Sensitivity of wintertime precipitation and soil hydrology simulation over the western United States to lower boundary specifications

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Abstract
This paper analyses the sensitivity of wintertime precipitation and soil hydrology over the western United States to coastal ocean sea surface temperature (SST), inland soil moisture and coastal topography representation, as simulated with a coupled limited‐area meterological model‐soil hydrology model. The period simulated is 1–30 January 1979, comprising nine storm episodes over the western United States. The main source of moisture feeding precipitation over the western United States is found to be the westerly advection of water vapour from the Pacific Ocean. Evaporation from coastal ocean waters is also significant, whereas land evaporation is of secondary importance. Coastal SST anomalies of ±1.5 K affect inland precipitation only weakly. However, for an anomaly of + 3 K, precipitation increases significantly. Because of the precipitation shadowing effect of the Sierra Nevada and Coastal ranges the precipitation enhancement mostly occurs over California and the Pacific Northwest. Regional precipitation averages increase by 10–20% there, with local effects being larger over coastal areas. Farther inland the effect is of the order of 5–7%. Precipitation is only minimally affected by variations in soil wetness and land evaporation (effects not exceeding 5%), but the efficiency of other components of the hydrologie budget, such as water infiltration to deep soil and surface runoff, strongly increases with soil water content. Problems related to the initialization of soil moisture for simulations of soil hydrology on scales of one to several weeks are examined. Finally, the precipitation simulation is sensitive to adjustments in the description of the complex western United States coastal topography, but effects are generally local and do not propagate far over the continental regions.