On the Role of Topography in the Description of Surface Atmosphere Exchange

Earth-Atmosphere interaction is generally understood to take place at the surface and to be determined by the abundance of energy, matter or momentum on the one hand, and by the efficiency of turbulent exchange on the other hand. Thus the characteristics of the atmospheric boundary layer (exchange efficiency) play a crucial role for the net exchange between the surface and the free troposphere. Processes related to this turbulent transport are usually parameterized in numerical models of all scales (except perhaps for very high-resolution Large Eddy Simulations). For sensible heat and trace gases such as water vapour, parameterizations employed are based on our knowledge on boundary layer processes from flat and horizontally homogeneous terrain even if the model is applied over topography. For momentum, however, it has long been recognized that these turbulence parameterizations are not even sufficient to reproduce mean flow magnitudes if relevant topography is present (nothing to say about local flow patterns) and therefore additionally, gravity wave drag parameterizations have been introduced in order to take into consideration the additional drag due to the effect of subgrid-scale topography.

In this contribution we hypothesize that also for the exchange of heat and mass over topographically influenced regions subgrid scale processes such as local circulations or geometrical effects may play an important role in altering the pure surface exchange due to turbulent transport. Some results from combined observational/ numerical studies will be reviewed in support of this hypothesis and possible consequences, e.g. for carbon budgeting in complex terrain will be discussed.