Examination of the coastal transition zone of hurricane frances

The structure of the coastal internal boundary layer (IBL) 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 mesoscales during a landfalling hurricane event. Changes in IBL structure are expected due to changes in coastal geometry and surface roughness, but other enhancements may also result from the passage of convective precipitation and associated downdrafts. Observations from the coastal transition zone in the onshore flow region of a hurricane are rare. The vast majority of surface observation platforms fail during landfall and only a limited number of mobile platforms are available for deployment by research groups. Logistical issues must also be taken into consideration in the immediate coastal area when attempting to safely operate manned mobile radars. Despite these challenges, a relatively comprehensive dataset was acquired from Hurricane Frances (2004).

Hurricane Frances made landfall near Port Saint Lucie, Florida as a Category 2 hurricane with estimated sustained maximum surface winds of 105 mph according to the National Hurricane Center. The landfall point in southeast Florida placed the Cape Canaveral area in onshore flow for the first portion of the event. Tower and high-resolution research radar data were collected during the event providing an excellent opportunity to examine coastal IBL structure.

Wind speed data were obtained from 33 instrumented towers at various locations on the Cape Canaveral Air Force Station and Kennedy Space Center (CCAFS/KSC) complex. The tower data include wind speed and direction recorded every five minutes at instrument heights ranging from nine to 150 m. Complete data records were collected by 24 towers, while the remaining nine failed at various times during the event. Raw and standardized mean wind speeds were determined for each site using the acquired wind records, as well as aerial and site photography to examine surface roughness characteristics. The tower data were coupled with single and dual-Doppler data sets collected by the Shared Mobile Atmospheric Research and Teaching (SMART) radars. The radars were located at Merrit Island Airport and Space Coast Regional Airport in Titusville, resulting in a baseline of 21.9 km, and placing much of the CCAFS/KSC tower network within dual-Doppler coverage. SMART radar data were collected using various scanning strategies through 1130 UTC on 5 September.

The complexity of the Cape Canaveral coastline and the highly convective nature of the outer portion of Hurricane Frances provide a useful examination of IBL structure during a hurricane landfall. Analysis will be presented from periods when stratiform and convective precipitation progressed over the Cape Canaveral mesonet and moved inland into the adjacent Florida peninsula.