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Three convectively active African easterly waves (AEWs) that propagated south of the African easterly jet were observed over the northeast Atlantic Ocean in September 2021. Their evolution is studied using a suite of theoretical frameworks, as well as the European Centre for Medium‐range Weather Forecast reanalyses and satellite‐derived brightness temperature observations. The environment of these AEWs was sampled during the Cloud–Atmospheric Dynamics–Dust Interactions in West Africa campaign near Cape Verde with the goal to assess their potential for developing into tropical cyclones. We highlight the processes that inhibited the development of the first AEW (which evolved into tropical disturbance Pierre‐Henri) and that played a role in the development of the later two into tropical storms Rose and Peter on September 19, 2021. The three AEWs developed a so‐called "marsupial protective" pouch. For Peter and Rose, the pouch was associated with a vertically aligned vortex at low levels and efficiently protected the convective systems inside from dry and dusty air intrusion. The development of this low‐level vortex is associated with an interaction with the monsoon trough for Rose and with a vorticity center associated with a wave propagating north of the African easterly jet (AEJ) in the case of Peter. The presence of a dust flux toward the convective core near the surface is highlighted for Rose and Peter in spite of the presence of the protective marsupial pouch. On the other hand, Pierre‐Henri interacted positively with both the monsoon trough and an AEW north of the AEJ but failed to develop into a tropical cyclone. The wave north of the AEJ brought Saharan air layer air masses inside the pouch that led to a drying of the circulation that may explain the decrease in convective activity. [ABSTRACT FROM AUTHOR]

During the boreal summer, mesoscale convective systems generated over West Africa propagate westward and interact with African easterly waves, and dust plumes transported from the Sahel and Sahara by the African easterly jet. Once off West Africa, the vortices in the wake of these mesoscale convective systems evolve in a complex environment sometimes leading to the development of tropical storms and hurricanes, especially in September when sea surface temperatures are high. Numerical weather predictions of cyclogenesis downstream of West Africa remains a key challenge due to the incomplete understanding of the clouds–atmospheric dynamics–dust interactions that limit predictability. The primary objective of the Clouds–Atmospheric Dynamics–Dust Interactions in West Africa (CADDIWA) project is to improve our understanding of the relative contributions of the direct, semidirect, and indirect radiative effects of dust on the dynamics of tropical waves as well as the intensification of vortices in the wake of offshore mesoscale convective systems and their evolution into tropical storms over the North Atlantic. Airborne observations relevant to the assessment of such interactions (active remote sensing, in situ microphysics probes, among others) were made from 8 to 21 September 2021 in the tropical environment of Sal Island, Cape Verde. The environments of several tropical cyclones, including Tropical Storm Rose, were monitored and probed. The airborne measurements also serve the purpose of regional model evaluation and the validation of spaceborne wind, aerosol and cloud products pertaining to satellite missions of the European Space Agency and EUMETSAT (including the Aeolus, EarthCARE, and IASI missions). [ABSTRACT FROM AUTHOR]

We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, exemplified for June 2011. Our approach, called AEROIASI, is based on the innovative retrieval of vertical profiles of the dust extinction coefficient from daily cloud‐free hyperspectral Infrared Atmospheric Sounder Interferometer (IASI) satellite observations. AEROIASI observations clearly agree with other widely used measurements (from lidar and radiometers). The 3D characterisation is focused on the dust maximum in June 2011, located in the central Sahara (17–23°N, 1–7°E) and linked to the major atmospheric dynamical drivers associated with the West African Monsoon (WAM) system. AEROIASI shows the near‐surface dust load to be dominated by five major emission events occurring every 3–4 days. These all occur when the study region is under the influence of northward bursts of the WAM and convection‐related cold pools, likely associated with orographic forcing by the Aïr Mountains. During the earliest (June 10) and the dustiest (June 17) cases, northward advection of moisture over the hotspot is favoured by the superposition of cyclonic circulations related to an extratropical disturbance northwest of the Sahara and to the Saharan heat low over Mauritania, respectively. Convection over the hotspot also triggers wave‐like disturbances that travel westwards. The three dustiest events are characterised by elongated dust fronts moving northwards, with a leading edge spanning 200–300 km horizontally and extending from the surface up to 2 km of altitude. Further south, the dust layer progressively elevates to 3.5 km along the slanted isentropes at the interface of the monsoon and the harmattan, increasingly losing contact with the ground. When northerlies blow over the study region, elevated dust layers at 3–5 km are observed, which are transported southwards within the Saharan air layer and westwards along the northern edge of the African easterly jet (after June 13). [ABSTRACT FROM AUTHOR]

Climate variability associated with the West African monsoon (WAM) has important environmental and socio-economic impacts in the region. However, state-of-the-art climate models still struggle in producing reliable climate predictions. An important cause of this low predictive skill is the sensitivity of climate models to different forcings. In this study, the mechanisms linking the WAM dynamics to the CO forcing are investigated, by comparing the effect of the CO direct radiative effect with its indirect effect mediated by the global sea surface warming. The July-to-September WAM variability is studied in climate simulations extracted from the Coupled Model Intercomparison Project Phase 5 archive, driven by prescribed sea surface temperature (SST). The individual roles of global SST warming and CO atmospheric concentration increase are investigated through idealized experiments simulating a 4 K warmer SST and a quadrupled CO concentration, respectively. Results show opposite and competing responses in the WAM dynamics and precipitation. A dry response (−0.6 mm/day) to the SST warming is simulated in the Sahel, with dryer conditions over western Sahel (−0.8 mm/day). Conversely, the CO increase produces wet conditions (+0.5 mm/day) in the Sahel, with the strongest response over central-eastern Sahel (+0.7 mm/day). The associated responses in the atmospheric dynamics are also analysed, showing that the SST warming affects the Sahelian precipitation through modifications in the global tropical atmospheric dynamics, reducing the importance of the regional drivers, while the CO increase reinforces the coupling between precipitation and regional dynamics. A general agreement in model responses demonstrates the robustness of the identified mechanisms linking the WAM dynamics to the CO direct and indirect forcing, and indicates that these primary mechanisms are captured by climate models. Results also suggest that the spread in future projections may be caused by unbalanced model responses to the CO direct and indirect forcing. [ABSTRACT FROM AUTHOR]

The Saharan heat low (SHL) plays a pivotal role in the West African monsoon system in spring and summer. The recent trend in SHL activity has been analysed using two sets of numerical weather prediction (NWP) model reanalyses and Atmospheric Models Intercomparison Project simulations from 15 climate models performed in the framework of the 5th Coupled Models Intercomparison Project (CMIP5) exercise. A local increase of temperature in the Sahara during the 90s is found in the two sets of NWP models temperature. This increase is stronger within the SHL region than over the surrounding areas. Using different temporal filters (under 25 days, 25-100 days and above 300 days), we show that this is accompanied by a slight but widespread increase of temperature, and a change in the filtered signal under 25 days during the transition period of the 90s. We also show that SHL pulsations occurring at different time scales impact the West Africa climate on a variety of spatial scales, from the regional scale (for the high band pass) to the synoptic scale (for the low band pass signal). Despite a large variability in the temporal trends for 15 climate models from the CMIP5 project, the warming trend in the 90s is observed in the models ensemble mean. Nevertheless, large discrepancies are found between the NWP models reanalyses and the climate model simulations regarding the spatial and temporal evolutions of the SHL as well as its impact on West African climate at the different time scales. These comparisons also reveal that climate models represent the West African monsoon interactions with SHL pulsations quite differently. We provide recommendations to use some of them depending on the time scales of the processes at play (synoptic, seasonal, interannual) and based on key SHL metrics (location, mean intensity, global trend, interaction with the West African monsoon dynamics). [ABSTRACT FROM AUTHOR]

