High Resolution Modeling of Lower Tropospheric Subsidence Warming in MCSs and its Role in Tropical Cyclogenesis

Much recent research has focused on clarifying the multiscale processes leading to tropical cyclogenesis (TC genesis). A converging consensus is that TC genesis cases occurs in conditions favorable for active and/or long lasting MCSs within large-scale disturbances. A deep tropospheric warming is needed to achieve falling surface pressure leading up to and during genesis. One possible factor contributing to hydrostatic surface pressure falls is hydrostatic subsidence warming in the presence of a series of multiple mesoscale convective systems in widespread stratiform rain region. This subsidence warming has been observed and shown in the Skew-T/Log-p profiles as “onion” soundings. Using dropsonde data from the early stages of Typhoons Fanapi and Megi of 2010 in the western North Pacific, Kerns and Chen (2015) referred to this as an underappreciated ingredient in tropical cyclogenesis. This study uses a high resolution (1.33 km) realistic WRF-ARW forecast of the genesis of Typhoon Fanapi to show that development of the deep tropospheric 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, though slightly offset (vertically tilted) in the center location in the early stage. The center of falling SLP is tracked hourly, and the columns in the immediate vicinity of the center (e.g., within 50 km) are compared with the pre-storm ambient conditions. Both ascent and subsidence are present in the model, in part due to gravity waves in and around MCSs. Nevertheless, a majority of the air parcels near the incipient center are subsiding and are warmer in the low-mid levels than the pre-storm profiles. Furthermore, the level of maximum warming in the low-mid levels descends from ~600 hPa to near 850 hPa as the surface pressure falls during TC genesis. Meanwhile, the system mean upward motion and positive low-level vorticity maximum is maintained by convective updrafts around of the central subsiding area.