A comparison of LEM simulations of trade wind cumulus clouds with aircraft observations taken during RICO

Trade wind cumulus clouds are prevalent over tropical oceans and form an important component of the global atmospheric and oceanic energy budget. The impact of precipitation from such clouds on the large-scale circulation in the Tropics and mid-latitudes remains poorly diagnosed, in part due to the crude representation of the warm rain process within current models. Therefore, realistic representations of the dynamical and thermodynamical properties, and the precipitation processes that occur in trade wind cumulus, are required in both global climate studies and Numerical Weather Prediction. The Rain In Cumulus over the Ocean (RICO) field campaign took place between November 2004 and January 2005 around the islands of Antigua and Barbuda. A wealth of instrumentation was deployed during the campaign, including three research aircraft, a research ship, sophisticated dual wavelength radar, and a surface site for chemistry and aerosol sampling. Observations from the campaign will provide an excellent tool for characterizing trade wind cumulus on all scales ranging from the microphysical to the statistical nature of the cloud field ensemble, therefore facilitating the further development of current models. This paper presents comparisons of some cloud field statistics derived from the research aircraft with those from numerical simulations performed with the Met Office Large Eddy Model (LEM). The variation of updraft velocities, cloud liquid water content, and precipitation loading as a function of altitude above cloud base are shown. Initial simulations suggest that whilst many of the features present in the statistics from the aircraft data are reasonably well represented in the model, the model underestimates the water content of precipitation sized droplets in the atmosphere. The impact of changes to the parameterization of precipitation within the model, such as how the fall speed of rain droplets varies with droplet size, and the use of a single or double moment precipitation scheme are investigated.