Evaluation of Weather Research and Forecasting Model Physics in Simulating West African Monsoon

Abstract

This research evaluates the ability of Weather Research and Forecasting model physics in simulating the West African monsoon. The purpose is to identify a possible model physics combinations in the WRF model whose outputs can be used to inform weather- and climate-related decision-making process at local to regional scale. In the study, the sensitivity of West African Monsoon (WAM) regimes to three model physics (i.e. Cumulus (CU), Microphysics (MP) and Planetary Boundary Layer (PBL) parameterization schemes) is assessed, performance of the model in representing the WAM dynamics is evaluated and impact of warming climate on WAM under the RCP8.5 scenario is also assessed. Twenty-seven (27) WRF simulations of the August-September 2007 monsoon regime at a 20-km grid over West Africa were evaluated to investigate the sensitivity of the WAM regime to the three model physics. The focus was on precipitation and surface temperature during the simulated period. The model’s precipitation was evaluated against the TRMM (reference), CMORPH and GPCP satellite rainfall products. Also, the surface temperature was evaluated against the ERA-Interim (reference), NCEP, MERRA, and GSAT. Results showed that all model physics combinations simulated the diurnal cycles of surface temperature better than the simulation of precipitation. A comparative model skill score was developed and used to identify that combination of WSM5-MYNN-nTDK and GD-MYJ-BMJ are best performing physics combinations in both temperature and precipitation. Also, the three WRF model physics combinations reproduced the characteristics of the region’s monsoon during selected normal (2007), wet (2008 and 2010) and dry (2001 and 2011) years. The dynamics of WAM such as monsoon flow, African Easterly Jet, and Tropical Easterly Jet, are replicated by most of the model combinations. Therefore, underscoring the strong potential impact of regional moisture, heat and momentum transport and redistribution on the monsoon dynamics as prescribed by the physics. Lastly, the Pseudo-Global Warming (PGW) simulation method perturbed by CESM1.0-CAM5.2 is employed to assess the impact of warming on WAM, the result shows a slight increase in precipitation amount (-2 to 16%) in the 2070s when compared with the current (reference) climate. This change is expected to be more pronounced in the Sahel, where the value is 16%, and less than 3% in the Guinea Coast. Furthermore, there is a decrease (increase) in both light and moderate (heavy) rainfall days. The outcomes of this research underscore the significance of WRF model as a potentially useful tool to investigate how future WAM seasons could vary in a changing climate. This provides relevant information to improve the understanding of the possible implications of such changes on economic activities such as agriculture, water resources, and other climate-related sectors, and to guide the design and implementation of climateresilient projects.