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

Christophe Bellisario,Luca Palchetti,Helen E Brindley,Gianluca Di Natale,Rolando Rizzi,Simon F B Tett,Giovanni Bianchini

doi:10.5194/acp-19-7927-2019

Abstract

Abstract. Far-infrared (FIR: 100cm-1<wavenumber, ν<667 cm−1) radiation emitted by the Earth and its atmosphere plays a key role in the Earth's energy budget. However, because of a lack of spectrally resolved measurements, radiation schemes in climate models suffer from a lack of constraint across this spectral range. Exploiting a method developed to estimate upwelling far-infrared radiation from mid-infrared (MIR: 667cm-1<ν<1400 cm−1) observations, we explore the possibility of inferring zenith FIR downwelling radiances in zenith-looking observation geometry, focusing on clear-sky conditions in Antarctica. The methodology selects a MIR predictor wavenumber for each FIR wavenumber based on the maximum correlation seen between the different spectral ranges. Observations from the REFIR-PAD instrument (Radiation Explorer in the Far Infrared – Prototype for Application and Development) and high-resolution radiance simulations generated from co-located radio soundings are used to develop and assess the method. We highlight the impact of noise on the correlation between MIR and FIR radiances by comparing the observational and theoretical cases. Using the observed values in isolation, between 150 and 360 cm−1, differences between the “true” and “extended” radiances are less than 5 %. However, in spectral bands of low signal, between 360 and 667 cm−1, the impact of instrument noise is strong and increases the differences seen. When the extension of the observed spectra is performed using regression coefficients based on noise-free radiative transfer simulations the results show strong biases, exceeding 100 % where the signal is low. These biases are reduced to just a few percent if the noise in the observations is accounted for in the simulation procedure. Our results imply that while it is feasible to use this type of approach to extend mid-infrared spectral measurements to the far-infrared, the quality of the extension will be strongly dependent on the noise characteristics of the observations. A good knowledge of the atmospheric state associated with the measurements is also required in order to build a representative regression model.

Highlights

Defined here as wavelengths above 15 μm or wavenumbers below 667 cm−1, the far-infrared (FIR) spectral band plays a key role in energetic exchanges between the Earth’s surface, atmosphere and space (Harries et al, 2008)
Given the constrained nature of the REFIR-PAD dataset, if the results show that the approach fails to capture the observed spectral behaviour, it would cast serious doubt on whether our ability to model the full infrared spectrum is sufficient for us to expect a similar approach to give a robust spectral prediction over a wider range of conditions and/or viewing geometries
150 cm−1, the predictor wavenumbers are scattered between 700 and 1400 cm−1, with low correlation values between 0.2 and 0.5. This can be explained by a high noise equivalent spectral radiance (NESR) at the edge of the REFIR-PAD detector, as seen in Fig. 1 in green

Summary

Introduction

Defined here as wavelengths above 15 μm or wavenumbers below 667 cm−1, the far-infrared (FIR) spectral band plays a key role in energetic exchanges between the Earth’s surface, atmosphere and space (Harries et al, 2008). Absorption in the FIR is dominated by water vapour such that typically very little FIR radiation emitted from the surface directly escapes to space. The very cold, dry conditions commonly found in polar regions simultaneously shift the peak of surface emission towards longer wavelengths and, under clear-skies, allow as much as 45 % of FIR radiation emitted from the ground to escape directly to space, making the clear-sky FIR outgoing longwave radiation sensitive to surface properties (Feldman et al, 2014). A corollary of this enhanced atmospheric transmissivity is the increased sensitivity of downward clear-sky FIR radiation at the surface to conditions at higher levels in the atmosphere. Bellisario et al.: Can downwelling FIR radiances be estimated from MIR information?

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