Coastal zone boundary layer transitions in off-shore and on-shore flows observed by Doppler profiler and radar during the landfall of Tropical Storm Gabrielle

Detailed observations of boundary layer structure were acquired on 13-14 September 2001, prior to and during the landfall of Tropical Storm Gabrielle. The UAH Mobile Integrated Profiling System (MIPS) and the Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) were co-located at the west Florida coast line near Venice, very close to the circulation center at landfall. The boundary layer prior to landfall was rendered stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall. The cool air mass was expansive, with an area within the 23 „aC surface temperature isotherm of about 50,000 km2. East-northeasterly surface flow transported this cool air off the west coast of Florida, toward the warm core of the Gabrielle, and promoted development of shallow fronts that were prominent during the landfall phase.

Airflow properties of the boundary layer around the coastal zone are examined using the MIPS/SMART-R data. Wind profiles exhibited temporal variability throughout the period of observations. The stable off-shore flow within stratiform precipitation exhibited a modest jet that descended from about 600 to 300 m within the 20 km zone centered on the coastline. Temporal variability was most likely related to gravity waves that were ducted by a stable layer capping the ABL. In contrast, the on-shore flow on the western side of the circulation center produced an unstable, more turbulent boundary layer that exhibited a well defined top varying between 400-1000 m MSL. Temporal variability in this flow regime was related to the presence of boundary layer rolls and a level of maximum wind that ascended from near to surface to 1.5 km after landfall occurred.. The horizontal variability of the each boundary layer is examined using high-resolution Doppler radar scans at locations up to 15 km on either side of the coastline, along the mean boundary layer flow direction. These analyses reveal that transitions in boundary layer structure for both the stable and unstable regimes were most substantial within 5 km of the coastline.