Observation of entrainment at the interface between monsoon flow and the Saharan Air Layer



During the AMMA experiment, the French ATR aircraft flew in the lower troposphere in south Niger, with instruments that allow fast measurement of wind, temperature and humidity. Stacked legs flown between early June and mid-August into the moist southwesterly monsoon flow and the dry northeasterly flow of the Saharan Air Layer (SAL) above, enable us to study the interaction between the two flows at the turbulence scale and the role of the planetary boundary layer (PBL) in the transfers between them. We describe the fluctuations observed at the top of the PBL and their evolution during the season. Dry tongues approximately 600 m wide and a few kilometres apart, intruding from the overlying SAL down into the PBL, are characteristic of the Sahelian region at that time of year. They are fully part of the entrainment process at the PBL top and responsible for large moisture fluxes in the upper PBL. They also skew the water vapour distribution which in turn impacts the water vapour scales. We evaluate the impact of the entrainment by use of a conditional analysis on combined fluctuations. The dry tongues play the largest role down to two thirds of the PBL depth. We find larger entrainment during early monsoon than during its active phase, and a large variability of the ratio of entrainment buoyancy flux over surface flux. Copyright © 2009 Royal Meteorological Society