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

• Published in: Geophysical research letters 2016-01, Vol.43 (1), p.482-488
• Main Authors: Hogan, Robin J., Hirahara, Shoji
• Format: Article
• Language: English
• Subjects: Amplitude; Atmosphere; Atmospheric models; Beams (radiation); Bias; Climate; Climate models; Climatology; Fluctuation; Fluctuations; Fluxes; Geophysics; Length; Mathematical models; Meteorology; Radiation; radiation scheme; Scaling; solar zenith angle; Specifications; Stratosphere; stratospheric temperature; Stratospheric temperatures; Temperature; Temperature effects; Trigonometric functions; Tropical environments; Wavelengths; Wavenumber; Weather; Zenith
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2015GL066868

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, μ0, 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 μ0 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 μ0 is averaged only over the sunlit part of the radiation time step that the temperature bias is removed. Key Points Infrequent radiation calls in models can lead to stratospheric temperature errors of 3–5 K They also cause erroneous patterns in time mean solar fluxes around a latitude circle Fix by averaging cosine of solar zenith angle over the daylit part of the radiation time step