29 Apr 2024
 | 29 Apr 2024
Status: this preprint is currently under review for the journal AMT.

The Far-INfrarEd Spectrometer for Surface Emissivity (FINESSE) Part II: First measurements of the emissivity of water in the far-infrared

Laura Warwick, Jonathan Murray, and Helen Brindley

Abstract. In this paper we describe a method for retrieving surface emissivity across the wavenumber range 400–1600 cm-1 using novel radiance measurements from the Far INfrarEd Spectrometer for Surface Emissivity (FINESSE) instrument. FINESSE is described in detail in part I of this paper. We apply the method to two sets of measurements of distilled water. The first set of emissivity retrievals is of distilled water heated above ambient temperature to enhance the signal to noise ratio. The second set of emissivity retrievals is of ambient temperate water at a range of viewing angles. In both cases the observations agree well with calculations based on compiled refractive indices across the mid and far-infrared. It is found that the reduced contrast between the up and downwelling radiation in the ambient temperature case degrades the performance of the retrieval. Therefore a filter is developed to target regions of high contrast which improves the agreement between the ambient temperature emissivity retrieval and the predicted emissivity. These retrievals are, to the best of our knowledge, the first published 10 retrievals of the emissivity of water that extend into the far-infrared and demonstrate a method that can be used for the in-situ retrieval of the emissivity of other surfaces in the field.

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Laura Warwick, Jonathan Murray, and Helen Brindley

Status: open (until 03 Jun 2024)

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Laura Warwick, Jonathan Murray, and Helen Brindley
Laura Warwick, Jonathan Murray, and Helen Brindley


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Short summary
We describe a method for measuring the emissivity of natural surfaces using data from the new FINESSE instrument. We demonstrate our method by making measurements of the emissivity of water. We then compare our results to the emissivity predicted using a model and find good agreement. The observations from FINESSE are novel because they allow us to determine surface emissivity at longer wavelengths than have been routinely measured before.