The Hydrological cycle in the Mediterranean eXperiment ( HyMeX) was held in autumn 2012 during which part of the observational effort was established on Menorca Island to characterize the upwind marine low-level flow. The ground-based Water-vapour Raman Lidar ( WALI), the airborne water-vapour lidar LEANDRE 2 and boundary-layer pressurized balloons were implemented during the first Special Observing Period and contributed to characterize water vapour variability in the vicinity of the Balearic Islands, together with the existing network of Global Positioning System receivers. Furthermore, analyses from regional and global numerical weather prediction ( NWP) models ( AROME-WMED, the ECMWF/ IFS NWP system and the Weather Research and Forecasting ( WRF) model) were also available over large domains encompassing part of or the entire western Mediterranean basin. We assess the consistency of water vapour mixing ratio ( WVMR) profiles and integrated water vapour contents ( IWVC) derived from the different datasets by comparing them to a common reference, the ground-based lidar WALI. We use consistency indicators such as root-mean-square errors, biases and correlations. Comparison between WVMR profiles from ground-based and airborne lidars (ground-based lidar and boundary-layer pressurized balloons) leads to a root-mean-square error lower than 1.6 g kg−1 (1.3 g kg−1) when the closest possible air masses are sampled. We observed a good agreement between the vertical WVMR profiles derived from WALI and the numerical models with correlations higher than 0.7 and root-mean-square errors lower than 2 g kg−1. Regarding IWVCs, the models exhibit biases less than 2 kg m−2, root-mean-square errors lower than 2.3 g kg−1 and correlations higher than 0.86 when compared to WALI. Finally, AROME-WMED 48 h forecasts were compared with WALI data composited over eleven 48 h periods. The quality of the forecast does not visibly degrade within the 48 h period from the initial analyses. [ABSTRACT FROM AUTHOR]

During the Intensive Observing Period 15b of the first Special Observation Period of the Hydrological Cycle in the Mediterranean Experiment ( HyMeX), a variety of mesoscale convective systems ( MCSs) impacted the Cevennes-Vivarais ( CV) target area leading to over 100 mm of 24 h accumulated rainfall on 20 and 21 October 2012. The CV area was first impacted by a V-shaped MCS developing over the Cevennes mountains, then by a MCS initiated on the eastern foothills of the Pyrenees and finally by three MCSs initiating over the sea. The MCSs initiated and propagated along a well-defined storm track ahead of an approaching upper-level trough, as observed with the 15 min resolution Spinning Enhanced Visible and Infrared Imager. The storm track was characterized by strong southeasterly winds over the Mediterranean and high integrated water vapour content ( IWVC), as derived from observations from the Moderate-resolution Imaging Spectroradiometer ( MODIS) and the Atmospheric Infrared Sounder ( AIRS). The ground-based Water-vapour Raman Lidar, located in the Balearic Islands, captured the increasing moistening of the free troposphere, up to 5 km, associated with the eastward propagation of the surface low from Gibraltar to a location west of the Balearic Islands. MODIS and AIRS observations, together with Weather Research and Forecasting ( WRF) model simulations, revealed the tropical origin of the high moisture content characterizing the storm track, with IWVC values on the order of 35 kg m−2, and enhanced moisture being observed below 500 hPa. The WRF simulations also showed that the MCS initiation offshore was very likely caused by low-level wind convergence and conditionally unstable air along the storm track, between North Africa and southern France. Low-level convergence resulted from the interaction between a strong southwesterly swirling flow around the low-pressure centre and an easterly low-level jet present along the southern France coastline. [ABSTRACT FROM AUTHOR]

The Sahel region of West Africa experiences decadal swings between periods of drought and abundant rainfall, and a large body of work asserts that these variations in the West African monsoon are a response to changes in the temperatures of the tropical Atlantic and Indian Oceans. However, here it is shown that when forced by SST alone, most state-of-the-art climate models do not reproduce a statistically significant upward trend in Sahelian precipitation over the last 30 years and that those models with a significant upward trend in rainfall seem to achieve this result for disparate reasons. Here the role of the Saharan heat low (SHL) in the recovery from the Sahelian drought of the 1980s is examined. Using observations and reanalyses, it is demonstrated that there has been an upward trend in SHL temperature that is coincident with the drought recovery. A heat and moisture budget analysis of the SHL suggests that the rise in temperature is due to greenhouse warming by water vapor, but that changes in water vapor are strongly dependent upon the temperature of the SHL: a process termed the Saharan water vapor-temperature (SWAT) feedback. It is shown that the structure of the drought recovery is consistent with a warming SHL and is evidence of a fundamental, but not exclusive, role for the SHL in the recent increase in Sahelian monsoon rainfall. [ABSTRACT FROM AUTHOR]

Airborne Leandre II differential absorption lidar (DIAL), S-band dual-polarization Doppler radar (S-Pol), and Goddard Lidar Observatory for Winds (GLOW) Doppler lidar data are used, in conjunction with surface mesonet and special sounding data, to derive the structure and dynamics of a bore and associated solitary wave train (soliton) that were generated in southwestern Kansas during the International H20 Project (IHOP_2002). Vertical cross sections of S-Pol reflectivity, S-Pol radial velocity, and DIAL water vapor mixing ratio show a stunning amplitude-ordered train of trapped solitary waves. DIAL data reveal that the leading wave in the soliton increasingly flattened with time as the soliton dissipated. A method is developed for using the GLOW Doppler winds to obtain the complex two-dimensional vertical circulation accompanying the dissipating soliton. The results show multiple circulations identical in number to the oscillations seen in the S-Pol and DIAL data. The leading updraft occurred precisely at the time that the bore passed over the GLOW facility, as well as when the photon count values suddenly ramped up (suggesting lifting of the low-level inversion by the bore). Additional evidence in support of the validity of the results is provided by the fact that layer displacements computed using the derived vertical motions agree well with those implied by the changes in height of the DIAL mixing ratio surfaces. The depth and speed of propagation of the bore seen in the DIAL and surface mesoanalyses were shown to be consistent with the predictions from bore hydraulic theory. Analysis of National Center for Atmospheric Research (NCAR) Integrated Sounding System (ISS) data shows that a highly pronounced curvature in the profile of bore-relative winds, related to the existence of a very strong low-level jet, effectively trapped the upward leakage of solitary wave energy below 3 km. This finding explains the trapped lee wave–type structures seen in the DIAL, GLOW, and S-Pol data. [ABSTRACT FROM AUTHOR]

A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg-1 and 1–2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1–2 g kg-1) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s-1 and variable horizontal widths of 2–5 km, although some were as wide as 7–8 km. These updrafts were up to 1.5 g kg-1 moister than the surrounding environment. Although models suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms over the intensive observation region (IOR). Possible reasons for this lack of convection are discussed. Strong capping inversion and moisture detrainment between the lifting condensation level and the level of free convection related to an overriding drier air, together with the relatively small near-surface moisture values (less than 10 g kg-1), were detrimental to CI in this case. [ABSTRACT FROM AUTHOR]

A cold-air outbreak over the Mediterranean, associated with a Tramontane event, has been simulated with the atmospheric non-hydrostatic model Meso-NH using a horizontal resolution of 2 km. Results are compared with in situ aircraft, airborne lidar and satellite measurements. On average, the mean and turbulent parameters simulated in the surface layer and mixed layer compared well with in situ measurements. The model was able to reproduce accurately the Foehn effect in the wake of Cape Creus, as well as the occurence of rolls in the coastal region in connection with cloud streets observed with AVHRR. Over the sea, the threshold value of turbulent kinetic energy defining the height of the atmospheric boundary-layer top in the model (defined as 25% of the maximum turbulent kinetic energy in the profile) enables the simulated atmospheric boundary-layer height to match the one retrieved from lidar measurements. Nevertheless, the model did not handle very well the abrupt gradients of all meteorological parameters observed at the top of the atmospheric boundary-layer. Reasons for this are investigated. [ABSTRACT FROM AUTHOR]

