A 22-GHz Mobile Microwave Radiometer (MobRa) for the Study of Middle Atmospheric Water Vapor

We present a new compact and automated ground- based microwave mobile radiometer dedicated to the study of middle atmospheric water vapor, which is convenient for measurement, intercomparison, and validation campaigns, particularly in remote places. The instrument detects the 6₁₆-5₂₃ H₂O transition line at 22.235 GHz by means of balanced beam-switching observation of the atmosphere. Tipping curves are performed to estimate the tropospheric opacity. The sky is used as a cold load for calibration, minimizing the use of liquid nitrogen. The main technical issues are caused by the small horn antenna (a beamwidth of ~12deg) inducing large bias when using pencil- beam approximation for the calculation of elevation-dependent parameters. Numerical methods have been developed to include this effect in the data calibration process. Preliminary vertical profiles are retrieved with the Microwave Odin Line Estimation and REtrieval radiative transfer and inversion tool from 25 to 55 km with a vertical resolution of 10-20 km increasing with height, for an integration time of ~12 h and a measurement error of ~15% (~0.8 ppmv). Preliminary comparisons with the spaceborne instrument Aura/Microwave Limb Sounder show a good agreement (~5%) in the 35-55-km altitude range, whereas a negative bias is detected below (up to -30% at 25 km).