PARAMETERIZATIONS FOR WATER-VAPOR IR RADIATIVE-TRANSFER IN BOTH THE MIDDLE AND LOWER ATMOSPHERES

Water vapor contributes a maximum of 1 degrees C/day to the middle atm ospheric thermal infrared (IR) cooling. This magnitude is small but no t negligible. Because of the small amount of mass involved and the ext remely narrow molecular absorption lines at pressures less than 1 mb, only a few existing parameterizations can compute accurately the water vapor cooling in this region. The accuracy and efficiency of two IR p arameterizations are examined in this study. One is the correlated-k d istribution method, and the other is the table look-up using precomput ed transmission functions. Both methods can accurately compute the coo ling rate from the earth's surface to 0.01 mb with an error of only a few percent. The contribution to the cooling rate at pressures < 1 mb comes from a very small fraction (<0.005) of the spectrum near the cen ters of the absorption bands, where the absorption coefficient varies by four orders of magnitude. It requires at least 100 terms of the k-d istribution function to accurately compute the cooling profile. The me thod of table look-up is, therefore, much faster than the correlated-k distribution method for computing the water vapor cooling profile inv olving both the middle and lower atmospheres.