Validation of the Vapor In-Cloud Profiling Radar

We present results of a recent validation of water vapor retrievals from the Vapor In-Cloud Profiling Radar (VIPR). VIPR uses the Differential Absorption Radar technique to provide high-vertical-resolution profiles of water vapor within cloudy and precipitating volumes. The instrument employs a frequency-modulated continuous-wave (FMCW) G-band DAR architecture unable from 167 to 174.8 GHz. Because of its precise high-resolution ranging capabilities the DAR technique is highly relevant to the Earth Science Decadal Survey Target Observable for Planetary Boundary Layer. 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. 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 g/m3, and median retrieval precision of 0.5 g/m³.