A hydraulic jump has been observed with the airborne backscatter lidar LEANDRE 1 (Lidar embarqué pour l'Etude des Aérosols, des Nuages, des interactions Dynamique-Rayonnement et du cycle de l'Eau) at the top of the atmospheric boundary layer (ABL) during a Tramontane event in the framework of the Pyréneés experiment. An analytical fluid mechanics model is used to interpret lidar observations in connection with in situ measurements and to study the sensitivity of the hydraulic jump triggering to the boundary conditions. This model, which generalizes the reduced-gravity shallow-water theory for two-dimensional stratified flows over a topograpy, is diagnostic (i.e., the reduced gravity g' = g Δ θv θv is prescribed) and uses boundary conditions defined in terms of Riemann invariants. Using inflow and outflow boundary conditions as well as the reduced gravity prescribed from in situ measurements, the model is able to diagnose the presence of a hydraulic jump at the location suspected from the lidar observations. The wind speed, ABL height and Froude number derived from the model are in good agreement with the observations (within about 20–30%). [ABSTRACT FROM AUTHOR]

During the Water Vapor Lidar Network Assimilation (WaLiNeAs) campaign, 8 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 spatio-temporal 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. This paper focuses on the WVMR profiling datasets obtained from three of the lidars managed by the French component of the WaLiNeAs team. These lidars were deployed in the towns of Coursan, Grau du Roi and Cannes. This measurement setup enabled monitoring of the water vapor content within the low troposphere along a period of three months over autumn – winter 2022 and four months in summer 2023. The lidars measured the WVMR profiles from the surface up to approximately 6–10 km at night, and 1–2 km during daytime; with a vertical resolution of 100 m and a time sampling between 15 – 30 min, 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, as well as the datasets given in NETcdf format. The final dataset is divided in two 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. [ABSTRACT FROM AUTHOR]

Atmospheric boundary layer (ABL) turbulent processes in the Paris area have been documented in the framework of the Étude de la Couche Limite en Agglomération Parisienne (ECLAP). Under anticyclonic conditions, simulations are made with a ’column‘ modelling approach, based on the three-dimensional version of the non-hydrostatic mesoscale model MERCURE restricted to a small domain. This ’column‘ model uses existing state-of-the-art surface-layer parameterizations (the addition of the convective velocity scale to the mean wind speed in near free convection periods, the prescription of the effective dynamical roughness length as well as a differentiation between dynamical and thermal roughness lengths). To ensure the representativeness of the comparison between measurements and simulations, the dynamical and thermal effective roughness lengths characterizing the experimental site are prescribed explicitly in the model, using sonic anemometer measurements. We show that the parameterizations implemented in MERCURE for this study enable a good description, by the three-dimensional model, of the observed complex ABL dynamics. We also show that in the region of Paris, the synoptic scale and mesoscale dynamics can have a dramatic impact on the ABL dynamics and turbulent processes at the local scale. This study is a first attempt at improving our ability to predict meteorological factors affecting urban air quality. [ABSTRACT FROM AUTHOR]

The internal thermal boundary layer developing over the Mediterranean during a cold-air outbreak associated with a Tramontane event has been studied by means of airborne lidar, in situ sensors, and a modelling approach that consisted of nesting the University of Washington (UW) planetary boundary-layer (PBL) model in an advective zero-order jump model. This approach bypasses some of the deficiencies associated with each model: the absence of the dynamics in the mixed layer for the zero-order jump model and the lack of an inversion at the PBL top for the UW PBL model. Particular attention is given to the parameterization of the entrainment flux at the PBL top. Values of the entrainment closure parameter derived with the model when matching PBL structure observations are much lower than those derived with standard zero-order jump models. They also are in good agreement with values measured in different meteorological situations by other studies. This improvement is a result of the introduction of turbulent kinetic energy production in the mixed layer. [ABSTRACT FROM AUTHOR]

The thickness of the entrainment zone at the top of the marine atmospheric boundary layer (MABL) has been documented by an airborne lidar on two consecutive days during a cold-air outbreak episode over the Mediterranean. In addition to the lidar observations, in situ turbulent flux measurements at three levels in the MABL were made by a second aircraft. The flights‘ tracks are broken down in segments 25–30 km long and the data are filtered for the parametrization of turbulent entrainment in the MABL at scales smaller than a few kilometres. The structural parameters of the entrainment zone are determined by lidar from the distributions of the instantaneous MABL top height. The average values Ph0 and Ph2 of the cumulative probability distributions are used to define the bottom and top heights of the entrainment zone h0 and h2, respectively. The parameters h0 and h2 are calculated by reference to a linear vertical buoyancy flux profile in the framework of a first-order jump model. The model is constrained by both lidar and in situ data to determine Ph0 and Ph2 and so h0 and h2. In unstable conditions the average fraction Ph0 is estimated to be 6.0 ± 1%. It is shown to be slightly sensitive to the presence of cloud at small cloud fractions. The mean value of the ratio of the inversion level buoyancy flux to the surface buoyancy flux ARv is found to range from 0.15 to 0.30 depending on the shear in the MABL. The average value is 0.22 ± 0.05. Our results are in good agreement with previous analysis at comparable spatial scales. In purely convective conditions, the value of ARv given by the parametrizations fitted to our results is about 0.10–0.12, a value smaller than the commonly accepted value of 0.2. When compared to previous parametrization results, our proportionality constant for the mechanical production of turbulent kinetic energy is also found to be scaled down, in good agreement with large-eddy simulation results. It is suggested that mesoscale organized motions in the MABL is the source of this difference. [ABSTRACT FROM AUTHOR]

Ground-based Global Navigation Satellite System (GNSS) measurements from nearly 50 stations distributed over the Caribbean arc have been analysed for the period 1 January–29 February 2020 in the framework of the EUREC 4 A (Elucidate the Couplings Between Clouds, Convection and Circulation) field campaign. The aim of this effort is to deliver high-quality integrated water vapour (IWV) estimates to investigate the moisture environment of mesoscale cloud patterns in the trade winds and their feedback on the large-scale circulation and energy budget. This paper describes the GNSS data processing procedures and assesses the quality of the GNSS IWV retrievals from four operational streams and one reprocessed research stream which is the main data set used for offline scientific applications. The uncertainties associated with each of the data sets, including the zenith tropospheric delay (ZTD)-to-IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the Barbados Cloud Observatory (BCO) and to the measurements from the operational radiosonde station at Grantley Adams International Airport (GAIA), Bridgetown, Barbados. A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (-2.9 kgm-2) and the GNSS results (-1.2 kgm-2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kgm-2), where the Vaisala RS41 measurements are moister than the GNSS retrievals. The IWV estimates from a collocated microwave radiometer agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to the soundings and to GNSS before that date. IWV estimates from the ECMWF fifth-generation reanalysis (ERA5) are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the GNSS-derived IWV peaks. Two successive peaks are observed on 22 January and 23–24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated. We classified the cloud organization for five representative GNSS stations across the Caribbean arc using visible satellite images. A statistically significant link was found between the cloud patterns and the local IWV observations from the GNSS sites as well as the larger-scale IWV patterns from the ECMWF ERA5 reanalysis. The reprocessed ZTD and IWV data sets from 49 GNSS stations used in this study are available from the French data and service centre for atmosphere (AERIS) (10.25326/79;). [ABSTRACT FROM AUTHOR]

