CLOUD-RESOLVING MODELING OF CLOUD SYSTEMS DURING PHASE-III OF GATE - PART II - EFFECTS OF RESOLUTION AND THE 3RD SPATIAL DIMENSION

Two- and three-dimensional simulations of cloud systems for the period of 1-7 September 1974 in phase III of the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) are performed us ing the approach discussed in Part I of this paper. The aim is to repr oduce cloud systems over the GATE B-scale sounding array. Comparison i s presented between three experiments driven by the same large-scale c onditions: (i) a fully three-dimensional experiment, (ii) a two-dimens ional experiment that is an east-west section of the three-dimensional case, and (iii) a high-resolution Version of the two-dimensional expe riment. Differences between two- and three-dimensional frameworks and those related to spatial resolution are analyzed. The three-dimensiona l experiment produced a qualitatively realistic organization of convec tion: nonsquall clusters, a squall line, and scattered convection and transitions between regimes were simulated. The two-dimensional experi ments produced convective organization similar to that discussed in Pa rt I. The thermodynamic fields evolved very similarly in all three exp eriments, although differences between model fields and observations d id occur. When averaged over a few hours, surface sensible and latent heat fluxes and surface precipitation evolved very similarly in ail th ree experiments and evaluated well against observations. Model resolut ion had some effect on the upper-troposheric cloud cover and consequen tly on the upper-tropospheric temperature tendency due to radiative fl ux divergence. When compared with the fully three-dimensional results, the two-dimensional simulations produced a much higher temporal varia bility of domain-averaged quantities. The results support the notion t hat, as long as high-frequency temporal variability is not of primary importance, low-resolution two-dimensional simulations can be used as realizations of tropical cloud systems in the climate problem and for improving and/or testing cloud parameterizations for large-scale model s.