Sensitivity of tropical storms simulated by a general circulation model tochanges in cumulus parametrization

A number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surfa ce temperatures. Many GCMs have demonstrated an ability to simulate some as pects of the observed interannual variability of tropical storms, in partic ular, variability in storm frequency. This has led to nascent attempts to u se GCMs as part of programs to produce operational seasonal forecasts of tr opical-storm numbers. In this study, the sensitivity of the statistics of GCM-simulated tropical storms to changes in the model's physical parametrizations is examined. Aft er preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identica l dynamical cores but different convective parametrizations were created. T he parametrizations examined included moist convective adjustment, two vari ants of the Arakawa-Schubert scheme, and several variants of the relaxed Ar akawa-Schubert (RAS) scheme; the impact of including a shallow-convection p arametrization was also examined. The simulated tropical-storm frequency, intensity, structure, and interannu al variability were all found to exhibit significant sensitivities to chang es in convective parametrization. A particularly large sensitivity was foun d when:he RAS and Arakawa-Schubert parametrizations were modified to place restrictions on the production of deep convection. Climatologies of the GCM tropical atmosphere and composites of tropical sto rms were examined to address the question of whether the tropical-storm sta tistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization-ind uced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropic;ll storms nevertheless produce a much more realistic interannual var iability of storms, apparently due to an improved tropical mean climate. A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to th e occurrence of tropical storms in the low-resolution GCMs, although this m ay not be the case with observations. If the tropical CAFE is too low, trop ical storms in the low-resolution GCMs cannot form with realistic frequency .