and Central Mediterranean[END_REF]. Under unsaturated conditions, i.e. RH < 100%, differences in diffusivity coefficients between H 2 O isotopologues [START_REF] Merlivat | Molecular diffusivities of H 2 16 O, HD 16 O and HD 18 O in gases[END_REF][START_REF] Barkan | Fractionation of oxygen and hydrogen isotopes in evaporating waters[END_REF]) induce out of equilibrium isotopic fractionations. As a result, the ratio between 2 H/ 1 H and 18 O/ 16 O in the rainfall reaching the ground differs from the one in the cloud vapour. Albeit to a lesser degree, evaporation of falling water drops may decrease the slope of the LMWL in southwest Europe, which is characterized by low RH in January (RH ≈ 50%) and July (RH ≈ 40%) according to data provided by Wypych et al. (2018). In addition, mild to warm temperatures prevail all year round with daily temperatures that can exceed 30°C during summer.

Finally, in the case of Europe, it is expected from the reasoning mentioned above that the slope S of the  Present-day GMWL and LMWL are most likely valid for the Holocene but risky to apply to older periods, especially glacial stages for which RH and prevailing winds may differ from today and have strong impacts at least on the intercept of linear equations [START_REF] Merlivat | Global climatic interpretation of the deuterium-oxygen 18 relationship for precipitation[END_REF]. Moreover, changes in continental ice volume have direct consequences on the  18 O of the global ocean [START_REF] Lambeck | Sea level and global ice volumes from the Last Glacial Maximum to the Holocene[END_REF] from which precipitations ultimately derive. The variations in slopes and intercepts of LMWL we observed over Europe also mean that the relationships between air temperature and  18 O of precipitation vary from one location to another. At a regional scale, robust linear relationships have been evidenced between mean air temperature (MAT) and oxygen isotope compositions of precipitation ( 2). Both locations share relative dry summers and rainy winters. Using equation ( 19) as a reference for Europe would allow mean air temperatures to be overestimated by a few degrees (between 3°C and 4°C) for Valentia, Ireland, while air temperatures calculated at La Coruna, Spain, would match that deduced from the local equation (Supplementary Table 2).

Wallingford, UK, is located in south England inland, the slope of its LMWL is higher (S = ≈ 6.6) than for the two previous mentioned IAEA stations, 2). Error in the estimate of air temperature using equation ( 19) would be only 1°C by reference to the local equation. Finally, eastward, Berlin, Germany, Kalinin and Arkhangelsk, Russia, are characterized by "continental-boreal" climatic conditions (Figure 1B) and they have the highest S of 7.97, 8.43 and 8.53 for their LMWL, respectively.  18 O mw display strong and positive correlations with air temperatures and weaker but positive correlations with precipitations (Supplementary Table 2). Those three locations share relative wet summers and less humid winters. Estimates air temperatures using equation ( 19) do not allow errors larger than 1°C by reference to local equations (Supplementary Table 2).

Reconstructions of air temperatures in Europe during the Holocene have been commonly performed on the basis of global or regional equations relating  18 O mw to air temperatures that were applied to the  18 O of lacustrine carbonates [START_REF] Hammarlund | Holocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden[END_REF][START_REF] Makhnach | Stable oxygen and carbon isotopes in Late Glacial-Holocene freshwater carbonates from Belarus and their palaeoclimatic implications[END_REF], speleothems [START_REF] Denniston | Integrating stalagmite, vertebrate, and pollen sequences to investigate Holocene vegetation and climate change in the southern Midwestern United States[END_REF]Lechleitner et al. 2018 and references therein), invertebrate remains such as aragonite from mollusc shells [START_REF] Baroni | Mollusca stable isotope record of a core from Lake Frassino, northern Italy: hydrological and climatic changes during the last 14 ka[END_REF][START_REF] Leng | Oxygen isotopes in Molluscan shell: applications in environmental archaeology[END_REF] and vertebrate remains of mammal teeth [START_REF] Iacumin | Stable isotope analyses of mammal skeletal remains of Holocene age from European Russia: a way to trace dietary and environmental changes[END_REF]Pilaar Birch et al. 2016). Fossil remains of invertebrates or vertebrates are generally retrieved from specific locations and their oxygen isotope compositions mostly record local climatic conditions. Moreover, some fossil remains such as rodent teeth (Royer et al. 2013) or earthworm calcite granules (Prud'homme et al. 2016) are known to only record the warmest months of the year. In areas characterized by a strong air temperature seasonality (e.g. Germany, Russia), errors in the calculation of air temperatures, which are based on the  18 O of either apatite phosphate or calcite, may exceed 5°C when using equation ( 19) (Supplementary Table 2).

