28 Sep 2021

28 Sep 2021

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

Above-aircraft cirrus cloud and aerosol optical depth from hyperspectral irradiances measured by a total-diffuse radiometer

Matthew S. Norgren1, John Wood2, K. Sebastian Schmidt1,3, Bastiaan van Diedenhoven4,5, Snorre A. Stamnes6, Luke D. Ziemba6, Ewan C. Crosbie6, Michael A. Shook6, A. Scott Kittelman3, Samuel E. LeBlanc7,8, Stephen Broccardo7,8,9, Steffen Freitag10, and Jeffery S. Reid11 Matthew S. Norgren et al.
  • 1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
  • 2Peak Design Ltd, Sunnybank House, Wensley Rd, Winster, Derbys, DE4 2DH, UK
  • 3Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, USA
  • 4SRON Netherlands Institute for Space Research, Utrecht, Netherlands
  • 5NASA Goddard Institute for Space Studies, New York, NY, USA
  • 6NASA Langley Research Center, Hampton, VA, USA
  • 7Bay Area Environmental Research Institute, Moffett Field, CA, USA
  • 8NASA Ames Research Center, Moffett Field, CA, USA
  • 9Universities Space Research Association, Columbia, Maryland, USA
  • 10Department of Oceanography, University of Hawaii at Manoa, Honolulu, HI, USA
  • 11Marine Meteorology Division, U.S. Naval Research Laboratory, Monterey, CA, USA

Abstract. This study develops the use of spectral total and diffuse irradiance measurements, made from a prototype hyperspectral total-diffuse Sunshine Pyranometer (SPN-S), to retrieve layer fine-mode aerosol (τaer) and total optical depths from airborne platforms. Additionally, we use spectral analysis in an attempt to partition the total optical depth it into its τaer and cirrus cloud optical depth (τcld) components in the absence of coarse-mode aerosols. Two retrieval methods are developed: one leveraging information in the diffuse irradiance, and the other using spectral characteristics of the transmitted direct beam, with each approach best suited for specific cloud and aerosol conditions. SPN-S has advantages over traditional sun-photometer systems including no moving parts and a low cost. However, a significant drawback of the instrument is that it is unable to measure the direct beam irradiance as accurately as sun-photometers. To compensate for the greater measurement uncertainty of the radiometric irradiances these retrieval techniques employ ratioed inputs or spectral information to reduce output uncertainty. This analysis uses irradiance measurements from SPN-S and the Solar Spectral Flux Radiometer (SSFR) aboard the National Aeronautics and Space Administration’s (NASA) P-3 aircraft during the 2018 deployment of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) campaign and the 2019 Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) mission to quantify above-aircraft cirrus τcld and derive vertical profiles of layer τaer. Validation of the τaer retrieval is accomplished by comparison with collocated measurements of direct solar irradiance made by the Sky-Scanning Sun-Tracking Atmospheric Research (4STAR) and in situ measurements of aerosol optical depth. For the aggregated 2018 ORACLES results, regression between the SPN-S based method and sun-photometer τaer values yield a slope of 0.96 with an R2 of 0.96, while the root-mean-square error (RMSE) is 3.0 × 10−2. When comparing the retrieved τaer to profiles of integrated in situ measurements of optical extinction, the slope, R2, and RMSE values for ORACLES are 0.90, 0.96, 3.4 × 10−2, and for CAMP2Ex are 0.94, 0.97, 3.4 × 10−2 respectively.

This paper is a demonstration of methods for deriving cloud and aerosol optical properties in environments where both atmospheric constituents may be present. With improvements to the low-cost SPN-S radiometer instrument, it may be possible to extend these methods to a broader set of sampling applications, such as ground-based settings.

Matthew S. Norgren et al.

Status: open (until 02 Nov 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-269', Anonymous Referee #1, 12 Oct 2021 reply

Matthew S. Norgren et al.

Matthew S. Norgren et al.


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Short summary
A new spectral instrument (SPN-S), with the ability to partition solar radiation into direct and diffuse components, is used in airborne settings to study the optical properties of aerosols and cirrus. SPN-S is low-cost and mechanically simple system but has higher measurement uncertainty than existing standards. This challenge is overcome by utilizing the unique measurement capabilities to develop new retrieval techniques. Validation is done with data from two NASA airborne research campaigns.