Wednesday, 17 January 2007
The Role of Precipitation in the Coupling of the Land Surface with the Atmosphere
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Although a strong coupling between the land surface and the atmosphere is often assumed, the strength of that linkage can vary significantly over time with changing ambient conditions. Using data collected in the Southern Great Plains of the United States as a part of the International H2O Project 2002, this study investigated the coupling between the land-surface and the atmosphere for five land cover types: bare soil, sand-sage shrubland, heavily grazed pasture, winter wheat cropland, and grassland. Specifically, the Decoupling Coefficient (W) of Jarvis and McNaughton was used as an index to estimate the degree of coupling between the land surface and the atmosphere. While all five sites were moderately coupled to the atmosphere (W @ 0.4), the strength of that coupling varied significantly with location and time. For example, it was found that the more sparsely vegetated surfaces typically exhibited a higher degree of coupling than the more heavily vegetated sites. It was also found that precipitation events were a key factor in controlling the degree of coupling. Following a rainfall event, the surface would become strongly decoupled (W approaching 1) then slowly relax to moderately coupled baseline state. It is suggested that this relationship is due to the influence of the near-surface soil moisture content; as the near-surface soil moisture soil moisture content increases, the degree of coupling decreases because atmospheric conditions, such as wind speed and humidity, exert a weaker influence than the available energy on the exchange of energy and moisture between the land-surface and the atmosphere.