Radiative convective equilibrium over a land surface

Radiative-convective equilibrium (RCE) describes an idealized state of the atmosphere in which the vertical temperature profile is determined by a balance between radiative and convective fluxes. While RCE has been applied extensively over oceans, its application over the land surface has been limited. The present study explores the properties of RCE over land using an atmospheric single column model (SCM) from the Laboratoire de Meteorologie Dynamique (LMD) General Circulation Model (LMDZ5B) coupled in temperature and moisture to a land surface model comprising a simplified bucket model with finite moisture capacity. Given the presence of a large-amplitude diurnal heat flux cycle, the resultant RCE exhibits multiple equilibria when conditions are neither strictly water- nor energy-limited. By varying top-of-the-atmosphere insolation (through changes in latitude), total system water content, and initial temperature conditions, the sensitivity of the land RCE states is assessed, with particular emphasis on the role of clouds. Based on this analysis, it appears that a necessary condition for the model to exhibit multiple equilibria is the presence of low-level clouds coupled to the diurnal cycle of radiation. In addition the simulated surface precipitation rate varies non-monotonically with latitude as a result of a tradeoff between in-cloud rain rate and subcloud rain re-evaporation, thus underscoring the importance of subcloud layer processes and unsaturated downdrafts. It is shown that clouds, especially at low levels, are key elements of the internal variability of the coupled land-atmosphere system through their feedback on radiation

The Added Value of Large-Eddy and Storm-Resolving Models for Simulating Clouds and Precipitation

More than one hundred days were simulated over very large domains with fine (0.156 km to 2.5 km) grid spacing for realistic conditions to test the hypothesis that storm (kilometer) and large-eddy (hectometer) resolving simulations would provide an improved representation of clouds and precipitation in atmospheric simulations. At scales that resolve convective storms (storm-resolving for short), the vertical velocity variance becomes resolved and a better physical basis is achieved for representing clouds and precipitation. Similarly to past studies we found an improved representation of precipitation at kilometer scales, as compared to models with parameterized convection. The main precipitation features (location, diurnal cycle and spatial propagation) are well captured already at kilometer scales, and refining resolution to hectometer scales does not substantially change the simulations in these respects. It does, however, lead to a reduction in the precipitation on the time-scales considered – most notably over the ocean in the tropics. Changes in the distribution of precipitation, with less frequent extremes are also found in simulations incorporating hectometer scales. Hectometer scales appear to be more important for the representation of clouds, and make it possible to capture many important aspects of the cloud field, from the vertical distribution of cloud cover, to the distribution of cloud sizes, and to the diel (daily) cycle. Qualitative improvements, particularly in the ability to differentiate cumulus from stratiform clouds, are seen when one reduces the grid spacing from kilometer to hectometer scales. At the hectometer scale new challenges arise, but the similarity of observed and simulated scales, and the more direct connection between the circulation and the unconstrained degrees of freedom make these challenges less daunting. This quality, combined with already improved simulation as compared to more parameterized models, underpins our conviction that the use and further development of storm-resolving models offers exciting opportunities for advancing understanding of climate and climate change

Clouds and convective self-aggregation in a multi-model ensemble of radiative-convective equilibrium simulations

The Radiative-Convective Equilibrium Model Intercomparison Project (RCEMIP) is an intercomparison of multiple types of numerical models configured in radiative-convective equilibrium (RCE). RCE is an idealization of the tropical atmosphere that has long been used to study basic questions in climate science. Here, we employ RCE to investigate the role that clouds and convective activity play in determining cloud feedbacks, climate sensitivity, the state of convective aggregation, and the equilibrium climate. RCEMIP is unique amongst intercomparisons in its inclusion of a wide range of model types, including atmospheric general circulation models (GCMs), single column models (SCMs), cloud-resolving models (CRMs), large eddy simulations (LES), and global cloud-resolving models (GCRMs). The first results are presented from the RCEMIP ensemble of more than 30 models. While there are large differences across the RCEMIP ensemble in the representation of mean profiles of temperature, humidity, and cloudiness, in a majority of models anvil clouds rise, warm, and decrease in area coverage in response to an increase in sea surface temperature (SST). Nearly all models exhibit self-aggregation in large domains and agree that self-aggregation acts to dry and warm the troposphere, reduce high cloudiness, and increase cooling to space. The degree of self-aggregation exhibits no clear tendency with warming. There is a wide range of climate sensitivities, but models with parameterized convection tend to have lower climate sensitivities than models with explicit convection. In models with parameterized convection, aggregated simulations have lower climate sensitivities than un-aggregated simulations