In the framework of the EUREC 4 A (Elucidating the role of clouds–circulation coupling in climate) campaign that took place in January and February 2020, integrated water vapour (IWV) contents were retrieved over the open tropical Atlantic Ocean using Global Navigation Satellite System (GNSS) data acquired from three research vessels (R/Vs): R/V Atalante , R/V Maria S. Merian and R/V Meteor. This paper describes the GNSS processing method and compares the GNSS IWV retrievals with IWV estimates from the European Centre for Medium-range Weather Forecasts (ECMWF) fifth reanalysis (ERA5), from the Moderate Resolution Imaging Spectroradiometer (MODIS) infrared products and from terrestrial GNSS stations located along the tracks of the ships. The ship-borne GNSS IWV retrievals from R/V Atalante and R/V Meteor compare well with ERA5, with small biases (-1.62 kgm-2 for R/V Atalante and +0.65 kgm-2 for R/V Meteor) and a root mean square (rms) difference of about 2.3 kgm-2. The results for the R/V Maria S. Merian are found to be of poorer quality, with an rms difference of 6 kgm-2 , which is very likely due to the location of the GNSS antenna on this R/V prone to multipath effects. The comparisons with ground-based GNSS data confirm these results. The comparisons of all three R/V IWV retrievals with MODIS infrared products show large rms differences of 5–7 kgm-2 , reflecting the enhanced uncertainties in these satellite products in the tropics. These ship-borne IWV retrievals are intended to be used for the description and understanding of meteorological phenomena that occurred during the campaign, east of Barbados, Guyana and northern Brazil. Both the raw GNSS measurements and the IWV estimates are available through the AERIS data centre (https://en.aeris-data.fr/ , last access: 20 September 2020). The digital object identifiers (DOIs) for R/V Atalante IWV and raw datasets are 10.25326/71 and 10.25326/74 , respectively. The DOIs for the R/V Maria S. Merian IWV and raw datasets are 10.25326/72 and 10.25326/75 , respectively. The DOIs for the R/V Meteor IWV and raw datasets are 10.25326/73 and 10.25326/76 , respectively. [ABSTRACT FROM AUTHOR]

The Etosha pan in northeastern Namibia is known to be one of the main dust sources in Austral Africa. In addition, the pan area is characterized by a high surface albedo, yielding the deepest and most turbulent planetary boundary layer (PBL) over the elevated, arid Namibian plateau during the Austral summer.In early September 2017, the Etosha pan region (and most of Namibia) was under the influence of a massive biomass burning aerosol plume originating from nearby Angola. The presence of large quantities of absorbing aerosols in and above the PBL disrupted the radiative balance over Etosha pan and impacted the PBL dynamics and thermodynamics diurnal cycle.In this study, we use a combination of ground-based remote sensing observations acquired in Windpoort, airborne observations from 5 flights performed with the SAFIRE Falcon 20 during the AEROCLO-sA field campaign (August-September 2017) and high resolution numerical mesoscale simulations to investigate the evolution of the PBL dynamics and thermodynamics over the Etosha pan as a function of the aerosol direct radiative forcing. The features of the PBL in the Etosha pan are also compared to those of surrounding areas characterized by darker surfaces. The impact of the PBL dynamics on aerosol distribution (dust and biomass burning particles) is also investigated using ground-based and airborne lidars, together with dropsondes released from the Falcon 20. Complementary observations include the suite of advanced airborne radiometers, airborne optical sensors for cloud and aerosol microphysics and a ground-based sunphotometer. [ABSTRACT FROM AUTHOR]

A specific instrumental synergy, associating ground-based lidar, airborne and spaceborne remote sensing measurements, was set up as part of the field campaign of the AErosol, RadiatiOn and CLOuds in southern Africa (AEROCLO-sA) project. The overarching objective of AEROCLO-sA is to improve our knowledge about the role of atmospheric aerosols on the specific climate of southern Africa. For this reason, the Henties Bay experimental site of the Research Centre of the University of Namibia (22° 6' S, 14° 17' E) in the Orongo region has been selected to implement a ground-based lidar between 22 August and 12 September, 2017. The geographical position of the site is favorable for such a study, it is bounded on its western side by the Atlantic Ocean and by the Namibia desert (~800 m amsl) on its eastern side. A big part of the year, as the west coast of South Africa, Henties Bay is covered with low clouds of stratocumulus type with very frequent fogs, and often overflown by air masses loaded with aerosol of various origins: forest fires, pollution, desert and/or ocean. Strong contributors to the aerosol load are the forest fires occurring during the dry season between August and October. Biomass burning aerosols exert a strong influence on the Earth's radiation budget by scattering and absorbing solar radiation, and influence the cloud formation and life time. The vertical distribution of absorbing aerosols is of paramount importance as it significantly influences the vertical profile of radiative heating in the atmosphere and, by this way, the stability of the atmosphere, thereby modifying convective and turbulent motions and clouds. The main vertical aerosol structures observed by lidar will be present and discussed. Passive (MODIS) and active (CALIPSO, CATS) space-borne observations coupled with back trajectory studies will allow for regionalized the local lidar observations. [ABSTRACT FROM AUTHOR]

The Sahara is the world's largest source of mineral dust in the atmosphere, which is a key player of the Earth system. Desert dust layers impact the radiative budget, atmospheric stability, cloud properties, chemical balances and air quality depending on the altitude at which they are transported. Over the Sahara, the three-dimensional (3D) distribution of dust is closely linked to dust uplifting mechanisms and its redistribution in the vertical, driven by the main atmospheric dynamical actors in the region. A better understanding of these mechanisms is very important for assessing the 3D budget of Saharan dust, which is currently highly uncertain. However, observations of the 3D distribution of desert dust are highly scarce and particularly over the Sahara where they exclusively rely on satellite remote sensing. Indeed, the two-dimensional (2D) distribution of dust is standardly observed: either its horizontal distribution in terms of aerosol optical depth from e.g. MODIS, OMI, IASI measurements and also in this last case mean dust layer altitudes or latitudinal transects of vertical profiles of dust from the CALIOP lidar. In order to overcome this limitation, we have developed a so-called AEROIASI new satellite approach for retrieving the 3D distribution of desert dust from thermal infrared spectra measured by IASI. This method uses full radiative transfer calculations from the KOPRA model and an auto adaptive Tikhonov-Philips-type regularization for deriving vertical profiles of dust extinction coefficient at 10 μm for most cloud-free IASI pixels, both over land (including bright desert surfaces) and ocean. The dust vertical distribution derived from AEROIASI agrees remarkably well with along track transects of CALIOP space-borne lidar vertical profiles as well as with aerosol optical depth derived from AERONET sun photometer measurements over West Africa.We use AEROIASI to investigate the role of the main dynamic actors in the vertical redistribution of dust over the Sahara during June 2011. We find a major role of the African easterly waves and cold air intrusions as those from the West African monsoon, affecting directly or indirectly the vertical distribution and propagation of major dust plumes. Cold pools generated by mesoscale convective systems are as well a major mechanism for uplifting dust over the Sahel and Southern Sahara. In the present study, we will present the full 3D distribution of dust observed by AEROIASI and the link with these major dynamic actors of the region. [ABSTRACT FROM AUTHOR]

Ground-based Global Navigation Satellite System (GNSS) measurements from nearly fifty stations distributed over the Caribbean Arc have been analysed for the period 1 January-29 February 2020 in the framework of the EUREC4A (Elucidate the Couplings Between Clouds, Convection and Circulation) field campaign. The aim of this effort is to deliver high-quality Integrated Water Vapour (IWV) estimates to investigate the moisture environment of mesoscale cloud patterns in the Tradewinds and their feedback on the large-scale circulation and energy budget. This paper describes the GNSS data processing procedures and assesses the quality of the GNSS IWV retrievals from four operational streams and one reprocessed research stream which is the main data set used for offline scientific applications. The uncertainties associated with each of the data sets, including the zenith tropospheric delay (ZTD) to IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the Barbados Cloud Observatory (BCO) and to the measurements from the operational radiosonde station at Grantley Adams international airport (GAIA). A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (-2.9 kg m-2) and the GNSS results (-1.2 kg m-2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kg m-2) where the Vaisala RS41 measurements are moister than the GNSS retrievals. The IWV estimates from a colocated microwave radiometer agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to the soundings and to GNSS before that date. IWV estimates from the ECMWF fifth generation reanalysis (ERA5) are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the GNSS derived IWV peaks. Two successive peaks are observed on 22 January and 23/24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated. We classified the cloud organisation for five representative GNSS stations across the Caribbean Arc and found that the environment of Fish cloud patterns to be moister than that of Flowers cloud patterns which, in turn, is moister than the environment of Gravel cloud patterns. The differences in the IWV means between Fish and Gravel were assessed to be significant. Finally, the Gravel moisture environment was found to be similar to that of clear, cloud-free conditions. The moisture environment associated with the Sugar cloud pattern has not been assessed because it was hardly observed during the first two months of 2020. The reprocessed ZTD and IWV data set from 49 GNSS stations used in this study are available from the AERIS data center (https://doi.org/10.25326/79; Bock (2020b)). [ABSTRACT FROM AUTHOR]

