An Introduction to A New Radiative Transfer Model of BCC_RAD

Beijing Climate Center Radiative Transfer Model (BCC_RAD) is introduced here. It includes three new part: treatments to gas overlapping bands; cloud and aerosol optical property calculations and radiative transfer algorithms.

Firstly, the correlated k-distribution (CKD) method is used to treat greenhouse gas absorptions. Five kinds of band division schemes are given for different needs. New optical approaches to treat overlapping bands and choose k-interval number were proposed and applied to the BCC_RAD. Then, we developed the optical parameterization schemes for aerosols such as sulfate, nitrate, black and organic carbon, dust and sea salt to match with above gas absorption band divisions. Of which, the optical properties of non-spherical dust-like aerosol and internally mixed aerosols are given in BCC_RAD. The cloud optics is an important component in radiation models, and the previous studies generally used band-mean water cloud optical parameterization, which ignores the correlation between gas absorption and cloud optical property within band; whereas we applied the CKD method for water cloud too to consider the correlation. A new ice cloud optical parameterization scheme that includes six kinds of ice crystal habits is given in the model. Different radiative transfer algorithms are used in BCC_RAD, including two-stream approximation, four-stream approximation and the new two- and four-stream combination approximations developed by the authors for readers to choose. Furthermore, this model is also compared with other radiation transfer models, and applied in the radiative forcing, global warming potential (GWP) and global temperature change potential (GTP) calculations; and climate simulations of Beijing Climate Center AGCM (BCC_AGCM).