Effect of Enhanced Moisture Triggers on Mean Precipitation and Winds in the Tropical East Pacific and the Caribbean

The ability of two climate model simulations to reproduce mean precipitation and winds in the Intra Americas Sea is compared using the NCAR Community Atmosphere Model version 3.1 with the relaxed Arakawa-Schubert convective scheme. One simulation has a minimum entrainment threshold implemented that increases the sensitivity of model precipitation to free tropospheric humidity. The two runs analyzed were defined as: “moisture trigger” and “no moisture trigger”. A comparison of observations to model results for the boreal summer (June-October) and boreal winter (November-April) is made. Results for the summer season show that the moisture trigger run makes precipitation weaker over the Caribbean, and reduces precipitation biases over the eastern Pacific and over land. Overall, higher moisture sensitivity results in a more accurate skill of the model to represent mean precipitation over the eastern Pacific and Caribbean. However, both model runs show excessive precipitation over land at the base of the Sierra Madre, likely due to the coarse resolution of the model. Further, both simulations have a relatively poor representation of precipitation over the Inter Tropical Convergence Zone (ITCZ) in the eastern Pacific during the winter.

When compared to observations, the low-level wind is not well represented along the coastlines in the models, likely due to problems in representing topography. The winds become more cyclonic over the eastern Pacific associated with increased mean precipitation there in the moisture trigger run, and enhanced easterlies over the Caribbean are associated with a decrease of precipitation.