Assessment of the Priestley-Taylor parameter value from ERA-interim global reanalysis data

The Priestley-Taylor parameter α of wet surface evaporation is investigated using daily, 0.75-degree ERA-Interim reanalysis data for the winter hemispheres of 2000-2001, 2006-2007, and 2012-2013. Published ERA-Interim sensible and latent heat fluxes over sea and land yield two distinct best-fit curves for α as a function of air temperature (Ta). When the wet land surface temperature (Tws) was estimated by an independent method, the two curves largely collapsed. Tropical land areas with low wind formed a subgroup of α distribution, yielding the lowest overall values of 1.06±0.03. The results for sea corroborate the widely accepted α value of 1.26±0.06 when Ta > 20 °C. At 0 °C the mean α value over sea is about 1.62±0.23. The α values for wet land surfaces with the independent Tws estimates display about the same mean but with larger variations. Published values of α scatter around the overlapping α vs Ta curves.