Using the ERA 40 ECMWF reanalyses experiment (1967-2001) and the operational analyses for recent years (2004-2006), we propose an automatically HL detection. The Low Levels Atmospheric Thickness (LLAT) defined by the difference between the 700 and 925 hPa geopotential heights at 0600 UTC appears to be adequate to identify low levels depression. The daily HL is defined as the largest area where the analyses-derived LLAT exceeds a threshold, defined by the 90 percent of the cumulated probability distribution function of LLAT of a domain covering West Africa.
At climatological time scale, we have analysed the seasonal migration of the HL: two prefered positions were found (1) west of the Darfour mountains during the winter season and (2) between the Hoggar and the Atlas in the summer season. During the transition period, a large and sudden thickening of the HL is observed (around mid-June), during the settling of the HL in its so called summer position. This increase in the LLAT seems to be associated with both an increase of the surface temperature along the western edge of the Hoggar mountains and with an increase of the geostrophic circulation in the wind field at 925 hPa. This phenomenon may be associated with the monsoon jump. The impact of horizontal advections (ventilation) of humidity and potential temperature on the seasonal behaviour of the HL have also been studied.
At intra seasonal time scale, the HL exhibits pulsations (as observed in size and intensity) of periods around 5, 20 and 30 days period. These pulsations could play an important role in the variations of the monsoon flux intensity and progression during the summer season.
Furthermore, the large dust plume events generally observed over the west Africa could modify dynamical stability of the atmosphere and thus influence the dynamics of the HL. A fist analysis of the impact of the dust on the HL dynamics has been conducted using the AMMA SOP dataset acquired in the summer of 2006.
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