As part of the FENNEC programme, a field campaign was conducted on the Mediterranean coast of southern Spain, near Gibraltar, from June to August 2011. Using a straightforward ground-based N2-Raman lidar, several aerosol optical properties were retrieved at 355 nm, including the linear particle depolarisation ratio (PDR), the lidar ratio (LR), and the aerosol backscatter and extinction coefficients. From continuous sampling over 58 nights, several periods were identified in which aerosol events exhibited optical thicknesses greater than 0.5. The primary drivers of these events are the incursions of Saharan dust mixed with local polluted and marine air masses. Pairing PDR and LR has been shown to be effective in identifying three distinct bulk aerosol classes: dust, carbonaceous and soluble (predominantly marine) aerosols. After processing the night-time data to ensure sufficient lidar range, the study demonstrates the effectiveness of lidar profiles in evaluating the reliability of the Copernicus Atmosphere Monitoring Service (CAMS) reanalyses of atmospheric aerosols up to approximately 7 km above mean sea level (a.m.s.l.). The two datasets show excellent consistency in terms of the optical thickness and vertical profile of the aerosol extinction coefficient in the Saharan dust aerosol layers. CAMS reproduces the temporal evolution well, with a correlation coefficient (COR) greater than 0.8. However, this is less accurate for the layer below 2 km a.m.s.l. (COR = 0.55), where CAMS tends to underestimate compared to ground-based lidar.

Abstract. The WaLiNeAs campaign took place along the north-western Mediterranean coast between October 2022 and January 2023. This period was marked by unusual weather conditions associated with dry autumn and winter. In such conditions and for the first time, eight ground-based stations equipped with water vapour Raman lidars were strategically deployed by four European countries. We studied the consistency of this network with the water vapour mixing ratio (WVMR) products derived from the Infrared Atmospheric Sounding Interferometer (IASI) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA5), which assimilate IASI radiances. The statistical metrics used in the comparison are the mean bias (MB, defined as lidar – IASI or ERA5), the root mean square error (RMSE) and the correlation coefficient (COR). A positive MB of approximately 0.9 g kg−1 (respectively 0.6 g kg−1) between 0.2 and 5 km above mean sea level (amsl) indicates a systematic underestimation of the WVMR by IASI (respectively ERA5). RMSE values range from 1 to 2 g kg−1 across all lidar stations for IASI and ERA5, while the measurement uncertainties of the lidars are typically below 0.4 g kg−1. COR presents little variation between stations, it ranges from 0.7 to 0.8 and remains almost constant between 0.2 and 5 km amsl. Both the IASI and the ERA5 products appear to accurately reproduce the temporal variability of the vertical structure of water vapour in the low troposphere. Nevertheless, they show MB and RMSE significantly above the uncertainties of lidar measurements.

Abstract. Biomass burning (BB) aerosols emitted over Southern Africa (SAF) and South America (SA) represent a major seasonal perturbation to the usually pristine atmosphere of the South-West Indian Ocean (SWIO) but remain understudied in this remote region. Following a multi-instrumental approach, we characterize BB plumes reaching Reunion Island (21° S, 55° E) during September 2017, combining ground-based measurements (sun-photometer, lidars, Fourier Transform Infrared spectrometer), spaceborne observations, CAMS EAC4 reanalysis, and the Lagrangian transport model FLEXPART. Aerosol optical depth at 550 nm over Reunion reached unusually high values (0.16–0.42) during the second half of September, with organic matter contributing up to 60 %. MODIS imagery revealed two large-scale smoke plumes originating from SAF and SA transported toward the SWIO, and CALIOP profiles showed smoke layers extending from 4 to 9 km of altitude above Madagascar. On September 19th, a single layer was identified over Reunion between 2.8 and 4.7 km with an Ångström exponent (Å₃₅₅/₅₃₂) of 1.32±0.23, consistent with moderately aged BB particles essentially originating from SAF. On September 25th, two vertically decoupled layers were identified: a lower layer (3.3–5 km, Å=1.45±0.12) associated with mixed aged aerosols of SAF and SA origin, and an upper, drier layer (5–9 km, Å=1.60±0.06) of potentially fresher SAF smoke, consistent with rapid convective uplift into the mid-troposphere. This study offers new insights concerning the dynamical processes that govern aerosol variability over Reunion Island and highlights its value as a strategic long-term observational site in the SWIO.

