J3.8
Contrasting land surface processes over basins of the Americas using Eta model forecasts
Ernesto Hugo Berbery, University of Maryland, College Park, MD; and Y. Luo, K. Mitchell, and A. K. Betts
NCEP’s Eta model forecasts are employed to discuss and contrast the soil moisture - precipitation feedbacks at monthly to seasonal time scales over selected river basins of the Americas. The Eta model is NCEP’s operational mesoscale model, and therefore has been subject to changes and upgrades along the years. Here we employ a 4-yr period (June 1998 - May 2002) after substantial upgrades to the land surface component were performed. In addition, the July 2000 version of the model has been employed at the University of Maryland to perform runs over South America. The Eta products are employed here to address the same type of feedbacks over the South America. Feedbacks are assessed from the degree of correlation between soil moisture and other variables (e.g., latent heat, sensible heat, evaporative fraction or Bowen ratio, lifting condensation level, precipitation) to develop the sequence that can explain the interactions among variables. In general, results can be summarized as follows: First, increased soil moisture is associated with an increase of the net longwave that is about 13% larger (in magnitude) than the reduction in net shortwave (due to increased cloud cover), resulting in a slight increase of net radiation at the surface. Second, latent heat also increases with soil moisture while sensible heat decreases, resulting in an almost linear increase of the evaporative fraction. Increased soil moisture is also associated with a lower lifting condensation level and an increase of observed precipitation. The overall results support the concept of a positive feedback in which increased soil moisture affects surface fluxes in such a manner that increased precipitation results. However, important basin-to-basin differences in the interactions are observed. Toward the eastern Mississippi, there are no well-defined land surface-atmosphere interactions: soil moisture is plentiful (compared to net radiation demand for evaporation), therefore its anomalies are not well correlated to monthly or seasonal precipitation anomalies, because the dry soil moisture anomalies are not usually large enough to notably reduce the surface evaporation. In consequence, the lifting condensation level (LCL), and convective precipitation are minimally influenced. Other effects, like the advection of moisture, may be more relevant for precipitation processes. The Columbia River basin has large soil moisture as a result of the orographically forced precipitation. Despite the significant advective processes, the soil moisture has strong links with most surface variables, while the much drier Colorado basin instead has weak links probably due to the large evaporation of precipitation before reaching the surface. A similar behavior is found over the core monsoon region. Finally, the South American monsoon region also reveals positive feedbacks, with the sensible heat being the stronger influence on the Bowen ratio and on precipitation. It is assumed that, as in the case of the eastern Mississippi basin, the values of soil moisture are high enough so that changes will not affect significantly the evaporation.
Joint Session 3, Land-Atmosphere Interactions (Joint with the 15th Symp. on Global Change and Climate Variations and 18th Conf on Hydrology; Room 609/610)
Tuesday, 13 January 2004, 8:30 AM-12:15 PM, Room 609/610
Previous paper Next paper