Model analysis of Coastal and Continental impacts on boundary layer meteorology over West Africa

Abstract

Data from the Weather Research and Forecasting (WRF) model and the regional climate model (REMO) of the Climate Service Center Germany (GERICS), combined with reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) were used to study the influence of the Atlantic Ocean and land surfaces on atmospheric boundary layer (ABL) over West Africa. The ABL is characterised by parameters such as: boundary layer height (BLH), temperature at 2-m, specific humidity at 925 hPa, surface latent and sensible heat flux and wind at 10-m. The diurnal, monthly, and seasonal variability of each parameter varies by location (i.e., land or oceanic sites). We see that in the two types of data studied (observed and simulated), the air temperature and wind speed are lower over the Atlantic Ocean than over land. In contrast, the specific humidity at 925 hPa is highest at 00 UTC, 06 UTC and 18 UTC from the Equator to 15° N, and low at 12 UTC, at 18 UTC it varies between 4 and 14 g/kg. We note that the variability of these parameters is lower in coastal areas (close to the Atlantic Ocean) than in continental areas (near the Sahara Desert). Statistical indicators such as bias and mean square error were calculated to assess the effectiveness of the model used in this study. The results show that simulated data produced good results. REMO forces with ECMWF interim reanalysis (ERA) interim simulations of ABL parameters for historical climate (1979–2017) were compared to ERA5 data using several descriptive statistics. Compared to the ERA5 reanalysis, the REMO models realistically reproduce the seasonal characteristics of 2-metre temperature, specific humidity at 925 hPa, BLH, and near-surface wind in most sub-regions of West Africa although notable biases still exist. The diurnal analysis of radiosonde on September 2, 2006 in Dakar and Niamey shows that dew temperature is lower than the ambient temperature. This explains the fact that the thickness of the ABL in Dakar is less than that observed in Niamey. The WRF model, provided by National Center for Atmospheric Research (NCAR), which is widely utilised in research and operation, also reproduces well the diurnal variation of near-surface parameter of the ABL over this region.