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The properties of the planetary boundary layer (PBL) are crucial for understanding convection and dynamics in the tropical atmosphere. In semi-arid regions, there are strong links between the PBL and soil moisture. This study demonstrates the link between PBL characteristics and antecedent rainfall via soil moisture variability on length-scales from 10 to 100 km. Spatial variability of the PBL in the west African Sahel is explored using observations from an aircraft for one day in the wet season, as part of the JET2000 experiment. Meteosat and surface station data provide information about rainfall patterns during the previous days.

The aircraft transect crossed several well-defined bands of low surface temperature and albedo, as determined by the satellite imagery. These bands indicate high soil moisture beneath the paths of recent storms. Observed atmospheric properties are closely linked to the surface gradients. Above moist soils, the PBL tends to be shallower, cooler, moister and less turbulent than in adjacent dry areas. The study shows that the characteristic large-scale gradients of temperature and humidity across the Sahel are concentrated in zones defined by recent rainfall.

Evidence is presented which is consistent with a positive feedback at the storm-scale between deep convection and soil moisture. Atmospheric profiles indicate that soil moisture plays an important role in determining the stability of the atmosphere to subsequent convection. It was found that in a region of low surface moisture covering 80 000 km(2), cold-cloud systems did not occur in the 48 hours following the flight. By contrast, mesoscale convective systems were observed in adjacent regions with wetter soils.