Before the Rain in Cumulus over the Ocean (RICO) field campaign that took place near Antigua and Barbuda from 2004-2005, it was thought that most trade wind cumulus clouds do not precipitate. The main goal of the RICO field campaign was to study rain in tropical marine cumuli and how this rain modifies trade wind clouds. One unexpected result that kept reappearing in satellite and radar data was the orientation of trade-wind cumuli that produced the majority of the precipitation. The majority of clouds that produced precipitation appeared to form on the outflow boundaries from previous convection in an arc-type orientation. The interactions of these outflow boundaries are thought to have a major impact on the location and duration of precipitation from shallow cumulus clouds. It was observed that clouds in the arc-shaped orientation typically reached heights of 3 km or deeper and moved 1-2 ms-1 faster than the mean wind. It may be that precipitation in the trade wind regime is significantly forced along outflow boundaries; however, at present it has not been extensively explored how or why these outflow boundaries occur where they do.
This study uses the large eddy simulation (LES) package of the Weather Research and Forecasting (WRF) model v3.3.1 to do idealized simulations of a cold pool outflow boundaries in a tropical marine cumulus environment. We are modeling cold air downdrafts and resulting surface cold pools that are thought to cause the arc-shaped cumulus clouds. In idealized sensitivity tests of tropical cumulus behavior, the environment (specified vertical wind shear and relative humidity) and imposed cooling (meant to mimic initial convective cooling) were varied to see what parameters have the largest effect in creating cloud formations similar to what has been observed. The modeling parameter study is designed to quantify and understand the role of layer moistening (on updrafts) and drying (on evaporative cooling and downdrafts) and subsequent behavior of cold pools and longevity and structure of convective arcs.