Retrievals of the Far Infrared Surface Emissivity Over the Greenland Plateau Using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS)

Abstract

The Tropospheric Airborne Fourier Transform Spectrometer measured near surface upwelling and downwelling radiances within the far infrared (FIR) over Greenland during two flights in March 2015. Here we exploit observations from one of these flights to provide in situ estimates of FIR surface emissivity, encompassing the range 80–535 cm−1. The flight campaign and instrumental setup are described as well as the retrieval method, including the quality control performed on the observations. The combination of measurement and atmospheric profile uncertainties means that the retrieved surface emissivity has the smallest estimated error over the range 360–535 cm−1 (18.7–27.8 μm), lying between 0.89 and 1 with an associated error that is of the order ±0.06. Between 80 and 360 cm−1, the increasing opacity of the atmosphere, coupled with the uncertainty in the atmospheric state, means that the associated errors are larger and the emissivity values cannot be said to be distinct from 1. These FIR surface emissivity values are, to the best of our knowledge, the first ever from aircraft‐based measurements. We have compared them to a recently developed theoretical database designed to predict the infrared surface emissivity of frozen surfaces. When considering the FIR alone, we are able to match the retrievals within uncertainties. However, when we include contemporaneous retrievals from the mid‐infrared (MIR), no single theoretical representation is able to capture the FIR and MIR behaviors simultaneously. Our results point toward the need for model improvement and further testing, ideally including in situ characterization of the underlying surface conditions.

FORUM: Unique Far-Infrared Satellite Observations to Better Understand How Earth Radiates Energy to Space

“…The results shown in Figure 8 indicate that under these conditions a satellite observation with an accuracy of 0.1 K in brightness temperature, as required to the FSI measurement, will have sufficient sensitivity to estimate the emissivity of frozen surfaces within the FIR to an accuracy better than 0.01. These estimates are consistent with retrievals made from aircraft measurements over the Greenland plateau (Bellisario et al 2017).…”

“…However, there is growing awareness that this assumption is unphysical (Chen et al 2014) and that may result in marked radiative biases in polar regions (Kuo et al 2018). Given the FIR enhanced atmospheric transparency in polar regions, the surface emissivity in the FIR can be directly measured with remote sensing measurements from airborne platforms (Bellisario et al 2017).…”

“…TAFTS has been successfully operated on a variety of different aircrafts during a number of measurement campaigns. Particular highlights include flights aimed at evaluating the FIR radiative impact of water vapor variability in clear-sky (Cox et al 2007), the radiative signature of cirrus across the MIR and FIR (Cox et al 2010), the assessment of water vapor FIR continuum spectroscopy (Green et al, 2012) and the first retrieval of FIR surface emissivity from airborne observations (Bellisario et al 2017). More recent work in direct support of FORUM preparation has exploited contemporaneous TAFTS and MIR hyperspectral measurements, covering the broad band required by the FSI (see an example in 5), to explore the benefit that FIR radiances may provide for water vapor retrievals and the ability of current ice bulk-scattering databases to capture observed cirrus radiances across the infrared.…”

Can downwelling far-infrared radiances over Antarctica be estimated from mid-infrared information?

“…A protective chimney separates the instrument from the outside temperature, and the ingress of wind and snow is prevented by a barrier on the rooftop. The instrument, fully described in Bianchini et al (2006), is composed of a Fourier transform spectroradiometer (Mach-Zehnder type) with an operating spectral bandwidth of 100-1400 cm −1 (100-7.1 µm) at a resolution of 0.4 cm −1 and with an acquisition time of 80 s. One calibrated spectrum is based on the average of four zenith observations for an overall measurement time of 6.5 min every 14 min. The noise equivalent spectral radiance (NESR) due to detector noise is approximately 1 mW m −2 sr −1 (cm −1 ) −1 at 400 cm −1 .…”

“…Aircraft and ground-based measurements available from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) (Canas et al, 1997) have been used to probe water vapour spectroscopy, upper tropospheric humidity, the radiative properties of cirrus and snow-ice surface emissivity (Green et al, 2012;Fox et al, 2015;Cox et al, 2007Cox et al, , 2010Bellisario et al, 2017). Balloon and ground-based observations from the Radiation Explorer in the Far InfraRed -Prototype of Applications and Development (REFIR-PAD; Bianchini et al, 2006) have been exploited to determine precipitable water (Bianchini et al, 2011), investigate the spectral signature of cirrus (Maestri et al, 2014) and provide simultaneous retrievals of water vapour, temperature and cirrus properties (Di Natale et al, 2017). The Far-InfraRed Spectroscopy of the Troposphere (FIRST) instrument (Mlynczak et al, 2006) has participated in both balloon and ground-based campaigns, providing a rigorous test of the ability of radiative transfer models to match the spectroscopic signals measured in the far infrared (Mlynczak et al, 2016;Mast et al, 2017).…”