Observational and Numerical Study of Mesoscale Land and Sea Breezes (Lsb) along the Guinea Coast of West Africa

Abstract

This study is in two parts. The first part utilized observational data to investigate the climatology of land and sea breezes (LSB) at seven coastline stations along the Guinea Coast of West Africa. The second part of the work studied the dynamics of LSB through a numerical simulation using the Weather Research and Forecasting (WRF) model. For the observational part, hourly data (covering the period 1983 – 2016) of wind speed and direction, precipitation and air temperature over land at selected stations were utilized. Daily Sea Surface Temperature (SST) data from adjacent Atlantic Ocean and daily precipitation data from the tropical rainfall measuring mission (TRMM) were also used. Ten (10) years reanalysis data (2003 – 2012) of ERA-Interim, Climate Forecast System Reanalysis version 2 (CFSRv2) and National Centers for Environmental Prediction reanalysis second version (NCEP2) were also used for comparison with the observations. A set of criteria based on the diurnal reversal of both surface and upper wind direction, the thermal gradient necessary to drive the wind circulation and sunshine duration were developed to identify land and sea breeze days. On station-wide basis, the results show that LSB is a seasonal phenomenon across the entire Guinea coast but much less frequent in the summer months (July and August) when the monthly mean temperature gradient is minimum at all stations. Results also show that ERA-Interim, CFSRv2 and NCEP2 reanalyses all under-estimated the monthly frequency and seasonal cycle of LSB in this region, although CFSRv2 is better in reproducing the observational patterns than other reanalyses. Based on wind speed and direction, wind roses revealed the existence of night/early-morning offshore winds (land breeze) at all stations, except Accra where there are no offshore winds or rather very weak. A particular result was that except in the months July – September, when the normal southwesterly monsoon winds prevail all day, sea breeze (SB) is observed from about 1100 LST at all stations, with a mean cessation time at 0100 LST, giving a mean duration of about fourteen (14) hours. However, at Abidjan SB is observed to occur at all hours of the day throughout the year. The vertical wind plots confirmed the wind direction reversals for LSB days and an indication of the depth and speed of LSB in different months. For instance, they provided some evidence that LSB height is influenced by the West African monsoon as the Intertropical Discontinuity (ITD) advances northwards from January. This was also seen in the LSB increased speed from December to August. The findings in this study have not been previously documented for West Africa. Diurnal hodograph analyses for LSB days reveal the existence of both clockwise and anticlockwise rotation of wind at all stations. This is related to the details of the local and synoptic influences on the given day. The hodographs for non-sea breeze days have indeterminate rotation as they show no clear diurnal rotation pattern of wind direction. A non-hydrostatic fully compressible numerical model (WRF) is used to simulate LSB circulation across the Guinea coast. Comparison of observed and simulated LSB patterns show that the WRF model reliably captures the LSB circulation in the region. A dynamical analysis performed by extracting individual forcing terms from the horizontal momentum equations at some selected regions within the study area showed that the direction of the wind rotation is a result of a complex interaction between surface and synoptic pressure gradients, advection, and horizontal and vertical diffusions forcings. However, hourly analysis of the rotation term suggests that surface gradient seems to dominate for the oceanic region while diffusion terms are more important for land area. This can likely be attributed to the variation of surface roughness due the landscape and urbanization. Therefore, it can be saying that this study has revealed the link between urbanization and LSB circulation in the Guinea coast of West Africa, where most important cities are located.