Radial profiles of infrared brightness temperature for 2405 different satellite observations from 14 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 each infrared (IR) brightness temperature (BT) profile were identified: coldest cloud top (CCT), first overshooting top (FOT), and lower (L45) and upper (U45) extent of the inner eyewall. Radial movement of the mean CCT point outward with increasing TC intensity, combined with subsequent warming of the mean L45 point with intensity, highlighted structure changes that are consistent with eye and eyewall development. When stratified by latitude and vertical wind shear, the CCT point moved radially outward for all cases, notably at higher intensities for lower-latitude TCs and at lower intensities for higher-latitude TCs. The majority of the warming of the L45 point with increasing intensity occurred for low-latitude and low-shear cases. Slopes of IR BT between the four critical points were statistically significantly negatively correlated with intensity, indicating that stronger (weaker) TCs had more (less) negative slopes of IR BT and more (less) vertical eyewall profiles. Furthermore, except in high-shear cases, the most negative correlations were found in the inner 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 most often at higher latitudes, providing evidence that changes in the secondary TC circulation may lead changes in the primary TC circulation for both strengthening and weakening TCs.

The role of Sumatra and adjacent topographic features in tropical cyclone (TC) formation over the Indian Ocean (IO) is investigated. Sumatra, as well as the Malay Peninsula and Java, have mountainous terrain that partially blocks low-level flow under typical environmental stratification. For easterly low-level flow, these terrain features often produce lee vortices, some of which subsequently shed and move westward from the northern and southern tips of Sumatra and thence downstream over the IO. Since Sumatra straddles the equator, extending in a northwest–southeast direction from approximately 6°N to 6°S, the lee vortices, while counter-rotating, are both cyclonic. Hence, they can serve as initial disturbances that eventually contribute to TC formation over the IO. In addition, low-level, equatorial westerly flow impinging on Sumatra is also typically blocked and diverges, at times contributing to cyclonic circulations over the IO, primarily near the southern end of the island. Data from two recent tropical campaigns, the 2008–10 Year of Tropical Convection (YOTC) and the 2011 Dynamics of the Madden–Julian Oscillation (DYNAMO), are used to study these phenomena. These datasets reveal the frequent occurrence of shed and nonshed terrain-induced cyclonic circulations over the IO, the majority of which occur during boreal fall and winter. During the 2.5 yr of the two campaigns, 13 wake vortices (13% of the shed circulations identified) were tracked and observed to subsequently develop into TCs over the northern and southern IO, accounting for 25% of the total TCs forming in the IO during that period.

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