Marine-AERI Retrievals of Sea Surface Emissivity and Skin Temperature from the ACAPEX Campaign

 

Authors

Jonathan Gero — University of Wisconsin
Matthew Hermann Westphall — University of Wisconsin, Madison
Robert O. Knuteson — University of Wisconsin
Nicholas R. Nalli — National Oceanic and Atmospheric Administration
William L. Smith — Hampton University

Category

Radiation

Description

The ARM Cloud Aerosol Precipitation Experiment (ACAPEX) was a joint DOE/NOAA field campaign in early 2015 to study atmospheric rivers in the Pacific Ocean and their impacts on the western United States. Coordinated measurements were made from ground-, aircraft- and sea-based platforms. The second ARM mobile facility (AMF2) was deployed on board the NOAA Ship Ronald H. Brown for this campaign, which included a new Marine Atmospheric Emitted Radiance Interferometer (M-AERI) to measure the atmospheric downwelling and reflected infrared radiance spectrum at the Earth's surface with high absolute accuracy. The M-AERI measures spectral infrared radiance between 520–3020 cm-1 (3.3–19 μm), at a resolution of 0.5 cm-1. A gold-coated rotating scene mirror allows the M-AERI to selectively view the atmospheric scene at zenith, and ocean/atmospheric scenes over a range of ±45° from the horizon. The M-AERI radiance spectra can be used to retrieve profiles of temperature and water vapor in the troposphere, as well as measurements of trace gases, cloud properties, surface emissivity, and sea surface skin temperature. The sea surface skin temperature (SSST) as well as emissivity can be retrieved by solving the radiative transfer equation for the viewing geometry from the ship, given certain simplifying assumptions. For the SSST, we use observations from certain spectral regions where the observed radiative temperature and the true thermodynamic temperature of the sea surface are nearly identical. The operational SSST product from the M-AERI has a demonstrated accuracy of better than 0.1 K. Sea surface emissivity can be retrieved from a combination of downlooking and uplooking measurements made at the same reflected and incident angle. We present retrieved sea surface emissivity results from the M-AERI for the range of wind speeds encountered during ACAPEX, and compare them with various models of sea surface emissivity. Due to the high accuracy of the M-AERI instrument, sea surface emissivity can be retrieved with an accuracy of 0.5% in the window region.