Lower Troposphere Subsidence Warming and its Contribution to Tropical Cyclogenesis

Tropical cyclogenesis (TC genesis) occurs in conditions favorable for long lasting MCSs within large-scale disturbances. One aspect of TC genesis is that the surface pressure needs to fall by a few mb relative to the surrounding environment, and these pressure falls need to be sustained. Warming aloft is associated with hydrostatic pressure falls at the surface. This study focuses on one factor contributing to hydrostatic surface pressure falls: subsidence warming associated with a series of multiple mesoscale convective systems with widespread stratiform rain. This type of warming commonly results in “onion” profiles in Skew-T/Log-p diagrams. Using dropsonde data from the pre-genesis stage of Typhoons Fanapi and Megi of 2010 in the western North Pacific, Kerns and Chen (2015) documented this and referred to it as an “underappreciated ingredient” in tropical cyclogenesis. Similar thermodynamic structures have been observed in other storms in the Atlantic. To study the development of the warm core during TC genesis, we use a realistic, high resolution (1.33 km) WRF-ARW forecast of the genesis of Typhoon Fanapi and track the center of falling SLP hourly in the model, comparing the columns in the immediate vicinity of the center (e.g., within 50 km) with the pre-storm ambient conditions. The developing warm core is associated with both the upper-level warm anomaly within the stratiform rain of the MCSs and subsidence warming below the melting level, which are misaligned initially but become vertically stacked as genesis proceeds. The system mean circulation is maintained mainly by convective updrafts; Nevertheless, many air parcels near the incipient center have a history of subsidence and are warmer in the low-mid levels than the pre-storm profiles. Additionally, the level of maximum “onion” subsidence warming in the low-mid levels descends from around 600 hPa to near 850 hPa as the surface pressure falls during TC genesis. The lower tropospheric warming contributes significantly to the surface pressure falls, which is a contribution to TC genesis.