85th AMS Annual Meeting

Wednesday, 12 January 2005
AGCM Biases in Evaporation Regime: Impact on Soil Moisture Memory and Land-Atmosphere Feedback
Sarith P. P. Mahanama, GEST and NASA/GSFC, Greenbelt, MD; and R. D. Koster
The "evaporation regime" of a land region is largely determined by the local relative availability of water (via precipitation) and energy (via net radiation). When energy is relatively plentiful, evaporation is controlled by water supply and varies strongly with soil moisture, whereas when water is relatively plentiful, evaporation is controlled more by atmospheric demand and is largely insensitive to soil moisture. The evaporative regime in turn has a strong impact on two critical aspects of the land surface's role in seasonal precipitation prediction: the timescale of initialized soil moisture anomalies and the degree to which precipitation can respond to these anomalies.

Precipitation and net radiation in an atmospheric general circulation model (AGCM) are typically biased relative to observations. As a result, the simulated evaporative regime of a region may be biased, with consequent negative effects on the AGCM's ability to translate initialized soil moisture anomalies into improved seasonal predictions. These potential problems are investigated through extensive offline analyses with the Mosaic land surface model (LSM). We first forced the LSM globally with a 0.5-degree, 6-hourly, 15-yr (1979-93) "observational" dataset, a dataset consisting of ECMWF reanalysis data corrected with observation-based precipitation and radiation. We then repeated the simulation after imposing a representative set of GCM climate biases onto the forcings - the observational forcings were scaled so that their mean seasonal cycles matched those simulated by the NSIPP-1 (NASA Global Modeling and Assimilation Office) AGCM over the same period (1979-93). Evaporative regime in both the "observational" and "GCM-biased" simulations is characterized, as in previous studies, by the slope of the relationship between evaporation efficiency (evaporation divided by net radiation) and soil moisture content.

The AGCM's climate biases do indeed lead to significant biases in evaporative regime in certain regions, with the expected impacts on soil moisture memory timescales. Furthermore, the offline simulations suggest that the biased forcing in the AGCM should lead to overestimates of feedback in certain parts of North America -- parts already identified in previous studies as having excessive feedback. The present study thus supports the notion that the reduction of climate biases in the AGCM will lead to an improved ability to translate soil moisture initialization into increased seasonal prediction skill.

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