SEQUENTIAL ASSIMILATION OF SOIL-MOISTURE FROM ATMOSPHERIC LOW-LEVEL PARAMETERS .1. SENSITIVITY AND CALIBRATION STUDIES

This paper and its companion report on the development of a sequential assimilation technique based upon optimum interpolation in order to i nitialize soil moisture in atmospheric models. A previous study by Mah fouf has demonstrated that it is possible to estimate soil moisture fr om the evolution of atmospheric temperature and relative humidity near the surface. The main purpose of this paper is to examine more precis ely the dependence of atmospheric low-level parameters upon soil moist ure and how this dependence is affected by various factors (soil chara cteristics, vegetation type, low-level wind). The sensitivity of atmos pheric parameters to soil moisture is expressed as the statistical qua ntities of the optimal interpolation. The importance of observation er rors, which define the relevance of the atmospheric parameters for the assimilation procedure, is also investigated. An analytical formulati on of the optimal interpolation coefficients is proposed. Finally, the usefulness and limitations of this work for soil moisture analysis in three-dimensional models are discussed.