Our study shows that it is tricky to apply regional or global-scale relationships between air temperature and  18 O mw to local isotopic records. Error estimates in air temperatures reconstructions may reach several degrees, either underestimating or overestimating real temperatures that prevailed during the life of animals. Therefore, we emphasize that distinct and appropriate equations must be applied, for example, to the Iberian Peninsula, for which a large surface is under the influence of the water lines (LMWL) determined for each station. For linear relationships defined over periods of at least five years and characterized by a coefficient of determination higher than 0.75, S increases with longitude  from ≈5 (Portugal) to ≈9 (Russia). S are positively correlated to relative air humidity, and continentality, which is quantified by the intra-annual amplitude of air temperatures. On the basis of our knowledge of isotopic fractionation that takes place during water phase changes, S close to 9 are expected to reflect hydrogen and oxygen isotope fractionation close to equilibrium during condensation of water vapour into liquid water or snow. Slopes that tend to decrease down to 5 are interpreted, in most cases, as the result of an increasing recycling of water vapour derived from the surface of the nearby oceanic waters ("Atlantic Side Effect"). In addition to this process, enrichment in heavy isotopes also took place in southwest Europe during the partial evaporation of water droplets during their fall in air of relative low humidity and cool to warm temperature.

Variations in slopes and intercepts of LMWL at a local scale over Europe also imply distinct linear equations between air temperature T and  The are grateful to Prof. Kazimierz Rozanski for preliminary discussions and his advice that contributed to improve the scientific content of this study. Ph. Boulvais and one anonymous reviewer are also thanked for their helpful and constructive comments and suggestions that helped us to improve the scientific content of this study. O slopes recorded at each station georeferenced by its latitude () and longitude (). IAEA stations that are under the influence of the Mediterranean climate, i.e. those located around the Mediterranean Sea, were not taken into account to build the database. The background map is extracted from Google Earth®. The correspondence between numbers reported on the map and city names is listed as follows:

1 -Valentia, Ireland. 2 -Inchnadamph, U.K. 3 -Plouzané, France. 4 -Altnabreac, U.K. 5 -Wallingford, U.K. 6 -Keyworth, U.K. 7 -Dax, France. 8 -Cestas-Pierroton, France. 9 -Fleam Dyke, U.K. 10 -Campistrous, France. 11 -Orléans-La-Source, France. 12 -Braakman, Netherlands. 13 -De Kooy, Netherlands. 14 -Gilze-Rijen, Netherlands. Russia. 61 Ukraine. Spain.

  precipitations (rain, snow) follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL: ). It was defined for the first time by

.

  London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from dry and cold continental air masses that interact with a marine basin characterized by high evaporation rates and relative high sea surface temperatures (≈ 20°C). Such air-sea interactions generate important sites of cyclogenesis, especially in the eastern part of the Mediterranean Sea. precipitation have been therefore compiled from European IAEA stations, excluding areas that are under the regional climatic influence of the Mediterranean basin. Those data allowed the calculation of the slopes of the corresponding  determined for each station. The IAEA stations are widespread over a This study aims to demonstrate that changes in S values of the local meteoric water line track the evolution of a humid air mass that progressively condenses eastward over Europe.

  constant and n is the number of moles of water vapour. A = 7.625, B = 243.04°C and C = 610.94 Pa. The Dew Point (T d ), expressed in °C, is the temperature a humid air mass should have to start condensing water. In other words, T d represents the temperature at which the actual water vapour pressure equals the saturated water vapour pressure (RH = 100%). Several empirical equations have been derived these last decades from Magnus (1844)'s equation. Various set of fitting parameters are available in the literature.

