Diurnal variation of clouds and precipitation over tropical South America: interactions between propagating and in-situ cloud systems

The diurnal variation of clouds and precipitation is determined by the net expression of interacting cloud systems spanning many time and space scales. Over tropical South America, recent studies have shown that propagating squall lines, local afternoon convection, and nocturnal stratiform clouds “compete”, causing the diurnal variation of clouds and rainfall to differ with region, time, and the spatial scale of interest.

This study exploits these differences to distinguish between the various mechanisms at work that lead to the daily changes of clouds and rain in Amazonia, via composite and case study analysis. The analysis is based on satellite, radar, sounding, and profiler observations of precipitating systems and cloudiness from the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM-LBA) and the coincident WET-season Atmospheric Mesoscale Campaign (WETAMC) field programs in the southwestern Amazon Basin during the early 1999 wet season.

A conceptual model of cloud system interactions, leading to the net observed diurnal cycle of clouds and rainfall in the western Amazon Basin, is developed. This conceptual model is summarized as follows. Following the collapse of locally generated afternoon (“noon balloon”) convection, nocturnal convective systems contribute to a post-midnight maximum in raining area and high cloudiness, and to a lesser extent rainfall. Many of these nocturnal convective events are traced to large-scale squall lines, which propagate westward thousands of kilometers from their point of origin along the northeast coast of Brazil. Nocturnal convective systems have the effect of weakening and delaying the onset of the following afternoon's convection by inhibiting the replenishment of the boundary layer. In addition, a previously undescribed nocturnal stratiform drizzle phenomenon, which covers half of the Amazon Basin at times, contributes significantly to nocturnal cloud cover.