Thermodynamical and dynamical aspects of tropical cloud clusters are studied using data from the GARP Atlantic Tropical Experiment (GATE). The data set used in this study is a three-dimensional gridded set of upper-air analyses constructed by Ooyama and Chu (Hurricane Research Division, AOML/NOAA and SSEC-University of Wisconsin) for wind data and Esbensen (Oregon State University) for thermodynamic data. The energy and momentum budgets are estimated on the scale of large cloud clusters. A strong upper-tropospheric heat source and middle-tropospheric drying are characteristic features of the mature stage of the observed cloud clusters. The heat source, moisture sink and the virtual heat flux for cloud clusters are larger than the corresponding quantities from GATE easterly-wave composites. The surface precipitation estimates produced from the vertically integrated moisture budget are consistent with direct observations. From the momentum budget study, the following conclusions are drawn concerning the cumulus momentum effects. In the growing stage, the mesoscale and cumulus scale effect tends to: 1) provide a vertically integrated net sink for westerly momentum around the cluster center; 2) induce a convergent circulation in the lower layer. In the mature stage, the effects are to: 1) induce a divergent circulation in the upper layer and maintain a vorticity couplet pattern; 2) maintain a weak convergent circulation in the lower layer; and 3) cause a relatively weak easterly acceleration in the upper layer at the center. A hypothesis is postulated to illustrate the convective dynamical effects. A simple barotropic non-divergent model was constructed to investigate the large-scale response to the hypothesized cumulus momentum forcing similar to that found in the GATE cloud-cluster momentum budget. The numerical results show that the cumulus momentum forcing is a plausible kinetic energy source for the mesoscale wavenumber spectrum. The sporadic nature of the convective mass flux does not have a significant effect on the large-scale dynamical response for physically realistic parameters in a barotropic non-divergent dynamical system. / Graduation date: 1987

Data from the GARP Atlantic Tropical Experiment (GATE) was analysed in an exploratory manner to discover the characteristics of mesoscale organization of convection and it's relationship to large-scale wind profiles. Automated methods were developed to identify the convective cells and their linear organization. These automated methods use a median high-pass filter to identify enhanced cells and a simple pattern recognition technique to ascertain the linear organization between them. Due to the simplified nature of the algorithm, the whole data set of the 21 day period from the phase 3 of GATE was processed in an economical manner obtaining a large data base which was used in the investigation of clusters and other associated phenomena. The mesoscale organization of convective cells and the widespread areas of lighter precipitation associated with them showed expected characteristics and compared satisfactorily with previous results. A large fraction of the rainfall (64%) fell from the widespread area. The total precipitation had a correlation of 0.94 with the fractional area of the widespread and a correlation of 0.89 with the fractional area of the clusters. The widespread precipitation had a correlation index of 0.97 with it's fractional area and the cluster precipitation had a strong linear relationship to it's area with a correlation of 0.99. These factors argue well for the parameterization of rainfall rate in tropical regions to a high accuracy by the area covered by organized convective cells and widespread areas. It was also seen that there was a good correlation with the number of clusters and number of cores with the total precipitation rate in a given area. These factors create a strong argument for identifying mesoscale systems consisting of convective cells and widespread precipitation as basic units of precipitation in tropical regions, having a characteristic life cycle of their own. The widespread and total precipitation showed very good correlation with upper-level vertical motion. Clusters tended to align parallel with the horizontal low-level wind shear and the degree of alignment appears to depend on the strength of the wind shear. / Graduation date: 1991