  precipitation have been compiled from European IAEA stations (Figure1A) along with the linear relationship for Europe illustrated in Supplementary Figure1. Local Meteoric Water Lines (LMWL) were generated using weighted averages of monthly precipitation over several years. The slopes S of the  London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from Several criteria were used to filter the database as follows:

  Amplitudes in temperature T A increase with  (Figure3B) according to equation (11): T A = 0.37(±0.01) + 14.23(±0.28) with R 2 = 0.91 (11)

  slopes range from about 6 to 10, although three stations (Odessa, Ukraine; Riga, Latvia and St.Petersburg, Russia) display quite large standard deviations around the mean, two of them, with the highest slope values close to 10, being outliers (Riga and St. Petersburg). In this data set, S regularly increases eastward (Figure4B) and is best described by a logarithmic fitting as follows: S = 2.45(±0.25)log 10 ( + 4.13±(0.35) with

  liquid water and vapour water[START_REF] Halas | About isotope equilibrium between liquid and vapour phases[END_REF]. A pioneering experimental study was performed by[START_REF] Majoube | Fractionnement en oxygene-18 et en deuterium entre l'eau et sa vapeur[END_REF] who proposed the two following isotopic fractionation equations determined in the temperature range 0°C-100°C:



  (ice-vap) = -94.5 + 16.289

  London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from

  increase as the clouds are progressively moving away from the Atlantic side. thermodynamic equilibrium during phase changes (liquid water  water vapour and water ice  water vapour) predict slopes in the range 8.5-10 as it observed in the easternmost part of Europe. London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from

  18 O mw ) for Europe (Mediterranean Basin excluded; Lécuyer 2013): MAT (°C) = 1.41(±0.09) 18 O mw + 22.02(±0.83) scale in the studied Europe area, we also observe some differences in mean annual and seasonal patterns of air temperatures between La Coruna, Valentia, Wallington, Berlin, Kalinin and Arkhangelsk (Figure 1A), which are IAEA stations selected for the demonstration, and located along a southwest-to-northeast gradient. La Coruna, Spain and Valentia, Ireland, are two cities located along the westernmost side of the Atlantic coast (Figure 1A). They are both characterized by low slope values of their LMWL, positive correlations between  18 O mw and monthly mean air temperatures, while strong negative correlations are observed between  18 O mw and monthly mean precipitations (Supplementary Table

18O

  mw are strongly and positively correlated with air temperatures while they are poorly and negatively correlated with precipitations (Supplementary

  London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from Mediterranean climate while the remaining one is exposed to the Atlantic seaside. As a general rule, equations differ between the southwest-west part of Europe with warm temperate and "oceanic" climate conditions and the north-northeast one that is characterized by cold temperate and continental or boreal climate conditions. Improved modelling of the past water cycle with an increased spatial resolution should provide the appropriate LMWL equations and those relating T to  18 O mw . It is of paramount importance if we want to bear in mind the objectives of connecting faunal assemblages, migration pathways, and the presence of human beings to climate dynamics during the Holocene. precipitation have been compiled from European IAEA stations, excluding areas that are under the regional climatic influence of the Mediterranean basin.Those data allowed the calculation of the slopes S of the corresponding 

18O

  of precipitation. Consequently, shifts in air temperature a few °C. Such isotopic properties of the water cycle put severe limitations on the use of regionally-based T- 18 O mw equations applied to the oxygen isotope compositions of biogenic calcium carbonates and phosphates that constitute the skeletons of most invertebrates and vertebrates. Distinct and appropriate equations must be established for sub-regions such as the Iberian Peninsula, the southwest and west Atlantic seaside and the east-northeast European areas.

  Figure captions

  93 -Leon/Virgen, Spain. 98 -Santander, Spain. B -Map of the European climate zones after Schneider et al. (2013). Empty arrow: direction of Arctic winds; Black arrow: direction of westerlies. Climatic zones: red = Mediterranean; bottle green = Temperate oceanic; green = temperate transitional; light green = temperate continental; pink = boreal; blue = polar.

Fig. 2 .

 2 Fig. 2. Monthly weighted  2 H reported against the monthly weighted  18 O of precipitation in Berlin

Fig. 3 .

