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 in- dices to quantify the coupling 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 , as well as other measures derived from model out- put were used. The aim of the study was to show that the quite complex coupling and feedback mechanisms can be described with these relatively simple measures. Model re- sults illustrate that the vegetation and the atmosphere are well coupled (expressed by a lower ) under conditions of a tall and dense canopy, as well as under strong dynamic forc- ing. This better aerodynamic coupling leads to an increase in evapotranspiration, as well as an increase in the evapora- tive fraction. This fact was also shown by the second cou- pling measure: the relative changes in daily model evapo- transpiration. This measure was inspired by the assumption that 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 sensi- tivity measure was used according to Jacobs and de Bruin (1992). It shows that the sensitivity of evaporative fraction to stomata resistance is much higher with a better aerodynamic coupling. The results of the factorare in a good agree- ment with the findings of Jacobs and de Bruin: they stress that it is a valuable strategy to group vegetation into two sim- ple categories (smooth and rough) for the understanding of vegetation-atmosphere coupling.