Retrieval of water vapor from AMSU-A and AMSU-B measurements

The NOAA-15 weather satellite carries the Advanced Microwave Sounding Units-A and -B (AMSU-A, AMSU-B) which measure thermal emission from an atmospheric oxygen band, two water lines, and several window frequencies. Relative-humidity profiles are retrieved from these measurements by an iterated algorithm which minimizes the a posteriori variance of retrieved parameters, given a priori second-order statistics of the parameters and the measured brightness temperatures. Relative humidity is converted into absolute humidity with use of the retrieved temperature profile. Two important issues in the retrieval problem are modeling of the surface and clouds. An a priori surface emissivity is computed on the basis of a preliminary classification algorithm, and the surface brightness spectrum is subsequently adjusted simultaneously with the moisture profile retrieval. Cloud liquid water is constrained by a condensation model that uses an extended definition of relative humidity as a parameter