Interaction mousson/Harmattan, échanges de petite échelle

C'est à l'échelle de la turbulence que se produit l'entraînement d'air sec du flux d'Harmattan à l'intérieur du flux humide de Mousson. Ce processus de petite échelle est analysé dans le cadre du programme AMMA (Analyse Multidisciplinaires de la Mousson Africaine) qui vise a mieux renseigner et prévoir la mousson de l'Afrique de l'ouest. Les mesures atmosphériques réalisées par l'avion de recherche français ATR-42 et la modélisation des grands tourbillons (LES) sont complémentaires et permettent de documenter la structure verticale, moyenne et turbulente de la couche limite Sahélienne, de d'écrire les intrusions d'air sec et leur contribution dans les transferts turbulents, de quantifier le processus d'entraînement, de tester les paramétrisations existantes et de les relier aux conditions de grande échelle, et aux caractéristiques de la couche limite et de ses interfaces, notamment du cisaillement de vent Mousson/Harmattan.Entrainment of dry air from the Harmattan flow inside the moist monsoon flow is a turbulent process. This process is analysed in the context of the AMMA (African Monsoon Multidisciplinary Analysis) campaign experiment, which aimed at better understanding and forecasting the African Monsoon. The atmospheric measurements made by the ATR-42 research aircraft, and large eddy simulations enable us in a complementary way to study the mean and turbulent vertical structure of the sahelian boundary layer, to describe the dry intrusions and their contributions in the turbulent transferts, to quantify the entrainment process, to test the existing parametrizations and to link the entrainment process with the conditions, and with the characteristics of the boundary layer and its interfaces, in particular the windshear between monsoon & Harmattan

Combining ground-based microwave radiometer and the AROME convective scale model through 1DVAR retrievals in complex terrain: an Alpine valley case study

Abstract. A RPG-HATPRO ground-based microwave radiometer (MWR) was operated in a deep Alpine valley during the Passy-2015 field campaign. This experiment aims to investigate how stable boundary layers during wintertime conditions drive the accumulation of pollutants. In order to understand the atmospheric processes in the valley, MWRs continuously provide vertical profiles of temperature and humidity at a high time frequency, providing valuable information to follow the evolution of the boundary layer. A one-dimensional variational (1DVAR) retrieval technique has been implemented during the field campaign to optimally combine an MWR and 1 h forecasts from the French convective scale model AROME. Retrievals were compared to radiosonde data launched at least every 3 h during two intensive observation periods (IOPs). An analysis of the AROME forecast errors during the IOPs has shown a large underestimation of the surface cooling during the strongest stable episode. MWR brightness temperatures were monitored against simulations from the radiative transfer model ARTS2 (Atmospheric Radiative Transfer Simulator) and radiosonde launched during the field campaign. Large errors were observed for most transparent channels (i.e., 51–52 GHz) affected by absorption model and calibration uncertainties while a good agreement was found for opaque channels (i.e., 54–58 GHz). Based on this monitoring, a bias correction of raw brightness temperature measurements was applied before the 1DVAR retrievals. 1DVAR retrievals were found to significantly improve the AROME forecasts up to 3 km but mainly below 1 km and to outperform usual statistical regressions above 1 km. With the present implementation, a root-mean-square error (RMSE) of 1 K through all the atmospheric profile was obtained with values within 0.5 K below 500 m in clear-sky conditions. The use of lower elevation angles (up to 5°) in the MWR scanning and the bias correction were found to improve the retrievals below 1000 m. MWR retrievals were found to catch deep near-surface temperature inversions very well. Larger errors were observed in cloudy conditions due to the difficulty of ground-based MWRs to resolve high level inversions that are still challenging. Finally, 1DVAR retrievals were optimized for the analysis of the IOPs by using radiosondes as backgrounds in the 1DVAR algorithm instead of the AROME forecasts. A significant improvement of the retrievals in cloudy conditions and below 1000 m in clear-sky conditions was observed. From this study, we can conclude that MWRs are expected to bring valuable information into numerical weather prediction models up to 3 km in altitude both in clear-sky and cloudy-sky conditions with the maximum improvement found around 500 m. With an accuracy between 0.5 and 1 K in RMSE, our study has also proven that MWRs are capable of resolving deep near-surface temperature inversions observed in complex terrain during highly stable boundary layer conditions

Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modeling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modeling to better understand the processes, suggest new parameterizations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modelling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modelling to better understand the processes, suggest new parameterisations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

The surface-boundary layer connection across spatial scales of irrigation-driven thermal heterogeneity: An integrated data and modeling study of the LIAISE field campaign

Irrigation in semi-arid regions induces thermal heterogeneity across a range of spatial scales that impacts the partitioning of energy at the surface, the development of the atmospheric boundary layer, and the bidirectional interactions between the atmosphere and the surface. In this analysis, we use data from the Land Surface Interactions with the Atmosphere in the Iberian Semi-Arid Environment (LIAISE) experiment combined with a coupled land–atmosphere model to understand the role of the scales of irrigation-induced, thermal heterogeneity on the surface fluxes and consequently, the development of the diurnal convective boundary layer. The surface heterogeneity is characterized by Bowen ratios that range from ∼0.01 in the irrigated areas to ∼30 in the non-irrigated areas; however, the observed boundary-layers dynamics in both locations are similar. In this analysis, we address the questions of how the surface fluxes impact the development of the boundary-layer dynamics and how the boundary layer influences the diurnal cycle of surface fluxes. To interpret the observations, we introduce a heterogeneity scaling scheme where length scales range from local scale (∼100 m) to regional scale (∼10 km) to investigate the role of scale on surface representation in numerical models and to address the discrepancy between surface observations and their representation in weather and climate models. We find that at the surface, both the available energy and its partitioning depend on spatial scale. The observed boundary-layer properties can be explained through the composite of surface fluxes at the regional scale. Surface fluxes at the local scales are unable to replicate the observed boundary layer — even when including large-scale contributions. We find that non-local boundary layer processes like advection are important for partitioning energy at the local scale. We explore the connection between surface fluxes and the development of the boundary layer and the potential non-local effects on boundary-layer development