Climate change is already leading to significant changes in rainfall patterns in the Mediterranean basin, and particularly in the south of France. The resilience of local population to extreme precipitation events requires investigative means of water vapor variability in the lower troposphere with high spatio-temporal resolution such as Raman lidar, associated to state-of-the-art weather models. This is the aim of the Water Vapor Lidar Network Assimilation (WaLiNeAs) research program, which was supported by France to carry out a demonstration campaign in autumn and winter 2022-2023.

From May to August 2020 and during summer 2024, aerosol backscatter and relative humidity profiles were measured near Paris region using the Water Vapour and Aerosol Lidar (WALI). The campaigns included observations on the Saclay plateau (48°42′42′′ N, 02°08′52′′ E) and in Paris (48°50′12′′ N, 02°20′10′′ E) during the 2024 Olympic Games. The high vertical (15 m) and temporal (15 min) resolution of WALI allow to study aerosol optical properties and water vapour profiles under stable atmospheric conditions. This study focuses on characterizing aerosol hygroscopic growth using lidar-derived aerosol backscatter coefficients as a function of relative humidity. Eight case studies were selected where the potential temperature gradient was neutral and the water vapor mixing ratio was constant with height. These include long range air pollution transport from the Benelux region, low hygroscopic aerosol events, and a sea salt-pollution mixture episode. Hygroscopicity was assessed using CAMS model analyses and reanalyses, allowing attribution of chemical composition to observed optical changes. Good agreement was found between lidar-derived hygroscopic properties and CAMS-inferred aerosol types. Lidar growth factors ranged from 0.3 to 1.5, with higher values linked to sea salt presence, consistent with literature values. Aerosols hygroscopicity is also studied using Mie's theory, as aerosols can be considered nearly spherical. Differences between extinction and backscatter-based growth retrievals are interpreted through Hänel's formalism. The results illustrate the potential of Raman lidar observations to provide valuable constraints on aerosol hygroscopicity, offering complementary information for chemistry-transport models and contributing to reduce uncertainties in aerosol-cloud interaction estimates.

The isotopic composition of water vapor can be used to track atmospheric hydrological processes and to evaluate numerical models simulating the water cycle. Accurate model-observation comparisons require understanding the spatial and temporal variability of tropospheric water vapor isotopes. The challenging task of obtaining highly resolved water vapor isotopic observations is typically addressed through airborne measurements performed aboard conventional aircraft, but these offer limited microscale insights. This study uses ultralight aircraft observations to investigate water vapor isotopic composition in the lower troposphere over southern France in late summer 2021. Combining observations with models, we identify key drivers of isotopic variability and detect short-lived, small-scale processes. The key findings of this study are that (i) at hourly and sub-daily scales, vertical mixing is the primary driver of isotopic variability in the lowermost troposphere above the study site; (ii) evapotranspiration significantly impacts the boundary layer water vapor isotopic signature, as revealed by the delta 18O-delta D relationship; and (iii) while water vapor isotopes generally follow large-scale humidity patterns, with separation distances that might range up to 100-300 km, they also reveal distinct small-scale structures (approximately hundreds of meters) that are not fully explained by humidity variations alone, highlighting sensitivity of water vapor isotopic composition to additional fine-scale processes. The latter are particularly evident for delta D, which also exhibit the largest differences in horizontal and vertical gradients. Combined with other airborne datasets, our results support a simple model driven by surface observations to simulate tropospheric delta D vertical profiles, improving surface-satellite comparisons.

