Structure and dynamics of the Saharan atmospheric boundary layer during the West African Monsoon onset: observations and analyses from the research flights of 14 and 17 July 2006

This paper presents the results of coordinated research flights over the Saharan heat low, conducted during July 2006 as part of the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Periods. The flights consisted of a morning transect on 14 July 2006 with the Falcon F20 aircraft of the Deutsches Zentrum für Luft- und Raumfahrt (DLR), using the doppler lidar WIND to observe the tropospheric winds, followed by an afternoon flight on 17 July with the BAe146 aircraft of the Facility for Airborne Atmospheric Measurments (FAAM) releasing a curtain of 16 dropsondes. Measurements from radiosondes at the Algerian stations of Tamanrasset and In-Salah, as well as the CALIPSO spaceborne lidar, are analysed also. It is shown that the daytime Saharan atmospheric boundary layer exhibits a remarkable split structure, with a well-mixed convective layer lying beneath a residual layer whose dynamics appear to be more nearly laminar. Observations from Saharan radiosonde stations confirm that the Saharan residual boundary layer is, on some days, a persistent rather than a transient feature, and that on occasion the residual layer can last right through the day. The broad features of the Saharan heat low thermodynamics and winds are successfully captured by the ECMWF operational analyses and the discrepancies are quantified here. The lidar winds of 14 July confirm that the analysis represents the main airflows accurately, apart from the zone to the southeast of a mid-level trough, for which the southerly component of winds was underestimated by some 8 m.s-1. On 17 July, the strengths of both the southwesterly monsoon and the northeasterly Harmattan winds were slightly underestimated also. The region of maximum boundary layer temperature over the desert was around 1 degree too far north, and too broad in the analysis, while the equivalent potential temperature in the Saharan CBL was around 2K lower than observed. The nearsaturated layer observed at the top of the Saharan atmospheric boundary layer was underestimated also in the analysis, in horizontal extent and altitude. The inter-tropical front of 17 July was coincident with strong surface contrasts in vegetation and fluxes. On each study day the inter-tropical front in the analyses was around 1 degree further north than observed.