SENSITIVITY OF LATENT-HEAT FLUX FROM PILPS LAND-SURFACE SCHEMES TO PERTURBATIONS OF SURFACE AIR-TEMPERATURE

In the PILPS Phase 2a experiment, 23 land-surface schemes were compare d in an off-line control experiment using observed meteorological data from Cabauw, the Netherlands. Two simple sensitivity experiments were also undertaken in which the observed surface air temperature was art ificially increased or decreased by 2 K while all other factors remain ed as observed. On the annual timescale, all schemes show similar resp onses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatur es and latent heat fluxes all increase while net radiation, sensible h eat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes an d, especially, the changes in the latent heat fluxes are nor linearly related to the change of temperature. Theoretically, the nonlinear rel ationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturat ion vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonl inear relationship is shown in the form that the increase of the laten t heat flux for a 2-K temperature increase is larger than its decrease for a 2-K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the +2-K experiment is smaller than the decrease of the latent hea t flux in the -2-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theo retical relationship and the Cabauw sensitivity experiments results (o r the asymmetry) is due to (i) the involvement of the beta(g), formula tion, which is a function of a series stress factors that limited the evaporation and whose values change in the +/-2-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of t he drag coefficient induced by the changes in stratification due to th e imposed air temperature changes (+/-2 K) in parameterizations of lat ent heat flux common in current land-surface schemes. Among all stress factors involved in the beta(g), formulation, the soil moisture stres s in the +2-K experiment induced by the increased evaporation is the m ain factor that contributes to the asymmetry.