Lagrangian and K‐theory approaches in modelling evaporation from sparse canopies

Abstract

AbstractAn evaporation model based on Lagrangian turbulent diffusion principles is developed and compared with simpler single‐ and dual‐source K‐theory models of evaporation. The performance of the Lagrangian model is assessed against observations of total evaporation by an eddy‐correlation instrument and found to be satisfactory for a millet crop in west Africa. Three versions of existing simpler models containing K‐theory descriptions of within‐canopy turbulence are described and their results also compared. The two K‐theory models that explicitly take into acccount the soil source perform better than the single‐source Penman‐Monteith model. A Lagrangian analysis does not seem to be necessary for this kind of crop because the low source‐density profile of the crop, associated with a low leaf‐area index, caused the near‐field effect to be very small. The overall difference between the evaporation estimates of the dual‐source and Lagrangian models is therefore small. It is concluded that, for practical purposes, K‐theory remains an adequate approximation of turbulent transport in sparse‐crop evaporation models.Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.