Investigation of the atmospheric environments associated with TC size changes

Tropical cyclones (TCs) have been observed to change size under a variety of environmental conditions. Several numerical studies conducted within the last decade have looked at the environmental conditions and associated physical mechanisms that likely cause TC size changes. Generally, their results have shown that high relative humidity and surface energy fluxes favor TC size expansion. Variables that negatively impact TC size and structure changes include high vertical wind shear and dry-air intrusion.

This study examines historical TC cases where the wind field made a significant expansion or contraction while maintaining an intensity of at least 50 kt during its lifetime. Size changes are evaluated using the Tropical Cyclone Extended Best Track Dataset which contains the maximum radial extent of the 64-, 50-, and 34-kt wind in four quadrants. The average 34-kt wind radius is used as an indicator of the size of the TC. For the purposes of this study the environment of a TC is investigated if the wind field either expanded or contracted in size at least 10 km radially in a 6-hour period.

Environmental quantities are derived from the 6-hourly, nominally 0.7 ERA-Interim global reanalysis dataset up to 120 h prior to the time of the size change. The fields analyzed in this study include relative humidity, specific humidity, atmospheric temperature, sea surface temperature, vertical wind shear, and surface energy fluxes. The goal of this study is to see if TC size changes correlate with changes in the surrounding environment similar to that shown in simulations and to analyze the mechanisms associated with those size changes. In this presentation we will compare results from the reanalysis with results previously obtained using numerical experiments.