Water vapour mixing ratio (WVMR) measurements were performed by three Raman lidars during the WaLiNeAs field campaigns in the western Mediterranean basin. Observations took place along the French Mediterranean coast in autumn and winter 2022-2023, and in southwestern France near Toulouse in summer 2023. These multiseasonal campaigns captured a variety of meteorological phenomena, including a dry winter, rainstorms, long-range aerosol transport, and a summer heatwave. The measured WVMR varied across the troposphere, ranging from less than 1 gkg(-1) to over 17 gkg(-1) in the planetary boundary layer, and from less than 1 gkg(-1) to approx. 15 gkg(-1) in the free troposphere. Lidar profiles have a vertical resolution of 100 m and a temporal resolution of one hour, matching the spatiotemporal resolution of the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) model outputs. The primary goal is to compare the ERA5 reanalysis with the lidar-derived WVMR profiles. Results show altitude-dependent differences in Pearson correlation coefficient (COR), mean bias (MB), and root-mean-squared deviation (RMSD), especially during high WVMR periods (>10 gkg(-1)). Over all cases, MB ranges from 0.1 to 3 gkg(-1), and RMSD from 0.6 to 3.7 gkg(-1). COR values range from 0.16 to 0.94, with the lowest values observed during warm episodes in the free troposphere. The statistical coefficients show the differences in the behaviour of the reanalysis model over different measurement periods and altitude ranges compared to lidar. However, in all cases the reanalysis underestimates the WVMR, whatever the altitude range. This article highlights the challenges faced by models during common meteorological events characterised by significant WVMR amounts. The results provide valuable insights into the performance of operational numerical weather prediction models. It highlights the necessity to refine the representation of the WVMR in the lower troposphere by assimilating ground-based lidar measurements.

Marine cold-air outbreaks (mCAOs) are a characteristic type of high-impact weather in the European Arctic and are characterized by an intense water cycle where polar cloud processes play an important role. Model simulations and weather forecasts of mCAO events are challenging and associated with poor predictability. One reason is that processes related to the water cycle interact with one another on a wide range of scales. In regional models, some of these processes are resolved and others are fully or partly parameterised. To test and improve numerical weather prediction models, additional observations and novel types of measurements of water vapour are highly demanded. Stable water isotopes are an increasingly available measurement, allowing to trace sub-grid scale processes, and providing the potential to constrain the mass budget of the atmospheric water cycle during mCAO events. During the ISLAS2022 field experiment (21 March to 10 April 2022), the stable isotope composition of water vapour and liquid samples, cloud structures, and other meteorological parameters were collected between Svalbard and Northern Scandinavia on various measurement platforms. Airborne survey flights to Svalbard provided the ocean evaporation signature and subsequent processing of water vapour during mCAO conditions. During a number of flights, mCAO airmasses were repeatedly sampled over a course of hours to days, allowing to characterize their thermodynamic evolution as clouds were first forming, then glaciating and precipitating. In addition, vapour isotope and sea water isotope measurements were taken continuously onboard R/V Helmer Hanssen between Tromsø and the Greenland west coast. Finally, coordinated land-based measurement activity over Northern Norway and Sweden allowed collection of precipitation samples, thus closing the mass budget of the mCAO events. Furthermore, using buoyancy-controlled meteorological balloons launched from Ny Ålesund, we additionally obtained continuous in-situ measurements of the boundary-layer evolution during the mCAO. We provide an overview over the airborne and ground-based measurement activities during the campaign and provide several examples to highlight the potential of the stable water isotope measurements to constrain the water budget of mCAOs in conjunction with traditional meteorological observations.

Observations from airborne field campaigns are used to study the interplay between boundary-layer thermals and clouds in the trades. The size distributions of thermal and cloud-base chords inferred from turbulence and horizontal lidar-radar measurements are robustly described by the sum of two exponentials. Analytical calculations and statistical simulations show that the merging of objects is sufficient to explain the two exponentials, representing, respectively, the populations of merged- and unmerged-object chords. They also show how circulations induced by convective objects facilitate the merging process. The observed day-to-day variability of these populations at cloud base can thus be tied to the variability of thermal merging across the depth of the subcloud layer. Clouds rooted in unmerged thermals are small and shallow while those rooted in merged thermals are wider and deeper. An intricate interplay between thermal- and cloud-merging arises: when thermal merging is weak, thermal number density is high and cloud bases merge easily, leading to strong mesoscale mass fluxes and “Gravel” shallow mesoscale organizations. In contrast, when thermal merging is strong, clouds are fed by sparser but wider thermals, leading to longer cloud lifetimes but weaker cloud merging, weaker mesoscale mass fluxes, and “Flower” mesoscale organizations. This interplay between thermal- and cloud-merging imposes an upper bound on cloud coverage and suggests a negative feedback on the growth of mesoscale circulations. Thermal merging also controls observed size distributions of thermals in deep convective regimes. The merging process thus appears to be a fundamental player in the mesoscale organization of convection.

