Surface response to rain events throughout the West African monsoon

F Lohou,F Timouk,J.-M Cohard,J Demarty,S Galle,D Ramier,A Boone,M Grippa,F Guichard,C M Taylor,L Kergoat,C Peugeot,B Cappelaere

doi:10.5194/acp-14-3883-2014

Abstract

Abstract. This study analyses the response of the continental surface to rain events, taking advantage of the long-term near-surface measurements over different vegetation types at different latitudes, acquired during the African Monsoon Multidisciplinary Analysis (AMMA) by the AMMA-CATCH observing system. The simulated surface response by nine land surface models involved in AMMA Land Model Intercomparison Project (ALMIP), is compared to the observations. The surface response, described via the evaporative fraction (EF), evolves in two steps: the immediate surface response (corresponding to an increase of EF occurring immediately after the rain) and the surface recovery (characterized by a decrease of EF over several days after the rain). It is shown that, for all the experimental sites, the immediate surface response is mainly dependent on the soil moisture content and the recovery period follows an exponential relationship whose rate is strongly dependent on the vegetation type (from 1 day over bare soil to 70 days over forest) and plant functional type (below and above 10 days for annual and perennial plants, respectively). The ALMIP model ensemble depicts a broad range of relationships between EF and soil moisture, with the worst results for the drier sites (high latitudes). The land surface models tend to simulate a realistic surface recovery for vegetated sites, but a slower and more variable EF decrease is simulated over bare soil than observed.

Highlights

The monsoon is the main source of precipitation over West Africa
Performed over a variety of land cover types and climate zones along a S–N transect in West Africa using observations, this study investigates the sensitivity of evaporative fraction (EF) dynamics to surface characteristics, comparing bare soil with annual and perennial vegetation types, alongside site latitude and soil type
Several empirical relationships have been derived from the African Monsoon Multidisciplinary Analysis (AMMA)-CATCH surface measurements to better describe and understand the way the surface fluxes respond to a rain event over a large range of surface characteristics along a S–N transect in West Africa and throughout the monsoon season

Summary

Introduction

The monsoon is the main source of precipitation over West Africa. It generates long-lived mesoscale systems which provide 80 to 90 % of the annual rainfall in the Sahel (D’Amato and Lebel, 1998). Land–atmosphere exchanges and surface fluxes are impacted by rainfall variability at all scales, either as a direct response to soil water availability or through vegetation changes, and have been identified as major influences on climate and weather in West Africa (Eltahir and Gong, 1996; Zeng et al, 1999; Koster et al, 2004; Taylor et al, 2011b). Recent results demonstrated that convection triggering, which is a critical process in the tropics, was significantly enhanced by mesoscale heterogeneity of surface soil moisture (Taylor et al, 2011a, 2012). Antecedent rain strongly influences the spatial structure of surface fluxes, with high latent heat flux and low sensible heat flux over recently wetted surfaces

Methods

Results

Conclusion