EMS Annual Meeting Abstracts
Vol. 18, EMS2021-401, 2021
https://doi.org/10.5194/ems2021-401
EMS Annual Meeting 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Cloud optical property benchmarking study

Kristian Pagh Nielsen
Kristian Pagh Nielsen
  • Danish Meteorological Institute, Copenhagen Ø, Denmark (kpn@dmi.dk)

A benchmarking study to compare various cloud optical property schemes is proposed. The transmittance, reflectance and absorptance of clouds of cloud optical property schemes that are widely used in weather and climate have in recent years been shown to give different results. Not much attention has been paid to this, probably due to the fact that errors in cloud cover have larger impacts on the shortwave and longwave radiative fluxes than the cloud optical properties. Here the optical properties are the mass extinction coefficient, the single scattering albedo and the asymmetry factor. Cloud optical property schemes are typically based on detailed theoretical calculations, which are then parametrized. Furthermore, spectral band averaging is performed to get optical properties that match the spectral bands of the radiative transfer scheme. Here the 14 shortwave and 16 longwave spectral bands of the RRTM scheme are mostly used presently. Both the equations used in the parametrizations and the spectral averaging matter. For the spectral averaging it must be considered that both the shortwave (solar) and longwave (thermal) spectra change following the circumstances in the atmospheric environment. Thus, a dry atmosphere has different spectral characteristics from a moist atmosphere. In the ultraviolet part of the spectrum ozone is also important to account for, as is the impact of carbon dioxide, methane, nitrous oxide and lesser greenhose gasses in the longwave part of the spectrum. This also affects the spectral distribution of the available radiative flux within individual spectral bands. Thus, the impact of the atmospheric environment must also be accounted for in the benchmarking study. The setup and an online framework for sharing benchmarking results will be presented.

How to cite: Nielsen, K. P.: Cloud optical property benchmarking study, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-401, https://doi.org/10.5194/ems2021-401, 2021.

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