Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during the DACCIWA field campaign

Biogenic fluxes from soil at a local and regional scale are crucial to study air pollution and climate. Here we present field measurements of soil fluxes of nitric oxide (NO) and ammonia (NH3) observed over four different land cover types, i.e. bare soil, grassland, maize field, and forest, at an inland rural site in Benin, West Africa, during the DACCIWA field campaign in June and July 2016. At the regional scale, urbanization and a massive growth in population in West Africa have been causing a strong increase in anthropogenic emissions. Anthropogenic pollutants are transported inland and northward from the megacities located on the coast, where the reaction with biogenic emissions may lead to enhanced ozone production outside urban areas, as well as secondary organic aerosol formation, with detrimental effects on humans, animals, natural vegetation, and crops. We observe NO fluxes up to 48.05 ngNm 2 s 1. NO fluxes averaged over all land cover types are 4:79 5:59 ngNm 2 s 1, and maximum soil emissions of NO are recorded over bare soil. NH3 is dominated by deposition for all land cover types. NH3 fluxes range between 6:59 and 4.96 ngNm 2 s 1. NH3 fluxes averaged over all land cover types are 0:911:27 ngNm 2 s 1, and maximum NH3 deposition is measured over bare soil. The observations show high spatial variability even for the same soil type, same day, and same meteorological conditions. We compare point daytime average measurements of NO emissions recorded during the field campaign with those simulated by GEOS-Chem (Goddard Earth Observing System Chemistry Model) for the same site and find good agreement. In an attempt to quantify NO emissions at the regional and national scale, we also provide a tentative estimate of total NO emissions for the entire country of Benin for the month of July using two distinct methods: upscaling point measurements and using the GEOS-Chem model. The two methods give similar results: 1:170:6 and 1.44 GgN month 1, respectively. Total NH3 deposition estimated by upscaling point measurements for the month of July is 0.21 GgN month1

