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

Effect of solar zenith angle specification on mean shortwave fluxes and stratospheric temperatures
Technical memorandum
Date Published
Secondary Title
ECMWF Technical Memoranda
Robin Hogan,
S. Hirahara,
AbstractA number of operational IFS configurations call the radiation scheme only every 3 h, and the cosine of the solar zenith angle (mu0) used in the radiation scheme is computed at the central time of this "radiation timestep". Comparing to model runs in which the radiation scheme is called every model timestep (treated as truth), it is found that calling radiation every 3 h leads to wavenumber-8 fluctuations around the tropics in annual-mean surface and top-of-atmosphere net shortwave fluxes of amplitude 1.6 and 0.9 W m-2, respectively. The total shortwave absorption by the atmosphere is overestimated by around 1.4 W m-2 in the tropics, which is associated with tropical stratospheric temperatures too warm by up to 3.4 K. Calculating mu0 used by the radiation scheme instead as the average value for the radiation timestep removes the wavenumber-8 fluctuations, but the stratosphere is still too warm by up to 2.3 K. This is found to be due to excessive solar absorption at dawn and dusk. By instead computing mu0 used by the radiation scheme as the average of only the sunlit part of the radiation timestep, stratospheric biases are reduced to only of order 0.5 K (compared to running the radiation scheme every timestep). Higher frequency fluctuations are also found in the incoming solar radiation at top-of-atmosphere, and removed by the new scheme, but it is found that both the wavelength and amplitude of these fluctuations decrease rapidly with shorter model timesteps and are negligible for the high-resolution model configurations used at ECMWF.