Water vapor estimates using simultaneous dual-wavelength radar observations

A technique for the estimation of humidity in the lower troposphere using simultaneous dual-wavelength radar observations is proposed and tested. The method compares the reflectivity from clouds and precipitation of a non-attenuated wavelength (S-band, 10 cm) and an attenuated wavelength (Ka-band, 8 mm) to compute the clear-air gaseous attenuation at the attenuated wavelength. These estimates are of total gaseous attenuation on radar ray segments that extend from the radar to a cloud/precipitation echo or from one echo to another. The attenuation estimates are then used to compute the path-integrated humidity, which is plotted at the midpoint of the ray segments. Using estimates at several elevation angles and different ranges, a profile of humidity through the lower troposphere can be retrieved. The retrieved humidity compared favorably to proximity in situ soundings with root mean square difference values between the retrieval and sounding ranging from 0.14 to 0.85 g m−3 (approximately 2–6% relative error, respectively).