Numerous field campaigns have been carried out to quantify the water vapour content of the atmosphere using vibrational Raman lidar technology. Each of them raises the question of calibration methods, in particular the reliability of this calibration over time. We present a study on the stability of the calibration of the WALI (Water vapour and Aerosol Lidar, now renamed Weather and Aerosol LIdar) lidar developed at Laboratoire des Sciences du Climat et de l'Environnement (LSCE) in France, over a period of 7 years (2016-2022) and across several field campaigns. A calibration method is applied mainly using radiosondes and, in a few cases, airborne meteorological probes. Complementing the previous approaches, we show that ground-based meteorological measurements can be of great value for lidar calibration under conditions of vertical stability in the lower troposphere and provide good knowledge of the lidar overlap function, with full overlap within the planetary boundary layer. Using statistical criteria, we emphasize that these three calibration approaches should remain consistent over time. The observation periods considered here allow us to sample a wide range of water vapour contents in the lower troposphere, from 0.5 g kg-1 to more than 10 g kg-1, characteristic of the variabilities expected over the mid-latitudes and even over the Arctic. From comparisons between lidar and in situ measurements (radiosondes and/or flights), we observe a variability of more than 10 % in the calibration constant between field experiments conducted with and without laser injection seeding. The root mean square error between the lidar and in situ reference measurements is between 0.23 and 0.6 g kg-1, mainly due to the atmospheric variability during the calibration. The bias is small, less than 0.08 g kg-1. For all the situations studied, the correlation coefficient remains high, above 0.75. The instrumental error is comparable to the 0.4 g kg-1 recommended by the World Meteorological Organization (WMO). Such a precision requires the use of a significant number of reference profiles, and the remaining limitation is due to the uncertainties associated with in situ weather sensors. We note that the use of ground-based measurements does not introduce any more uncertainty in the lidar calibration coefficient than vertical profiles obtained by radiosondes or airborne means. Furthermore, the use of reanalyses can be an interesting option for calibration if the lidar profiles are not used in the models themselves, e.g. by means of assimilation.

Abstract. Numerous field campaigns have been carried out to quantify the water vapour content of the atmosphere using vibrational Raman lidar technology. Each of them raises the question of calibration methods, in particular the reliability of this calibration over time. We present a study on the stability of the calibration of the WALI lidar developed at Laboratoire des Sciences du Climat et de l’Environnement in France (LSCE), over a period of 7 years (2016–2022) and across several field campaigns. A calibration method is applied that mainly use radiosondes and, in a few cases, airborne meteorological probes. Complementing the previous approaches, we show that ground-based meteorological measurements can be of great interest for lidar calibration under conditions of vertical stability in the lower troposphere and of good knowledge of the lidar overlap function, with full overlap within the planetary boundary layer. We emphasize that these three calibration approaches remain consistent over time. The observation periods considered here allow us to sample a wide range of water vapour contents in the lower troposphere, from 0.5 g kg-1 to more than 10 g kg-1 characteristic of the variabilities expected over the mid-latitudes and even over the Arctic. We observe a variability of more than 10 % in the calibration constant between field experiments conducted with and without laser injection seeding. The root mean square error is between 0.23 and 0.6 g kg-1, mainly due to the atmospheric variability during the calibration. The bias is small, less than 0.08 g kg-1. For all the situations studied, the correlation coefficient remains high, above 0.75. The instrumental error is comparable to the 0.4 g kg-1 recommended by the World Meteorological Organization (WMO). Such a precision requires the use of a significant number of reference profiles and the remaining limitation is due to the uncertainties associated with in situ weather sensors. We note that the use of ground-based measurements does not introduce any more uncertainty in the lidar calibration coefficient than vertical profiles obtained by radiosondes or airborne means. Furthermore, the use of re-analyses can be an interesting option for calibration when there are no operational constraints.

