Landfall is one of the major causes of the demise of tropical cyclones. It is often the moment at which the cyclone presents the major hazard to life and property. Observational studies have shown there may be important changes in the distribution of the low level winds at landfall, while modeling studies suggest larger scale changes in the cyclone can begin to occur when it is several hundred kilometres off shore. These changes are all due to the changed surface characteristics at landfall, yet little has been done to examine the details of the boundary layer response to such surface contrasts. Indeed, where these have been considered, the approach has been largely to apply the well-developed theory of internal boundary layers and ignore the cyclone-scale changes in the dynamics.

Here, we present the initial results of some studies of the changes in boundary layer wind field produced by landfall. It is shown how previous work describing the motion-induced asymmetry in the cyclone wind field may be applied to this case. At landfall, the theory predicts marked asymmetries in both the surface winds, in the height and strength of the low level jet, and in the slope of the radius of maximum winds with height. It is also shown that significant asymmetries may be produced in the eyewall at some distance from islands. The results will be supported by comparison with observations of Hurricane Danny, and the depiction of Typhoon Utor in the Bureau of Meteorology's operational tropical cyclone prediction system, TC-LAPS.