Graduate Thesis Or Dissertation
 

An observational study of the wind fields associated with GATE cloud clusters

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  • The wind and thermodynamic fields associated with eastern Atlantic cloud clusters are studied using radiosonde data from the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE). These data are from the gridded set of winds prepared by Dr. Katsuyuki Ooyama (AOML-NOAA) and Dr. Jan-Hwa Chu (SSEC, University of Wisconsin, Madison) using an objective analysis scheme designed by Dr. Ooyama. Similarly-analysed thermodynamic data prepared by Dr. Steven Esbensen (Oregon State University) are also used. Case studies of the vorticity budget for the 4 September squall and the 5 September cluster show significant contributions by all budget terms including the residual, which is interpreted as the effects of cumulus convection, mesoscale cloud lines, or other small-scale circulations. The residual is particularly large in the boundary layer and upper troposphere. The fields of winds and vorticity budget terms in the upper troposphere of the two systems are similar. However, at the level of the mid-tropospheric jet, the small-scale production of vorticity is significantly different in the two systems. A technique for compositing clusters using satellite-derived cloud-top data is applied to GATE data from Phase 3. Vertical motions within and below the anvils are in accord with previous studies. Frictionally-induced inflow is found to be of secondary importance to the development and maintenance of cluster circulations. The composited vorticity budget residuals are again large in the boundary layer and upper troposphere. A large-magnitude asymmetric vorticity couplet observed in the upper troposphere of individual clusters and in composite results is examined. Cluster-scale twisting and motions at meso- or smaller scales are found to produce the couplet, while cluster-scale divergence is the primary destructive agent. A deceleration of the strong easterlies at this level produces these couplets. Possible mesoscale and cumulus sources of this deceleration are discussed. Present schemes that parameterize vorticity production by cumulus convection are found to be inconsistent with similar schemes that parameterize momentum production. Furthermore, these vorticity parameterizations cannot describe the production of the upper-tropospheric couplets discussed above. An alternate parameterization, formed by taking the curl of the parameterized momentum source, does qualitatively describe the couplets.
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