Thermodynamics and tubulence during cold air outbreaks over the Gulf of Lion

En période hivernale, le golfe du Lion est sujet à des conditions de vents régionaux forts (Mistral et/ou Tramontane) qui transportent des masses d'air continentales froides et sèches au dessus de la mer. Ces événements, les Cold Air Outbreaks (CAO) , conduisent à d'intenses échanges air-mer et donc à un pompage de chaleur qui favorise la formation d'eaux denses et le déclenchement de la convection océanique profonde. La bonne représentation de ces échanges air-mer intenses dans les modèles de climat et de prévision numérique du temps reste à l'heure actuelle une problématique majeure. Elle est au cœur du projet ASICS-MED centré sur compréhension des mécanismes de formation d'eaux denses en Méditerranée et qui s'inscrit dans le cadre de la thématique " Échanges air-mer intenses " du programme HyMeX dédié à l'étude du cycle de l'eau en Méditerranée. Les processus qui s'opèrent au sein de la couche limite atmosphérique marine (CLAM) et de la couche de mélange océanique (CMO) interagissent entre eux à différentes échelles spatiales et temporelles. La compréhension de l'évolution globale de la CLAM mais également des mécanismes locaux nécessitent la prise en compte de l'ensemble des processus. L'étude présentée ici est consacrée à la structure moyenne et turbulente de la CLAM en conditions de vents forts. L'objectif est de déterminer comment l'organisation du champ turbulent est impactée lors d'épisodes de CAO et d'estimer les flux de surface associés à ces conditions de vents forts. La méthodologie adoptée est basée sur l'utilisation conjointe d'observations aéroportées collectées lors de la campagne de mesure SOP2 d'HyMeX et de simulations numériques. La campagne de mesure SOP2 d'HyMeX qui a eu lieu au cours de l'hiver 2013 dans le golfe du Lion a permis de documenter grâce à l'avion de recherche ATR42 la structure moyenne et turbulente de la CLAM lors de 11 épisodes de CAO. Une analyse spectrale s'appuyant sur un modèle analytique a été réalisé sur 181 paliers (i.e. segments de vol rectilignes et stabilisés en altitude). Les profils verticaux des échelles turbulentes caractéristiques ainsi que la forme du spectre de la vitesse verticale ont permis de mettre en évidence un allongement des structures énergétiques dans l'axe du vent moyen associé à l'organisation du champ turbulent sous la forme de rouleaux longitudinaux. Une description unidirectionnelle du champ turbulent tridimensionnel peut conduire à une représentativité limitée des structures cohérentes au sein des échantillons. Cependant, la connaissance des profils de flux sur toute l'épaisseur de la CLAM est nécessaire pour l'estimation des échanges air-mer. Une méthode de correction des flux turbulents calculés par eddy correlation a été appliqué afin de prendre en compte les erreurs systématique et aléatoire relatives à la mesure et au traitement de données. Cette correction a permis de déterminer les meilleures estimations possibles des flux extrapolés à la surface avec une marge d'incertitude pour les 11 épisodes de CAO documentés lors de la campagne SOP2 d'HyMeX. La comparaison de ces estimations aéroportées aux autres sources d'information dérivées de paramétrisations des flux a permis de mettre en évidence une sous-estimation systématique du flux de chaleur latente en conditions de vents forts. Une approche numérique a permis de compléter l'analyse de la structure moyenne et turbulente de la CLAM lors d'épisodes de CAO.During winter, local strong winds (Mistral or Tramontana) occurred in the Gulf of Lion which bring cold and dry continental air over a warmer sea. Those events, the cold air outbreaks, can lead to intense air-sea interactions which favour dense water formation and deep oceanic convection. The representation of air-sea exchanges is a fundamental aspect of of climate modelling and numerical weather forecasting. The ASICS-MED project aims to identify fine-scale processes leading to dense water formation and is a part of the "Intense air-sea exchanges" topic of the HyMeX program devoted to hydrological cycle in the Mediterranean. The processes occurring within the marine atmospheric boundary layer (MABL) and the oceanic mixing layer (ML) interact with one another at different spatial and temporal scales. Understanding the overall evolution of the MABL but also the local mechanisms requires taking into account all the processes. The study presented here is devoted to the mean and turbulent structure of the MABL under strong wind conditions. The objective is to determine how the organization of the turbulent field is impacted during CAO events and to estimate the surface fluxes associated with these strong wind conditions. The methodology adopted is based on the joint use of airborne observations collected during the HyMeX-SOP2 field campaign and numerical simulations. The HyMeX-SOP2 field campaign took place during the winter of 2013 in the Gulf of Lion. The research aircraft ATR42 was operated to document the mean and turbulent structure of the MABL during 11 CAO events. A spectral analysis based on an analytic model was carried out on 181 legs (i.e. stacked straight and level runs stabilized in altitude). The vertical profiles of the turbulent characteristic scales as well as the shape of the vertical velocity spectrum revealed an elongation of the energy structures in the mean wind direction associated with the organization of the turbulent field into longitudinal rolls. A unidirectional sampling of the three-dimensional turbulent field may lead to a limited representativeness of the coherent structures within the samples. However, knowledge of kinematic fluxes profiles over the entire thickness of the CLAM is necessary to estimate air-sea exchanges. A correction method was applied to turbulent fluxes calculated by eddy correlation in order to take into account systematic and random errors related to measurement and data processing. This correction made it possible to determine the best possible estimates of the extrapolated surface fluxes with a margin of uncertainty for the 11 CAO events documented during the HyMeX-SOP2 field campaign. The comparison of these airborne estimates with the other sources of information derived from bulk parameterizations show a systematic underestimation of the latent heat flux under strong wind conditions. A numerical approach allowed to complete the analysis of the mean and turbulent structure of the MABL during CAO events. The numerical study, based on the non-hydrostatic Meso-NH model, focuses on an episode of strong Tramontana with winds greater than 25m/s documented during the HyMeX-SOP2 field campaign. In a first step, a one-dimensional framework made it possible to understand the forcing terms necessary to reproduce in a realistic way the development of the observed MABL. This reference configuration allowed, in a second time, a Large-Eddy Simulation of the CAO event. This simulation has been validated using airborne data and has allowed to deepen the description of the turbulent field as well as the evolution of the coherent structures oriented in the axis of the mean wind

