SECHIBA, a New Set of Parameterizations of the Hydrologic Exchanges at the Land-Atmosphere Interface within the LMD Atmospheric General Circulation Model

Abstract

A simple parameterization of the hydrologic exchanges between the soil-vegetation system and the atmosphere (SECHIBA) has been developed for use within atmospheric general circulation models (AGCM). For each grid box of the model, eight land surface types (bare soil plus seven vegetation classes) are defined, each of them covering a fractional area of the grid box and allowed to be found simultaneously. Over each of these covers the transfers are computed: evaporation from soil, transpiration from plants through a resistance defined by the concepts of stomatal resistance and architectural resistance, and interception loss from the water reservoir over the canopy. These fluxes are then averaged over the grid box to derive the total amount of water vapor that is transferred to the first atmospheric level of the AGCM. Parameterization of soil water allows for the moistening of an upper layer, of variable depth, during a rainfall event. This new scheme is quite simple and requires prescription of a restricted number of parameters: seven for each class of vegetation and four for the soil. Nevertheless, it is demonstrated that the latent heat fluxes it simulates are quite comparable to the ones simulated by the Biosphere-Atmosphere Transfer Scheme or calculated by Shuttleworth over the tropical rainforest of the Reserva Ducke (Amazon), with no tuning involved.