F. Guichard, J.-P. Lafore and J.-L. Redelsperger

Centre National de Recherches Météorologiques/GAME (CNRS & Météo-France), France

Quarterly Journal of the Royal Meteorological Society, 1997, vol. 123, pp.2297-2324.

Summary: A 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.