Subsidence-induced warming in the genesis of Tropical Storm Cindy (2017)
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The formation of tropical cyclones in unfavorable large-scale environments remains a challenge for researchers and forecasters. This thesis provides an investigation of the genesis of Tropical Storm Cindy (2017) using a combination of aircraft measurements collected during the Convective Processes EXperiment (CPEX) field campaign and a fully-coupled atmosphere-wave-ocean numerical simulation. Prior to TC genesis, a shallow cyclonic vortex is embedded in a deep layer of W/SW winds associated with an upper-level trough. Despite vertical wind shear greater than 20 m/s, low mid-tropospheric relative humidity and poorly organized convection, Cindy formed at 1800 UTC 20 June 2017 in the central Gulf of Mexico. At the time of genesis, the dropsondes and the numerical simulation reveal the presence of subsidence within the developing storm. Descent produces a temperature and moisture perturbation maximized in the 2-2.5 km layer. In the hours prior to TC genesis, 2500 m temperature increases by 5 Â°C and the dew point temperature depression grows larger than 8 Â°C. At TC genesis, the subsidence-induced virtual temperature perturbation in the 1.5-3.5 km layer accounts for 50% of the sea-level pressure perturbation and for more than 65 % of the sea-level pressure drop during the genesis stage. A backward trajectory analysis indicates that the air parcels warm by 14 Â°C in response to a 1.8 km descent along the upshear flank of the storm, with an estimated subsidence rate of nearly 6 cm/s.
- Atmospheric sciences