Evaluation of soil moisture- precipitation feedback theories: Statistical approach using a convection permitting model over Indian subcontinent

Soil moisture is an important geophysical parameter that controls the convective rainfall by influencing land-atmosphere interaction processes. The purpose of this study is to test the applicability of various existing soil moisture-precipitation feedback theories (Findell and Eltahir 2003a, Koster et al 2004, Taylor et al 2012) over the Indian sub-continent during the monsoon season. To test these theories we have evaluated the relationships between soil moisture, fluxes, wind convergence, orography and rainfall in a convection-permitting atmospheric model (UK Met Office Unified Model), run with 4km grid-spacing. Four different sub-domains of the Indian subcontinent have been chosen, which fulfil various topographic and soil moisture conditions. Various statistical analyses have been performed in the immediate vicinity of a region that has received afternoon rainfall, using i) antecedent soil moisture, ii) topography, iii) the Convective Triggering Potential-Humidity Index (CTP-HI) framework (Findell and Eltahir 2003a) and iv) wind convergence. From soil moisture analysis it has been found that afternoon convective initiations are preferred over dry soil with comparatively flat orography. Using generalized soil moisture conditions in the CTP-HI framework, it seems there is a requirement for a more generalized or modified framework, to have universal threshold values to distinguish between dry and wet advantage rain events. The CTP-HI analysis along with subjective wind-field convergence analysis also shows that the 1-D boundary-layer evaluation is not sufficient to address all orographic and atmospheric conditions. Further analysis of wind convergence plots and orography over central India shows that at mesoscale, land atmosphere coupling is weaker over central India because it is orographically driven.

References:- Findell, K. L and Eltahir, E. A.B, 2003a: Atmospheric controls on soil moisture – boundary layer interactions. Part I: Framework development. J. Hydrometeor., 4, 552–569. Koster, R. D., and Coauthors, 2004: Regions of strong coupling between soil moisture and precipitation. Science, 305, 1138–1140. Taylor C.M.,De Jeu R.A.M., Guichard F.,Harris P.P., and Dorigo W.A. (2012): Afternoon rain more likely over drier soils. Nature,489, 423-426.