DISTRIBUTION OF SURFACE FRICTION IN HURRICANES

Abstract

Terms of the horizontal equations of motion are evaluated from surface analysis of three hurricanes and three nonhurricane cases in order to investigate the components of friction (tangential to and normal to the anemometer wind direction) in the boundary layer. A method of moisture-flux analysis is developed to study the variation of height of the inflow layer which in turn yields a clue concerning the variation of stress profile in time and space. While the magnitude of friction increases toward the hurricane center, it is clear that it is not a simple function of wind speed or surface roughness. It is also shown that (1) the component of friction normal to the surface wind direction is almost always significant, (2) accelerations are quite small and are not correlated with the down-stream pressure force, (3) accelerations and angle of cross-isobar flow are correlated, and (4) total friction is not correlated with angle of cross-isobar flow. It is suggested that a compensating mechanism characterizes the boundary layer which changes the angle of cross-isobar flow (thus changing the down-stream pressure force) to compensate a tendency toward acceleration brought about by changes of momentum flux from upper layers as well as changes of surface stress. This compensation in turn provides a variable component of friction normal to the surface wind which is almost always significant.