Examination of the coastal transition zone of Hurricane Frances (2004)

Understanding the structure of the coastal internal boundary layer (IBL) during the landfall of a tropical cyclone has important ramifications on operational forecasting, structural design, and post-storm damage assessment. Despite these important issues, it is unclear how the structure of the IBL evolves at the coastline on micro- and meso-scales during a landfalling hurricane. Knowledge of the vertical kinematic structure within tropical cyclones over water has improved greatly through aircraft reconnaissance missions and the advent of GPS dropsondes and the Stepped Frequency Microwave Radiometers. Unfortunately, reconnaissance and research aircraft are limited to over-water missions resulting in a poor understanding of vertical kinematic structure near the coastal interface where changes in IBL structure are expected due to changes in coastal geometry and surface roughness. Additionally, IBL structure may evolve due to the passage of convective precipitation and associated downdrafts.

An innovative observational dataset was collected from the coastal transition zone in the onshore flow region of Hurricane Frances (2004) over Cape Canaveral, FL. Single- and dual-Doppler radar data collected by the Shared Mobile Atmospheric Research and Teaching radars provide the ability to discern horizontal and vertical mean IBL structure over a complex coastal interface while assessing the influence of a variable underlying surface. The underlying surface is characterized using the NOAA Coastal Change Analysis Program and the USGS National Elevation Dataset digitized databases.

Radar and tower data analyses reveal that though IBL mean structure is prevalent over the Cape Canaveral region during the landfall of Hurricane Frances, IBL growth is much less than expected when compared to several empirical growth models. Additionally, transient convective gusts commonly perturb the mean structure at the top of the IBL, though the higher momentum associated with these gusts is typically not able to descend to the surface within an established IBL.