An Approach for the Representation of Surface Heterogeneity in Land Surface Models. Part I: Theoretical Framework

Abstract

In this paper, a theoretical framework is described for the representation of surface heterogeneity within complex biophysical surface schemes for use in climate models. The methodology adopts aspects of the mosaic approach and the statistical?dynamical approach. A grid cell is subdivided into fractional areas covered by basic surface types, that is, vegetated, bare soil, snow-covered, and impermeable surfaces, which separately exchange momentum, energy, and water vapor with the overlying atmosphere. Fractional precipitation areas within a grid box, and fractional rainfall and snowfall areas within the precipitation area, can also be specified. Within each surface type, heterogeneity is described by assuming that surface temperatures and soil water content follow continuous analytical probability density functions (PDFs) and by integrating relevant nonlinear terms over the appropriate PDF. Linear and symmetric PDFs are chosen since they allow ready analytical partial and full integration. This heterogeneity representation is implemented within the framework of a surface package, including a multilayer soil model, a one-layer vegetation model, a multilayer snow model, a two-layer impermeable surface model, and a surface hydrology model. The companion paper by Giorgi describes an extensive set of experiments carried out to validate the model and assess its sensitivity to relevant parameters.