Thermodynamique et turbulence dans les épisodes de vent fort sur le Golfe du Lion

During winter, local strong winds (Mistral or Tramontana) occurred in the Gulf of Lion which bring cold and dry continental air over a warmer sea. Those events, the cold air outbreaks, can lead to intense air-sea interactions which favour dense water formation and deep oceanic convection. The representation of air-sea exchanges is a fundamental aspect of of climate modelling and numerical weather forecasting. The ASICS-MED project aims to identify fine-scale processes leading to dense water formation and is a part of the "Intense air-sea exchanges" topic of the HyMeX program devoted to hydrological cycle in the Mediterranean. The processes occurring within the marine atmospheric boundary layer (MABL) and the oceanic mixing layer (ML) interact with one another at different spatial and temporal scales. Understanding the overall evolution of the MABL but also the local mechanisms requires taking into account all the processes. The study presented here is devoted to the mean and turbulent structure of the MABL under strong wind conditions. The objective is to determine how the organization of the turbulent field is impacted during CAO events and to estimate the surface fluxes associated with these strong wind conditions. The methodology adopted is based on the joint use of airborne observations collected during the HyMeX-SOP2 field campaign and numerical simulations. The HyMeX-SOP2 field campaign took place during the winter of 2013 in the Gulf of Lion. The research aircraft ATR42 was operated to document the mean and turbulent structure of the MABL during 11 CAO events. A spectral analysis based on an analytic model was carried out on 181 legs (i.e. stacked straight and level runs stabilized in altitude). The vertical profiles of the turbulent characteristic scales as well as the shape of the vertical velocity spectrum revealed an elongation of the energy structures in the mean wind direction associated with the organization of the turbulent field into longitudinal rolls. A unidirectional sampling of the three-dimensional turbulent field may lead to a limited representativeness of the coherent structures within the samples. However, knowledge of kinematic fluxes profiles over the entire thickness of the CLAM is necessary to estimate air-sea exchanges. A correction method was applied to turbulent fluxes calculated by eddy correlation in order to take into account systematic and random errors related to measurement and data processing. This correction made it possible to determine the best possible estimates of the extrapolated surface fluxes with a margin of uncertainty for the 11 CAO events documented during the HyMeX-SOP2 field campaign. The comparison of these airborne estimates with the other sources of information derived from bulk parameterizations show a systematic underestimation of the latent heat flux under strong wind conditions. A numerical approach allowed to complete the analysis of the mean and turbulent structure of the MABL during CAO events. The numerical study, based on the non-hydrostatic Meso-NH model, focuses on an episode of strong Tramontana with winds greater than 25m/s documented during the HyMeX-SOP2 field campaign. In a first step, a one-dimensional framework made it possible to understand the forcing terms necessary to reproduce in a realistic way the development of the observed MABL. This reference configuration allowed, in a second time, a Large-Eddy Simulation of the CAO event. This simulation has been validated using airborne data and has allowed to deepen the description of the turbulent field as well as the evolution of the coherent structures oriented in the axis of the mean wind.En période hivernale, le golfe du Lion est sujet à des conditions de vents régionaux forts (Mistral et/ou Tramontane) qui transportent des masses d'air continentales froides et sèches au dessus de la mer. Ces événements, les Cold Air Outbreaks (CAO) , conduisent à d'intenses échanges air-mer et donc à un pompage de chaleur qui favorise la formation d'eaux denses et le déclenchement de la convection océanique profonde. La bonne représentation de ces échanges air-mer intenses dans les modèles de climat et de prévision numérique du temps reste à l'heure actuelle une problématique majeure. Elle est au cœur du projet ASICS-MED centré sur compréhension des mécanismes de formation d'eaux denses en Méditerranée et qui s'inscrit dans le cadre de la thématique " Échanges air-mer intenses " du programme HyMeX dédié à l'étude du cycle de l'eau en Méditerranée. Les processus qui s'opèrent au sein de la couche limite atmosphérique marine (CLAM) et de la couche de mélange océanique (CMO) interagissent entre eux à différentes échelles spatiales et temporelles. La compréhension de l'évolution globale de la CLAM mais également des mécanismes locaux nécessitent la prise en compte de l'ensemble des processus. L'étude présentée ici est consacrée à la structure moyenne et turbulente de la CLAM en conditions de vents forts. L'objectif est de déterminer comment l'organisation du champ turbulent est impactée lors d'épisodes de CAO et d'estimer les flux de surface associés à ces conditions de vents forts. La méthodologie adoptée est basée sur l'utilisation conjointe d'observations aéroportées collectées lors de la campagne de mesure SOP2 d'HyMeX et de simulations numériques. La campagne de mesure SOP2 d'HyMeX qui a eu lieu au cours de l'hiver 2013 dans le golfe du Lion a permis de documenter grâce à l'avion de recherche ATR42 la structure moyenne et turbulente de la CLAM lors de 11 épisodes de CAO. Une analyse spectrale s'appuyant sur un modèle analytique a été réalisé sur 181 paliers (i.e. segments de vol rectilignes et stabilisés en altitude). Les profils verticaux des échelles turbulentes caractéristiques ainsi que la forme du spectre de la vitesse verticale ont permis de mettre en évidence un allongement des structures énergétiques dans l'axe du vent moyen associé à l'organisation du champ turbulent sous la forme de rouleaux longitudinaux. Une description unidirectionnelle du champ turbulent tridimensionnel peut conduire à une représentativité limitée des structures cohérentes au sein des échantillons. Cependant, la connaissance des profils de flux sur toute l'épaisseur de la CLAM est nécessaire pour l'estimation des échanges air-mer. Une méthode de correction des flux turbulents calculés par eddy correlation a été appliqué afin de prendre en compte les erreurs systématique et aléatoire relatives à la mesure et au traitement de données. Cette correction a permis de déterminer les meilleures estimations possibles des flux extrapolés à la surface avec une marge d'incertitude pour les 11 épisodes de CAO documentés lors de la campagne SOP2 d'HyMeX. La comparaison de ces estimations aéroportées aux autres sources d'information dérivées de paramétrisations des flux a permis de mettre en évidence une sous-estimation systématique du flux de chaleur latente en conditions de vents forts. Une approche numérique a permis de compléter l'analyse de la structure moyenne et turbulente de la CLAM lors d'épisodes de CAO

