Sensitivity of orographic precipitation to changing soil moisture and ambient conditions

Understanding the physical processes that lead to orographic precipitation has its importance in domains such as weather and climate prediction or water management. Mountainous areas affect atmospheric circulation over a wide range of scales, contributing to an uneven repartition of moisture and rain in space and time. Understanding the link between temporal and spatial variations of rainfall distribution over complex terrain is therefore a great challenge. The runoff production of mountainous watersheds will be very sensitive to this distribution. Based on the analysis of radar images and on the simulation of a shallow convection case organized in bands above the Cévennes – Vivarais region (France), Miniscloux et al. (2001), Cosma et al. (2002) and Anquetin et al. (2003) have shown that the small scale orographic features of the Massif Central focus and intensify the precipitation due to the convergence of low level air masses within the succession of oriented east-west ridges and penetrating valleys. Following these previous works and within the general context of the Hydrometeorological Observatory of Cévennes – Vivarais, this study is based on ideal atmospherical simulations that aim to understand and highlight the main processes that lead to orographic precipitation organized in bands. The interaction between the local modification of the flow (wind speed and direction) and the resulting precipitation are investigated in terms of localisation of the rain patterns and in terms of water depth at catchment scale. The role of initial soil moisture is also investigated. The main results show that the orographic precipitation distribution depends mainly on the ambient forcing (flux direction, vertical shear). The modification of the ground fluxes due to the soil moisture slightly modifies the amount of cumulated rainfall leading to wider orographic rain bands.

Anquetin, S., F. Miniscloux, J.-D. Creutin, and S. Cosma, 2003, Numerical simulation of orographic rainbands, J. of Geophysical Research, 108(D8), 8386,doi:10.1029/2002JD001593. Cosma S., Richard E., Miniscloux F., 2002: The role of small-scale orographic features in the spatial distribution of precipitation. Q. J. R. Meteorol. Soc., 128, 1-18. Miniscloux, F., J.D. Creutin and S. Anquetin, 2001, Geostatistical analysis of orographic rainbands. J. of Applied Meteor., 40, 11, 1835-1854.