Vibrational Raman lidar measurements of the water vapour mixing ratio (WVMR) were conducted during the WaLiNeAs (Water Vapor Lidar Network Assimilation) field campaigns in the western Mediterranean during autumn and winter 2022–2023 and in southwestern France (Toulouse) between June and September 2023. These campaigns, which spanned different seasons and geographical locations, provided an opportunity to sample various meteorological phenomena, including a dry winter, rainstorms, long-range aerosol transport, and an intense heat wave. Consequently, the water vapour content recorded in the lower troposphere showed significant variability during WaLiNeAs, ranging from less than 1 g kg-1 to more than 17 g kg-1. For operational purposes, a vertical resolution of 100 m and a temporal resolution between 15 and 60 min have been chosen. These resolutions are aligned with the spatio-temporal resolution of the ERA5 dataset from ECMWF's Integrated Forecasting System (IFS) global numerical weather prediction models. The processing of the lidar data has resulted in a scientific publication explaining the methods used to invert the lidar data and recover various atmospheric parameters. Lidar measurements address a critical gap left by operational instruments, which struggle to capture the diurnal cycle of water vapour from the planetary boundary layer to the lower free troposphere. The primary aim of this study is to compare ERA5 data with lidar-derived WVMR profiles. The results reveal altitude-dependent differences in Pearson correlation coefficient (COR), mean bias (MB), and root mean square deviation (RMSD), particularly during periods of high-water vapour content (> 10 g kg⁻¹). Over all periods the MB ranges from 0.1 to 3 g kg⁻¹, and the RMSD varies between 0.6 and 3.7 g kg⁻¹. COR ranges from 0.16 to 0.94, with lower values observed in the free troposphere during warmer periods. These variations underline the differences in the performance of the reanalysis model over different periods and altitudes when compared to lidar profiles. We show that the reanalysis constantly underestimated the WVMR at all altitudes. This study highlights the importance of scrutinising WVMR and the challenges faced by models during high water vapour meteorological events. The results provide valuable insights into the performance of operational numerical weather prediction models and highlight the need to refine their representation of WVMR vertical profiles in the lower troposphere by incorporating ground-based lidar measurements.We give special thanks to the ANR grant #ANR-20-CE04-0001 for its contribution to the WaLiNeAs programme, to Meteo-France for its help with the measurements in Toulouse, and to the CNRS INSU national LEFE programme for its financial contribution to this project.

During the Water Vapor Lidar Network Assimilation (WaLiNeAs) campaign, eight lidars specifically designed to measure water vapor mixing ratio (WVMR) profiles were deployed on the western Mediterranean coast. The main objectives were to investigate the water vapor content during case studies of heavy-precipitation events in the coastal western Mediterranean and assess the impact of high spatiotemporal WVMR data on numerical weather prediction forecasts by means of state-of-the-art assimilation techniques. Given the increasing occurrence of extreme events due to climate change, WaLiNeAs is the first program in Europe to provide network-like, simultaneous and continuous water vapor profile measurements over a period of 3-4 months. This paper focuses on the WVMR profiling datasets obtained from three of the lidars run by the French part of the WaLiNeAs team. These three lidars were deployed in the cities of Coursan, Le Grau-du-Roi and Cannes. This measurement setup enabled monitoring of the water vapor content of the lower troposphere over periods of 3 months in fall and winter 2022, with some interruptions, and 4 months in summer 2023. The lidars measured the WVMR profiles from the surface up to approximately 6-10 km at nighttime and 1-2 km during daytime. They had a vertical resolution of 100 m and a time resolution between 15 and 30 min, and they were selected to meet the needs of weather forecasting with an uncertainty lower than 0.4 g kg(-1). The paper presents details about the instruments, the experimental strategy and the datasets provided. The final dataset (10.25326/537; Chazette et al., 2023) is divided into two sub-datasets: the first with a time resolution of 15 min, which contains a total of 26 423 WVMR vertical profiles, and the second with a time resolution of 30 min to improve the signal-to-noise ratio and signal altitude range.

