Processes involved in the diurnal cycle of the Planetary Boundary Layer in the frame of the West African Monsoon

We present an experimental analysis of the Sahelian lanetary Boundary Layer (PBL) processes in the context of the AMMA (African Monsoon Multidisciplinary Analysis) programme and its extensive observational deployment in 2006.

The analysis of wind profilers and radiosondes data in Niamey (Niger) showed a consistent jet developping during the night all along the year. It is a north-easterly Harmattan) flow or south-westerly (monsoon) flow during the dry and wet season respectively, that occurs below 1000 m agl, with a maximum of 10 to 20 ms-1 at around 400 m agl. The transition between both flows, which corresponds to the passage of the Intertropical Discontinuity (ITD) in the considered area, starts a 2-3 month period of moistening of the low troposphere before the monsoon onset in mid-July when the Inter Tropical Convergence Zone moves to the north and brings deep convection in the Sahelian band with frequent intense rainshowers. The beginning of this moistening period is associated with a pronounced diurnal signal of the water vapour, with horizontal advection during the night through the low level jet, and vertical transport from the low to the mid-troposphere with dry convection and turbulence during the day.

During summer 2006, from early June to the end of August, the intensive exploration of the lower troposphere in the vicinity of Niamey area by the French ATR aircraft with instruments that measure wind, temperature and humidity at high rate allowed us to study the daytime turbulent processes that redistribute water over the vertical. We studied the interaction between the moist monsoon flow and the overlying dry Harmattan flow, through entrainment of dry tongues penetrating deep into the PBL. The role of the convective PBL, the contribution of the dry tongues, and estimates of the entrainment processes are investigated using the observations of several flights made (i) during the moistening period closely preceding the Monsoon onset and (ii) during the active phase of the Monsoon. Thus, we highlight the evolution of the nature and importance of the interaction between the monsoon and Harmattan in the context of the larger scale West African Monsoon processes.