Effect of solar zenith angle specification in models on mean shortwave fluxes and stratospheric temperatures

Many weather and climate models call their radiation schemes only every 3 h, which we show can lead to a stratospheric temperature overestimate of 3–5 K and wavenumber 8 fluctuations in top‐of‐atmosphere (TOA) net shortwave flux around the tropics of amplitude 1.6 W m −2 . Solving this problem while retaining a 3h radiation time step requires careful treatment of the cosine of the solar zenith angle, μ 0 , which appears twice in the calculation of shortwave fluxes, scaling the following: (1) TOA incident flux and (2) the path length of the direct solar beam through the atmosphere. If μ 0 is calculated as the average over the radiation time step, rather than at the central time, then the fluctuations are removed, but the stratosphere is still too warm by 2–3 K. It is only if the second μ 0 is averaged only over the sunlit part of the radiation time step that the temperature bias is removed.