Quantifying the coupling degree between land surface and the atmospheric boundary layer with the coupled vegetation-atmosphere model HIRVAC

In the present study, the ability of different indices to quantify the coup ling degree between a vegetated surface and the atmospheric boundary layer is tested. For this purpose, a one-and-a-half dimensional atmospheric bound ary layer model, including a high resolved vegetation canopy, was applied ( HIRVAC) and indices, such as the decoupling factor Omega. as well as other measures derived from model output were used. The aim of the study was to s how that the quite complex coupling and feedback mechanisms can be describe d with these relatively simple measures. Model results illustrate that the vegetation and the atmosphere are well coupled (expressed by a lower Omega) under conditions of a tall and dense canopy, as well as under strong dynam ic forcing. This better aerodynamic coupling leads to an increase in evapot ranspiration, as well as an increase in the evaporative fraction. This fact was also shown by the second coupling measure: the relative changes in dai ly model evapotranspiration. This measure was inspired by the assumption th at these changes are primarily dependent on the coupling degree between the surface and the atmosphere, if the other boundary conditions in the model are fixed. A third sensitivity measure was used according to Jacobs and de Bruin (1992). It shows that the sensitivity of evaporative fraction to stom ata resistance is much higher with a better aerodynamic coupling. The resul ts of the factor Omega are in a good agreement with the findings of Jacobs and de Bruin: they stress that it is a valuable strategy to group vegetatio n into two simple categories (smooth and rough) for the understanding of ve getation-atmosphere coupling.