F. Guichard, J.-L Redelsperger and J.-P. Lafore
Centre National de Recherches Météorologiques/GAME (CNRS & Météo-France), France
Quarterly Journal of the Royal Meteorological Society, 2000, vol. 126, pp. 3067-3096.
Summary: 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.