INTERACTIONS BETWEEN THE LAND-SURFACE AND MESOSCALE RAINFALL VARIABILITY DURING HAPEX-SAHEL

The Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahe l) was designed to investigate land-atmosphere interactions in the sem iarid conditions of southwest Niger. During the intensive observation period (IOP) in 1992, a pronounced mesoscale rainfall gradient develop ed over the Southern Super Site (SSS). Measurements from a high-resolu tion rain gauge network indicate that over a distance of 9 km, cumulat ive rainfall in the final 7 weeks of the wet season (31 July-18 Septem ber) ranged from 224 mm in the south to 508 mm in the north. The extre me rainfall gradient is not apparent in other years and evolves throug h persistent local intensification of convection in passing large-scal e storms, This paper assesses the influence of the rainfall variabilit y on the surface and atmosphere, and explores the possibility of a lan d surface feedback on rainfall at this scale. Soil moisture estimates across the SSS illustrate the importance of rainfall on the water bala nce and indicate that gradients of soil moisture deficit are likely th roughout the IOP. Observations from the three dominant vegetation type s reveal the sensitivity of available energy and evaporative fraction to antecedent rainfall. This arises from the high coverage of bare soi l and the growth response of Sahelian vegetation to soil moisture. A b road range of evaporation rates are found, while sensible hear fluxes are generally less sensitive to antecedent rainfall. Surface and airbo rne measurements of temperature and humidity show that rainfall-induce d surface variability across the SSS leads to mesoscale gradients in p roperties of the planetary boundary layer (PBL). On a day with light w inds, a thermally induced area of PBL convergence associated with ante cedent rainfall conditions is observed. A surface feedback mechanism h as been proposed to explain the organization of rainfall on scales of about 10 km. Typical Sahelian surface conditions generate large anomal ies of low-level moist static energy following mesoscale rainfall even ts, This variability influences the development of individual convecti ve cells within subsequent larger-scale disturbances, The anomalous ra infall pattern at the SSS is linked to typical spatial scales of a con vective cell and the preferred direction of travel of Sahelian squall lines. This hypothesis is supported by the temporal variability of the rainfall anomalies. Differences in precipitation across the SSS show a pronounced diurnal cycle in phase with PBL anomalies and are largest during periods when surface variability is high. A case study is also presented from an isolated convective storm over the SSS. This highli ghts the sensitivity of deep convective instabilities to PBL anomalies of the magnitude that were measured throughout the experiment.