The upper troposphere / lower stratosphere (UTLS) is a focal region of interest since exhibits a maximum at the base of the stratosphere above TCs. Variability of PV in the UTLS is related to updrafts in the TC. A strong diurnal cycle in was found at 100 hPa for TC Marco, which was of strength category 1 (C1) or less for its duration, with a maximum in at 7 am local time. Marco was a unique case among the Gulf Coast cases in that the storm never made any major landfall while categorized as a tropical depression (TD), tropical storm (TS), or C1. The pronounced diurnal cycle in 100 hPa for TC Marco, with a sunrise maximum, and the warm homogeneous sea surface temperatures of the Gulf of Mexico, are similar to those described for TC Yasa in the Southeast Pacific by Hitchman and Rowe (2023).
The composite study Gulf Coast TCs during 2020-2022 showed a strong diurnal cycle in 100 hPa for storms categorized as C2 or higher, with a statistically significant peak in near local midnight. For storms of category C1 or weaker, a diurnal cycle is also observed, but with somewhat reduced statistical significance. The peak for weaker storms occurs near local sunrise. This result supports the concept that strong axisymmetric storms have a diurnal cycle which peaks near midnight, perhaps due to cloud-top radiative destabilization, while developing storms have a diurnal cycle which peaks near sunrise, with the delay related to extra forcing from the contrast between cloudy and clear air (e.g., Ruppert and Hohenegger 2018). The effects of landfall are also examined in the composite, and work is related to that of Jiang et al. (2011) and Gray and Jacobson (1977).
This work contributes toward understanding the dynamical adjustment process in high resolution models of TCs.
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