ORGANIZED CONVECTIVE SYSTEMS IN THE TROPICAL WESTERN PACIFIC AS A PROCESS IN GENERAL-CIRCULATION MODELS - A TOGA COARE CASE-STUDY

We examine the large-scale effects of organized convective systems in the tropical western Pacific observed served during the Tropical-ocean Global-Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). In a case-study approach, we examine realizations of a supercl uster, associated with the onset of the December 1992 westerly wind bu rst, in the T213 operational medium-range weather forecasting model of the European Centre for Medium-Range Weather Forecasts (ECMWF). We id ealize a supercluster as a hierarchy of three interacting scales, name ly organized cumulonimbus C-1, mesoscale convective systems C-2, and t he supercluster component C-3. It is shown that the ECMWF model repres ents this hierarchy as a C-3-like surrogate whose influence dominates the effect of parametrized convection. This causes over-prediction of the model tendencies which, in the case of zonal momentum, is explaine d in elementary terms. The structure of the resolved-scale momentum fl ux is explained by Moncrieff's (1992) archetypal theory of organized c onvection which has been verified against observations and cloud-resol ving model data-sets. The parametrization of subgrid-scale convective momentum-nux in the ECMWF model, based on a momentum mixing concept, p roduces subgrid-scale tendencies that are physically different from tr ansports associated with cumulonimbus convection in a shear now. We ou tline a strategy for parametrizing the momentum nux by the Cel compone nt based on the archetypal model. The C-2 component, which is part-res olved and part-parametrized, is at odds with the assumptions of scale separation underpinning parametrization. It is argued that this compon ent should be represented as part of the prognostic treatment of conve ctively generated cirrus. Finally, we suggest cloud-resolving modellin g studies to further quantify the structure and large-scale impact of superclusters in a westerly-wind-burst environment, ranging from ideal ized models to models having data assimilation capability.