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

Abstract

A one-week convective period of the Coupled Ocean-Atmosphere Response Experiment (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 and moisture sink 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 mode results to observational uncertainties, results are analysed 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.