Forest systems are the main carbon sink after the oceans. However, due to climate change, an alarming number of tree species of the Northern Hemisphere are at risk of migrating northwards or becoming extinct. This is the case for the downy oak (Quercus pubescens), one of the main species constituting the forests close to the Mediterranean Sea in France. Our aim is to retrieve aboveground carbon (AGC) and underground root carbon (UGC) stocks of the downy oak forest at the Observatoire de Haute-Provence (OHP), located about 80 km north of Marseille, in order to provide a baseline against which to assess the effect of climate change on this model species. The study presented here is based on airborne lidar observations performed on May 2012 and field measurements from 2012, 2018 and 2023 in the OHP forest. The OHP forest consists of similar to 75 % downy oak, which is highly sensitive to global warming. Field measurements indicate minimal changes in tree growth and density between 2012 and 2023, and thus its carbon storage efficiency remains stationary. As retrieved by lidar measurements, tree top heights (TTHs) are mostly between 5 and 12 m, with an uncertainty of around 1 m. The slow evolution of trees at the OHP site makes it appropriate to use lidar data recorded in 2012 to assess the carbon stock trapped in current forest biomass. By coupling allometric laws established from field measurements with lidar observations, we show that the quantities of carbon trapped in aboveground biomass are double those trapped in the root system. Over an area of similar to 24 ha, mean values of 15 +/- 14 tC ha-1 are assessed for the aerial biomass against 8-10 +/- 3-7 tC ha-1 for the roots of diameter larger than 1 cm for low and high assessments. These values depend heavily on the height of the sampled trees themselves, as well as on their location on the OHP plateau (smaller trees, 5-6 m) or on the slope (tallest trees, 10-12 m). Using a Monte Carlo approach, the relative uncertainties in AGC were calculated to be of the order of 17 % and 11 % for trees 5-6 m and 10-12 m tall, respectively. For UGC, the relative uncertainties were calculated as 8 % and 5 % for the same tree heights, but the assumptions of the allometric model are associated with biases that can easily reach 100 %. Although the surface footprints are different, we show that there is a reasonable agreement between our airborne lidar measurements and the level 2B (TTH) and (aboveground biomass) operational products of the Global Ecosystem Dynamics Investigation (GEDI) mission on the International Space Station for data acquired between 2019 and 2022.

(1) Laboratoire des Sciences du Climat et de l’Environnement, Laboratoire mixte CEA-CNRS-UVSQ, CEA Saclay, 91191 Gif-sur-Yvette, (2) LEOSPHERE, 48 rue de Clignancourt, 75018 Paris, France., (3) Laboratoire de Météorologie Dynamique, Ecole Polytechnique, 91128 Palaiseau, France, (4) Laboratoire Atmosphères Milieux Observations Spatiales, Laboratoire mixte CNRS-UVSQ-UPMC, Université Paris 6, 4 Place Jussieu 75252 Paris, France.

eye-safe lidar is based on a Nd:YAG laser giving pulses of 16 mJ at 355 nm with a frequency of 20 Hz. The CEA and LEOSPHERE have recently upgraded this instrument into a three detection channels lidar measuring the two elastic cross-polarizations and the Raman-N2 backscatter signal at 387 nm. It is able to retrieve aerosol optical properties (extinction, backscatter coefficients and depolarization ratio) and atmospheric structures (boundary layer height and clouds) with a resolution of 1.5 m along the line-of-sight in analog mode and 15 m in photon-counting mode. This new lidar is particularly well-adapted to air quality survey thanks to a full overlap reached at 150 m. This compact (90x50x20 cm) and light (less than50 kg) instrument has been integrated into the Mobile Aerosol Station (MAS) onboard a small truck and enables mobile measurements. We will here present and analyze some results obtained around Paris area with this Raman-N2 lidar.

Cloud observations in the Arctic are still rare, which requires innovative observation techniques to assess ice crystal properties. We present an original approach using the Raman lidar measurements applied to a case study in northern Scandinavia. The vertical profiles of the optical properties, the effective radius of ice crystals and ice water content (IWC) in Arctic semi-transparent clouds were assessed using quantitative ground-based lidar measurements at 355 nm performed from 13 to 26 May 2016 in Hammerfest (north of Norway, 70∘39′48′′ N, 23∘41′00′′ E). The field campaign was part of the Pollution in the ARCtic System (PARCS) project of the French Arctic Initiative. The presence of low-level semi-transparent clouds was noted on 16 and 17 May. The cloud base was located just above the atmospheric boundary layer where the 0 ∘C isotherm reached around 800 m above the mean sea level (a.m.s.l.). To ensure the best penetration of the laser beam into the cloud, we selected case studies with cloud optical thickness (COT) lower than 2 and out of supercooled liquid pockets. Lidar-derived multiple scattering coefficients were found to be close to 1 and ice crystal depolarization around 10 %, suggesting that ice crystals were small and had a rather spherical shape. Using Mie computations, we determine effective radii between ∼7 and 25 µm in the clouds for ice water contents between 1 and 8 mg m−3, respectively. The uncertainties regarding the effective radii and ice water content are on average 2 µm and 0.65 mg m−3, respectively.