Intéraction Mousson/Harmattan, échanges de petite échelle

Entrainment of dry air from the Harmattan flow inside the moist monsoon flow is a turbulent process. This process is analysed in the context of the AMMA (African Monsoon Multidisciplinary Analysis) campaign experiment, which aimed at better understanding and forecasting the African Monsoon. The atmospheric measurements made by the ATR-42 research aircraft, and large eddy simulations enable us in a complementary way to study the mean and turbulent vertical structure of the sahelian boundary layer, to describe the dry intrusions and their contributions in the turbulent transferts, to quantify the entrainment process, to test the existing parametrizations and to link the entrainment process with the conditions, and with the characteristics of the boundary layer and its interfaces, in particular the windshear between monsoon & Harmattan.C'est a l'échelle de la turbulence que se produit l'entraînement d'air sec du flux d'Harmattan a l'intérieur du flux humide de Mousson. Ce processus de petite échelle est analysé dans le cadre du programme AMMA (Analyse Multidisciplinaires de la Mousson Africaine) qui vise a mieux renseigner et prévoir la mousson de l'Afrique de l'ouest. Les mesures atmosphériques réalisées par l'avion de recherche français ATR-42 et la modélisation des grands tourbillons (LES) sont complémentaires et permettent de documenter la structure verticale, moyenne et turbulente de la couche limite Sahélienne, de d'écrire les intrusions d'air sec et leur contribution dans les transferts turbulents, de quantifier le processus d'entraînement, de tester les paramétrisations existantes et de les relier aux conditions de grande échelle, et aux caractéristiques de la couche limite et de ses interfaces, notamment du cisaillement de vent Mousson/Harmattan

Ciel typique de foehn sous un lever de soleil flamboyant

Ciel typique de foehn sous un lever de soleil flamboyant. Le franchissement d'un obstacle (ici les Pyrénées) par une masse d'air provoque la formation d'onde dont cette barre de foehn est un expression. intérêt de l'image: L'image illustre la diversité des ciels, résultant de multiples phénomènes atmosphériques. Le foehn est un phénomène atmosphérique engendré par le franchissement d'un obstacle (ici, les Pyrénées) par une masse d'air. La masse d'air, pour franchir cet obstacle, peut soit le contourner, soit se soulever pour passer au-dessus. Le soulèvement en amont du relief et la descente en aval engendrent divers phénomènes: des précipitations en amont, des ondes au-dessus, et le phénomène de foehn en aval, associé à un réchauffement et un assèchement de l'air, et de la turbulence. Ce phénomène peut provoquer la formation d'un "trou de foehn" puis d'une "barre de foehn", sous le vent de l'obstacle, comme on peut l'observer sur cette image. contexte de la recherche: Le Centre de Recherches Atmosphériques de Lannemezan est un large espace ouvert permettant de déployer facilement une instrumentation variée parfois encombrante. Une grande variété de ciels peut y être observée. objectif poursuivi: De nombreuses mesures sont faites de façon continue au Centre de Recherches Atmosphériques de Lannemezan, constituant une base de données à long terme pour les études de phénomènes atmosphériques

Ciel typique de foehn sous un lever de soleil flamboyant

Ciel typique de foehn sous un lever de soleil flamboyant. Le franchissement d'un obstacle (ici les Pyrénées) par une masse d'air provoque la formation d'onde dont cette barre de foehn est un expression. intérêt de l'image: L'image illustre la diversité des ciels, résultant de multiples phénomènes atmosphériques. Le foehn est un phénomène atmosphérique engendré par le franchissement d'un obstacle (ici, les Pyrénées) par une masse d'air. La masse d'air, pour franchir cet obstacle, peut soit le contourner, soit se soulever pour passer au-dessus. Le soulèvement en amont du relief et la descente en aval engendrent divers phénomènes: des précipitations en amont, des ondes au-dessus, et le phénomène de foehn en aval, associé à un réchauffement et un assèchement de l'air, et de la turbulence. Ce phénomène peut provoquer la formation d'un "trou de foehn" puis d'une "barre de foehn", sous le vent de l'obstacle, comme on peut l'observer sur cette image. contexte de la recherche: Le Centre de Recherches Atmosphériques de Lannemezan est un large espace ouvert permettant de déployer facilement une instrumentation variée parfois encombrante. Une grande variété de ciels peut y être observée. objectif poursuivi: De nombreuses mesures sont faites de façon continue au Centre de Recherches Atmosphériques de Lannemezan, constituant une base de données à long terme pour les études de phénomènes atmosphériques

Interaction mousson-Harmattan, échanges de petite échelle

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

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