A GCSS model intercomparison for a tropical squall line observed during TOGA-COARE. II: Intercomparison of single-column models and a cloud-resolvingmodel

This paper presents single-column model (SCM) simulations of a tropical squ all-line case observed during the Coupled Ocean-Atmosphere Response Experim ent of the Tropical Ocean/Global Atmosphere Programme. This case-study was part of an international model intercomparison project organized by Working Group 4 'Precipitating Convective Cloud Systems' of the GEWEX (Global Ener gy and Water-cycle EXperiment) Cloud System Study. Eight SCM groups using different deep-convection parametrizations participa ted in this project. The SCMs were forced by temperature and moisture tende ncies that had been computed from a reference cloud-resolving model (CRM) s imulation using open boundary conditions. The comparison of the SCM results with the reference CRM simulation provided insight into the ability of cur rent convection and cloud schemes to represent organized convection. The CR M results enabled a detailed evaluation of the SCMs in terms of the thermod ynamic structure and the convective mass flux of the system, the latter bei ng closely related to the surface convective precipitation. It is shown tha t the SCMs could reproduce reasonably well the time evolution of the surfac e convective and stratiform precipitation, the convective mass flux, and th e thermodynamic structure of the squall-line system. The thermodynamic stru cture simulated by the SCMs depended on how the models partitioned the prec ipitation between convective and stratiform. However, structural difference s persisted in the thermodynamic profiles simulated by the SCMs and the CRM . These differences could be attributed to the fact that the total mass Aux used to compute the SCM forcing differed from the convective mass Aux. The SCMs could not adequately represent these organized mesoscale circulations and the microphysical/radiative forcing associated with the stratiform reg ion. This issue is generally known as the 'scale-interaction' problem that can only be properly addressed in fully three-dimensional simulations. Sensitivity simulations run by several groups showed that the time evolutio n of the surface convective precipitation was considerably smoothed when th e convective closure was based on convective available potential energy ins tead of moisture convergence. Finally, additional SCM simulations without u sing a convection parametrization indicated that the impact of a convection parametrization in forced SCM runs was more visible in the moisture profil es than in the temperature profiles because convective transport was partic ularly important in the moisture budget.