Since its launch by the European Space Agency in 2018, the Aeolus satellite has been using the first Doppler wind lidar in space to acquire three-dimensional atmospheric wind profiles around the globe. Especially in the tropics, these observations compensate for the currently limited number of other wind observations, making an assessment of the quality of Aeolus wind products in this region crucial for numerical weather prediction. To evaluate the quality of the Aeolus L2B wind products across the tropical Atlantic Ocean, 20 radiosondes corresponding to Aeolus overpasses were launched from the islands of Sal, Saint Croix, and Puerto Rico during August–September 2021 as part of the Joint Aeolus Tropical Atlantic Campaign. During this period, Aeolus sampled winds within a complex environment with a variety of cloud types in the vicinity of the Intertropical Convergence Zone and aerosol particles from Saharan dust outbreaks. On average, the validation for Aeolus Rayleigh-clear revealed a random error of 3.8–4.3 m s -1 between 2 and 16 km, and 4.3–4.8 m s -1 between 16 and 20 km, with a systematic error of -0.5±0.2 m s -1. For Mie-cloudy, the random error between 2 and 16 km is 1.1–2.3 m s -1 and the systematic error is -0.9±0.3 m s -1. It is therefore concluded that Rayleigh-clear winds do not meet the mission's random error requirement, while Mie winds most likely do not fulfil the mission bias requirement. Below clouds or within dust layers, the quality of Rayleigh-clear observations are degraded when the useful signal is reduced. In these conditions, we also noticed an underestimation of the L2B estimated error. Gross outliers, defined as large deviations from the radiosonde data, but with low error estimates, account for less than 5 % of the data. These outliers appear at all altitudes and under all environmental conditions; however, their root cause remains unknown. Finally, we confirm the presence of an orbital-dependent bias observed with both radiosondes and European Centre for Medium-Range Weather Forecasts model equivalents. The results of this study contribute to a better characterisation of the Aeolus wind product in different atmospheric conditions and provide valuable information for further improvement of the wind retrieval algorithm. [ABSTRACT FROM AUTHOR]

In the framework of the EUREC4A (Elucidating the role of clouds-circulation coupling in climate) campaign that took place in January and February 2020, integrated water vapour (IWV) contents were retrieved over the open Tropical Atlantic Ocean using Global Navigation Satellite Systems (GNSS) data acquired from three research vessels (R/Vs): R/V Atalante, R/V Maria S. Merian, and R/V Meteor. This paper describes the GNSS processing method and compares the GNSS IWV retrievals with IWV estimates from the European Center for Medium-range Weather Forecast (ECMWF) fifth ReAnalysis (ERA5), from the Moderate-Resolution Imaging Spectroradiometer (MODIS) infra-red products, and from terrestrial GNSS stations located along the tracks of the ships. The ship-borne GNSS IWVs retrievals from R/V Atalante and R/V Meteor compare well with ERA5, with small biases (-1.62 kg m-2 for R/V Atalante and +0.65 kg m-2 for R/V Meteor) and a root mean square (RMS) difference about 2.3 kg m-2. The results for the R/V Maria S. Merian are found to be of poorer quality, with RMS difference of 6 kg m-2 which are very likely due to the location of the GNSS antenna on this R/V prone to multipath effects. The comparisons with ground-based GNSS data confirm these results. The comparisons of all three R/V IWV retrievals with MODIS infra-red product show large RMS differences of 5-7 kg m-2, reflecting the enhanced uncertainties of this satellite product in the tropics. These ship-borne IWV retrievals are intended to be used for the description and understanding of meteorological phenomena that occurred during the campaign, east of Barbados, Guyana and northern Brazil. Both the raw GNSS measurements and the IWV estimates are available through the AERIS data center (https://en.aeris-data.fr/). The digital object identifiers (DOIs) for R/V Atalante IWV and raw datasets are https://doi.org/10.25326/71 (Bosser et al., 2020a) and https://doi.org/10.25326/74 (Bosser et al., 2020d), respectively. The DOIs for the R/V Maria S. Merian IWV and raw datasets are https://doi.org/10.25326/72 (Bosser et al., 2020b) and https://doi.org/10.25326/75 (Bosser et al., 2020e), respectively. The DOIs for the R/V Meteor IWV and raw datasets are https://doi.org/10.25326/73 (Bosser et al., 2020c) and https://doi.org/10.25326/76 (Bosser et al., 2020f), respectively. [ABSTRACT FROM AUTHOR]

The Aerosol, Radiation and Clouds in southern Africa (AEROCLO-sA) project investigates the role of aerosols on the regional climate of southern Africa. This is a unique environment where natural and anthropogenic aerosols and a semipermanent and widespread stratocumulus (Sc) cloud deck are found. The project aims to understand the dynamical, chemical, and radiative processes involved in aerosol–cloud–radiation interactions over land and ocean and under various meteorological conditions. The AEROCLO-sA field campaign was conducted in August and September of 2017 over Namibia. An aircraft equipped with active and passive remote sensors and aerosol in situ probes performed a total of 30 research flight hours. In parallel, a ground-based mobile station with state-of-the-art in situ aerosol probes and remote sensing instrumentation was implemented over coastal Namibia, and complemented by ground-based and balloonborne observations of the dynamical, thermodynamical, and physical properties of the lower troposphere. The focus laid on mineral dust emitted from salty pans and ephemeral riverbeds in northern Namibia, the advection of biomass-burning aerosol plumes from Angola subsequently transported over the Atlantic Ocean, and aerosols in the marine boundary layer at the ocean–atmosphere interface. This article presents an overview of the AEROCLO-sA field campaign with results from the airborne and surface measurements. These observations provide new knowledge of the interactions of aerosols and radiation in cloudy and clear skies in connection with the atmospheric dynamics over southern Africa. They will foster new advanced climate simulations and enhance the capability of spaceborne sensors, ultimately allowing a better prediction of future climate and weather in southern Africa. [ABSTRACT FROM AUTHOR]

Kinematic and thermodynamic structures of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP) are examined with dropsondes and airborne instrumentation that includes Doppler radars, a differential absorption lidar, and in situ sensors. Intensive observations were collected along a 125-km segment of the front, with coverage of both the cold front leading edge and the post- and prefrontal areas. Whereas the first part of this two-part series of papers focused on across-front kinematic and moisture characteristics, the study herein investigates alongfront structures relevant for convection initiation. A northeast–southwest-oriented cold front moved into the observational domain from the northwest, but its motion slowed to less than 1 m s-1 in the early afternoon. In the late afternoon it was intersected by a north-northeast–south-southwest-oriented reflectivity thin line that was advected from the southwest, and another boundary that is an extension of a large-scale dryline paralleling the thin line but located farther to the east. Doppler wind synthesis suggests an increase in low-level horizontal wind shear across the cold front leading edge with the approach and intersection of the boundaries causing an increase in low-level convergence (up to ∼1 × 10-3 s-1), positive vertical vorticity (up to ∼0.5 × 10-3 s-1), and upward motion (up to ∼1 m s-1). An organized pattern of misocyclones (vertical vorticity maxima <4 km) and enhanced updrafts with a spacing of ∼5–8 km were observed at the cold front leading edge. At the same time vortex lines manifested as horizontal vorticity maxima were observed within the cold air oriented perpendicular to the cold front leading edge and on top of the vertical wind shear layer. The analysis suggests that inflection point instability was the dominant mechanism for their development. Low Richardson number (0.3–0.4), short lifetime (<2 h), horizontal wavelength of 3–6 km, and collocation with strong horizontal and vertical wind shear are characteristics that support the hypothesis that these instabilities were Kelvin–Helmholtz waves. Towering cumulus developed along the cold front forming a convective cell close to the intersection of the cold front, dryline, and reflectivity thin line. [ABSTRACT FROM AUTHOR]

