Impact of Soil Moisture on Regional Spectral Model Simulations for South America


A two-year (March 1997 - March 1999) regional simulation using the regional spectral model (RSM) with a 50-km grid covering South America, bounded and initialized by the NCEP/NCAR 28 vertical levels T62 spectral resolution reanalyses is described.

Initial experiments showed that the RSM had a severe dry bias in the soil moisture that adversely impacted the precipitation. The dry bias was presumably a response to the positive feedback between imperfect model parameterization schemes and the soil moisture module in the model. This dry bias was corrected in two additional experiments by modifying the deep soil moisture in the model during the integration. The first experiment prescribed the daily soil moisture in the deep second layer from the driving reanalysis; the second experiment utilized observed precipitation, to interactively correct the deep soil moisture during the integration. Both experiments had less bias than the reanalysis precipitation. However, regional precipitation temporal variations were only as good as, if not marginally better than, the reanalysis precipitation when validated against observed 0.5-degree gridded data. The experiment with interactively corrected soil moisture provided the best simulation and had better anomaly threat scores at all precipitation intensities than the one with the specified soil moisture values from the reanalysis. Until current coupled land-atmosphere regional model soil moisture simulation biases can be substantially reduced, they can at least be corrected by using observed precipitation.

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