Interactions entre processus de surface et convection profonde sur les continents tropicaux (représentation dans un modèle de climat)

Le but de cette thèse est de montrer que les mécanismes de couplage entre la convection profonde et son environnement local, au travers des processus sous nuageux et de surface, affectent son déclenchement, et par là non seulement le cycle diurne mais aussi la variabilité inter-diurne de la convection nuageuse. L'analyse de ces couplages à travers le prisme l'équilibre radiatif convectif, d'une part, et leur meilleure représentation par l'introduction d'une paramétrisation stochastique du déclenchement, d'autre part, sont exclusivement menées au moyen du GCM unicolonne du LMD (LMDZ). L'Equilibre Radiatif Convectif (ERC) nous permet d'identifier le signature climatique de la nouvelle paramétrisation. On pose en premier la question de savoir quelle est la sensibilité de la convection profonde aux conditions de surface à l'équilibre. On travaille d'abord dans un cadre idéalisé en Equilibre Radiatif Convectif et sans couplage avec le rayonnement et le sol (les conditions de surface sont prescrites en température et en coefficient d'évapo-transpiration). Ce cas très simplifié nous permet d'évaluer comment évolue le poids relatif de chaque processus sous-maille selon le partitionnement entre flux sensible et flux latent. Ensuite on se place dans un cadre plus réaliste, avec un ERC continental comportant un cycle diurne, un rayonnement et une surface couplés à l'atmosphère, et enfin une hydrologie simplifiée. Une analyse de sensibilité de la convection continentale à l'humidité de surface menées dans le SCM (Single Column Model) et comparée avec des données satellites montre un bon accord entre l'ERC continental et les observations. On se penche ensuite sur les interactions entre la convection profonde et les processus sous-nuageux aux échelles de temps courtes: en particulier sur la problématique de la transition vers la convection profonde sur continents. D'aprés l'analyse statistique d'une simulation LES d'un cas de déclenchement d'orage isolé sur sol semi-aride (cas AMMA), on propose une formulation stochastique du déclenchement. Cette dernière est ensuite intégrée, sous la forme d'une paramétrisation, au GCM du LMD (LMDZ) puis testée au travers de cas d'études 1D variés, sa valeur ajoutée par rapport à l'ancienne paramétrisation est aussi discutée. Enfin cette nouvelle paramétrisation est analysée dans le cadre de l'ERC continental, et on y retrouve le même effet sur la variabilité intra-diurne et inter-diurne de la convection que celui diagnostiqué avec les cas réalistes 1D et le modèle 3D. L'ERC contiendrait donc l'empreinte climatique de cette nouvelle paramétrisation.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Cold Pools Observed during EUREC<SUP>4</SUP>A: Detection and Characterization from Atmospheric Soundings

International audienceA new method is developed to detect cold pools from atmospheric soundings over tropical oceans and applied to sounding data from the Elucidating the Role of Cloud-Circulation Coupling in Climate (EUREC4A) field campaign, which took place south and east of Barbados in January-February 2020. The proposed method uses soundings to discriminate cold pools from their surroundings: cold pools are defined as regions where the mixed-layer height is smaller than 400 m. The method is first tested against 2D surface temperature and precipitation fields in a realistic high-resolution simulation over the western tropical Atlantic Ocean. Then, the method is applied to a dataset of 1068 atmospheric profiles from dropsondes (launched from two aircraft) and 1105 from radiosondes (launched from an array of four ships and the Barbados Cloud Observatory). We show that 7% of the EUREC4A soundings fell into cold pools. Cold-pool soundings coincide with (i) mesoscale cloud arcs and (ii) temperature drops of ∼1 K relative to the environment, along with moisture increases of ∼1 g kg-1. Furthermore, cold-pool moisture profiles exhibit a “moist layer” close to the surface, topped by a “dry layer” until the cloud base level, and followed by another moist layer in the cloud layer. In the presence of wind shear, the spreading of cold pools is favored downshear, suggesting downward momentum transport by unsaturated downdrafts. The results support the robustness of our detection method in diverse environmental conditions and its simplicity makes the method a promising tool for the characterization of cold pools, including their vertical structure. The applicability of the method to other regions and convective regimes is discussed

