Comparisons of δ-two-stream and δ-four-stream radiative transfer schemes in RRTMG for solar spectra

Abstract

Five δ-two-stream and δ-four-stream schemes are compared in solar spectra using the Rapid Radiative Transfer Model for General Circulation Models Applications (RRTMG). By calculating the flux and heating rate in various atmosphere, it is found that, in accuracy, the δ-four-stream schemes overwhelmingly outperform the δ-two-stream schemes. The precision of adding algorithm of the δ-four-stream spherical harmonic expansion (δ-4SDA) is comparable to that of adding algorithm of the δ-four-stream discrete ordinates method (δ-4DDA). Furthermore, the accuracy of the adding algorithm of δ-Eddington approximation (δ-2SDA) is close to that of δ-two-stream approximation with Practical Improved Flux Method (δ-PIFM), while adding algorithm of δ-two-stream discrete ordinates method (δ-2DDA) produces the poorest results among the five approximate schemes. For the RRTMG model with radiative transfer calculation, the computational time of δ-4SDA is about 1.5 times that of δ-two-stream schemes, and the computational time of δ-4SDA is about 88% that of δ-4DDA.