Sensitivity of model-simulated summertime precipitation over the Mississippi River Basin to the spatial distribution of initial soil moisture

Using a numerical model, the Regional Atmospheric Modeling System (RAMS), we simulate July precipitation over parts of the Mississippi River Basin and surroundings for each of three years, 1995 1997, with six different initial soil moisture patterns: three (control, dry, and wet) with a realistic (observationally based) spatial distribution, and three (control, dry, and wet) with a horizontally homogeneous distribution. Our goal is to determine the impact on future simulated precipitation of changing the initial soil moisture spatial distribution. The spatially homogeneous initial soil moisture pattern represents, in effect, a "wet west/dry east" anomaly imposed on the realistic soil moisture pattern (that reflects the west-to-east climatological gradient . The impact of this anomaly, i.e., increasing soil moisture in the western half and decreasing it in the eastern half of the simulation domain, is most pronounced for the dry experiments and weakens nonlinearly with increasing domain-average initial soil moisture. In the dry regime, the impact is to enhance the total monthly precipitation in both the west and east. We examine the various terms in the atmospheric moisture budget to interpret these results. The changes in precipitation in the runs with a homogeneous compared to realistic initial soil moisture spatial pattern are consistent with enhanced evaporation in the western half of the model domain accompanied by enhanced west-to-east horizontal moisture transport that helps restore the initially depleted soil moisture in the east. In this manner, the zonal moisture flux acts toward re-establishing the initial climatological soil moisture pattern of the region, thus acting as a negative feedback mechanism. In addition, the soil moisture anomaly generally produces diminished meridional moisture transport into the simulation domain from the south through a decrease in the low-level meridional wind speed. This decrease in meridional flux acts in the same direction as the zonal flux change in the west, and in the opposite direction to the zonal flux change in the east. Since this change is most pronounced in the west, it therefore also contributes to the overall negative feedback of the atmospheric dynamics on the initial soil moisture. The persistence timescale of the impact of this particular soil moisture anomaly pattern on precipitation is on the order of 3 months in the dry regime. Sensitivity of the results to a change in convection scheme is also explored.