Rains and Showers in OTREC; Weak Temperature Gradient Modeling

Rainfall in the tropics has been shown to be produced either by isolated but intense convective systems (showers regime) or widespread but weaker systems (rains regime). We examine significant rainfall systems observed in the OTREC project (Organization of Tropical East Pacific Convection) in order to tease out the physical mechanisms differentiating these two regimes. We find that rains occur in very moist environments, typically with weak conditional instability. In contrast, showers develop in drier environments with larger instability. Spectral weak temperature gradient numerical calculations show that showers are associated with episodic rainfall separated by significant quiescent periods, whereas rains produce continuous simulated rainfall after a spinup period. Mass flux profiles of showers and rains are very different, resulting in different effects on the large scale environment. Rain can occur in intense bursts (showers) or in extended periods of less intense rainfall (rains). The latter regime often produces more total rain than the former. Using observations obtained over the tropical east Pacific and the southwest Caribbean, we have isolated the atmospheric conditions that occur for each of these rainfall regimes. In particular, rains require a very moist atmosphere but can occur with weak atmospheric instability, whereas showers occur in drier atmospheres but require stronger instability. Numerical simulations of rainfall systems confirm these results and reproduce the intense bursts of rain in showers versus the steady but weaker rainfall in rains. "Showers" produce intense but localized rainfall; "rains" are weaker but widespread, producing more average precipitation Rains occur in very moist environments with weak instability while showers favor drier but more unstable environments Showers are associated with periodic bursts of rain in numerical simulations, rains produce continuous precipitation