Microphysical and dynamical characteristics in the stratiform region of Tropical Storm Gabrielle at landfall

On 14 September 2001 an extensive stratiform rainband associated with Tropical Storm (TS) Gabrielle was sampled by a 915 MHz wind profiler and the Shared Mobile Atmosphere Research and Teaching Radar (SMART-R) C-band Doppler radar before the storm made landfall around the west coast of Florida. An intense bright band and moderate rainfall rates were observed during the 0500-0630 UTC period of stratiform precipitation. The melting layer plays a potentially important dynamical and microphysical role in changing internal structure and precipitation property of stratiform system. Hence, the stratiform rainband of Gabrielle was studied with a focus on the melting layer.

Vertical air motion and horizontal divergence profiles were obtained by various methods, including the Extended Velocity Azimuth Display (EVAD) and the profiler divergence integration (Quasi-VAD). The use the SMART-R in combination with a 915-MHz wind profiler provided a qualitatively good agreement of vertical air motion and divergence profiles between them. A quantitative analysis of the melting layer is conducted by processing the 915-MHz profiler reflectivity profiles in order to investigate aggregation and breakup which affect the microphysics within and below the melting layer. With a change in horizontal winds, spectral widths near the top of the melting layer are found very informative in analyzing aggregation and breakup intensity around the melting layer.

Below the melting layer, the vertical variations of raindrop size distributions from the 915-MHz profiler spectra are examined with regard to aggregation and breakup, and rainfall rate. Correlations among precipitation parameters and their relationship with the bright band intensity (i.e. aggregation) will also be presented.