 3 Fig. 3. A -Hela index K of continentality (see equation (2) for a definition) reported against the

Fig. 4 .

 4 Fig. 4. A -Slope S of the MWL recorded at each IAEA station as a function of their longitude . Two

  , the mean annual Dew Point values decrease over highly contrasted for IAEA stations located far inland (see for example Berlin, Germany and Arkhangelsk, Russia) while they are rather limited (a few degrees in the positive values) for western coastal locations such as La Coruna, Spain, or islands such as Valentia, Ireland.

	Downloaded from	http://sp.lyellcollection.org/	at University College London on July 7, 2020
	summer) are
			U S C R I P T
			A N
		M
	C C E P T E D
	A
	Europe along a southwest-northeast gradient, from about 15°C in Portugal down to about -20°C in
	Russia. Along this gradient, the mean air temperature (MAT) ranges from +17°C to -1°C and RH

values increase from about 35% up to 75% in July and from about 50-55% to 85-90% in January

(NCARS 2017; Wypych et al. 2018)

. Seasonal variations in T d values (negative in winter, positive in

at University College London on July 7, 2020 http://sp.lyellcollection.org/ Downloaded from

This slope ranges from about 6 to 9 after the removal of three stations (Odessa, Ukraine; Riga, Latvia and St. Petersburg, Russia) that display quite large standard deviations around the mean value, including two outliers (Riga and St. Petersburg). Fig. 5. Slopes S of the MWL calculated as a function of temperature T (°C) on the basis of the experimentally-determined oxygen isotope fractionation factors  between liquid water (l) and water vapour (v) (green curves) and between water ice (i) and water vapour (v) (purple curve). Plain green curve was computed by combining equations ( 13) and ( 14) determined by [START_REF] Majoube | Fractionnement en oxygene-18 et en deuterium entre l'eau et sa vapeur[END_REF]; dotted green curve was computed by combining equations ( 15) and ( 16) determined by [START_REF] Horita | Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature[END_REF]; purple curve was computed by combining equations ( 17) and ( 18) determined by [START_REF] Majoube | Fractionation factor of 18 O between water vapour and ice[END_REF] and [START_REF] Merlivat | Fractionnement isotopique lors des changements d'état solide-vapeur et liquide-vapeur de l'eau à des températures inférieures à 0°C[END_REF].


Madougou Saïdou

Saïd Frederique
email: frederique.said@aero.obs-mip.fr

Campistron Bernard
email: bernard.campistron@aero.obs-mip.fr

Cheikh Fadel Kebe

Low Level Jet Wind Shear in the Sahel

Keywords: Low level jet, Wind shear, Wind shear direction, Aircraft

In the Sahel, a vertical wind shear appears in the dry and in the wet seasons. In Niamey, Niger, during the dry season, the period of strong shears is clearly linked to the Nocturnal Low Level Jet (LLJ) since it occurs in a narrow time period around 06H00 UTC at 60% of the cases reach shears which require an alert to the pilots (higher than 4 ms -1 per 100 m). The majority of cases occur during the night with a wind shear direction between 90 and 150° per 100 m, which is shown that it is dangerous for aircraft. In Bamako, Mali, high wind shears represent (higher than 4 ms -1 per 100 m) only 16-22% of the cases and can occur at any time of the day. There are, however, 8% of the cases, the whole day long, when the wind shear can reach more than 6 ms -1 per 100 m. Most of the wind shear directions are also between 0 and 90° per 100 m during the night. This is why the Agency for the safety of aircraft navigation in Africa and Madagascar (ASECNA) has put in 2004 at Bamako airport an UHF wind profiler radar for monitoring nocturnal strong Low Level Jet wind shear which occur regularly in this airport.

