Abstract
Wintertime Arctic surface emissivities are retrieved from Advanced Technology Microwave Sounder (ATMS) passive microwave measurements at 88.2, 165.5, and 183.31 GHz. Surface emitting layer temperatures are simultaneously retrieved at 183.31 GHz. Random errors in emissivities are estimated to be 2.0%, 2.0%, and 3.5% at 88.2, 165.5, and 183.31 GHz, respectively, and the random errors in surface emitting layer temperatures are 4.3 K. A series of tests on the retrieved products reveal that land and sea ice are Lambertian reflectors and ocean is a specular reflector. The retrieved emissivities show broad agreement with products from published databases, with differences partly due to the uncertainties in surface emitting layer temperatures. The geographical distribution of 165.5/183.31 GHz surface reflectance ratios over land and sea ice, which is important for the retrieval of microwave satellite water vapor column (WVC), is presented. Neglecting the geographical variations leads to random errors in retrieved wintertime Arctic WVCs of approximately 1.8% and 25% in the mid (1.5 – 9 kgm−2) and extended (8 – 15 kgm−2) slant column retrieval regimes, respectively. Choosing specular instead of Lambertian reflection in the surface emissivity retrievals over land and sea ice causes systematic WVC retrieval errors of up to −4.1%.
Highlights
S ATELLITE-BORNE microwave radiometers provide three decades of brightness temperature measurements around the 183.31 GHz water vapor emission line that may be used to monitor polar humidity [1], [2]
Brightness temperatures measured by the Advanced Technology Microwave Sounder (ATMS) aboard the Suomi National Polar-orbiting Partnership (NPP) satellite [12] are used in this article
The systematic bias between Microwave Integrated Retrieval System (MIRS) and this article may be explained by differences in the surface emitting layer temperatures that were used in the respective retrievals
Summary
S ATELLITE-BORNE microwave radiometers provide three decades of brightness temperature measurements around the 183.31 GHz water vapor emission line that may be used to monitor polar humidity [1], [2]. We use existing techniques to retrieve surface emissivities from microwave satellite brightness temperatures for the purposes of microwave WVC measurements. Two-channel microwave retrievals have been developed that simultaneously obtain emissivities and surface emitting layer temperatures using brightness temperature measurements at widely spaced frequencies [20]. Brightness temperatures measured by the Advanced Technology Microwave Sounder (ATMS) aboard the Suomi National Polar-orbiting Partnership (NPP) satellite [12] are used in this article. The impact of surface assumptions on microwave satellite humidity measurements is assessed using forward simulations and a WVC retrieval described in our previous work [2]. The specular version of the WVC retrieval is used in this article to gauge the response of WVC measurements to variations in surface emissivities. Pan-Arctic measurements of emissivities, surface emitting layer temperatures, and reflectance ratios are given, and compared with products from available databases.
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