Above-aircraft Cirrus Cloud and Aerosol Optical Depth from Hyperspectral Irradiances Measured by a Total-Diffuse Radiometer

Abstract. This study develops the use of spectral total and diffuseirradiance measurements, made from a prototype hyperspectral total-diffusesunshine pyranometer (SPN-S), to retrieve layer fine-mode aerosol (τaer) and total optical depths from airborne platforms. Additionally, weuse spectral analysis in an attempt to partition the total optical depthinto its τaer and cirrus cloud optical depth (τcld)components in the absence of coarse-mode aerosols. Two retrieval methods aredeveloped: one leveraging information in the diffuse irradiance and theother using spectral characteristics of the transmitted direct beam, witheach approach best suited for specific cloud and aerosol conditions. The SPN-Shas advantages over traditional sun photometer systems, including no movingparts and a low cost. However, a significant drawback of the instrument isthat it is unable to measure the direct-beam irradiance as accurately assun photometers. To compensate for the greater measurement uncertainty inthe radiometric irradiances, these retrieval techniques employ ratioed inputsor spectral information to reduce output uncertainty. This analysis usesirradiance measurements from the SPN-S and the solar spectral flux radiometer(SSFR) aboard the National Aeronautics and Space Administration's (NASA) P-3aircraft during the 2018 deployment of the ObseRvations of Aerosols aboveCLouds and their intEractionS (ORACLES) campaign and the 2019 Cloud, Aerosoland Monsoon Processes Philippines Experiment (CAMP2Ex) mission toquantify above-aircraft cirrus τcld and derive vertical profiles oflayer τaer. Validation of the τaer retrieval isaccomplished by comparison with co-located measurements of direct solarirradiance made by the Sky-Scanning Sun-Tracking Atmospheric Research(4STAR) and in situ measurements of aerosol optical depth. For theaggregated 2018 ORACLES results, regression between the SPN-S-based methodand sun photometer τaer values yields a slope of 0.96 with anR2 of 0.96, while the root mean square error (RMSE) is 3.0×10-2. When comparing the retrieved τaer to profiles ofintegrated in situ measurements of optical extinction, the slope, R2,and RMSE values for ORACLES are 0.90, 0.96, and 3.4×10-2, and forCAMP2Ex they are 0.94, 0.97, and 3.4×10-2, respectively. This paper is a demonstration of methods for deriving cloud and aerosoloptical properties in environments where both atmospheric constituents maybe present. With improvements to the low-cost SPN-S radiometer instrument,it may be possible to extend these methods to a broader set of samplingapplications, such as ground-based settings.