The International H2O Project (IHOP_2002) was designed to sample the three-dimensional time-varying moisture field to better understand convective processes. Numerous research and operational water vapor measuring systems and retrievals, via in situ and remote sensing techniques, were operated in the U.S. Southern Great Plains from 13 May to 25 June 2002. This was done in combination with more traditional observations of wind and temperature. Convection initiation (CI) sampling strategies were designed to optimally employ the array of ground-based and airborne sensors to observe the processes leading to the development of deep, moist convection. This case study examines several clear-air features and their impact on CI on 12 June 2002. The supercells that developed produced damaging winds and hail. The clear-air, preconvective features included (i) a mesoscale low pressure region, (ii) a dryline, (iii) an old outflow boundary, (iv) the intersection of (ii) and (iii), (v) internal gravity waves, and (vi) horizontal convective rolls. A unique combination of instruments was positioned to sample the preconvective environment on 12 June 2002. The Lidar pour l’Etude des Interactions Aérosols Nuages Dynamique Rayonnement et du Cycle de l’Eau (LEANDRE II) water vapor differential absorption lidar (DIAL), the airborne Electra Doppler Radar (ELDORA), and the Navy Research Laboratory (NRL) P3 aircraft in situ measurements provided information on the moisture and vertical velocity distribution within the boundary layer. Radiosondes, dropsondes, wind profilers, and an Atmospheric Emitted Radiance Interferometer (AERI) provided temperature, moisture, and wind profiling information. Although other ground-based sensors (i.e., S-band dual-polarization Doppler radar, Mobile Integrated Profiling System) were 50–150 km west of the CI area, they were useful for illustrating the boundary layer kinematics and reflectivity fields. Results suggest that the mesolow and mesoscale boundaries, respectively, acted to enhance the low-level moisture advection and convergence in the CI region. While internal gravity waves were present and appeared to modulate water vapor along the old outflow boundary, they did not play an obvious role in CI in this case. Horizontal convective rolls were observed beneath the new storms that initiated and may have helped to focus the CI in this case. [ABSTRACT FROM AUTHOR]

A wide array of ground-based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP). This study, the first of two parts, is focused on describing structures in the across-front dimension. Coarsely resolved observations from the operational network and dropsondes deployed over a 200-km distance centered on the front are combined with higher-resolution observations from in situ sensors, Doppler radars, a microwave radiometer, and a differential absorption lidar that were collected across a ∼40-km swath that straddled a ∼100-km segment of the front. The northeast–southwest-oriented cold front moved toward the southeast at ∼8–10 m s-1 during the morning hours, but its motion slowed to less than 1 m s-1 in the afternoon. In the early afternoon, the cold front separated cool air with a northerly component flow of 2–4 m s-1 from a 10-km-wide band of hot, dry air with 5 m s-1 winds out of the south-southwest. The average updraft at the frontal interface was ∼0.5 m s-1 and slightly tilted back toward the cool air. A dryline was located to the southeast of the front, separating the hot, dry air mass from a warm, moist air mass composed of 10 m s-1 southerly winds. Later in the afternoon, the warm, moister air moved farther to the northwest, approaching the cold front. The dryline was still well observed in the southwestern part of the observational domain while it vanished almost completely in the northeastern part. Low-level convergence (∼1 × 10-3 s-1), vertical vorticity (∼0.5 × 10-3 s-1), and vertical velocity (∼1 m s-1) increased. The strong stable layer located at ∼2.0–2.5 km MSL weakened in the course of the afternoon, providing a basis for the development of isolated thunderstorms. The applicability of gravity current theory to the cold front was studied. There was evidence of certain gravity current characteristics, such as Froude numbers between 0.7 and 1.4, a pronounced feeder flow toward the leading edge, and a rotor circulation. Other characteristics, such as a sharp change in pressure and lobe and cleft structures, remain uncertain due to the temporally and spatially variable nature of the phenomenon and the coarse resolution of the measurements. [ABSTRACT FROM AUTHOR]

The evolution and finescale structure of a dryline that initiated a line of thunderstorms is presented. Both the along-line variability and mean vertical structure were examined using data collected by an airborne Doppler radar and a water vapor differential absorption lidar (DIAL). The initiation of convection appeared to result from the diurnally induced easterly flow in the maritime air east of the dryline that typically develops late in the day. This flow increased the low-level convergence and allowed rising parcels of air to reach the level of free convection. The along-line variability was largely attributed to numerous misocyclones distorting the thin line of radar reflectivity by advecting dry (moist) air across the dryline south (north) of the misocyclone. The misocyclones also influenced the location of the updrafts, with most of the peak values positioned north of the circulations. As a result, these updrafts were fortuitously positioned in regions of high mixing ratio where the first convective cells initiated. [ABSTRACT FROM AUTHOR]

Laser active remote sensing of tropospheric water vapor is a promising technology to complement passive observational means in order to enhance our understanding of processes governing the global hydrological cycle. In such a context, we investigate the potential of monitoring both water vapor H216O and its isotopologue HD16O using a differential absorption lidar (DIAL) allowing for ground-based remote measurements at high spatio-temporal resolution (150 m and 10 min) in the lower troposphere. This paper presents a sensitivity analysis and an error budget for a DIAL system under development which will operate in the 2 µm spectral region. Using a performance simulator, the sensitivity of the DIAL-retrieved mixing ratios to instrument-specific and environmental parameters is investigated. This numerical study uses different atmospheric conditions ranging from tropical to polar latitudes with realistic aerosol loads. Our simulations show that the measurement of the main isotopologue H216O is possible over the first 1.5 km of atmosphere with a relative precision in the water vapor mixing ratio of <1 % in a mid-latitude or tropical environment. For the measurement of HD16O mixing ratios under the same conditions, relative precision is found to be slightly lower but still sufficient for the retrieval of range-resolved isotopic ratios with precisions in δD of a few per mil. We also show that expected precisions vary by an order of magnitude between tropical and polar conditions, the latter giving rise to poorer sensitivity due to low water vapor content and low aerosol load. Such values have been obtained for a commercial InGaAs PIN photodiode, as well as for temporal and line-of-sight resolutions of 10 min and 150 m, respectively. Additionally, using vertical isotopologue profiles derived from a previous field campaign, precision estimates for the HD16O isotopic abundance are provided for that specific case. [ABSTRACT FROM AUTHOR]

The diurnal cycle of the atmospheric boundary layer (ABL) has been documented on 8 August 1998 in the framework of the Étude et Simulation de la QUalité de l'air en Ile-de-France (ESQUIF) experiment that took place in the Paris area. The ABL structure was documented by means of a ground-based lidar, surface meteorological stations and soundings. The interaction between the residual layer and the convective boundary layer is investigated using the collected data as well as mesoscale modelling. As opposed to the generally accepted concept, we find evidence of entrainment at the top of the residual layer. High temporal simulations of the 8 August 1998 case made with the mesoscale atmospheric model Meso-NH also show evidence of mixing at the top of the residual layer (RL). This mixing is believed to be related to the presence of convective (gravity) waves in the RL. [ABSTRACT FROM AUTHOR]

A criterion, allowing one to assess conditions likely to generate gap flows and/or hydraulic jumps in stratified flows over a mountain ridge or a mountain pass, is derived. It is based on the one-dimensional reduced-gravity shallow-water theory generalized to a three-dimensional orography with moderate streamwise variations by introducing a variable effective flow cross-section. In this way, the hydraulic jump and gap flow are accommodated within the same model. The resulting steady hyperbolic problem is shown to require the boundary conditions to be expressed in terms of Riemann invariants. The latter yield the flow between two given sites in a unique way. In particular, it is possible to relate unambiguously the existence of a hydraulic jump/gap flow and its energy discontinuity to the boundary conditions. A simple method of flow interpolation and energy discontinuity calculation between two sites is presented. [ABSTRACT FROM AUTHOR]

