The impact of soil moisture initialization on seasonal precipitation in West Africa

The objective of the study is to investigate the impact of soil moisture on seasonal precipitation in the West African Sahel. The relationship between late spring soil moisture and summer precipitation is examined based on data analysis and model simulations.; An Empirical Orthogonal Function (EOF) analysis was conducted of twenty-four years (1981-2004) of monthly average soil moisture data from the National Centers for Environmental Prediction-Department of Energy Atmospheric Model Intercomparison Project (NCEP-DOE AMIP-II) Reanalysis (R-2). From the EOF analysis, only non-El Nino/non-La Nina (neutral) years with contrasting soil moisture anomaly fields were examined as possible case studies for what will be referred to as realistic model integrations.; The model integrations were four months long initialized from May 1 st 2004 over a domain of 17.5°W-25°E, 10°S-20°N. For the realistic model integrations the soil moisture fields from 2004 and 1990 were selected for the control and perturbation runs respectively. 2004 was chosen as the year for the control model integration where all atmosphere and all land states would be from that specific year. For the perturbation run, the atmospheric and land states were from 2004 (the same as the control run), however the initial soil moisture was obtained from the 1990 soil moisture field. 1990 and 2004 resembled the first two leading modes of soil moisture variability from the EOF analysis. Additionally, idealized model integrations were conducted to assess the model sensitivity, that is, how the model simulated the behavior of surface fields such as sensible and latent heat fluxes, humidity, temperature and precipitation as a result of perturbations in initial soil moisture. For the idealized model integrations simplified cases were used where one of the model integrations was initialized with minimum soil water content and the other initialized with maximum soil water content.; The results from the realistic and idealized model rations were compared for four sub-regions in the model domain to assess the impact of the initial soil moisture perturbation on various surface fields and precipitation. In examining the results it appears that the four sub-regions show different sensitivity to the various soil moisture initialization cases.