P. Bechtold¹, J. L. Redelsperger², I. Beau², M. Blackburn³, S. Brinkop⁴, J.-Y. Grandpeix⁵, A. Grant⁶, D. Gregory⁷, F. Guichard², C. Hoff² and E. Ioannidou³

¹ Laboratoire d'Aérologie, Observatoire Midi-Pyrenees, Toulouse, France
² CNRM/GAME (CNRS & Météo-France), Toulouse, France
³ University of Reading, Dept. of Meteorology, Reading, United Kingdom
⁴ DLR, Oberpfaffenhofen, Germany
⁵ LMD, Paris, France
⁶ UK Met Office/JCMM, Reading, United Kingdom
⁷ ECMWF, Reading, United Kingdom

Quarterly Journal of the Royal Meteorological Society, 2000, vol. 126, pp. 865-888.

Summary: The single column model (SCM) simulations of a tropical squall line case observed during TOGA­COARE are presented. This case study is part of an international model intercomparison project organized by the working group 4 ''Precipitating convective cloud systems'' of GCSS (GEWEX Cloud System Study). 8 SCM groups using different deep convection parameterizations participated in this project. The SCMs are forced by temperature and moisture tendencies that have been computed from a reference cloud resolving model (CRM) simulation using open boundary conditions. The comparison of the SCM results with the reference CRM simulation provided insight into the ability of current convection and cloud schemes to represent organized convection. The CRM results allowed for a detailed evaluation of the SCMs in terms of the thermodynamic structure and the convective mass flux of the system, the latter being closely related to the surface convective precipitation. It is shown that the SCMs can reasonably reproduce the time evolution of the surface convective and stratiform precipitation, the convective mass flux, and the thermodynamic structure of the squall line system. The thermodynamic structure simulated by the SCMs depends on how the models partition between convective and stratiform (grid­scale) precipitation. However, structural differences persist in the thermodynamic profiles simulated by the SCMs and the CRM. These differences could be attributed to the fact that the total mass flux used to compute the SCM forcing differed from the convective mass flux. The SCMs could not adequately represent these organized mesoscale circulations and microphysical transformations associated with the stratiform region. This issue is generally known as the ''scale­interaction'' problem that can only be properly addressed in fully 3D simulations. Sensitivity simulations run by several groups showed that the time evolution of the surface convective precipitation is considerably smoothed when the convective closure is based on the convective available potential energy instead of the moisture convergence. Finally, additional SCM simulations without using a convection parameterization indicated that the impact of a convection parameterization in forced SCM runs is more visible on the moisture profiles than on the temperature profiles as convective transport is particularly important in the moisture budget.