Diurnal Variability of Precipitation: Multiple Modes & Ambiguities

Numerous physical mechanisms have been proposed for explaining diurnal and semi-diurnal variations that have been long-observed in precipitation throughout the world's continents and oceans. Some mechanisms address local scale modes, others large scale modes. Some modes tend to be phase-locked to particular solar times whereas other modes exhibit maximum-amplitude phase propagation in conjunction with horizontal propagation of the precipitation itself. All but perhaps the tidal modes exhibit meandering phase behavior that may extend over periods of a few hours to as many as six hours. The numerous choices of mechanisms, the lack of clarity in diurnal signals as diagnosed from observations, and the fact that more than a single mode may be simultaneously operating within a specific region has made the interpretation and modeling of precipitation's diurnal variability a difficult and debatable topic. However, given that there are now an abundance of satellite precipitation data that can consistently observe precipitation's daily cycles, and that there also exist a number of high-quality cloud-resolving models that can resolve the precipitation process, it seems that the time is ripe for renewed emphasis on investigation of some of the more salient issues concerning how and why diurnal cycling of precipitation occurs, under what geographic and atmospheric conditions, and with what amplitude and phase properties. This talk seeks to draw attention to the main themes of these research issues -- with particular emphasis on the multiple-mode problem and its associated ambiguities.