Global gray-sky dT (Version 2): inverse of the surface psychrometric constant parameter in the SSEBop evapotranspiration model

The surface psychrometric constant (spc) is a key model parameter in actual evapotranspiration (ET) modeling using the Operational Simplified Surface Energy Balance (SSEBop) model for establishing model boundary limits for the dry/bare and wet/vegetated surface conditions. The inverse of the constant (1/spc) represents the temperature difference (dT) between the bare/dry surface and the air temperature at the canopy level. The main output of the SSEBop model is an ET fraction (0-1) and, when combined with reference (“maximum”) ET, it produces an actual ET estimate from satellite-observed land surface temperature. The dT is determined using net radiation inputs under gray-sky radiations from the ERA-5 datasets, i.e., Surface Solar Radiation Downwards instead of clear-sky assumptions used for the previous dT versions. The dT estimates account for elevation effects using established equations for air density variations with altitude. In the updated dT version 2.0, adjustments are made to algin the seasonality of dT with that of air temperature because of the phase shift between net radiation and surface temperature, especially in regions with strong seasonality, i.e., winter to spring transitions.