Organized Turbulence in a Cold-Air Outbreak: Evaluating a Large-Eddy Simulation with Respect to Airborne Measurements

International audienceCold-air outbreaks (CAO) lead to intense air–sea interactions, the appropriate representation of which are fundamental for climate modelling and numerical weather forecasting. We analyze a CAO event with low-level wind speeds of approximately 25 m s−1 observed in the north-western Mediterranean Sea. The marine atmospheric boundary layer (MABL) was sampled with an aircraft equipped for turbulence measurements, revealing the organization of the MABL flow in coherent structures oriented along the mean wind direction, which was then simulated in two steps. First, a one-dimensional simulation enabled the determination of the forcing terms (particularly horizontal advection) required to adequately reproduce the vertical structure of the MABL flow. These terms were computed from a limited-area forecast model in operation during the entire field campaign. Then, a large-eddy simulation (LES) was performed during the well-established phase of the CAO event. The LES output is validated with respect to airborne data, not only with respect to the mean wind-speed and thermodynamic profiles, but also the turbulence statistics and coherent structures. The validated LES results enable description of the turbulent field as well as the coherent structures. The main discrepancy is a considerable underestimation of the simulated evaporation (computed with a parametrization of the turbulent surface fluxes), and hence of the moisture fluctuations throughout the boundary layer. Several possible explanations may explain this underestimation. The structure of the boundary layer is nonetheless well reproduced by the LES model, including the organized structures and their characteristic scales, such as the structure wavelength, orientation, and aspect ratio, which closely agree with observations. A conditional-sampling analysis enables determination of the contribution of the coherent structures to the vertical exchange. Although they occupy a limited fractional area, organized structures are the primary contributors to the turbulent exchange

