A New Highly Stable Multi-Decade Satellite Climate Data Set Derived from Polar Hyperspectral Infrared Sensors
Authors: Hepplewhite Chris , Strow Larrabee , Motteler Howard , de Souza-Machad Sergio , Buczkowski Steven 
Published: March 2020

&lt;p&gt;NASA's Atmospheric Infrared Sounder (AIRS) started the continuous measurement of the Earth's upwelling infrared radiation at high spectral resolution in Sept. 2002 in a 13:30 polar orbit. &amp;#160;The AIRS record was supplemented by the CrIS sensor flying on the NASA SNPP platform, also in the 13:30 polar orbit, in 2012. &amp;#160;In 2018 a second CrIS sensor on NOAA's JPSS-1 platform (NOAA-20) began operation, also in the 13:30 orbit. &amp;#160;Two more CrIS sensors are presently being procured for the JPSS-2 and 3 satellites, which will extend this record from 2002 through ~2040. &amp;#160;EUMETSAT's METOP-A/B/C provide very similar hyperspectral observations starting with the IASI sensors in the 09:30 orbit, starting in 2007, which will be continued with METOP-SG for years to come. &amp;#160;&lt;/p&gt;&lt;p&gt;Inter-calibration of all of the operating sensors shows agreement generally to 0.2K or better in brightness temperature. &amp;#160;More importantly, we have shown that the radiometric stability of the AIRS sensors is in the 0.002 K/year range or 0.02K/decade, based on measurements of CO2 and SST trends. &amp;#160; Similar stability is expected for CrIS and IASI. &amp;#160;Community consensus suggests that direct radiance trending, followed by conversion of these trends to geophysical quantities will yield the most accurate climate trends. &amp;#160;&lt;/p&gt;&lt;p&gt;Here we introduce a new satellite hyperspectral infrared radiance product we call the &quot;Climate Hyperspectral InfraRed Product (CHIRP)&quot; that combines AIRS, CrIS, and IASI into a homogeneous Level 1 radiance product with a common spectral response and channel centers for all three satellites. &amp;#160;This grid is equivalent to an interferometer with optical path differences of 0.8/0.6/0.4 cm for the long-wave/mid-wave/short-wave spectral bands. &amp;#160;This corresponds to a virtual instrument with the same spectral resolution of the JPSS-1 CrIS sensor in the long-wave, with 25/50% degradation in spectral resolution in the mid-wave/short-wave. &amp;#160;This choice allows accurate conversion of the long AIRS record to an equivalent interferometer record. &amp;#160;Conversion of IASI to CHIRP is trivial. &amp;#160;Conversion of all sensors to the CHIRP spectra grid permits simple adjustments of inter-satellite radiometric bias differences since all measurements are first converted to a common spectral grid. &amp;#160;Multiple methods (SNOs, statistical inter-comparisons) indicate these adjustments can be made to the 0.03K level or better. &amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;A sample application of CHIRP to climate trending will be given by showing multi-decade anomalies of temperature, humidity, and ozone profiles retrieved from CHIRP radiance anomalies, a retrieval that requires almost no a-priori information. &amp;#160;This data set should yield definitive measurements of water-vapor feedback and heavily contribute to our understanding of both tropospheric and stratospheric temperature trends. &amp;#160; Initial production of CHIRP radiances that combine AIRS and CrIS are expected to begin in late 2020. &amp;#160;&lt;/p&gt;