Water Vapor Tomography With Two Microwave Radiometers

The present study shows how two microwave radiometers can be used to derive a 2-D water vapor field by means of a tomographic technique. For this purpose, synthetic measurements are simulated with a radiative transfer model applied to water vapor fields. These fields are obtained from a Large Eddy Simulation model, producing realistic atmospheric boundary layer structures. To derive the water vapor field from the microwave measurements, an iterative procedure based on the optimal estimation technique is used. The synthetic measurements are calculated for various measurement geometries. The comparison of the measurement geometries from one versus two radiometers shows that the standard deviation obtained from measurements with the two radiometers is smaller by 15%, and the degrees of freedom for signal are simultaneously larger by 61%. For deriving the best possible water vapor field, the spatial distribution of the measurement angles and the angular resolution of the scan are important. The angles are optimally distributed when most measurements originate from regions with a high variability in the water vapor field.