Introduction

In the Sahelian band, the year is characterized by for four seasons: the dry season, the pre-monsoon, the monsoon and the post-monsoon. These seasons are linked to the northward migration of the Intertropical Convergence Zone (ITCZ) with the seasonal motion of the sun. South of the Sahara desert, there is a dry season in winter with dry, warm easterly to northerly winds (the Harmattan) associated with high pressure over the Sahara whereas, during summer, the monsoon winds blow from the south-west in the low layers, carrying cooler and moister air associated with rainfall. The migration of the ITCZ induces a meridional variability in rainfall. The diurnal cycle of the wind is marked and the vertical profiles do not follow the usual logarithmic profile close to the ground due to the existence of a strong nocturnal low level jet. The regularity of this diurnal cycle and the repetitivity of the wind structure are others factors which define the characteristics of the seasons. The Sahel is an area where the nocturnal low level jet occurs regularly. The strength, level and direction of these LLJs vary during the daytime [START_REF] Saïdou | Results of two eight-month, UHF radar observations of the nocturnal low-level jet for wind energy applications[END_REF].

Wind shears occurring near the ground can be due to low level jets, downbursts of thunderstorms or the presence of large obstacles on the ground. The wind shear between two levels is the difference between the two horizontal wind vectors. It is thus defined both in magnitude and in direction. Wind shears are known to disturb air properties around airplanes by generating sudden disturbance of the relative wind speed or by producing turbulence. They therefore constitute major risks during take-off and landing of the aircraft [START_REF] Blackadar | Boundary layer and maxima and their significance for the growth of nocturnal inversions[END_REF]. They are known to have been directly or indirectly responsible for many aircraft disasters between 1964 and 1986 in USA and some parts of the world. The aim of this paper is to present the nocturnal low level jet wind shear in the Sahel, which occurs near the ground and constitutes major risks during the take-off and landing of aircraft.

Materials and Methods

Our data were obtained from two wind profiler radars installed at Bamako in 2005 and at Niamey in 2006 and cover two eight-months of observations collected during the dry and wet seasons of the African Monsoon Multidisciplinary Analysis (AMMA) field campaign in West Africa. These wind profiler radars provided continuously vertical profiles of the wind with a good time resolution associated with the documentation of the vertical profile of the wind. and at 380 m asl in south west Mali (Figure 1). The radar installed at Bamako is an UHF PCL 1300 (Figure 2a) to monitor the nocturnal strong low-level wind shears that regularly compromise the safety of aircraft on landing and take-off. This radar was operated during 2005 from March 4 th until December 29 th without interruption except for an inactive period from May to June due to technical problems. This radar suffered from technical problems after December 2005, even after repair, the data were no longer recorded. UHF radar measures wind vertical profiles and backscattered signal strength between about 0.1 and 3-5 km depending on the conditions of the atmosphere. It operates by transmitting electromagnetic energy into the atmosphere and measuring the strength and frequency of the backscattered energy and can work in clear air or during precipitation events. The radars provide vertical profiles of the three components of the mean wind, turbulence parameters, Atmospheric Boundary Layer (ABL) top Zi and qualitative information on precipitation and air humidity. In clear air, the atmospheric signal is due to the backscattering by spatial irregularities of the air refractivity index at the scale of half the radar wavelength when the source of the fluctuations comes from turbulent mixing which follows the behavior of the Kolmogorov inertial subrange. Air refractivity depends on temperature and humidity, the latter dominating in the lower atmosphere. The observation made by those two Doppler profilers was based on a repetitive sequence cycling over the five beams, of 5.23-min duration in Niamey and 2.5-min in Bamako, and comprising a high and low collection mode. Vertical profiles of the wind are deduced from the Doppler spectra obtained at each level gate of the five radial directions for each cycle of measurements. The three wind components are deduced from the mean Doppler radial velocity measured on all beams with the assumption of local horizontal field homogeneity. Only the low mode will be used here. The difficulty of operating with profiler radar used in clear air is the fact that the meteorological echoes (produced by the wind) are very faint relative to the noise. So the data processing is rather complex.

