Soil Moisture Variability and Scale-dependency of Nonlinear Parameterizations in Coupled Land±atmosphere Models

Many of the relationships used in coupled land±atmosphere models to describe interactions between the land surface and the atmosphere have been empirically parameterized and thus are inherently dependent on the observational scale for which they were derived and tested. However, they are often applied at scales quite di€erent than the ones they were intended for due to practical necessity. In this paper, a study is presented on the scale-dependency of parameterizations which are nonlinear functions of variables exhibiting considerable spatial variability across a wide range of scales. For illustration purposes, we focus on parameterizations which are explicit nonlinear functions of soil moisture. We use data from the 1997 Southern Great Plains Hydrology Experiment (SGP97) to quantify the spatial variability of soil moisture as a function of scale. By assuming that a parameterization keeps its general form the same over a range of scales, we quantify how the values of its parameters should change with scale in order to preserve the spatially averaged predicted ¯uxes at any scale of interest. The ®ndings of this study illustrate that if modi®cations are not made to nonlinear parameterizations to account for the mismatch of scales between optimization and application, then sig-ni®cant systematic biases may result in model-predicted water and energy ¯uxes.