09 Aug 2021

09 Aug 2021

Review status: this preprint is currently under review for the journal AMT.

Differential absorption lidar measurements of water vapor by the High Altitude Lidar Observatory (HALO): Retrieval framework and validation

Brian J. Carroll1, Amin R. Nehrir2, Susan Kooi3, James Collins3, Rory A. Barton-Grimley2, Anthony Notari2, David B. Harper2, and Joseph Lee2 Brian J. Carroll et al.
  • 1NASA Postdoctoral Program Fellow, NASA Langley Research Center, Hampton, VA, USA
  • 2NASA Langley Research Center, Hampton, VA, United States
  • 3Science Systems and Applications, Inc., Hampton, VA, United States

Abstract. Airborne differential absorption lidar (DIAL) offers a uniquely capable solution to the problem of measuring water vapor (WV) with high precision, accuracy, and resolution throughout the troposphere and lower stratosphere. The High Altitude Lidar Observatory (HALO) airborne WV DIAL was recently developed at NASA Langley Research Center and was first deployed in 2019. It uses four wavelengths at 935 nm to achieve sensitivity over a wide dynamic range, and simultaneously employs 1064 nm backscatter and 532 nm high spectral resolution lidar (HSRL) measurements for aerosol and cloud profiling. A key component of the WV retrieval framework is flexibly trading resolution for precision to achieve optimal data sets for scientific objectives across scales. A technique for retrieving WV in the lowest few hundred meters of the atmosphere using the strong surface return signal is also presented.

The five maiden flights of the HALO WV DIAL spanned the tropics through midlatitudes with a wide range of atmospheric conditions, but opportunities for validation were sparse. Comparisons to dropsonde WV profiles were qualitatively in good agreement, though statistical analysis was impossible due to systematic error in the dropsonde measurements. Comparison of HALO to in situ WV measurements onboard the aircraft showed no substantial bias across three orders of magnitude, despite variance (R2 = 0.66) that may be largely attributed to spatiotemporal variability. Precipitable water vapor measurements from the spaceborne sounders AIRS and IASI compared very well to HALO with R2 > 0.96 over ocean and R2 = 0.86 over land.

Brian J. Carroll et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-229', Anonymous Referee #1, 23 Aug 2021
  • RC2: 'Comment on amt-2021-229', Anonymous Referee #2, 07 Sep 2021

Brian J. Carroll et al.

Brian J. Carroll et al.


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Short summary
HALO is a recently developed lidar system that demonstrates new technologies and advanced algorithms for profiling water vapor as well as aerosol and cloud properties. These high-resolution, high-accuracy measurements have unique advantages within the suite of atmospheric instrumentation, such as directly trading water vapor measurement resolution for precision. This paper provides some validation of HALO water vapor, showing excellent agreement with in situ and spaceborne sounder measurements.