Tuesday, 13 January 2004: 5:00 PM
Impact of the Boundary Layer Processes on Simulated Tropical Rainfall
The impact of the boundary layer (BL) processes on simulated tropical precipitation has been studied using the National Centers for Environmental Prediction (NCEP) Medium-Range Forecast (MRF) model with a resolution of T62L28. A new nonlocal mixing concept of Noh et al. after the Troen and Mahrt has successfully been incorporated into the MRF model. In this study, simulations with 10 member ensembles have been conducted for boreal summers of a normal, El Niņo and La Niņa years, respectively. In particular, the effect of the BL scheme depending on two different cumulus parameterization schemes - Simplified Arakawa-Schubert (SAS) and Relaxed Arakawa-Schubert (RAS) - on the simulation of tropical rainfall is focused on. According to the simulation result, the new scheme improves the precipitation over the tropics by reducing the rainfall in the central and eastern equatorial Pacific Ocean. The reduction of rainfall in that region is a direct effect of the new BL scheme resulting in a less mixing of the heat and moisture. This effect is a common irrespective of the convection scheme. Meanwhile, the effect of the BL processes over the western Pacific is rather indirect, and is related to the change of the Walker Circulation. As well, this highly depends on the convection scheme selected. That is, the model with the SAS convection scheme improves the precipitation over the western Pacific by enhancing the rainfall with intensification of the Walker Circulation when the new scheme is used, whereas the RAS scheme suppresses the rainfall activity in that region. As a result, the improvement due to the new BL processes is clearer in the case of the SAS convection scheme than in the RAS convection scheme being employed.