Case Study of Coastal Fog Events in Senegal Using LIDAR Ceilometer

Abstract

This study aims to examine the atmospheric conditions characterising fog phenomena on the Senegalese coast focusing on two specific instances that occurred on April 3 and April 30, 2023. These events were detected by the LIDAR Ceilometer installed at LPAOSF/ESP/UCAD and confirmed on the METARs of the meteorological stations at Dakar and Diass airports. The LIDAR's backscatter signal showed that the fog of April 3 started around midnight with a vertical extension at 100 m altitude and dissipated around 10 a.m. The April 30 event characterized by a good vertical extension from the surface up to 300 m above sea level, was triggered just after 2 a.m. and lasted around 3 hours. The results showed that a decrease in temperature, accompanied by an increase in humidity and light wind, is favorable for the triggering and persistence of fog. Sea Level Pressure (SLP) anomaly fields show two distinct configurations. The April 3 event was characterized by a zonal dipole of SLP anomalies between the Sahara and the northern Senegalese coast, while the April 30 event was characterized by a meridional dipole between the Sahara and the Gulf of Guinea area as far as the equatorial Atlantic. A weakening of the pressure around the study area was observed in both cases, allowing moisture advection to favor the onset of fog. The hovmoller diagrams of relative humidity and wind show that a good vertical extension of humidity associated with a westerly wind in the lower layers plays an important role in the formation and persistence of fog. The presence of dry air associated with a weak easterly wind in the middle layers could explain the low vertical extension of the fog on April 3. A strong wind in the lower layers would be responsible for the premature dissipation of the April 30 fog.