Rapid Intensification of Tropical Cyclones: A Comparison of Flight Level and Surface Winds

Rapid intensification can occur at various points in the development of a tropical cyclone. It is defined as an increase in the maximum sustained wind speed of 25 knots in 24 hours in this study. A Primary Rapid Intensification Period (PRIP) is one that transforms a tropical storm or minimal hurricane into a major of near major hurricane. Between 1991 and 2008 33 tropical cyclones underwent PRIPs over the Atlantic Ocean. In some cases the pressure began to decrease rapidly prior to the onset of a rapid increase in the wind speed at the surface. In these cases an increase in the horizontal scale pressure gradient force may have been the cause of the rapid intensification. However, in over half the cases the rapid increase in surface wind speed preceded the rapid decrease of surface pressure. Subsequent research indicated that a decrease in the frictional force at higher wind speeds may contribute to rapid intensification, but it is not likely to cause it.

An alternative hypothesis may be that wind speeds increase rapidly above the boundary layer and then some process transports the kinetic energy and stronger winds speeds to the surface. In storms with stronger winds at flight level, downward motion in the eyewall and rainbands near the center of a tropical cyclone may transport the faster moving air to the surface. This could produce a rapid increase in surface wind speed in some tropical cyclones. Aircraft reconnaissance data are analyzed in this study in order to determine if a rapid increase in wind speed occurred at flight level prior to the rapid increase of wind speed at the surface. Examples are presented for storms in which the flight level winds increased prior to the onset of rapid intensification and for cases when the flight level winds did not increase prior to rapid intensification.