Validation of a G-band differential absorption cloud radar for humidity remote sensing

Differential absorption radar (DAR) offers an active remote sensing solution to the problem of measuring humidity profiles with high vertical and horizontal resolution in hydrometeor layers. The Vapor In-cloud Profiling Radar (VIPR) is a frequency-modulated continuous-wave (FMCW) G-band DAR tunable from 167 to 174.8 GHz being developed at the Jet Propulsion Laboratory (JPL). Here we describe ground-based measurements from VIPR performed at the Department of Energy’s Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site for humidity product validation. Two distinct measurement capabilities are investigated: (1) Humidity profiles inside of cloudy volumes with 180 m vertical resolution, and (2) integrated water vapor (IWV) between the surface and cloud base. High radar sensitivity permits detection of upper-tropospheric clouds and retrieval of humidity profiles above 10 km in height. We develop an improved humidity retrieval algorithm based on a regularized least-squares method that includes detailed accounting of measurement covariances and systematic error sources. This regularization mitigates high-spatial-frequency humidity biases that arise from frequency-dependent hydrometeor scattering, which is an important limitation for DAR systems. Through comparisons with over 20 coincident radiosondes, we find close agreement between in-situ and remotely sensed humidity profiles, with a correlation coefficient of r = 0:96, root-mean-square error (RMSE) of 0.8 gm􀀀3, and median retrieval precision of 0.5 gm􀀀3. Using a merged radiosonde and Raman lidar product for surface-to-cloud-base IWV, we demonstrate precise column sounding capabilities with r = 1:00, RMSE of 1.2 mm, and median retrieval precision of 0.25 mm.