A quantitative method for estimating cloud cover over tropical cyclones from satellite data

Abstract

A photometric method for quantifying cloud cover over tropical cyclones as observed from satellite photographs is presented.

Two gridded photographs of tropical cyclones are analyzed by this method. On each photograph, nine concentric circles are drawn. The observed or reported centre of the cyclones is used as the centre for each set of concentric circles. Photometric estimates of cloud cover are made along the nine concentric circles. The principle of harmonic analysis is applied to the cloud cover estimates along the circumference of the circles. The distribution of cloud cover estimates N (measured in tenths) and the orientation and development of the cloud patterns are quantitatively described by certain parameters and also by the phase angles of the harmonic components considered. These quantities are computed as functions of radial distances. The parameters allow for the process of symmetricalization of tropical cyclones to be studied as the cyclones develop and also provide an objective means of studying the changes in the areal extent of the cloud cover of the cyclones. The first two harmonic components are found to account for most of the cloud cover estimates along the circles. The curves joining the phase positions along the nine circles, for the first two components for each cloud pattern analyzed, provide a means for studying the changes in the orientation of the cloud structures.

By using conventional data to investigate the vertical structure of a tropical cyclone and by comparing the results with those from cloud analysis of the same cyclone, it is demonstrated that the cloud patterns of tropical cyclones often reflect the thermodynamic processes going on beneath the cloud patterns.