Comparisons of emissivity observations from satellites and the ground at the CRCS Dunhuang Gobi site

Abstract

Two sets of field-measured hyperspectral resolution infrared (IR) emissivity spectra were taken from the China Radiometric Calibration Sites (CRCS) Dunhuang site in China, with one representing daytime and the other representing nighttime. Comparisons of the two sets show that the daytime emissivity is smaller than the nighttime emissivity in almost the entire spectrum between 7.5 and 14 μm, strong field evidence to support emissivity diurnal variations, which have been reported in previous studies using satellite observations. These emissivities are used as a reference to evaluate three different emissivity products from the same site: the Atmospheric Infrared Sounder (AIRS) operational emissivity products, the Moderate Resolution Imaging Spectroradiometer (MODIS) operational emissivity products, and the University of Wisconsin-Madison Hyperspectral Resolution IR emissivity (UWIREMIS) database. The AIRS emissivity does not agree as well with the field measurements when compared to that from MODIS and the UWIREMIS despite the fact that AIRS is hyperspectral; the likely cause for the disagreement is cloud contamination due to AIRS' large footprint. MODIS has the advantage of high spatial resolution and visible/near-infrared channels to help the cloud mask and is therefore less affected by cloud contamination. The V4.1 MODIS emissivity agrees better with the field measurement than the UWIREMIS, while the V5 does not do as well. The UWIREMIS emissivity, on the other hand, has the advantage of hyperspectral resolution, which makes it more useful for applications. The temporal analysis of the three satellite-based emissivity products is also presented.