The Association of Tall Eyewall Convection with Tropical Cyclone Intensification




Kelley, Owen A.

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Despite decades of research, operational weather agencies still find it difficult to predict change in the intensity of a tropical cyclone's surface wind. This dissertation examines whether precipitation cells in a tropical cyclone eyewall provide information about change in wind intensity. Nine years of tropical cyclone overflights are studied with the first radar in space capable of resolving the detailed three-dimensional structure of precipitation. This radar is the Precipitation Radar on the Tropical Rainfall Measuring Mission (TRMM) satellite. With a single satellite overflight, the presence or absence of a tall eyewall cell correctly identifies whether or not intensification is occurring 62% of the time (166 out of 269 overflights). The empirical association of tall eyewall cells and wind intensification is also examined in a decade of tropical cyclones near landfall observed with the WSR-88D ground radars along the U.S. coast. With at least a 3 hour period of continuous ground radar observation, the frequent occurrence of tall eyewall cells (at least 1 in 3 volume scans) correctly identifies whether or not wind intensification is occurring 83% of the time (24 out of 29 observation periods). These results suggest that the near-continuous nature of ground radar data compensates for the ground radar's coarser vertical resolution compared with the TRMM satellite radar. Physical mechanisms for the observed association are discussed. Based on an upper bound calculation, the energy released during a 9 hour long burst of tall eyewall cells could warm a tropical cyclone's eye by at most 4 K if the eye's radius were small (20 km). If the eye's radius were large (40 km), the same cells could warm the eye by at most 1 K. These upper bounds are based on the amount of latent heat released by the burst of tall cells. From the upper bound to the eye warming, an upper bound can be estimated for the maximum wind intensification that might be caused by the tall eyewall cells.



Tropical cyclone, Intensity, TRMM, Hot tower