The question of how the hurricane surface and boundary layer wind field changes as a tropical cyclone approaches the coastline is of utmost importance to hurricane forecasters, emergency managers responsible for coastal evacuations, and a host of public and private businesses that drive the growing coastal economy. The hurricane surface wind field may be relatively well known from aircraft observations offshore. However, the manner in which the wind field changes near the coast is largely unknown. Modification of the underlying roughness, which affects the surface wind and hurricane planetary boundary layer (HPBL) wind profile, may gradually begin offshore as the wave field transitions from long, large waves to short, choppy waves with decreasing water depth. At the coast, the abrupt change in surface from water to a variety of natural coastal habitats and man-made coastal structures may rapidly and dramatically alter the surface wind and vertical HPBL wind profile. Better understanding of this process is required for improved forecasts of coastal winds and management of coastal response.

Due to the lack of observations within the HPBL at landfall during past tropical cyclones, the nature and spatial dimensions of boundary layer wind transitions near the coastline are poorly understood. Hurricane observing programs consisting of portable and mobile equipment and regional coastal mesoscale observing networks are currently filling this gap in observations.

In an effort to improve our understanding of hurricane coastal wind transitions, a unique array of portable meteorological instrumentation coupled with coarsely spaced automatic surface weather stations and a portable Doppler radar is used to diagnose the transition in boundary layer onshore winds during the landfall of Hurricane Lili (2002) along the central Louisiana coast. Observations from five portable meteorological towers with instrumentation at multiple levels, three portable meteorological observing stations, forty-two fixed automatic observing stations, and one portable Doppler radar are used in the analysis.

Preliminary investigation of the data set reveals the existence of both frictionally-induced and thermodynamically-induced changes in the boundary layer downwind of the coastline within the onshore flow, outside of the inner core. These changes appear similar to internal boundary layers observed in other situations where the flow is propagating over an abrupt change in surface. Preliminary results show that the 1-minute average surface wind decreased by 10 percent within 10 km from the coast outside Lili's radius of maximum winds during onshore flow. The rate of decrease was an exponential function of distance from the coast and can be considered representative of the native vegetation types in the region.