A rapid atmospheric transmittance algorithm for microwave sounding channels

A rapid transmittance algorithm for NOAA´s Advanced Microwave Sounding Units A and B and possible future instruments has been devised. Window channels, water vapor channels, and oxygen-band channels are considered separately; each uses tabular or polynomial approximations to line-wing or near-line absorption from water vapor or oxygen as appropriate. Absorption by cloud liquid water is also included. For oxygen-band channels that sound the atmosphere above 40 km, water vapor and clouds are not significant, but Zeeman splitting produced by the terrestrial magnetic field is. Because magnetic field strength varies over the Earth, channels within the Zeeman region use an algorithm in which transmittance is parameterized as a function of magnetic field strength B add angle θ_B (with respect to the direction of propagation) at a frequency resolution of B 2.22 kHz/μT; then the average over channel passbands is done on-line. In tests, the rapid algorithm required thirty times less computation than a line-by-line algorithm, and reproduced the line-by-line calculation of brightness temperatures with accuracy comparable to or better than the channel sensitivities