A numerical study of ocean surface‐layer response to atmospheric shallow convection: Impact of cloud shading, rain, and cold pools

International audienceAbstract The response of the oceanic surface layer to atmospheric shallow convection is explored using realistic atmospheric large eddy simulations coupled with an oceanic 1D model with high vertical resolution. The effects of cloud shading, rain, and enhanced heat loss due to gust fronts on the edge of cold pools and their interactions are investigated in a case study of the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation experiment in the tropical Indian Ocean, during a suppressed phase of the Madden–Julian Oscillation. Conditions of low surface wind and strong solar heating result in diurnal warming of the oceanic surface of 2 °C over a depth of 1 m. Analysis of specific periods covering the diurnal cycle shows the contrasting effects of cloud shading, rain, and turbulent heat fluxes under the cold pools on the sea temperature at the surface and below. On the one hand, decreasing the solar radiation (cloud shading) results in slight cooling extended horizontally and penetrating down to 1–2 m depth, depending on the time of the day. On the other hand, turbulent heat fluxes enhanced up to 300 Wm by gusts and freshwater lenses due to rain act together and more locally. They isolate and strongly cool a thin inner layer at the surface, which eventually destabilizes the surface layer and propagates the cooling downward. The exact relative part and efficiency of these processes depend on the time evolution of the thermal stratification and vertical turbulent mixing in the oceanic upper layer. Surface cooling of up to 0.5 °C may occur in a few tens of minutes and last for several hours, significantly mitigating the effects of diurnal warming over large extents

A case‐study of the coupled ocean–atmosphere response to an oceanic diurnal warm layer

International audienceA modelling case-study based on observations from the Dynamics of the Madden–Julian Oscillation field campaign is presented and discussed. It aims at investigating the ocean–atmosphere coupling and the marine atmospheric boundary-layer structure over an oceanic diurnal warm layer. This case corresponds to the development of a diurnal warm layer characterized by a sea surface temperature diurnal cycle of ∼2°C. A 1D oceanic model with high vertical resolution is used to investigate the mechanisms responsible for the establishment and decay of the diurnal warm layer highlighting competing impact of the absorption of the solar radiation, the turbulent transport and the surface heat loss. An atmospheric large-eddy simulation coupled to the 1D oceanic model is then presented and extensively evaluated against the numerous observations available for this case. The simulation is able to reproduce the surface fluxes and the main boundary-layer structures. This study thus provides a case to investigate the ability of parametrizations to handle the ocean–atmosphere coupling and its impact on the atmospheric boundary layer

On the imprint of the mesoscale organization of tradewind clouds at cloud base and below

International audienceTrade-wind clouds can exhibit different patterns of mesoscale organization. These patterns were observed during the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field campaign that took place in Jan-Feb 2020 over the western tropical Atlantic near Barbados: while the HALO aircraft was observing clouds from above and was characterizing the large-scale environment with dropsondes, the ATR-42 research aircraft was flying in the lower troposphere, characterizing clouds and turbulence with horizontal radar-lidar measurements and in-situ probes and sensors. By analyzing these data for different cloud patterns, we investigate the extent to which the cloud organization is imprinted in cloud-base properties and subcloud-layer heterogeneities. The implications of our findings for understanding the roots of the mesoscale organization of tradewind clouds will be discussed

