A Satellite Study of Convective Development and Moisture Variability on Hourly to Daily Time Scales

In this study the moisture variability associated with convective development is explored on a time scale comparable to the convective life cycle. To make this possible with satellite measurements, Aqua AMSR-E and Cloud CPR data are projected into the time domain composited with respect to TRMM PR detected convection, exploiting the overpass time difference between sun-synchronous and asynchronous orbits that drifts over time. The composite vertical structure of CloudSat cloudiness is found useful for differentiating a variety of convective systems ranging from shallow convection to highly organized systems. A systematic moistening prior to convection and a subsequent drying is evident for all types of convective systems (see Figure below). An organized cloud system is preceded by a larger magnitude of moisture buildup than an isolated system, and a convective system of the same kind accompanies a greater amplitude of moistening in a humid environment than in a dry environment. The moisture source (moisture convergence plus evaporation) is diagnosed based on a simple water budget consideration. It is found that the ratio of precipitation to the moisture source stays nearly invariant for 24 hrs before convection peaks regardless of any convective systems until it begins to vary and diverge as convection decays. This suggests that some tight, universal constraint coupling precipitation to the moisture source may exist during the developing stage of convection. The known relationship of precipitation with column relative humidity (CRH) is also examined in light of the current findings. This relationship is extremely non-linear over an individual convective life cycle, while the non-linearity weakens to some degree when different convective systems under a wide range of humidity conditions are averaged together into a climatological relationship.