The clear‐sky downwelling long‐wave radiation at the surface in current and future climates

Abstract

Abstract Downwelling long‐wave radiation is a crucial component of the energy balance of the land and ocean surface. Here we develop a semi‐analytic model for the downwelling long‐wave dependent on five governing parameters: the near‐surface air temperature, the near‐surface specific humidity, the surface air pressure, the e‐folding height‐scale of water vapour, and the CO concentration. The model predicts the hourly clear‐sky long‐wave in the ERA5 reanalysis product with a global mean error of 8.2 Wm, and on average captures 97% of the temporal variation at individual locations. We show that the model may be used to calculate clear‐sky downwelling long‐wave from only surface observations of temperature and humidity by using the time‐mean water vapour height‐scale from the ERA5, interpolated to the location of the observation. Using this method replicates sub‐hourly observations from individual sites having a range of climates with errors of 12–25 Wm. Furthermore, the inclusion of CO allows the model to be used to study changes in downwelling long‐wave at the surface as CO concentrations vary. We validate the model's representation of CO by comparison with five CMIP5 climate models. Our model thus provides a simple yet accurate framework to understand the key parameters controlling downwelling long‐wave and its variability in the current and future climates.

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