Results and Discussion

Nocturnal Low-Level Jet Description: Figure 3 shows the monthly vertical profiles of wind speed observed at Bamako and Niamey at 00H00, 06H00, 12H00, 18H00 and 21H00 UTC during the four characteristic periods: April at Bamako and May at Niamey for the pre-monsoon period, August, October and December for the monsoon, post-monsoon and dry season respectively at both sites. These figures show an increase of wind speed between 200 and 800 m with a well marked maximum between 00H00 and 06H00 UTC, especially during the dry or pre-monsoon periods. This Nocturnal Low Level Jet is much less marked during the monsoon period. In general, the jet is not yet formed at 18H00 UTC and sometimes not even at 21H00 UTC, as was shown by Malcher and Kraus [START_REF] Malcher | Low-level jet phenomena described by an integrated dynamical PBL model[END_REF]. The maximum jet (wind speed of about 14 ms -1 ) occurs during the dry season at 06H00 UTC. The maximum of the jet is not located at a constant level in the different figures: the jet peak corresponds to the top of the stable nocturnal layer [START_REF] Blackadar | Boundary layer and maxima and their significance for the growth of nocturnal inversions[END_REF] and its evolution is linked to the evolution of the stability conditions. In our data set, most of the time the jet level increases between 00H00 and 06H00 UTC which is in agreement with the stability evolution. Despite the variability of profiles, the repetition of the structure of the jet in the low troposphere has enabled a composite day to be developed, corresponding to the average of profiles every half-hour for each month. The height-time cross sections in Figure 4 show the composite days of the horizontal wind in August and December. During the rainy season (Figures 4a and4b), the African Easterly Jet (AEJ) is positioned at the latitudes of Niamey and Bamako, and blows steadily from east to west, spreading from 3 to 5.5 km agl and peaking around 15 ms -1 at both locations, as shown by Kalapureddy et. al. [START_REF] Kalapureddy | Wind profiler analysis of the African Easterly Jet in relation with the Boundary-layer and the Saharan Heat Low[END_REF]. The Inter Tropical Discontinuity (ITD) is located between 15 and 20° north, and the low layers are affected by a weak monsoon flow (less than 3 ms -1 ), blowing from the south-west.

The interface between the monsoon and the Harmattan is roughly at 1000 m. A cell of stronger wind appears at night: it corresponds to the LLJ which brings moisture from lower latitudes into a thin layer close to the surface as described in Lothon et. al. [START_REF] Lothon | Observation of the diurnal cycle in the low troposphere of West Africa[END_REF]. It only reaches 5-7 ms -1 on average during the rainy season. This jet is much more marked during the dry season (Figure 4c and4d) when it reaches 13 ms -1 . Its direction is then north-east, which is also the direction of the Harmattan, which blows up to 2-3 km. Above 4 km, we find the Subtropical Westerly Jet. The only difference between Bamako and Niamey is the intensity of the Harmattan in the middle of the day. In both locations, the LLJ disappears with the morning onset of turbulence and the wind weakens in Niamey in the low layers (5-6 ms -1 ) as described in Madougou et. al. [START_REF] Madougou | Nine months of uhf wind profiler radar observation of low atmosphere at Bamako and Niamey[END_REF].

An example of the characteristics of the lower troposphere is presented in Figure 5 and In the early morning, the jet can reach 10 ms -1 and its depth can grow to 1200 m. This jet is roughly centered at 06H00 UTC and disappears shortly after 10H00 UTC. During this period, the monsoon blows from the south-west, advecting moisture at low levels and generating a high wind shear and a strong gradient of humidity at its top, above which the Harmattan blows. These shears and humidity gradients produce reflectivity maxima, which are well marked in Figures 5b and6b. In these figures, some reflectivity maxima appear in the night. Those correspond to the upper part of the jet. After sunrise, the thermal turbulence grows and gradually destroys the jet. The development of the atmospheric boundary layer (the top of the ABL is indicated by the maximum of the reflectivity, symbolized by black dots in the figures) starts slowly in the morning (before 10H00 UTC) and accelerates once the wind weakens. The top of the atmospheric boundary layer can reach 4-5 km (April 26 th at Bamako, May 4 th at Niamey). The development of the ABL also appears between 12H00 and 18H00 UTC in Figures 5c and6c, where the dissipation rate of turbulent energy is represented. On the other hand, the high nocturnal values of dissipation remain unexplained so far. Wind Shear: The wind shear between two levels is the difference between the two horizontal wind vectors. It is thus defined both in magnitude and in direction:

2 / 1 2 2                           z v z u WS mag (1)                              z v z u arctg WS dir (2)
Wind shears are known to disturb air properties around airplanes by generating sudden disturbance of the relative wind speed or by producing turbulence. They therefore constitute major risks during take-off and landing [START_REF] Blackadar | Boundary layer and maxima and their significance for the growth of nocturnal inversions[END_REF]. They are known to have been directly or indirectly responsible for many aircraft disasters between 1964 and 1986 in some parts of the world and especially in the USA, where traffic is very heavy (see the statistics of aircraft accidents related to wind shear in the Federal Aviation Administration circular) [START_REF]Pilot wind shear guide: advisory circular AC N°00-54[END_REF].

