Cloud Evolution during Atlantic Tropical Cyclone Formation as Revealed by TRMM PR

Cloud evolution during tropical cyclogenesis was examined using the TRMM PR products from 3 days before to 1 day after genesis. It was found that precipitation increases substantially within 36 hours before genesis. Stratiform clouds, mid-level and deep convection all contribute to the increasing precipitation. Stratiform precipitation accounts for ~80% of precipitating pixels. Convective precipitation, owing to its large rain rate per pixel, or conditional rain rate, makes a contribution to the total precipitation comparable to stratiform precipitation. The contribution by stratiform precipitation is attributed to its increasing areal coverage while its conditional rain rate changes little from Day -3 to Day +1. The contribution by mid-level and deep convection comes from both their increasing areal coverage and intensifying rain rates. Among the three types of convection, deep convection has the largest conditional rain rate, but mid-level convection occurs most frequently and makes the largest contribution to the total precipitation. This suggests that tropical cyclone formation may be an outcome of the collective contribution by different types of precipitation. The importance of different types of precipitation is further examined through the analysis of the TRMM diabatic heating products.