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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 12
Atmos. Meas. Tech., 11, 6511-6523, 2018
https://doi.org/10.5194/amt-11-6511-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Meas. Tech., 11, 6511-6523, 2018
https://doi.org/10.5194/amt-11-6511-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Dec 2018

Research article | 06 Dec 2018

Boundary-layer water vapor profiling using differential absorption radar

Richard J. Roy, Matthew Lebsock, Luis Millán, Robert Dengler, Raquel Rodriguez Monje, Jose V. Siles, and Ken B. Cooper Richard J. Roy et al.
  • Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

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 183GHz 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 200m resolution and typical statistical uncertainty of 0.6gm−3 out to around 2km. 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.

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The measurement of water vapor profiles inside clouds with high spatial resolution represents an outstanding problem in atmospheric remote sensing. Here we present measurements from a proof-of-concept millimeter-wave (170 GHz) cloud radar aimed at filling this observational gap, and demonstrate the ability to retrieve in-cloud water vapor profiles with high precision and resolution. This technology could meaningfully impact future satellite-based measurements of water vapor.
The measurement of water vapor profiles inside clouds with high spatial resolution represents an...
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