83rd Annual

Tuesday, 11 February 2003: 9:00 AM
The sensitivity of simulated central U.S. summer precipitation and atmospheric moisture budget to both the spatial distribution and the amount of initial soil moisture
M. Georgescu, Rutgers University, New Brunswick, NJ; and C. P. Weaver, R. L. Walko, and R. Avissar
Recent regional numerical modeling work has explored the role of feedbacks with soil moisture in warm season continental precipitation. The focus has largely been on how domain-average initial soil moisture amount conditions future precipitation on month ly to seasonal timescales (with corresponding implications for precipitation predictability). While these investigations have greatly increased our understanding of the relevant feedback pathways, the role of initial soil moisture spatial distribution, r ather than simply total amount, has been given less attention.

Here we quantify the relative influences of initial soil moisture amount and initial soil moisture spatial distribution on future simulated precipitation, and the other components of the atmospheric water vapor budget, over the central U.S. A series of Regional Atmospheric Modeling System (RAMS) simulations have been made for a domain covering the U.S. Great Plains and southwest for July 1995, 1996, and 1997. Control simulations are initialized with soil moisture and temperature from the NCEP reanalysis, as well as with soil texture and soil hydraulic properties from the LDAS database, and are validated against various datasets including precipitation observations from the Arkansas-Red Basin River Forecast Center (ABRFC). 10 additional RAMS simulations for each of the three Julys investigate the relative sensitivity of simulated evaporation, precipitation, horizontal atmospheric moisture flux and flux divergence, and atmospheric storage during that month to both the initial domain-average soil moisture amount and the spatial distribution of that initial soil moisture (i.e., spatially homogeneous vs. realistic initial distributions). In addition we investigate the further sensitivity to the choice of two convective parameterizations.

We find that regional hydrometeorology is highly sensitive to both the spatial pattern and the amount of initial soil moisture, because changes in spatial variability produces changes in, and feedbacks on, large-sc ale dynamical factors such as zonal and meridional moisture transport. These three-dimensional dynamical effects interact with the one-dimensional (vertical thermodynamic) convective feedbacks. This sensitivity is greatest in relatively drier initial soil moisture regimes. In addition, the results are significantly sensitive to the choice of convective parameterization.

Improved understanding of these mechanisms should impact our ability to make more accurate predictions of hydrometeorological conditions (e.g., droughts and floods) on seasonal (and longer) timescales. In this context, a key finding of this study is that, in the domain considered, certain soil moisture anomaly patterns would be expected to have greater persistence than others. Specifically, a soil moisture anomaly pattern that looks similar to the "dry west/wet eastâ€* climatological soil moisture distribution is expected to have greater persistence than the opposite ("wet west/dry east") anomaly pattern.

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