Observations and simulation of a Saharan air layer event with a midtropospheric dust layer at Dakar, Senegal, 6-7 July 2010

On 3 July 2010 a large dust event was initiated over the Saharan Desert in Southern Algeria with the westward movement of an intensifying thermal low. Increases in Aerosol Optical Depth (AOD) in Algeria, Niger, Senegal and Cape Verde show the progression of the dust event with an AOD of 3.5 to 4 found in Senegal on 7 July. The surface total and direct shortwave radiative fluxes are reduced by more than 200 W m(-2) and 300 W m(-2) respectively compared to the previous day. Furthermore, there is an increase in diffuse shortwave radiation of 50-150 W m(-2) when compared to the previous day. On 7 July, satellite based lidar observations confirm a uniform dust layer at approximately 5 km near the top of the Saharan Air layer where relative humidity values are greater than 50%. A WRF-GOCART simulation show additional sources of dust perturbed over Eastern and Western Mauritania during 6 and 7 July with elevated dust concentrations aloft in concert with satellite measurements on the morning of 7 July. The warm temperatures are associated with the thermal low creating sloping isentropes to the south, which allows the SAL air mass and dust aerosols to rise above the monsoon layer over Senegal.