Laser active remote sensing of tropospheric water vapor is a promising technology to complement passive observational means in order to enhance our understanding of processes governing the global hydrological cycle. In such context, we investigate the potential of monitoring both water vapor H216O and its isotopologue HD16O using a differential absorption lidar (DIAL) allowing for ground-based remote measurements at high spatio-temporal resolution (150 m and 10 min) in the lower troposphere. This paper presents a sensitivity analysis and an error budget for a DIAL system under development which will operate in the two-micron spectral region. This numerical study uses different atmospheric conditions ranging from tropical to polar latitudes with realistic aerosol loads. Our simulations show that the measurement of the main isotopologue H216O is possible over the first 1.5 km of atmosphere with a relative precision in the water vapor mixing ratio of < 1 % in a mid-latitude or tropical environment. For the measurement of HD16O mixing ratios under the same conditions, relative precision is shown to be of similar order, thus allowing for the retrieval of range-resolved isotopic ratios. We also show that expected precisions vary by an order of magnitude between tropical and polar conditions, the latter giving rise to reduced precision due to low water vapor content and low aerosol load. Such values have been obtained for a commercial InGaAs PIN photodiode, as well as temporal and line-of-sight resolutions of 10 min and 150 m, respectively. Additionally, using vertical isotopologue profiles derived from a previous field campaign, precision estimates for the HD16O isotopic abundance are provided. [ABSTRACT FROM AUTHOR]

Direct and semi‐direct radiative effects of biomass burning aerosols (BBA) from southern and central African fires are still widely debated, in particular because climate models have been unsuccessful in reproducing the low single scattering albedo in BBA over the eastern Atlantic Ocean. Using state‐of‐the‐art airborne in situ measurements and Mie scattering simulations, we demonstrate that low single scattering albedo in well‐aged BBA plumes over southern West Africa results from the presence of strongly absorbing refractory black carbon (rBC), whereas the brown carbon contribution to the BBA absorption is negligible. Coatings enhance light absorption by rBC‐containing particles by up to 210%. Our results show that accounting for the diversity in black carbon mixing state by combining internal and external configurations is needed to accurately estimate the optical properties and henceforth the shortwave direct radiative effect and heating rate of BBA over southern West Africa. Plain Language Summary: Extensive seasonal fires over southern and central Africa result in the transport of massive amounts of biomass burning aerosols over huge areas of the eastern Atlantic Ocean. Recent field observations highlight that biomass burning aerosols transported from the coast of southern Africa to the far north over southern West Africa were characterized by low single scattering albedo. This finding is of paramount interest, because radiative heating within the absorbing aerosol layer is hypothesized to affect the low cloud deck over this specific region and may ultimately influence the large‐scale circulation. However, debate remains about the causes of the low single scattering albedo by biomass burning aerosols, causing ambiguous parameterizations of their optical properties in climate models. Here we present simultaneous airborne measurements of the composition and optical properties of biomass burning aerosols transported over southern West Africa. We show that black carbon particles dominated the light absorption by biomass burning aerosols at mid‐visible wavelengths. Our findings indicate that the black carbon mixing state plays a significant role in the aerosol optical properties and may be an important modulator to be considered in climate models for simulating direct and semi‐direct radiative effects of biomass burning aerosol over southern West Africa. Key Points: The low single scattering albedo in biomass burning aerosol (BBA) over southern West Africa results from the presence of refractory black carbonThe brown carbon contribution to the aerosol absorption is negligible in these well‐aged BBA plumesAccounting for the diversity in black carbon mixing state is needed to accurately estimate BBA optical properties [ABSTRACT FROM AUTHOR]

From 23 January to 13 February 2020, 20 manned research flights were conducted over the tropical Atlantic, off the coast of Barbados (13∘30′ N, 58∘30′ W), to characterize the trade-wind clouds generated by shallow convection. These flights were conducted as part of the international EUREC 4 A (Elucidating the role of cloud–circulation coupling in climate) field campaign. One of the objectives of these flights was to characterize the trade-wind cumuli at their base for a range of meteorological conditions, convective mesoscale organizations and times of the day, with the help of sidewards-staring remote sensing instruments (lidar and radar). This paper presents the datasets associated with horizontal lidar measurements. The lidar sampled clouds from a lateral window of the aircraft over a range of about 8 km, with a horizontal resolution of 15 m, over a rectangle pattern of 20 km by 130 km. The measurements made possible the characterization of the size distribution of clouds near their base and the presence of dust-like aerosols within and above the marine boundary layer. This paper presents the measurements and the different levels of data processing, ranging from the raw Level 1 data (10.25326/57; Chazette et al., 2020c) to the Level 2 and Level 3 processed data that include a horizontal cloud mask (10.25326/58; Chazette et al., 2020b) and aerosol extinction coefficients (10.25326/59; Chazette et al., 2020a). An intermediate level, companion to Level 1 data (Level 1.5), is also available for calibrated and geolocalized data (10.25326/57; Chazette et al., 2020c). [ABSTRACT FROM AUTHOR]

Atmospheric circulation over southern Africa and South Atlantic is characterised by the complex interaction of cloud cover and aerosol distribution, affecting the regional radiative budget in a way that is not fully understood yet. During austral winter, a compact stratocumulus low cloud deck over South Atlantic contrasts clear sky over the continent, where forest fires triggered by dry conditions emit large amount of biomass burning aerosols dominating the mid troposphere aerosol distribution. The characterisation of the tropospheric distribution of aerosols – biomass burning, as well as dust and sea salt aerosols – is a basic step towards the understanding of the complex cloud-aerosol interactions in the region.In this study, a characterisation of the synoptic variability of aerosols in southern Africa and South Atlantic during austral winter is presented. By analysing data from ECMWF reanalysis products, a weather regime classification of the region is constructed and used to characterise the distribution of biomass, dust and sea salt aerosols in the period 1979-2017. The identified aerosol-circulation patterns are then used to discuss specific biomass burning and dust aerosol occurrences observed during the AEROCLO-sA campaign in August-September 2017 in Namibia, showing a robust association between synoptic circulation and aerosol measures. [ABSTRACT FROM AUTHOR]

To study key processes of the water cycle, two special observation periods (SOPs) of the Hydrological cycle in the Mediterranean experiment (HyMeX) took place during autumn 2012 and winter 2013. The first SOP aimed to study high precipitation systems and flash flooding in the Mediterranean area. The AROME-WMED (western Mediterranean) model is a dedicated version of the mesoscale Numerical Weather Prediction (NWP) AROME-France model, which covers the western Mediterranean basin providing the HyMeX operational center with daily real-time analyses and forecasts. These products allowed for adequate decision-making for the field campaign observation deployment and the instrument operation. Shortly after the end of the campaign, a first reanalysis with more observations was performed with the first SOP operational software. An ensuing comprehensive second reanalysis of the first SOP, which included field research observations (not assimilated in real time) and some reprocessed observation datasets, was made with AROME-WMED. Moreover, a more recent version of the AROME model was used with updated background error statistics for the assimilation process. This paper depicts the main differences between the real-time version and the benefits brought by HyMeX reanalyses with AROME-WMED. The first reanalysis used 9 % additional data and the second one 24 % more compared to the real-time version. The second reanalysis is found to be closer to observations than the previous AROME-WMED analyses. The second reanalysis forecast errors of surface parameters are reduced up to the 18 and 24 h forecast range. In the middle and upper troposphere, fields are also improved up to the 48 h forecast range when compared to radiosondes. Integrated water vapor comparisons indicate a positive benefit for at least 24 h. Precipitation forecasts are found to be improved with the second reanalysis for a threshold up to 10 mm (24 h) -1. For higher thresholds, the frequency bias is degraded. Finally, improvement brought by the second reanalysis is illustrated with the Intensive Observation Period (IOP8) associated with heavy precipitation over eastern Spain and southern France. [ABSTRACT FROM AUTHOR]

