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

A one-week convective period of the Coupled Ocean-Atmosphere Response Exper iment (10-17 December 1992), prior to a westerly wind burst, has been simul ated with a cloud-resolving model. Large-scale advection derived from obser vations is used to force the model, in the same way as usually done in sing le-column models. Our aim is to evaluate this explicit simulation against o bserved large-scale thermodynamic and radiative fields, and to investigate the sensitivity of model results to observational uncertainties. Precipitat ion, apparent heat source and moisture sink are fairly well reproduced by t he model as compared to those diagnosed from observations. Temperature (T) and moisture (q(v)) fields are also reasonably well captured except for a m oderate cold and moist bias. Simulated moist static energy is too high belo w 6 km and too low above, possibly because convection is slightly less acti ve in the model than observed. In order to investigate the sensitivity of model results to observational u ncertainties, results are analysed with the moist static energy budget toge ther with independent observational radiative datasets. This analysis sugge sts that the atmospheric radiative rate that is in equilibrium with the app lied large-scale advection and observed surface fluxes is too weak and that its diurnal cycle is not realistic. The most likely reason for this proble m is found to be related to uncertainties in the large-scale advection diag nosed from observations. This analysis also indicates that the simulated hi gh-cloud cover is too large in the model. It is greatly improved by increas ing the ice-crystal fall speed. Additional tests show a large sensitivity o f the simulated moist static energy, and thus T and q(v), to the range of u ncertainties previously found for large-scale advection. The vertical struc ture of the model bias is not significantly modified by changing the intens ity of these forcings, but it is most sensitive to their vertical structure s. It is argued that it is crucial to get some insights into the range of unce rtainties of external forcings (large-scale advection, surface fluxes and a tmospheric radiative-heating rate) so as to assess the relevance of any eva luation of simulated temperature and moisture when a model, either resolvin g clouds or parametrizing them, is forced with large-scale advection deduce d from observations.