Modeling the Vegetation Atmospheric Exchange with a Transilient Model

Abstract

The interaction between the atmosphere and the soil-vegetation system in ecological models is often modeled with the Penman-Monteith (PM) approach (Monteith 1980) which is based on the energy balance equation. This method describes the interaction with the surface by a resistance approach and relies on the constant flux layer approach which is equivalent to the classical K approach (see, e.g., Stull 1988; Kramm 1995). The PM method is mainly forced by radiation and includes some simplification which does not generally allow the use of short averaging periods in the order of one hour. The constant flux layer assumption is not valid to describe the exchange processes for a forest canopy (Shaw et al. 1974; Raupach et al. 1991; Kaimal and Finnigan 1994), with typical effects like counter gradients (Denmead and Bradley 1985), coherent structures (Amiro 1990) or mixing layers (Raupach et al. 1996). The big leaf approach or the PM method cannot describe the effects of physical processes behind such features. Models using more suitable descriptions of energy transfer, thermodynamics and turbulence (e.g. higher order closure formulations, see Kurata (1982); Meyers and PawU 1986,1987) yield results that better agree with measured data from the field.