Deep Convection Triggering by Boundary Layer Thermals. Part I: LES Analysis and Stochastic Triggering Formulation

International audienceThis paper proposes a new formulation of the deep convection triggering for general circulation model convective parameterizations. This triggering is driven by evolving properties of the strongest boundary layer thermals. To investigate this, a statistical analysis of large-eddy simulation cloud fields in a case of transition from shallow to deep convection over a semiarid land is carried out at different stages of the transition from shallow to deep convection. Based on the dynamical and geometrical properties at cloud base, a new computation of the triggering is first proposed. The analysis of the distribution law of the maximum size of the thermals suggests that, in addition to this necessary condition, another triggering condition is required, that is, that this maximum horizontal size should exceed a certain threshold. This is explicitly represented stochastically. Therefore, the new formulation integrates the whole transition process from the first cloud to the first deep convective cell and can be decomposed into three steps: (i) the appearance of clouds, (ii) crossing of the inhibition layer, and (iii) deep convection triggering

A Physically Based Definition of Convectively Generated Density Currents: Detection and Characterization in Convection‐Permitting Simulations

International audienceIn this study, a conceptual model to define convective density currents is proposed. Based on theory, observations and modeling studies, we define convective density currents as 3D coherent structures with an anomalously cold core, an adjacent wind gust, and a vertical structure made of two layers: a well-mixed one near the surface and a stratified one above. With this definition, a methodology is proposed to identify and label individual convective density currents in convection-permitting simulations. The method is applied to four distinct cloud scenes taken from a convection-permitting simulation. Our methodology reveals new dynamic, thermodynamic, and geometric features related to the density currents’ imprint on the planetary boundary layer. The method is found to be (i) relevant in distinct convective regimes, (ii) relevant in land and oceanic situations, and (iii) adapted to both Cloud Resolving Models and Large Eddy Simulations. It also provides proxies such as the number, the spatial coverage, the mean radius, and the mean velocity of convective density currents, from which a detailed analysis of their role in the life-cycle and spatial organization of convection could be performed in the near future

Deep Convection Triggering by Boundary Layer Thermals. Part II: Stochastic Triggering Parameterization for the LMDZ GCM

International audienceThis paper presents a stochastic triggering parameterization for deep convection and its implementation in the latest standard version of the Laboratoire de Météorologie Dynamique–Zoom (LMDZ) general circulation model: LMDZ5B. The derivation of the formulation of this parameterization and the justification, based on large-eddy simulation results, for the main hypothesis was proposed in Part I of this study.Whereas the standard triggering formulation in LMDZ5B relies on the maximum vertical velocity within a mean bulk thermal, the new formulation presented here (i) considers a thermal size distribution instead of a bulk thermal, (ii) provides a statistical lifting energy at cloud base, (iii) proposes a three-step trigger (appearance of clouds, inhibition crossing, and exceeding of a cross-section threshold), and (iv) includes a stochastic component.Here the complete implementation is presented, with its coupling to the thermal model used to treat shallow convection in LMDZ5B. The parameterization is tested over various cases in a single-column model framework. A sensitivity study to each parameter introduced is also carried out. The impact of the new triggering is then evaluated in the single-column version of LMDZ on several case studies and in full 3D simulations.It is found that the new triggering (i) delays deep convection triggering, (ii) suppresses it over oceanic trade wind cumulus zones, (iii) increases the low-level cloudiness, and (iv) increases the convective variability. The scale-aware nature of this parameterization is also discussed

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