Radiation, surface temperature and evaporation over wet surfaces

Predicting evaporation from wet surfaces (water, wet soil and canopy surfaces) has long been of major interest in hydrological, meteorological and agricultural communities. In practical applications of the existing models/theories of wet surface evaporation (e.g. the Priestley–Taylor model), net radiation (R_n) and/or surface temperature (T_s; or near-surface air temperature) are considered to be independent external forcings that determine the evaporation rate. However, neither R_n nor T_s are independent of evaporation, since R_n directly depends on T_s via the outgoing long-wave radiation. In this study, we use monthly data for the global ocean to investigate the relation between radiation, evaporation and surface temperature. We use a new theoretical formulation to show that as T_s increases, a greater fraction of R_n is partitioned to evaporation (i.e. higher evaporative fraction) but R_n declines because of an increase in outgoing long-wave radiation. The consequence is that a maximum evaporation rate emerges naturally from that trade-off. We find that this maximum corresponds to the actual evaporation over global ocean surfaces at both local and global scales. In addition, the maximum in evaporation defines a T_s that corresponds to independent estimates of sea-surface temperature. These results suggest that the concept of maximum evaporation reported here is a natural attribute of an extensive wet evaporating surface.