Rapid Intensification of Tropical Cyclones and Changes in the Drag Coefficient

Prediction of rapid intensification of tropical cyclones remains a difficult problem. In this research a Rapid Intensification Period (RIP) is defined as an increase in the maximum sustained wind speed of 30 knots or more within a 24 hour period. RIPs can occur at different times during the existence of a tropical cyclone. An early RIP occurs when a tropical depression or tropical storm intensifies rapidly into a hurricane. The primary RIP occurs when a strong tropical storm or Category 1 hurricane intensifies rapidly. Secondary RIPs can occur when internal or external factors, such as eyewall replacement cycles, cause temporary, large fluctuations in the intensity of a tropical cyclone in an otherwise favorable environment.

The fact that RIPs can occur at different stages during the development of a tropical cyclone suggests that there may be multiple processes that can trigger them. Wind speed began to increase rapidly prior to the start of a rapid decrease in pressure in over half of the RIPs studied. This seems to indicate that an increase in the large scale pressure gradient force at the surface is not the trigger of those RIPs. Powell, et al. (2003) presented empirical evidence for a decrease in the drag coefficient that occurs when the wind speed in a tropical cyclone reaches 40 m s-1. If it occurs, that decrease in the drag coefficient may coincide with the onset of a primary RIP. This study examines changes in the drag coefficient that may occur as the wind speed increases to see if a reduction in friction at higher wind speeds could trigger a RIP.