VARIABILITY IN BOUNDARY-LAYER STRUCTURE DURING HAPEX-SAHEL WET-DRY SEASON TRANSITION

The variability of the Sahelian boundary layer has been studied with s treamline analyses, rainfall measurements, and upper air soundings dur ing its transition from wet to dry season. The 1992 rainy season ended prematurely because of the early arrival of westerly troughs over Wes t Africa. The change in the circulation is related to global-scale atm ospheric circulations as successive westerly troughs over this region can be traced back upstream on a planetary scale. Once the upper level easterlies changed to westerlies, the large-scale circulation brought the surface northeasterly flow southward, which led to the retreat of the Southwest Monsoon in Niger. The boundary layer responded quickly to this transition of synoptic events from wet to dry seasons. During the wet period, the boundary layer was relatively cool and moist becau se evapotranspiration dominated, keeping the surface cool and preventi ng significant direct sensible heating of the boundary layer. During t he transition period, less extensive showery weather allowed the bound ary layer to have more time to recover from rainfall episodes, leading to a warming and drying trend. During the dry period, soil moisture c ontents dropped rapidly. With more sensible heat flux made available f or boundary layer heating and energetics, the boundary layer reached i ts maximum temperatures and minimum moisture contents during the Hydro logical Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) intens ive observational period. Budget calculations indicate that the horizo ntal advection and vertical flux divergence terms were most important during the wet period, whereas during the dry period, the subsidence a nd vertical flux divergence terms were most important. From wet to dry seasons, the vertical wind shear of the zonal wind was reduced from 2 3 m s(-1) to 16 m s(-1) consistent with vertical wind shear difference s between wet and dry years as reported in the literature. Similaritie s and differences with the First ISLSCP Field Experiment (FIFE-89) bou ndary layer are also examined. It is hypothesized that the retreat of the southwesterly monsoon could be upheld by a sustained secondary cir culation if the wet season rainfall pattern imprints an organized sout h to north soil moisture gradient maintaining a concurrent reverse gra dient in surface sensible heating. The boundary layer circulation that would be established in response to the heating gradient would reinfo rce surface southwesterlies. as well as reinforcing mid-level easterli es of which the African Easterly Jet is a part, and thus help uphold t he intrusion of westerlies and the monsoon retreat. Such a mechanism. whose effectiveness would be a function of how distinct the south-nort h soil moisture gradient develops from the wet season precipitation pa ttern, could help explain the large interannual variability of rainfal l over the Sahel.