Global gray-sky dT: the 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 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, produces an actual ET estimate from satellite-observed land surface temperature. This 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 are adjusted for elevation effects using established equations.