CLIMCAPS context

CLIMCAPS is a multi-platform retrieval system designed to generate a long-term continuous record of satellite soundings

CLIMCAPS is configured to retrieve soundings (vertical atmospheric profiles) from AIRS/AMSU on Aqua and CrIS/ATMS on Suomi-NPP as well as JPSS-1 (with the capability to continue its record of CrIS/ATMS retrievals from instruments launched on JPSS-2, JPSS-3 and JPSS-4 in future). CLIMCAPS retrieves multiple atmospheric variables from a single cloud-cleared infrared (IR) radiance measurement using a sequential Optimal Estimation approach (Smith and Barnet, 2019, 2020). 

AIRS platform

AIRS (Atmospheric Infrared Sounder; Aumann et al., 2003) is a grating interferometer with 2378 spectral channels, each a pair of detectors – “A” and “B”. AIRS has 17 arrays of detectors and it is possible for individual detectors to be bad or compromised. If both detectors are good then their spectral values are averaged, and their noise reduced to the square root of the two channel noise values.

AIRS detector characteristics can change over time (colloquially known as “popping”) and especially where events caused the instrument to shut-down and warm up again. AIRS spectral resolution changes with wavenumber but is roughly equal to the wavenumber, $\nu$, divided by 1200, and the spectral equivalent to $\nu$/2400.

Each AIRS channel and each field-of-view (FOV) can be considered an independent measurement. There are 9 × FOVs collocated within each AMSU footprint. The CLIMCAPS cloud clearing step combines these 9 FOVs into a single cloud-cleared radiance, known as a field-of-regard (FOR) with coarser spatial resolution, reduced instrument noise and the radiative effects of clouds removed from each cloud-sensitive channel.

CrIS platform

CrIS (Cross-track Infrared Sounder; Glumb et al., 2002; Strow et al., 2013) is an interferometer that measures 9 × FOVs measured simultaneously for three spectral bands – short-wave (645–1210 cm^-1), mid-wave (1210–2000 cm^-1) and long-wave (2000–2760 cm^-1). CrIS radiances are unapodized in the Level 1 product files, hence the presence of duplicate ‘guard’ channel at the ends of each band. In CLIMCAPS, we apply Hamming apodization according to the method described in (Barnet et al., 2000; Gambacorta and Barnet, 2011). In Table 1 we exclude guard channels from the total number of channels reported for CrIS.

AIRS and CrIS compared

Table 1: Overview of AIRS and CrIS, the two hyperspectral infrared (IR) instruments on low-Earth orbiting satellite platforms for which CLIMCAPS is configured.

Table notes:
1Atmospheric Infrared Sounder
2Cross-track Infrared Sounder
3Field of View
4Field of Regard (CLIMCAPS retrieval footprint)
5Longwave infrared
6Midwave infrared
7Shortwave infrared

CrIS on SNPP was first transmitted at nominal spectral resolution (NSR) but later switched to full spectral resolution (FSR), so we list values for both. These instruments measure the top of atmosphere radiance with spatial footprints known as fields of view (FOV). CLIMCAPS retrieves soundings from every array of 3 × 3 FOVs knowns as the Field of Regard (FOR), which we refer to as retrieval 'footprints'.

Instrument noise is given in brightness temperature units as the noise equivalent delta temperature (NEDT) for a scene at 250K. This table was originally published in (Smith and Barnet, 2019). Note that we exclude mention of the two CrIS duplicate ‘guard’ channels for each spectral band.