To study key processes of the water cycle, two special observation periods (SOPs) of the Hydrological cycle in the Mediterranean experiment (HyMeX) took place during the autumn 2012 and winter 2013. The first SOP aimed to study high precipitation systems and flash-flooding in the Mediterranean area. The AROME-WMED (West-Mediterranean) model (Fourrié et al., 2015) is a dedicated version of the mesoscale Numerical Weather Prediction (NWP) AROME-France model 5 which covers the western Mediterranean basin providing the HyMeX operational centre with daily real-time analyses and forecasts. These products allowed adequate decision-making for the field campaign observation deployment and the instrument operation. Shortly after the end of the campaign, a first re-analysis with more observations was performed with the first SOP operational software. An ensuing comprehensive second re-analysis of the first SOP which included field research observations (not assimilated in real-time), and some reprocessed observation datasets, was made with AROME-WMED. Moreover, a more recent version of the AROME model was used with updated background error statistics for the assimilation process. This paper depicts the main differences between the real-time version and the benefits brought by HyMeX re-analyses with AROME-WMED. The first re-analysis used 9% of additional data and the second one 24% more compared to the real-time version. The second re-analysis is found to be closer to observations than the previous AROME-WMED analyses. The second re-analysis forecast errors of surface parameters are reduced up to the 18-h or 24-h forecast range. In the mid and in the upper troposphere, upper-level fields are also improved up to the 48-h forecast range when compared to radiosondes. Integrated Water Vapour comparisons indicate a positive benefit for at least 24 hours. Precipitation forecasts are found to be improved with the second re-analysis for a thresholds up to 10 mm/24-h. For higher thresholds, the frequency bias is degraded. Finally, improvement brought by the second re-analysis is illustrated with the Intensive Observation Period (IOP 8) associated with heavy precipitation over Eastern Spain and South of France. [ABSTRACT FROM AUTHOR]

The deployment of special instrumentation for the Hydrological Cycle in the Mediterranean Experiment (HyMeX) provides a valuable opportunity to investigate the spatio‐temporal variability of atmospheric water vapour across scales in relationship with the occurrence of Heavy Precipitation Systems (HPSs) in the north Western Mediterranean (WMed) during the Intensive Observation Period (IOP12), which is the focus of this investigation. High‐resolution convection‐permitting COSMO simulations complement the observational network and allow the calculation of on‐line trajectories. In addition to the presence of a favourable large‐scale situation and low‐level convergence, atmospheric moisture changes resulting in conditionally unstable air are identified as responsible for convective initiation (CI). All HPSs within the north‐WMed form in periods/areas of maximum integrated water vapour (IWV; 35–45 kg/m2) after an increase of about 10–20 kg/m2. The most intense events receive moisture from different sources simultaneously and show a sudden increase of about 10 kg/m2 between 6 and 12 h prior to the event, whereas in the less intense events the increase is larger, about 20 kg/m2, over a period of at least 24–36 h. Changes in the lower (∼900 hPa) and mid‐troposphere (∼700 hPa) control the evolution of the atmospheric moisture and the instability increase prior to CI. Spatial inhomogeneities in the lower boundary layer determine the timing and location of deep convection, whereas enhanced moisture in the mid‐troposphere favours intensification. Moister and deeper boundary layers, with updraughts reaching up to 2 km are identified in those pre‐convective environments leading to HPS, whereas dry, shallow boundary layers are found everywhere else. The build‐up time and vertical distribution of the moisture changes are found to be crucial for the evolution and severity of the HPSs rather than the amount of total column atmospheric moisture. This article investigates the spatio‐temporal variability of atmospheric water vapour across scales in relation to the occurrence of heavy precipitation systems in the northwestern Mediterranean. Synergies between different instruments, Global Positioning System (GPS) stations, radiosondes, airborne and ground‐based lidars and space‐borne observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), among others, are used. High‐resolution convection‐permitting simulations complement the observational network. Daily mean spatial distribution of total column integrated water vapour (IWV) from MODIS (background) and GPS (circles) observations. Simulated COSMO‐7km and COSMO‐2.8km 950 hPa winds at 1200 UTC are superimposed. [ABSTRACT FROM AUTHOR]

Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air-sea interactions and convective organization. [ABSTRACT FROM AUTHOR]

Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (~6.5°) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa. [ABSTRACT FROM AUTHOR]

In this study we aim to assess the WRF-Chem model capacity to reproduce dust transport over the eastern Mediterranean. For this reason, we compare the model aerosol optical depth (AOD) outputs to observations, focusing on three key regions: North Africa, the Arabian Peninsula and the eastern Mediterranean. Three sets of four simulations have been performed for the 6-month period of spring and summer 2011. Each simulation set uses a different dust emission parametrisation and for each parametrisation, the dust emissions are multiplied with various coefficients in order to tune the model performance. Our assessment approach is performed across different spatial and temporal scales using AOD observations from satellites and ground-based stations, as well as from airborne measurements of aerosol extinction coefficients over the Sahara. Assessment over the entire domain and simulation period shows that the model presents temporal and spatial variability similar to observed AODs, regardless of the applied dust emission parametrisation. On the other hand, when focusing on specific regions, the model skill varies significantly. Tuning the model performance by applying a coefficient to dust emissions may reduce the model AOD bias over a region, but may increase it in other regions. In particular, the model was shown to realistically reproduce the major dust transport events over the eastern Mediterranean, but failed to capture the regional background AOD. Further comparison of the model simulations to airborne measurements of vertical profiles of extinction coefficients over North Africa suggests that the model realistically reproduces the total atmospheric column AOD. Finally, we discuss the model results in two sensitivity tests, where we included finer dust particles (less than 1 μm) and changed accordingly the dust bins' mass fraction. [ABSTRACT FROM AUTHOR]

In this study we aim to assess the WRF-Chem model capacity to reproduce dust transport over the eastern Mediterranean. For this reason, we compare the model aerosol optical depth (AOD) outputs to observations, focusing on three key regions: North Africa, the Arabian Peninsula and the eastern Mediterranean. Three sets of four simulations have been performed for the six-month period of spring and summer 2011. Each simulation set uses a different dust emission parametrisation and for each parametrisation, the dust emissions are multiplied with various coefficients in order to tune the model performance. Our assessment approach is performed across different spatial and temporal scales using AOD observations from satellites and ground-based stations, as well as from airborne measurements of aerosol extinction coefficients over the Sahara. Assessment over the entire domain and simulation period shows that the model presents temporal and spatial variability similar to observed AODs, regardless of the applied dust emission parametrisation. On the other hand, when focusing on specific regions, the model skill varies significantly. Tuning the model performance by applying a coefficient to dust emissions may reduce the model AOD bias over a region, but may increase it in other regions. In particular, the model was shown to realistically reproduce the major dust transport events over the eastern Mediterranean, but failed to capture the regional background AOD. Further comparison of the model simulations to airborne measurements of vertical profiles of extinction coefficients over North Africa suggests that the model realistically reproduces the total atmospheric column AOD. Finally, we discuss the model results in two sensitivity tests, where we included finer dust mode (less than 1 μm) and changed accordingly the dust bins mass fraction. [ABSTRACT FROM AUTHOR]

Two deep cyclones occurred in the Mediterranean between 25 and 31 October 2012, during the first Special Observation Period (SOP1) of the Hydrological cycle in the Mediterranean Experiment (HyMeX). Both cyclones were associated with extreme rainfall covering a large part of the western Mediterranean Sea, where 24 h accumulated precipitation measurements exceeded 150 mm. We combine complementary observations from airborne radar and lidar systems, ZEUS lightning detection network and meteorological surface stations along with satellite diagnostics on deep convection, for a detailed microphysics and (thermo-)dynamical analysis of the two extreme rainfall cases. In addition, we use operational analysis data from the European Centre for Medium-range Weather Forecasts (ECMWF) for analysing the synoptic conditions and diagnosing strongly ascending air masses in the vicinity of the cyclones, so-called warm conveyor belts (WCBs). The analysis revealed the different physical characteristics of the two cyclones responsible for the extreme rainfalls. Both cyclones were associated with a WCB and a comma cloud, but deep convection, intense lightning and very cold cloud tops occurred only for the first case while the second cyclone was mostly associated with stratiform rainfall, a strong WCB, and only a few embedded cells of deep convection. [ABSTRACT FROM AUTHOR]