Katherine Mitchell

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 compared 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 artificially increased or decreased by 2 K while all other factors remained as observed. On the annual timescale, all schemes show similar responses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatures and latent heat fluxes all increase while net radiation, sensible heat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes and, especially, the changes in the latent heat fluxes are not linearly related to the change of temperature. Theoretically, the nonlinear relationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturation vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonlinear relationship is shown in the form that the increase of the latent heat flux for a 2-K temperature increase is larger than its decrease for a 2K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the 12-K experiment is smaller than the decrease of the latent heat flux in the 22-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theoretical relationship and the Cabauw sensitivity experiments results (or the asymmetry) is due to (i) the involvement of the bg formulation, which is a function of a series stress factors that limited the evaporation and whose values change in the 62-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of the drag coefficient induced by the changes in stratification due to the imposed air temperature changes (62 K) in parameterizations of latent heat flux common in current land-surface schemes. Among all stress factors involved in the bg formulation, the soil moisture stress in the 12-K experiment induced by the increased evaporation is the main factor that contributes to the asymmetry.