Simulating the Characteristics of West African Monsoon Using the Mpas Model

Abstract

This research evaluates the Model for Prediction Across Scale (MPAS) to simulate the West African monsoon. The research aims to investigate how well the MPAS model simulates the characteristics of the West African monsoon. The specific objectives are to examine the performance of the MPAS model in reproducing observed spatial pattern and seasonality of the West African Monsoon (WAM) rainfall, examine the capability of MPAS to reproduce the rainfall producing system of the West African Monsoon and to investigate the capability of MPAS to simulate temperature and precipitation based ex-treme events over West Africa.The West African monsoon (WAM) plays a crucial role in the West African climate system. It is the primary process for transporting moisture from the Atlantic Ocean to land masses during the boreal summer. The ability of the Model for Prediction Across Scales-Atmosphere (MPAS-A) with a mesh with a quasi-uniform resolution of 60 km to reproduce the WAM rainfall-producing system was eval-uated. The model is run from 1981 to 2010 using the "mesoscale reference" physics par-ametrization. The model results are compared with satellite-derived datasets (Climate Hazards Group InfraRed Precipitation with Stations, CHIRPS), gridded observation da-tasets (Climate Research Unit, CRU), and reanalysis datasets (Climate Forecast System Reanalysis, CFSR; the National Oceanic and Atmospheric Administration, NOAA; and the European Centre for Medium-Range Weather Forecasts version 5, ERA5). MPAS reproduces the rainfall pattern over West Africa and the different phases of the monsoon dynamics, including the position of the Inter-tropical Discontinuity (ITD). However, MPAS shows weaknesses in reproducing the orographic rainfall, maximum over the Guinea Coast, Jos Plateau, and Mount Cameroon; likewise, the magnitudes of the verticalvelocity and zonal wind are underestimated. In addition, MPAS shows a cold bias in the temperature gradient. Also, a large ensemble of 51 simulations with the MPAS has been applied to assess its ability to reproduce extreme temperatures and heat waves in the area of West Africa and the Eastern Sahel. With its global approach, the model avoids transi-tion errors influencing the performance of limited area climate models. The MPAS sim-ulations were driven with SST and sea ice extent as the only boundary condition. The results reveal moderate cold biases in the range from -0.6° to -0.9° C for the daily mean temperature and -1.4°to -2.0° C for the area mean of the daily maximum temperature. The bias in the number of tropical nights ranges from +3 to -10 days. An underestimation by up to 50% is also present regarding the number of summer days. The heat wave dura-tion index is underestimated regionally by 10% to 60%. Compared to the reanalyzes, the biases revealed by the MPAS simulations are generally smaller than with measured ob-servational reference. The results from long-term runs and from short-term runs with se-lected SST years are similar. The shortcomings in the reproduction of the temperatures and precipitation found in the present investigation with the MPAS approach are similar to that of regional climate models.