Moisture and precipitation evolution during tropical cyclone formation as revealed by the SSM/I-SSMIS retrievals

The simultaneous precipitation and column water vapor retrievals from the SSM/I and SSMIS passive microwave instruments were used to examine the convective and moisture evolution during tropical cyclone formation. Using a wave-pouch-track dataset, composites of precipitation and column water vapor were constructed with more than 2000 satellite overpasses for a 3-day time period prior to genesis. It was found that high column water vapor occurs near the pouch center and starts to increase about 42 h prior to genesis while a substantial increase in precipitation occurs within 24 h prior to genesis. These features are consistent with a recently proposed two-stage conceptual model for tropical cyclone formation, in which gradual moisture preconditioning precedes an abrupt transition to sustained deep convection leading up to genesis. The relationship between precipitation and saturation fraction (SF) is examined for the developing waves and compared with the general tropical NorthAtlantic. Precipitation rate is found to increase at the same exponential rate above the same critical point of SF in the two groups, but convection in the developing waves has a higher probability of occurrence near and above criticality. This can be attributed to the positive feedback between convection and the low-level moisture convergence, which counteracts the negative feedback of convection on water vapor and makes convection in a developing tropical cyclone more sustainable.