A NUMERICAL STUDY OF THE EFFECTS OF LAND PROXIMITY AND CHANGES IN SEA-SURFACE TEMPERATURE ON HURRICANE TRACKS

A three-layer shallow-water model with a convection parametrization is used to study the track deflections of slow-moving hurricanes on a be ta-plane as they approach straight coastlines of different orientation s with respect to north, or as they cross to a cooler or warmer body o f water. In the landfall experiments, the and surface is level and is characterized by an abrupt increase of the drag coefficient and zero e vaporation. The tracks of a 72 h integration on a beta-plane are compa red with an ocean control experiment. It is found that the divergent f low in the boundary layer in regions of offshore winds is responsible for the observed track deflections. In this region middle-layer air de scends into the boundary layer and increases the positive potential-vo rticity anomaly in the middle layer. In the case of land conditions to the west of the vortex, the maximum potential-vorticity anomaly exten ds less to the north of the vortex centre and results in a more southe rly track than in the ocean control case. If land conditions are speci fied in the north, the positive potential-vorticity-asymmetry maximum extends more to the north-west and results in a northward deflection o f the track. On the other hand, land conditions to the north-west of t he initial vortex centre do not significantly change the location of t he asymmetry, and therefore the track deflection is small. The magnitu de of the track de;lections, as well as the drift speed, increases whe n 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 hurric anes, track deflections of model hurricanes which drift into a region of different sea surface temperature (SST) are smaller and are a resul t of intensity changes of the vortex circulation. The maximum wind spe ed at the time of landfall increases if a region of higher SST is loca ted adjacent to the coastline.