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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2019-426
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-426
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Dec 2019

Submitted as: research article | 11 Dec 2019

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This preprint is currently under review for the journal AMT.

Development of on-site self-calibration and retrieval methods for sky-radiometer observations of precipitable water vapor

Masahiro Momoi1,2, Rei Kudo3, Kazuma Aoki4, Tatsuhiro Mori5, Kazuhiko Miura5, Hiroshi Okamoto1, Hitoshi Irie1, Yoshinori Shoji3, Akihiro Uchiyama6, Osamu Ijima7, Matsumi Takano8,7, and Teruyuki Nakajima9 Masahiro Momoi et al.
  • 1Center for Environmental Remote Sensing, Chiba University, Chiba, 263-8522, Japan
  • 2Graduate School of Science, Tokyo University of Science, Tokyo, 162-8601, Japan
  • 3Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, 305-0052, Japan
  • 4Graduate School of Science and Engineering, University of Toyama, Toyama, 930-8555, Japan
  • 5Faculty of Science Division I, Tokyo University of Science, Tokyo, 162-8601, Japan
  • 6Narional Institute for Environmental Studies, Tsukuba, 305-0053, Japan
  • 7Aerological Observatory, Japan Meteorological Agency, Tsukuba, 305-0052, Japan
  • 8Osaka Regional Headquarters, Japan Meteorological Agency, Osaka, 540-0008, Japan
  • 9Earth Observation Research Center, Japan Aerospace Exploration Agency, Tsukuba, 305-8505, Japan

Abstract. The Prede sky-radiometer, whose aerosol channels are calibrated by on-site measurements (the Improved Langley method), has been used for continuous long-term observation of aerosol properties. However, continuous long-term observation of precipitable water vapor (PWV) by sky-radiometer remain challenge, because the water vapor channel is generally calibrated by the standard Langley method at limited observation sites (e.g., the Mauna Loa Observatory). In this study, we developed SKYMAP, a new onsite self-calibration method for the water vapor channel of the Prede sky-radiometer using diffuse radiances normalized by direct solar irradiance. The SKYMAP algorithm consists of three steps. First, aerosol optical and microphysical properties are retrieved using direct solar irradiances and the normalized diffuse radiances at aerosol channels. The aerosol optical properties at the water vapor channel are interpolated from those at aerosol channels. Second, the transmittance of PWV is retrieved using the diffuse radiance normalized to the direct solar irradiance at the water vapor channel, which does not need the calibration constant. Third, the calibration constant at the water vapor channel is estimated from the transmittance of PWV and aerosol optical properties. Intensive sensitivity tests of SKYMAP using simulated data of the sky-radiometer showed that the calibration constant is retrieved reasonably well for PWV < 2 cm, indicating that SKYMAP can calibrate the water vapor channel on-site in dry conditions. Then SKYMAP was applied to actual measurements in the dry season at two sites (Tsukuba and Chiba, Japan). Because the SKYMAP algorithm is useful for clear-sky and low PWV (< 2 cm) conditions, the water vapor channel was calibrated for the dry season. After determining the calibration constant, PWV is able to be retrieved using direct solar irradiances for the whole year. The retrieved PWV values correspond well to those derived from a Global Navigation Satellite System (GNSS)/Global Positioning System (GPS) receiver, a microwave radiometer, and a AERONET sun-sky radiometer at both sites (correlation coefficient γ > 0.96), indicating that the Prede sky-radiometer provides both aerosol and PWV data based on its unique on-site calibration methods.

Masahiro Momoi et al.

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Masahiro Momoi et al.

Masahiro Momoi et al.

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Short summary
The water vapor channel of sunphotometers, such as a sky-radiometer, has been calibrated at limited observation sites (e.g. Mauna Loa) in previous studies, but our procedure has made on-situ calibration possible by using sky radiances in addition to direct solar irradiance. The retrieved precipitable water vapor values correspond well to those derived from a Global Navigation Satellite System / Global Positioning System receiver, a microwave radiometer, and a AERONET sun-sky radiometer.
The water vapor channel of sunphotometers, such as a sky-radiometer, has been calibrated at...
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