Optimized Alternate Mapping Correlated K‐Distribution Method for Atmospheric Longwave Radiative Transfer.

Radiative transfer models are widely applied in climate models to simulate vertical temperature perturbations caused by external radiative forcings. A large part of radiative transfer models is the infrared gaseous spectral transmittance scheme, which quantifies the longwave atmospheric absorption. A rapid infrared gaseous spectral transmittance scheme, called the Optimized alternate Mapping Correlated K‐Distribution model (OMCKD), is introduced in this paper. To improve the accuracy of our scheme without increasing pseudo‐monochromatic calculations, we introduce the optimal iteration method to automatically tune the equivalent absorption coefficients in the cumulative probability subspace. In addition, a new expression weighted by black‐body radiation is introduced to calculate the equivalent absorption coefficient. The OMCKD simulates heating rate and radiation flux with errors of less than 0.12 K d−1 and 0.35 W m−2, respectively, below stratopause for standard atmospheric profiles. The OMCKD is also evaluated and compared with the rapid radiative transfer model for general circulation models (RRTMG) in realistic atmospheric profiles. We found that OMCKD can accurately produce heating rates and generally captures radiative forcings associated with large perturbations to the concentrations of main greenhouse gases. Furthermore, the number of pseudo‐monochromatic calculations in OMCKD is 11.4% less than that in RRTMG, which indicates less computational cost. Plain Language Summary: Radiative models are practical tools for studying the greenhouse effect. The thermal infrared radiation absorbed by greenhouse gases is quantified by a specially designed program called the radiation scheme. The scheme divides the thermal infrared band into hundreds of intervals and calculates each interval's gaseous radiation absorption ability separately. Generally, increasing the number of intervals improves simulation accuracy, but the computational cost is proportional to the number of intervals. We have developed a rapid infrared gaseous spectral transmittance scheme called the Optimized alternate Mapping Correlated K‐Distribution model (OMCKD). Our scheme introduces an optimal method to adjust the radiation absorption ability of each interval with the line‐by‐line integration scheme (accurate but extremely time‐consuming) as the standard. We succeeded in improving the accuracy of our scheme without increasing the number of intervals. The OMCKD simulates heating rate and radiation flux with errors of less than 0.12 K d−1 and 0.35 W m−2, respectively, below stratopause for standard atmospheric conditions. The OMCKD generally captures the changes in the net fluxes and heating rates associated with perturbations to the concentrations of five main greenhouse gases. The stability of OMCKD implies its potential to improve the accuracy of climate predictions under higher CO2 concentration scenarios. Key Points: A rapid infrared gaseous spectral transmittance scheme based on the correlated K‐distribution method and the optimal method is developedThe optimal iteration method can significantly improve the accuracy of heating rates simulated with the correlated K‐distribution modelOur infrared gaseous spectral transmittance scheme maintains high accuracy across various carbon dioxide concentration scenarios [ABSTRACT FROM AUTHOR]