Multi-wavelength radar observations of tropical cyclone rainbands over south Florida

This study takes advantage of a unique opportunity to observe rainbands from three tropical cyclones as they pass over a field experiment site in South Florida in August and September 2008. Multiple wavelength radars were operating in an upward facing configuration to allow for a characterization of the vertical structure of these bands and the development of a conceptual model of rainbands over land. The main instruments used in this study are a vertically pointing X-band radar, a vertically pointing W-band radar, and a 915 MHz wind profiler that provide a high resolution vertical mapping of reflectivity, Doppler velocities, and winds throughout the column above. To complement the radar data set, rawinsonde launches were made 2-3 times daily along with radiative and turbulence flux measurements, rain gauges, and standard meteorological observations. These observations are being made in combination with the radar data to provide a more complete understanding of tropical cyclone rainbands and their influence on surface conditions. An analysis of NWS Miami WSR-88D images during the observing period indicate that 20 rainbands passed the site during Tropical Storm Fay, 2 during Hurricane Gustav, and 2 during Hurricane Ike, for a total of 24 distinct rainbands. Because of this large data set, we sampled rain bands associated with storms of varying strength (tropical storm to Category 4 hurricane), bands at distances from 100-800 km from the storm center, bands approaching the site from a wide range of directions, and bands lasting anywhere from 15 minutes to over 9 hours depending on whether they were observed cross-band or along-band.

Based on the appearance of band structure from the University of Miami X-band radar, all of the bands are observed to be shallow with reflectivity returns limited to below 10 km, often with a melting layer present between 4.5-5 km. Using these structures, the bands are classified into six band types: Classic (3 bands) where the convection had a clear starting and stopping point with consistent activity; Along-band (4 bands) where the band passed over the site length-wise; Lead by anvil and convection (3 bands) where the band started with low convection and an anvil before the classic structure appeared; Convection with breaks (4 bands) ; Stratiform (6 bands); and Other (3 bands). Only one of the 24 bands was not sampled by the X-band radar, and that band is excluded from classification. Initial results on the stratiform bands show a bright band just below the melting level and a peak in spectrum width in the transition region where ice and water both exist. The similarities and differences between the structures for the different band classifications will be used to explain how the band structure affects meteorological variables below the melting layer and at the surface and create a high resolution vertical mapping of rainband characteristics over land. Work is in progress to 1) characterize the vertical structure of the bands including organization, dynamics and rainfall rate; 2) study how the band passage modifies the wind and moisture fields in the boundary layer; 3) explain the melting layer processes and how they affect drop size distributions; 4) examine the momentum transports by each band; and 4) showcase the influence of the bands on surface meteorological conditions.