Analysis of the Thermodynamic Properties of Developing and Nondeveloping Tropical Disturbances Using a Comprehensive Dropsonde Dataset

A dropsonde dataset is analyzed to quantify the necessary thermodynamic conditions for tropical cyclogenesis by evaluating the properties that distinguish developing tropical disturbances from nondeveloping disturbances, and by describing the temporal evolution of the developing inner core. The dataset consists of 2204 dropsonde observations from 12 developing disturbances and 245 from four nondeveloping disturbances. These disturbances are the cases with the best pregenesis sampling from field programs between 2005 and 2010, and include those investigated by three coincident field programs during 2010: the NASA Genesis and Rapid Intensification Processes (GRIP) and NCAR/NSF Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiments, as well as NOAA's Intensity Forecast Experiment (IFEX). Composite analyses indicate clear differences between developing and nondeveloping disturbances: developing disturbances exhibit greater moisture and a higher humidity at midlevels (above 800 hPa) than nondeveloping, and while the developing inner core experiences some midlevel moistening and stabilization as genesis nears, nondeveloping disturbances become progressively drier and more convectively unstable. Developing disturbances also exhibit some important characteristics in their inner core within 2 days of genesis: the low to midtroposphere (below 500 hPa) approaches near-saturation, a mid- to upper-level warm temperature anomaly develops and progressively deepens toward the low levels, and a low-level (below 900 hPa) cool, dry anomaly develops and is removed by the day of genesis. Overall the results support one proposed pathway to tropical cyclone formation in which an initially stronger midlevel vortex, in a moist, humid environment, precedes primarily low-level intensification within a day of genesis.