Interactions between the Land Surface and Mesoscale Rainfall Variability during HAPEX-Sahel

Abstract

The Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) was designed to investigate land?atmosphere interactions in the semiarid conditions of southwest Niger. During the intensive observation period (IOP) in 1992, a pronounced mesoscale rainfall gradient developed over the Southern Super Site (SSS). Measurements from a high-resolution rain gauge network indicate that over a distance of 9 km, cumulative rainfall in the final 7 weeks of the wet season (31 July?18 September) ranged from 224 mm in the south to 508 mm in the north. The extreme rainfall gradient is not apparent in other years and evolves through persistent local intensification of convection in passing large-scale storms. This paper assesses the influence of the rainfall variability on the surface and atmosphere, and explores the possibility of a land surface feedback on rainfall at this scale. Soil moisture estimates across the SSS illustrate the importance of rainfall on the water balance and indicate that gradients of soil moisture deficit are likely throughout the IOP. Observations from the three dominant vegetation types reveal the sensitivity of available energy and evaporative fraction to antecedent rainfall. This arises from the high coverage of bare soil and the growth response of Sahelian vegetation to soil moisture. A broad range of evaporation rates are found, while sensible heat fluxes are generally less sensitive to antecedent rainfall. Surface and airborne measurements of temperature and humidity show that rainfall-induced surface variability across the SSS leads to mesoscale gradients in properties of the planetary boundary layer (PBL). On a day with light winds, a thermally induced area of PBL convergence associated with antecedent rainfall conditions is observed. A surface feedback mechanism has been proposed to explain the organization of rainfall on scales of about 10 km. Typical Sahelian surface conditions generate large anomalies of low-level moist static energy following mesoscale rainfall events. This variability influences the development of individual convective cells within subsequent larger-scale disturbances. The anomalous rainfall pattern at the SSS is linked to typical spatial scales of a convective 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 highlights the sensitivity of deep convective instabilities to PBL anomalies of the magnitude that were measured throughout the experiment.