Relationships between TC intensity and eyewall structure: radial profiles of inner-core IR brightness temperature

Radial profiles of infrared brightness temperature for fourteen western North Pacific tropical cyclones (TCs) from the 2012 season were analyzed and compared to intensity and changes in intensity. Four critical points along the inner core of 2405 different infrared (IR) brightness temperature (BT) profiles were identified: coldest cloud top (CCT), first overshooting top (FOT), and lower (L45) and upper (U45) extent of the inner eyewall, where L45 and U45 were defined as the first 45-degree upturn and the first 45-degree downturn, respectively, of the exponential BT curve. Radial movement of the CCT point outward with increasing TC intensity, combined with subsequent warming of the L45 point, highlighted structure changes that may relate the primary and secondary TC circulations. Slopes of IR BT between the four critical points were strongly negatively correlated with intensity, confirming prior work showing that stronger TCs have more vertical eyewall profiles. Furthermore, the most negative correlations were found in the lower eyewall, consistent with results from recent studies based on radar reconnaissance data. Finally, 12-h changes in slope were found to lead 12-h changes in intensity, providing evidence that changes in the secondary TC circulation may lead changes in the primary TC circulation for both strengthening and weakening TCs. Given that these results were found using readily available geostationary satellite data, they may be useful to improve current TC intensity estimation techniques, particularly in basins without routine aircraft reconnaissance.