Influence of initial soil moisture in a regional climate model study over West Africa – Part 1: Impact on the climate mean

Brahima Koné,N'Datchoh Evelyne Touré,Adama Bamba,Arona Diedhiou,Adama Diawara,Arsene Toka Kobea,Sandrine Anquetin

doi:10.5194/hess-26-711-2022

Abstract

Abstract. The impact of soil moisture initial conditions on the mean climate over West Africa was examined using the latest version of the regional climate model of the International Centre for Theoretical Physics (RegCM4) at a 25 km horizontal resolution. The soil moisture reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis of the 20th century (ERA-20C) was used to initialize the control experiment, while its minimum and maximum values over the entire domain were used to establish the respective initial dry and wet soil moisture conditions (hereafter referred to as dry and wet experiments, respectively). For the respective control, wet and dry experiments, an ensemble of five runs from June to September was performed. In each experiment, we analyzed the two idealized simulations most sensitive to the dry and wet soil moisture initial conditions. The impact of soil moisture initial conditions on precipitation in West Africa is linear over the Central and West Sahel regions, where dry (wet) experiments lead to a rainfall decrease (increase). The strongest precipitation increase is found over the West Sahel for wet experiments, with a maximum change value of approximately 40 %, whereas the strongest precipitation decrease is found for dry experiments over the Central Sahel, with a peak change of approximately −4 %. The sensitivity of soil moisture initial conditions can persist for 3–4 months (90–120 d) depending on the region. However, the influence on precipitation is no longer than 1 month (between 15 and 30 d). The strongest temperature decrease is located over the Central and West Sahel, with a maximum change of approximately −1.5 ∘C in wet experiments, whereas the strongest temperature increase is found over the Guinea coast and Central Sahel for the dry experiments, with a maximum change of around 0.6 ∘C. A significant impact of soil moisture initial conditions on the surface energy fluxes is noted: in the wet (dry) experiments, a cooling (warming) of the surface temperature is associated with a decrease (increase) in sensible heat flux, an increase (decrease) in latent heat flux and a decrease (increase) in the boundary layer depth. Part 2 of this study (Koné et al., 2022) investigates the influence of soil moisture initial conditions on climate extremes.

Highlights

In the climate system, soil moisture is a crucial variable that influences water balance and surface energy components through latent surface fluxes and evaporation
We set up three numerical experiments with RegCM4: on the first day (1 June), we applied a control soil moisture initial condition, a wet soil moisture initial condition and a dry soil moisture initial condition
We present the results of the two JJAS 2003 and JJAS 2004 runs most impacted by soil moisture dry and wet initial conditions, respectively, in order to avoid the effects of initial soil moisture internal forcing

Summary

Introduction

Soil moisture is a crucial variable that influences water balance and surface energy components through latent surface fluxes and evaporation. Soil moisture impacts the development of weather patterns and precipitation. The strength of soil moisture impacts on land– atmosphere coupling varies with location and season. Koster et al (2004) sustained that improving the simulation of the atmospheric response to the slow variations in land and ocean surface conditions may be important for seasonal climate prediction. Schär et al (1999) sustained that the role of soils may be comparable to that of the oceans. The solar energy received by the oceans is stored in summer and used to heat the atmosphere in winter.

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