Intercomparison of methods of coupling between convection and large-scale circulation: 1. Comparison over uniform surface conditions

As part of an international intercomparison project, a set of single column models (SCMs) and cloud-resolving models (CRMs) are run under the weak temperature gradient (WTG) method and the damped gravity wave (DGW) method. For each model, the implementation of the WTG or DGW method involves a simulated column which is coupled to a reference state defined with profiles obtained from the same model in radiative-convective equilibrium. The simulated column has the same surface conditions as the reference state and is initialized with profiles from the reference state. We performed systematic comparison of the behavior of different models under a consistent implementation of the WTG method and the DGW method and systematic comparison of the WTG and DGW methods in models with different physics and numerics. CRMs and SCMs produce a variety of behaviors under both WTG and DGW methods. Some of the models reproduce the reference state while others sustain a large-scale circulation which results in either substantially lower or higher precipitation compared to the value of the reference state. CRMs show a fairly linear relationship between precipitation and circulation strength. SCMs display a wider range of behaviors than CRMs. Some SCMs under the WTG method produce zero precipitation. Within an individual SCM, a DGW simulation and a corresponding WTG simulation can produce different signed circulation. When initialized with a dry troposphere, DGW simulations always result in a precipitating equilibrium state. The greatest sensitivities to the initial moisture conditions occur for multiple stable equilibria in some WTG simulations, corresponding to either a dry equilibrium state when initialized as dry or a precipitating equilibrium state when initialized as moist. Multiple equilibria are seen in more WTG simulations for higher SST. In some models, the existence of multiple equilibria is sensitive to some parameters in the WTG calculations

Water vapour variability within a convective boundary layer assessed by Large Eddy Simulations and IHOP_2002 observations

This study presents a comprehensive analysis of the variability of water vapour in a growing convective boundary-layer (CBL) over land, highlighting the complex links between advection, convective activity and moisture heterogeneity in the boundary layer. A Large-eddy Simulation (LES) is designed, based on observations, and validated, using an independent data-set collected during the International H2O Project (IHOP_2002) field-experiment. Ample information about the moisture distribution in space and time, as well as other important CBL parameters are acquired by mesonet stations, balloon soundings, instruments on-board two aircraft and the DLR airborne water-vapour differential-absorption lidar. Because it can deliver two-dimensional cross-sections at high spatial resolution (140 m horizontal, 200 m vertical), the airborne lidar offers valuable insights of small-scale moisture-variability throughout the CBL. The LES is able to reproduce the development of the CBL in the morning and early afternoon, as assessed by comparisons of simulated mean profiles of key meteorological variables with sounding data. Simulated profiles of the variance of water-vapour mixing-ratio were found to be in good agreement with the lidar-derived counterparts. Finally, probability-density functions of potential temperature, vertical velocity and water-vapour mixing-ratio calculated from the LES show great consistency with those derived from aircraft in situ measurements in the middle of the CBL. Downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature The observed water-vapour variability exhibits contributions from different scales. The influence of the mesoscale (larger than LES domain size, i.e. 10 km) on the smaller-scale variability is assessed using LES and observations. The small-scale variability of water vapour is found to be important and to be driven by the dynamics of the CBL. Both lidar observations and LES evidence that dry downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature and vertical velocity. In particular, intrusions of drier free-troposphere air from above the growing CBL impose a marked negative skewness on the water-vapour distribution within it, both as observed and in the simulation. Copyright © 2005 Royal Meteorological Society

Cloud-resolving simulation of convective activity during TOGA-COARE: Sensitivity to external sources of uncertainties

International audienceSummary: A one-week convective period of COARE (10-17 December 1992), prior to a westerly wind burst, has been simulated with a cloud resolving model. Large-scale advection derived from observations is used to force the model, in the same way as usually done in single column models. Our aim is to evaluate this explicit simulation against observed large-scale thermodynamic and radiative fields, and to investigate the sensitivity of model results to observational uncertainties.Precipitation, apparent heat source (Q1) and moisture sink (Q2) are fairly well reproduced by the model as compared to those diagnosed from observations. Temperature (T) and moisture (qv) fields are also reasonably well captured except for a moderate cold and moist bias. Simulated moist static energy is too high below 6 km and too low above, possibly because convection is slightly less active in the model than observed. In order to investigate the sensitivity of model results to observational uncertainties, results are analyzed with the moist static energy budget together with independent observational radiative datasets. This analysis suggests that the atmospheric radiative rate that is in equilibrium with the applied large-scale advection and observed surface fluxes is too weak and that its diurnal cycle is not realistic. The most likely reason for this problem is found to be related to uncertainties in the large-scale advection diagnosed from observations. This analysis also indicates that the simulated high cloud cover is too large in the model. It is greatly improved by increasing the ice crystal fall speed. Additional tests show a large sensitivity of the simulated moist static energy, and thus T and qv, to the range of uncertainties previously found for large-scale advection. The vertical structure of the model bias is not significantly modified by changing the intensity of these forcings, but it is most sensitive to their vertical structures. It is argued that it is crucial to get some insights into the range of uncertainties of external forcings (large-scale advection, surface fluxes and atmospheric radiative heating rate) so as to assess the relevance of any evaluation of simulated temperature and moisture when a model, either resolving clouds or parametrizing them, is forced with large-scale advection deduced from observations

Thermodynamical impact and internal structure of a tropical convective cloud system

International audienceA three-dimensional cloud-resolving model is used to simulate a cloud system, observed during the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment, corresponding to the development of shear parallel convective lines and characterized by the absence of large-scale ascent. The system life cycle includes different types of clouds interacting in both space and time. The thermodynamical impact as well as statistical properties of the system are analysed using a partition of the total domain into several (6 to 12) internal areas. In-cloud temperature excess is weak as observed, whereas water vapour excess is significant and correlated with vertical velocity. However, buoyancy deviations are extremely small, indicating an equilibrium of density, involving thermodynamics and microphysics. Decomposition of budgets highlights the mechanisms of compensation occuring between the precipitating system and its environment. Moisture convective transports are extremely intense and complex to analyse. A decomposition into vertical and horizontal parts shows that horizontal exchanges are important, in particular to explain moistening at upper levels. The effective part of vertical fluxes (after removing the compensating parts) occurs in active shallow and deep clouds, at very fine scales. These results question some basic hypotheses assumed in existing convective parametrizations

Use of a Radar Simulator on the Output Fields from a Numerical Mesoscale Model to Analyze X-Band Rain Estimators

International audienceA full radar simulator, which works with the 3D output fields from a numerical mesoscale model, has been developed. This simulator uses a T-matrix code to calculate synthetic radar measurements, accounts for both backscattering and propagation effects, and includes polarimetric variables. The tool is modular to allow several options in the derivation of the synthetic radar variables. A measurement uncertainty can be taken into account on both the simulated reflectivities and the differential phase shift. A scheme can also be switched on to allow for the gate-to-gate variability of the rain drops size distribution or, also, their oblateness. This work was done in the framework of the installation in West Africa of a polarimetric X-band radar for the observation of tropical rain. Accordingly, the first objective pursued with this simulation setup is a detailed analysis of X-band polarimetric rain retrieval algorithms. Two retrieval schemes, a simple R–KDP formula and a profiler that uses both reflectivity and DP, are tested. For that purpose the simulator is run on a model case study of an African squall line, then the two schemes are used to retrieve the rain rates from the synthetic radar variables and compare them to the original. The scores of the schemes are discussed and compared. The authors analyze the sensitivity of the results to the measurement uncertainty and also to several aspects of drop size distribution and drop shape variability