Impact of Boundary Layer Processes on Simulated Tropical Rainfall

Abstract

The impact of boundary layer (BL) processes on simulated tropical precipitation was studied using the National Centers for Environmental Prediction (NCEP) Medium-Range Forecast (MRF) Model. A new BL scheme, which is a nonlocal mixing concept of Noh et al. after Troen and Mahrt, was successfully incorporated into the MRF Model. In this study, simulations with 10-member ensembles were conducted for boreal summers of normal, El Niño, and La Niña years, respectively. In particular, the authors focused on the impact on tropical rainfall of the new BL scheme when two different convection schemes are utilized respectively in the model. The new BL scheme improves simulated tropical precipitation overall and in particular reduces the simulated rainfall in the central and eastern equatorial Pacific Ocean. This reduction over the eastern Pacific is a direct effect of the new BL scheme resulting in less mixing of heat and moisture and is irrespective of the convection scheme. The effect of BL processes over the western Pacific, however, is indirectly related to the change of the Walker circulation and highly dependent on the convection scheme selected.