Shears occurring near the ground can be due to low level jets, downbursts of thunderstorms or the presence of large obstacles on the ground (mountains, large buildings, etc.). In the Sahel lower troposphere, a vertical wind shear also appears between the monsoon layer and the Harmattan layer, as was seen (Figure 7) between 1000 m and 1500 m in August. This shear is not dangerous, because it is not too close to the surface and secondly, because the wind intensity is very low in this area. In contrast, the shear generated by the low level jet is far more dangerous.

Some studies [START_REF] Abdou | The diurnal cycle of lower boundary layer wind in the West African monsoon[END_REF] calculated the magnitude of the wind shear between 30 and 300 m, from SODAR observations installed in the framework of AMMA on sites located about sixty kilometers east of Niamey. The first author is a forecaster at the National Meteorology Directorate of Niamey and he knows the standards of safety procedures for aircraft taking off or landing at an airport. In practice, an alert is triggered when the wind speed measured by the radiosonde at 600 m is above 30 knots (15 ms -1 ). Considering that the wind shear is maximum between 0 and 300 m, from the observations they made between the beginning of June and mid-August, this threshold of 15 ms -1 corresponds to a wind shear of 5 ms -1 per 100 m. Kaplan et. al. [START_REF] Kaplan | A terminal area PBL prediction system at Dallas-Fort Worth and its application in simulating diurnal PBL jets[END_REF], who described a series of measurements made near Dallas airport in the Great Plains of the USA, indicates that the average shear is 2.9 ms -1 per 100 m at 200-300 m, i.e. below the maximum LLJ, located at 500 m on average.

Wind Shear Characteristics in the Sahel and its Consequences: Figure 7 shows the frequency distributions of wind shear magnitude and wind shear direction versus time, between 150 and 300 m at both sites, during the dry season and pre-monsoon period. The magnitude occurrences are spaced 2 ms -1 per 100 m and the direction occurrences 30° per 100 m.

In Niamey during the dry season, the period of strong shears is clearly linked to the LLJ since it occurs in a narrow time period around 06H00 UTC: 60% of the cases reach shears which require an alert to the pilots (higher than 4 ms -1 per 100 m) (Figure 7 (b)). Such high shears can also be found at the same time in 20% of the occurrences in the 300-500 m layer (not shown). These high shear values are within the Harmattan wind but, since the LLJ is stronger during this season, with a peak level closer to the surface, the shear magnitude is stronger. The majority of cases occur during the night with a wind shear direction between 90 and 150° per 100 m (Figure 7 (d)), which is also dangerous for aircraft (cross-wind). In Bamako, high wind shears (higher than 4 ms -1 per 100 m) represent only 16-22% of the cases and can occur at any time of the day (Figure 7 (a)). There are, however, 8% of the cases, the whole day long, when the wind shear can reach more than 6 ms -1 per 100 m. Most of the wind shear directions are also between 0 and 90° per 100 m (Figure 7 Abdou et. al. [START_REF] Abdou | The diurnal cycle of lower boundary layer wind in the West African monsoon[END_REF] found higher wind shears in magnitude between 30 and 300 m during a period spanning May 31 st to August 17 th (i.e. from the pre-monsoon to the monsoon period) with 18 to 38% of wind shears greater than 4 ms -1 per 100 m. The discrepancy between their results and ours probably stems from the fact that higher wind shears also occur between 30 m and 150 m, in a range that cannot be reached by the UHF radar.

