Characterization of WRF Simulated Low-Level Jets over Southern West Africa

Abstract

Low-level jets are mainly nocturnal phenomena with relatively strong horizontal winds in the lower section of the troposphere. The development and evolution of deep convection are linked to the low-level jets, which are vital for both horizontal and vertical fluxes of temperature and moisture. The focus of this study is to characterize low-level jets simulated with the WRF model in the planetary boundary layer (PBL) over southern West Africa. The simulation's forcing data were Global Forecast System datasets (GFS) and boundary conditions were specified in the WRF runs at 3-hourly temporal resolution. The performance of the simulation was tested with observational datasets from the European Center for Medium-range Weather Forecast (ECMWF). The study examined low-level jet characteristics such as frequency, intensity, height of occurrence, and direction of propagation. Three bioclimatic regions namely West Sudanian Savanna, Guinean, and Guineo-Congolian that can be found in southern West Africa were used as the basis for the analysis. The study employed a relative and absolute detection criteria for low-level jets which requires that the lowest maximum of a vertical wind speed profile in the lowest 1800 m of the atmosphere is at least 2 m/s and 25% stronger than the next minimum above. It was found that low-level jets are more frequent in the Guineo-Congolian than, West Sudanian Savanna and Guinean regions and about 90% occurring between the hours of 000 UTC and 060 UTC with south-westerlies dominating. The maximum average intensities of low-level jets in the Guineo-Congolian, West Sudanian Savanna and Guinean regions are 8.1 m/s, 12.9 m/s and 8.9 m/s respectively. Low-level jets observed do frequently occur between the height of 700 – 750 m above ground level, an average of 725 m. The study recommends more observational and modeling case study on low-level jets over southern West Africa to emphasize conclusions made in the study.