Observations and Simulations of the Stable Water Isotope Signature of Convective Updrafts and Downdrafts Associated with Different Mesoscale Organization Patterns of Shallow Trade Wind Clouds

International audienceWater molecules containing a heavy hydrogen or oxygen atom have lower saturation vapor pressures and diffusivities than their light counterparts, which leads to a change in their relative abundance during phase transitions. Each process controlling the water vapor budget of shallow cumulus clouds in the trade wind region such as ocean evaporation, convective and turbulent mixing, condensation and evaporation of hydrometeors is associated with a characteristic isotopic fingerprint. In a recent study, different transport regimes in the trades have been shown to be associated with distinct mesoscale organization patterns and water vapor isotope signals in the sub-cloud layer. The reported isotopic differentiation between various mesoscale organisation patterns is most likely due to variations in the relative contribution of the processes mentioned above to the water vapor budget along the flow. To test this hypothesis and assess the timescales over which the pattern-specific anomalies in water vapor isotopes emerge, we use a combination of airborne measurements of stable water vapor isotopes and high-resolution simulations with the isotope-enabled numerical model COSMOiso. The evolution of the stable isotope signature along the trade wind flow is assessed using three-dimensional back-trajectories calculated based on COSMOiso wind fields. The isotope measurements in water vapor were performed on the French aircraft ATR-42 during 18 flights from 25 January to 13 February 2020 as a part of the international EUREC4A field campaign. The flights were conducted over the tropical ocean near Barbados with the aircraft staying predominantly at the height of cloud base. Complementary cloud radar and lidar data as well as in-situ turbulence and microphysics measurements are used to characterise the physical environment of the studied cloud fields. Our isotope measurements reveal substantial differences between flights, which are related to contrasting large-scale flow situations and mesoscale cloud organizations. This study provides promising process-based insights into the cloud-circulation coupling conundrum and demonstrates the potential of water isotopes to identify relevant processes

Observations and Simulations of the Stable Water Isotope Signature of Convective Updrafts and Downdrafts Associated with Different Mesoscale Organization Patterns of Shallow Trade Wind Clouds

International audienceWater molecules containing a heavy hydrogen or oxygen atom have lower saturation vapor pressures and diffusivities than their light counterparts, which leads to a change in their relative abundance during phase transitions. Each process controlling the water vapor budget of shallow cumulus clouds in the trade wind region such as ocean evaporation, convective and turbulent mixing, condensation and evaporation of hydrometeors is associated with a characteristic isotopic fingerprint. In a recent study, different transport regimes in the trades have been shown to be associated with distinct mesoscale organization patterns and water vapor isotope signals in the sub-cloud layer. The reported isotopic differentiation between various mesoscale organisation patterns is most likely due to variations in the relative contribution of the processes mentioned above to the water vapor budget along the flow. To test this hypothesis and assess the timescales over which the pattern-specific anomalies in water vapor isotopes emerge, we use a combination of airborne measurements of stable water vapor isotopes and high-resolution simulations with the isotope-enabled numerical model COSMOiso. The evolution of the stable isotope signature along the trade wind flow is assessed using three-dimensional back-trajectories calculated based on COSMOiso wind fields. The isotope measurements in water vapor were performed on the French aircraft ATR-42 during 18 flights from 25 January to 13 February 2020 as a part of the international EUREC4A field campaign. The flights were conducted over the tropical ocean near Barbados with the aircraft staying predominantly at the height of cloud base. Complementary cloud radar and lidar data as well as in-situ turbulence and microphysics measurements are used to characterise the physical environment of the studied cloud fields. Our isotope measurements reveal substantial differences between flights, which are related to contrasting large-scale flow situations and mesoscale cloud organizations. This study provides promising process-based insights into the cloud-circulation coupling conundrum and demonstrates the potential of water isotopes to identify relevant processes