Abstract
A three‐layer shallow‐water model with a convection parametrization is used to study the track deflections of slow‐moving hurricanes on a β‐plane as they approach straight coastlines of different orientations with respect to north, or as they cross to a cooler or warmer body of water. In the landfall experiments, the land surface is level and is characterized by an abrupt increase of the drag coefficient and zero evaporation. the tracks of a 72 h integration on a β‐plane are compared with an ocean control experiment.
It is found that the divergent flow in the boundary layer in regions of offshore winds is responsible for the observed track deflections. In this region middle‐layer air descends into the boundary layer and increases the positive potential‐vorticity anomaly in the middle layer. In the case of land conditions to the west of the vortex, the maximum potential‐vorticity anomaly extends less to the north of the vortex centre and results in a more southerly track than in the ocean control case. If land conditions are specified in the north, the positive potential‐vorticityasymmetry maximum extends more to the north‐west and results in a northward deflection of the track. On the other hand, land conditions to the north‐west of the initial vortex centre do not significantly change the location of the asymmetry, and therefore the track deflection is small. the magnitude of the track deflections, as well as the drift speed, increases when the roughness of the land is increased. It is significant that track deflections commence about 20 h before landfall, reaching a value of about 150 km at the time of landfall.
Compared with landfalling hurricanes, track deflections of model hurricanes which drift into a region of different sea surface temperature (SST) are smaller and are a result of intensity changes of the vortex circulation. the maximum wind speed at the time of landfall increases if a region of higher SST is located adjacent to the coastline.