Abstract
Abstract. Remote sensing of water vapor in the presence of clouds and precipitation constitutes an important observational gap in the global observing system. We present ground-based measurements using a new radar instrument operating near the 183 GHz H2O line for profiling water vapor inside of planetary-boundary-layer clouds, and develop an error model and inversion algorithm for the profile retrieval. The measurement technique exploits the strong frequency dependence of the radar beam attenuation, or differential absorption, on the low-frequency flank of the water line in conjunction with the radar's ranging capability to acquire range-resolved humidity information. By comparing the measured differential absorption coefficient with a millimeter-wave propagation model, we retrieve humidity profiles with 200 m resolution and typical statistical uncertainty of 0.6 g m−3 out to around 2 km. This value for humidity uncertainty corresponds to measurements in the high-SNR (signal-to-noise ratio) limit, and is specific to the frequency band used. The measured spectral variation of the differential absorption coefficient shows good agreement with the model, supporting both the measurement method assumptions and the measurement error model. By performing the retrieval analysis on statistically independent data sets corresponding to the same observed scene, we demonstrate the reproducibility of the measurement. An important trade-off inherent to the measurement method between retrieved humidity precision and profile resolution is discussed.
Highlights
We discuss the implementation of differential absorption radar (DAR) for measuring humidity profiles inside of boundary-layer clouds (Lebsock et al, 2015; Millán et al, 2016)
Important to the validity of this DAR method is the dominance of gaseous differential absorption over particulate differential absorption, since we assume that βpart is frequencyindependent
The architecture is similar to that presented in an earlier work (Cooper et al, 2018) which demonstrated the DAR technique between 183 and 193 GHz, but modified to transmit in the 167 to 174.8 GHz band, in which transmission is not prohibited by international regulations (NTIA, 2015), to perform narrow-bandwidth frequency chirps, and to provide a 5 MHz offset of the zero-range radar signal from zero frequency within the intermediate frequency (IF) band
Summary
We discuss the implementation of differential absorption radar (DAR) for measuring humidity profiles inside of boundary-layer clouds (Lebsock et al, 2015; Millán et al, 2016). Millimeter-wave transparency in clouds allows for airborne or spaceborne measurements of lower tropospheric humidity in cloudy scenes, while DIAL systems typically cannot measure inside boundary-layer clouds due to high optical thickness. The retrieved profiles constitute the first active remote sensing measurements of water vapor profiles inside of clouds, and open up possibilities for a variety of scientific studies, including investigation of in-cloud humidity heterogeneity and the coupled relationship between boundary-layer clouds and thermodynamic profiles
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