The Mediterranean Rim, and more particularly the western Mediterranean area, is one of the most sensitive regions to climate change. The associated environmental changes are already evident through periods of drought and intense rainfall. The predictions of these phenomena are a major societal issue, which led us to use lidar systems to constrain regional modelling. The Raman lidars HORUS-1 and -2 are each composed of two telescopes of 15 cm diameter. For each telescope N2 and H2O channels are associated. Lidars components have been specifically defined for this task and put into operation during the international Water vapor Lidar Network Assimilation (WaLiNeAs) campaign led by French research teams. Among the three stations managed by the LSCE team, two of them were equipped with HORUS lidar systems at the Port Camargue (43.52 N 4.13 E) and Coursan (43.23 N 3.06 E) sites. The main difference between the two HORUS lidars is the laser used. For HORUS-1 we used an ULTRA laser (optimally pumped by a flash lamp at 30 mJ/20Hz) which showed a good reliability since the beginning of the lidar installation. However, the MERION-C laser (optimally pumped by diodes at 30 mJ/100 Hz) installed in HORUS-2 did not live up to our expectations with several failures, to the point of stopping the measurements in Coursan. We will nevertheless discuss the relative interest of these two lasers in projection of future Raman lidar networks. Observations available from these two lidar systems will be presented and discussed with respect to the meteorological processes encountered during their operating periods.We give a special acknowledgment to the ANR grant #ANR-20-CE04-0001 for the contribution to the WaLiNeAs program and a special acknowledgment to Meteo France and to the CNRS INSU national LEFE program for their financial contribution for this project. The CEA is acknowledged for the provision of its staff and facilities.

The vertical aerosol layering of the troposphere is poorly documented in mountainous regions, particularly in the Alpine valleys, which are influenced by valley and mountain winds. To improve our knowledge of particulate matter trapped in the Annecy valley, synergetic measurements performed by a ground-based meteorological Raman lidar and a Rayleigh-Mie lidar aboard an ultralight aircraft were implemented as part of the Lacustrine-Water vApor Isotope inVentory Experiment (L-WAIVE) over Lake Annecy. These observations were complemented by satellite observations and Lagrangian modeling. The vertical profiles of aerosol optical properties (e.g., aerosol extinction coefficient (AEC), lidar ratio (LR), particle linear depolarization ratio (PDR)) are derived from lidar measurements at 355 nm during the period between 13 and 22 June 2019. The background aerosol content with an aerosol optical thickness (AOT) of 0.10 ± 0.05, corresponding to local–regional conditions influenced by anthropogenic pollution, has been characterized over the entirety of Lake Annecy thanks to the mobile ultralight payload. The aerosol optical properties are shown to be particularly variable over time in the atmospheric column, with mean LRs (PDRs) varying between 40 ± 8 and 115 ± 15 sr (2 ± 1 and 35 ± 2%). Those conditions can be disturbed by air masses that have recirculated over the valley, as well as by contributions from neighboring valleys. We have observed an important disruption in the atmospheric aerosol profiles by the arrival of an exceptionally dry air mass (RH ~ 30%), containing aerosols identified as coming from the Great Western Erg (AOT ~ 0.5, LR = 65 ± 10 sr, PDR = 20–35%) in the Sahara. These desert dust particles are shown to influence the entire atmospheric column in the Annecy valley. Such an experimental approach, coupling upward and downward lidar and spaceborne observation/Lagrangian modelling, was shown to be of significant interest for the long-term monitoring of the evolution of aerosol loads over deep valleys. It allows a better understanding of the influence of dust storms in the presence of severe convective weather processes.