Conclusion

This study shows that the strongest values of LLJ are encountered during winter, which corresponds to the dry season or during the transition period between the dry and the wet seasons. Associated with these strong values of LLJ, large wind shear, either in magnitude or direction, appear in the vicinity of the nocturnal jet. The probabilities of strong wind shear are most important between 150 and 270 m. In the LLJ period, in Niamey it found 60% of cases of shear higher than 4 ms -1 per 100 m between 150 and 270 m and 20% between 270-500 m. Shears higher than 4 ms -1 per 100 m, appear in Bamako, where the pre-monsoon wind shear frequencies vary from 10 to 20% at 4-6 -1 per 100 m, at the LLJ time (21H00 to 06H00). There are even cases, but less probable, where the wind shear can reach 6-8 ms -1 per 100 m. In Niamey, the period to worry is between 03H00 and 06H00, which corresponds to the maximum of the LLJ. These shears of low layers are the most dangerous for aircraft. Both profilers were located in the airport area, which enabled accurate information to be provided to the pilots, who otherwise received conflicting measurement indications from their on-board instruments and the control tower.

Figure 1

 1 Figure 1 AMMA field observation extends from the edge of Atlantic Ocean in West Africa to the coast of Central Africa, including Lake Chad and the Gulf of Guinea. The measurements presented here were acquired in Niamey and Bamako (red dots) which are located in the Sahel band (red dashed lines)

Figure

  Figure 2a Bamako UHF PCL 1300 Wind profiler radarThe second radar (Figure2b) was installed at Niamey airport in March 2006 during the AMMA EOP in the framework of the US ARM program (Atmospheric Radiation Measurements) and provided data from April 4 th , 2006 to January 6 th , 2007, except for an inactive period from June 17 th to July 2 nd .

Figure 2b

 2b Figure 2b Niamey UHF Wind profiler Doppler radar

Figure 3

 3 Figure 3 Vertical profiles of wind speed observed at night (00H00; 06H00; 18H00 and 21H00 UTC), at Bamako in 2005 and (b) Niamey in 2006 (i) Pre-monsoon, (ii) Monsoon, (iii) Post-monsoon, (iv) Dry season.

Figure 4

 4 Figure 6 as height-time cross sections of horizontal wind, reflectivity and dissipation rate of turbulent kinetic energy, observed at Bamako from April 26 th to 29 th , 2005 and at Niamey from May 3 rd to 6 th , 2006. Figure 5a and 6a clearly show the LLJ. These periods correspond to the time when the ITD is located near Bamako and Niamey. It results in an alternation between the two wind regimes in the lower layers, the monsoon at night and the Harmattan during daytime, because the ITD moves southwards during the day and northwards during the night in connection with the diurnal cycle of the Saharan depression heat. Composite days of the horizontal wind in August (a) at Bamako in 2005 and (b) at Niamey in 2006, and in December (c) at Bamako in 2005 and (d) at Niamey in 2006. The color scale shows the intensity of the wind speed

Figure 5 Figure 6

 56 Height-time cross sections of (a) horizontal wind, (b) reflectivity and (c) dissipation rate of turbulent kinetic energy, obtained at Bamako from April 26 th to 29 th , 2005. Black dots indicate the top of the inversion layer at night or the top of the ABL during daytime Height-time cross sections of (a) horizontal wind, (b) reflectivity and (c) dissipation rate of turbulent kinetic energy, obtained at Niamey from May 3 rd to 6 th , 2006. Black dots indicate the top of the inversion layer at night or the top of the ABL during daytime

  (c)) during the night.

Figure 7

 7 Figure 7 Frequency distributions of wind shear versus time: (a, b, e, f) in magnitude and (c, d, g, h) in direction observed in Bamako (left) or Niamey (right), during the dry or pre-monsoon periods

Figure 8

 8 Figure 8 Frequency distributions of wind shear versus time: (a, b) in magnitude and (c, d) in direction observed in Bamako (left) or Niamey (right), during the dry or pre-monsoon periods

Acknowledgments

The authors of this work thank the University Abdou Moumouni (Niamey, Niger) and the INSU-CNRS authorities. We would like to give special thanks to ASECNA for providing us the UHF profiler data in Bamako. We also thank the US Atmospheric Radiation Measurement program for operating the UHF wind profiler in Niamey and providing us the data.