A Cloud‐Free Approach to Modeling Daily Downwelling Longwave Irradiance at Global Scale.

Parametric modeling of downwelling longwave irradiance under all‐sky conditions (LW↓) typically involves "correcting" a clear‐ (or non‐overcast) sky model estimate using solar‐irradiance‐based proxies of cloud cover in lieu of actual cloud cover given uncertainties and measurement challenges of the latter. While such approaches are deemed sound, their application in time and space is inherently limited. We report on a correction model free of solar irradiance‐derived cloud proxies that is applicable at the true daily (24 hr) and global scales. The new "cloud‐free" correction model demonstrates superior performance in a range of environments relative to existing cloud‐free modeling approaches and to corrections based on solar‐derived cloudiness proxies. Literature‐based performance benchmarking indicates a performance that is often comparable to—and in some cases superior to—performances yielded by conventional parametric modeling approaches employing locally or regionally calibrated parameters, as well as to performances of satellite‐based algorithms. Plain Language Summary: Downwelling longwave irradiance (LW↓) is a major component of Earth's surface energy budget and is often modeled indirectly using routinely measured surface meteorological variables. In overcast (or "all‐sky") conditions, LW↓ is typically estimated by correcting an estimate valid for non‐overcast (or "clear‐sky") conditions using information about cloud cover or cloudiness proxies derived from solar irradiance variables. Cloud cover information is often unavailable and uncertain, and proxies based on solar irradiance variables inherently limit the correction to the daytime. Here, we present a correction model that circumvents the dependence on cloud variables or their proxies and document its performance when applied under a variety of conditions. Key Points: A model free of cloud variables or their solar‐based proxies is proposed to correct clear‐sky longwave irradiances under all‐sky conditionsModel performance is weakly sensitive to the underlying clear‐sky model when jointly parameterizedThe model yields daily errors of 3.9%–4.5% over ocean and 5.5%–7% over land, depending on the error metric and underlying clear‐sky model [ABSTRACT FROM AUTHOR]