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Differences in shortwave radiative effects driven from different treatments of the transition zone

Babak Jahani, Ph.D. student of the Environmental Physics research group, Dr. Josep Calbó and Dr. Josep‐Abel González have published the paper entitled “Transition Zone Radiative Effects in Shortwave Radiation Parameterizations: Case of Weather Research and Forecasting Model” in the Journal of Geophysical Research Atmospheres. This study was conducted within the “Clouds, Radiation to the Atmosphere and Climate” research line of the group.

Babak Jahani, PhD student of the Environmental Physics research group, Dr. Josep Calbó and Dr. Josep‐Abel González have published the paper entitled Transition Zone Radiative Effects in Shortwave Radiation Parameterizations: Case of Weather Research and Forecasting Model in the Journal of Geophysical Research Atmospheres.

In this paper, the authors explain that a number of studies have stated that the shift from a cloud‐free to cloudy atmosphere (and vice versa) contains an additional phase, named “Transition (or twilight) Zone”. However, the information available about radiative effects of this phase is very limited. Consequently, in most meteorological and climate studies, the area corresponding to the transition zone is considered as an area containing aerosol or optically thin clouds. This study investigates the differences in shortwave radiative effects driven from different treatments of the transition zone. To this aim, three of the shortwave radiation parameterizations (NewGoddard, Rapid Radiative Transfer Model for Global circulation models, and Fu‐Liou‐Gu) included in the Advanced Research Weather Research and Forecasting Model (WRF‐ARW) were isolated and adapted for one‐dimensional vertical simulations. These parameterizations were then utilized to perform simulations under ideal “cloud” and “aerosol” modes, for different values of (i) cloud optical depths resulting from different sizes of ice crystals or liquid droplets and mixing ratios; and (ii) different aerosol optical depths combined with various aerosol types. The resulting shortwave broadband total, direct, and diffuse irradiances at the Earth surface were analyzed. The uncertainties originated from different assumptions of a situation regarding to the transition zone are quite substantial for all the parameterizations. For all the parameterizations, direct and total irradiances are the least and most sensitive irradiances to different treatments of the transition zone, respectively. Differences in the radiative effects of transition zone dominantly result from the difference between the radiative effects of clouds and aerosols (different types), not from cloud type or droplet/crystal size.

This research has been developed within the “Clouds, Radiation to the Atmosphere and Climate” research line of the Environmental Physics research group.

Babak Jahani is beneficiary of the FI‐ AGAUR PhD grant (2018FI_B_00830) provided by the Generalitat de Catalunya (Secretaria d'Universitats i Recerca) and the European Union.

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