Rapid radiative transfer model for AMSU/HSB channels

The atmospheric transmittance model for AMSU and HSB channels on the Aqua spacecraft uses a polynomial approximation to the temperature dependence of oxygen-band opacity within atmospheric layers. It uses look-up tables to calculate local water-vapor line intensity and pressure-broadening parameters as well as contributions to absorption from the water-vapor continuum, distant lines, and cloud liquid water. The algorithm includes water-line self-broadening and the magnetic-field effect on AMSU channel 14. The microwave surface emission model is based on a preliminary classification of the surface type, with subsequent adjustments to the emissivity spectrum which are obtained from the retrieval algorithm. A simple approximate correction for surface non-specularity is included. 1. Atmospheric transmittance model a) For channels near a water line or in a window, at most one Lorentzian resonance factor is computed. Look-up tables are used to obtain the temperature dependence of: self and foreign continuum, self and foreign line broadening, line intensity, cloud-liquid absorption, fixed-gas opacity. b) Channels in the oxygen band use a third-order polynomial function of temperature to compute fixed-gas opacity. Channel 14 has a magnetic-field correction. Water-vapor and cloud-liquid opacity are computed as for a window channel. 2. Surface brightness model An a priori surface-brightness spectrum is chosen by a classification algorithm that operates on AMSU window channels. There are 7 classes: 0 Coastline 1 Land 2 Ocean 3 First-year sea ice or mixed ice/land/water 4 Multi-year sea ice or mixed ice/water 5 Snow over land 6 Glacier The mw retrieval algorithm subsequently adds a smooth 3-parameter correction curve to the a priori spectrum. Later stages of the retrieval can adjust the surface brightness curve uniformly (1 degree of freedom). ___________________________________________________________ 3. The radiative-transfer calculation uses an empirically determined correction factor of 1.15 for reflected-sky path length, to account for surface non-specularity. 4. Tests a. Comparison with line-by-line calculations Approximation errors are smaller than the instrument noise. b. Comparison of brightness temperature computed from radiosondes with NOAA-15 AMSU measurements Adjustments to data: Sidelobe corrections calculated by T. Mo Temperature profiles extended to 1 mb using retrieved profile, but the raob profile must extend to an altitude where transmittance to space = 0.9