2002 Annual

Tuesday, 15 January 2002
Soil moisture-rainfall feedback on the North American Monsoon system
Jianjun Xu, New Mexico Institute of Mining & Technology, Socorro, NM; and E. E. Small
A positive soil moisture-rainfall feedback exists when above-normal soil moisture increases the likelihood of future precipitation, and vice versa for dry soil. Observations and modeling studies suggest that this feedback may be important in magnifying and prolonging hydroclimatic anomalies in a variety of regions. Our preliminary modeling experiments show that the soil moisture-rainfall feedback is strong in the North American Monsoon System (NAMS) region and may contribute to variability of summertime precipitation in this area. However, this result is based on sensitivity experiments using extreme forcing - soil moisture was held at field capacity or wilting point throughout season long simulations. Here we use the MM5 model linked to the OSU land surface scheme to assess the strength of soil moisture-rainfall feedbacks in the NAMS region that result from realistic soil moisture forcing. Simulations are driven by NCEP reanalysis. The horizontal resolution of the finest grid is 30 km. Ensemble experiments begin on June 1 and end on October 1. First, we use the coupled MM5-OSU model to simulate NAMS climate and soil moisture in wet (1999) and dry (2000) monsoon seasons. Second, we repeat these two experiments but constrain the soil moisture field so that it approximates the mean state. This is accomplished by scaling the simulated precipitation at each point so that it is equal to mean observed precipitation at that location. This modification preserves the temporal variability of soil moisture that is characteristic of the NAMS region. We compare the atmosphere and land surface state in the control and sensitivity experiments. This isolates the effects of soil moisture anomalies on NAMS climate in both a dry and wet year. We check that the simulated soil moisture and surface energy balance state in the control and sensitivity simulations is reasonable via comparisons to both field observations and remotely sensed data.

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