REPRESENTATION OF HETEROGENEITY EFFECTS IN EARTH SYSTEM MODELING - EXPERIENCE FROM LAND-SURFACE MODELING

The land surface is characterized by pronounced spatial heterogeneity that spans a wide range of scales. This heterogeneity affects the surf ace energy and water budgets, as well as the land-atmosphere exchanges of momentum, heat, water and other constituents, through a number of highly nonlinear processes. The resolution of present-day Earth (or cl imate) system models is still too coarse to explicitly capture the eff ects of surface heterogeneity, which therefore needs to be parameteriz ed within the framework of complex and nonlinear land surface process schemes. The effects of surface heterogeneity are here grouped in two categories, which we define as ''aggregation'' and ''dynamical'' effec ts. Models of aggregation effects attempt to calculate the contributio n of different subgrid scale surface types to the grid box average ene rgy and water budgets and surface-atmosphere exchanges. Such models ha ve been based on discrete approaches, whereby heterogeneity is describ ed in terms of a finite number of subgrid ''tiles'' or ''patches,'' an d on continuous approaches, in which heterogeneity is described in ter ms of probability density functions. Subgrid scale aggregation has bee n shown to especially affect the surface latent and sensible heat flux es, the simulation of snow, and the dynamics of soil moisture and runo ff. Dynamical heterogeneity effects are associated with microscale and mesoscale circulations induced by heterogeneous surfaces. These circu lations can influence boundary layer structure, cloud formation, preci pitation, and vertical transfer of momentum, energy, and water up to t he midtroposphere. In the last decade or so, the importance of land su rface heterogeneity representation has been increasingly recognized in a large number of new studies. This paper reviews and critically disc usses different approaches that have been proposed to represent aggreg ation and dynamical effects of surface heterogeneity and their incorpo ration in land surface process schemes. Some of the methodologies disc ussed in this paper are of general nature and therefore can be of inte rest for problems of subgrid scale process description in other geophy sical disciplines.