Transition Zone Radiative Effects in Shortwave Radiation Parameterizations: Case of Weather Research and Forecasting Model

Abstract

A number of studies have stated that the shift from a cloud‐free to cloudy atmosphere (and viceversa) contains an additional phase, named'Transition (or twilight) Zone'. However, the informationavailable about radiative effects of this phase is very limited. Consequently, in most meteorological andclimate studies, the area corresponding to the transition zone is considered as an area containing aerosol oroptically thin clouds. This study investigates the differences in shortwave radiative effects driven fromdifferent 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 inthe Advanced Research Weather Research and Forecasting Model (WRF‐ARW) were isolated and adaptedfor one‐dimensional vertical simulations. These parameterizations were then utilized to performsimulations under ideal'cloud'and'aerosol'modes, for different values of (i) cloud optical depths resultingfrom different sizes of ice crystals or liquid droplets and mixing ratios; and (ii) different aerosol optical depthscombined with various aerosol types. The resulting shortwave broadband total, direct, and diffuseirradiances at the Earth surface were analyzed. The uncertainties originated from different assumptions of asituation regarding to the transition zone are quite substantial for all the parameterizations. For all theparameterizations, direct and total irradiances are the least and most sensitive irradiances to differenttreatments of the transition zone, respectively. Differences in the radiative effects of transition zonedominantly result from the difference between the radiative effects of clouds and aerosols (different types),not from cloud type or droplet/crystal size