FEEDBACKS IN THE LAND-SURFACE AND MIXED-LAYER ENERGY BUDGETS

A mixed-layer model of the surface energy budget and the planetary bou ndary layer (PBL) is developed, based on the prognostic equations for soil temperature, mixed layer potential temperature and specific humid ity and the growth and abrupt collapse of the PBL. Detailed parameteri zations of the longwave radiative fluxes are included. The feedbacks i n the uncoupled (i.e. surface energy budget with non-responding PBL) a nd coupled land surface and atmospheric mixed-layer energy budgets are examined. A simplified, time continuous, version of the model, in whi ch the specific humidity budget is the balance of evapotranspiration a nd dry-air entrainment, and the PBL height is given by the lifted cond ensation level, is shown to be in good agreement with the complete mod el. By forcing the simplified model with daily mean rather than period ic solar radiation, an equilibrium model state is achieved where the f luxes are in close agreement with the daily mean fluxes corresponding to the periodic forcing. The model also agrees favorably with measurem ents from the FIFE field experiment. Feedbacks are examined using the equilibrium model state. The uncoupled and coupled model sensitivities with respect to the minimal stomatal resistance and the atmospheric s pecific humidity not only differ in magnitude, but in sign as well. Th is results puts into question the extent to which uncoupled land-surfa ce models that are forced with atmospheric variables may be used in se nsitivity studies.