Analysis of a Tropical Depression Based on Radar Data

Abstract

A tropical depression approached the Lesser Antilles on 26 July 1969 and remained under radar surveillance for more than 12 hr. Radar echoes observed from Barbados were tracked and used to define a motion field. This motion field was compared to winds observed by instrumented aircraft. Echoes were found to move to the left of the wind directions reported at 1500 m and faster than the winds measured at 3000 m. The field of motion derived from the radar echoes, which is not the wind field, was analyzed in terms of vorticity and divergence. These quantities were found to be correlated as expected, and different centers of maximum vorticity dominated the field throughout the duration of the study. A process of reformation toward the north was observed after 1400 GMT. In an experiment with the tendency equation for absolute vorticity as applied to the field of motion inferred from the echoes, the local time derivative, the horizontal advection and the divergence term were computed directly. All other terms were lumped into a residual. This residual was found to be as large as the first two terms, while the divergence term was consistently smaller. However, large values of the divergence term were found in areas of maximum vorticity. Large values of the residual, on the other hand, appear to be related to areas of intense convection.