THE EFFECTS OF LINE WING CUTOFF ON RADIATIVE CALCULATIONS

There are three basic methods in atmospheric radiative transfer calculations, which are line-by-line (LBL) integration, correlated k-distribution method, and band model. The LBL integration is the most accurate one of all. It is divided into two approaches named as integration by lines and by sampling points in the calculated spectral regions according to different integration methods when calculating atmospheric transmissions. Because LBL integration is the most expensive of all, it is necessary and important to deduce calculation time but increase calculation speed when it is put to use in the daily operation in atmospheric remote sensing and atmospheric sounding. A simplified LBL method is given in this paper on the basis of integration by lines, which increases computational speed greatly with keeping the same accuracy. Then, it discusses the effects of different schemes of line-wing cutoff on atmospheric absorption coefficient, transmittance and cooling rate under accurate and simplified LBL methods with a great detail. It is shown by this numerical study that the way to cut spectral line wing has a great effect on the accuracy and speed of radiative calculations. The relative errors of the calculated absorption coefficients are the largest for the method of CUTOFF 2 under different pressures, while for the method of CUTOFF 1, they are less than 2% at most of sampling points and for the method of CUTOFF 3 or 4, they are almost less than 5% in the calculated spectral region, however, the calculation time is deduced apparently. We find in this study that the transmittance at the lower atmosphere is not sensitive to the different approaches for accurate or simplified and the different cutoff methods to the line wing that given in this work. Whereas for the higher atmosphere, the differences of transmittance results between the method OF CUTOFF 2 and other three cutoff methods are the biggest of all no matter for the accurate LBL or for the simplified LBL integrations. By comparison, the best scheme of line-wing cutoff is given in this paper at last.