Development of a Land Surface Model. Part II: Data Assimilation

Abstract

Part I described a land surface model, its implementation in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5), and some model evaluation results. Part II describes the indirect soil moisture data assimilation scheme. As described in Part I, the land surface model includes explicit soil moisture, which is based on the Interactions between Soil, Biosphere, and Atmosphere (ISBA) model, and three pathways for evaporation: soil evaporation, evaporation from the wet canopy, and vegetative transpiration. The data assimilation scheme presented here also follows similar work on data assimilation for ISBA and uses model biases of the 2-m air temperature and humidity against observed analyses to nudge soil moisture. An important difference from the ISBA schemes is that the nudging strengths are computed from model parameters such as solar radiation, temperature, leaf area, vegetation coverage, and aerodynamic resistance rather than from statistically derived functions. The rationale is that nudging soil moisture according to model biases in air temperature and humidity should depend on the degree of coupling across the landatmosphere interface. Thus, nudging strengths are designed to reflect the potential for the surface and root-zone soil moisture to affect near-surface air temperature and humidity. Model test cases are used to examine relationships between the nudging strengths and modeled physical parameters and then to demonstrate the effects of the nudging scheme on model results.