5.1 micrometers and IASI

September 11th, 2022 |

A previous post discussed the 5.1 micrometer channel that might be part of the imager (GXI) to fly on GeoXO, the next-generation satellite (beyond GOES-R) to be launched in the 2030s. That previous post, however, used Cross-track Infrared Sounder (CrIS) data, and CrIS observes close to, but not at, 5.1 micrometers. However, the Infrared Atmospheric Sounding Interferometer (IASI) that is part of the payload on Metop-B and Metop-C (it was on Metop-A as well!) does take observations at 5.1 micrometers. The animation shows a Metop-A IASI observations at 5.1, 6.19, 6.9 and 7.3 micrometers (or wavenumbers 1950, 1615, 1438 and 1369) from a granule over Europe. Of note is apparent moisture aloft — shown as cooler brightness temperatures especially at 6.9 micrometers — that has no apparent signal at 5.1 micrometers because of less sensitivity to mid-tropospheric water vapor at the shorter wavelength. Indeed, 5.1 micrometer brightness temperatures are much warmer than the other channels.

Metop-A observations at 5.1, 6.19, 6.9 and 7.3 micrometers, 1140 UTC on 15 January 2007. (Click to enlarge)

A greyscaled image of 5.1 micrometers, below, shows how the boundary layer influences the signal in clear air.

5.1 micrometer image from IASI data (click to enlarge)

What did geostationary imagery look like on this day? Meteosat-8 imagery at 6.19 and 7.3 micrometers is shown below.

Meteosat-8 Bands 5 and 6 (6.2 and 7.3 micrometers) at 1930 UTC on 15 January 2007 (Click to enlarge)

How dry was this airmass over the Mediterranean? Soundings from 0000 and 1200 UTC on 15 January 2007 (link, from the Wyoming Sounding site) show total precipitable water values between 10 and 14 mm.


The addition of the 5.1 micrometer channel is designed to aid in convective forecasting: gradients in water vapor very low in the atmosphere should be apparent.

5.1 micrometers

September 2nd, 2022 |
CrIS Brightness Temperatures, wavenumber 1755 (5.71 µm) and 1370 (7.3 µm) at 1855 UTC on 15 August 2022. The same color enhancement is used in both, with red temperatures being warmest. Color scale runs from 240 to 270 K (Click to enlarge)

GOES-R Satellites include GOES-16 (the present GOES-East), GOES-17 and GOES-18 (both scanning as GOES-West), and GOES-U, which will become GOES-19 after achieving geostationary orbit after launch (scheduled to occur in 2024). The follow-on to the GOES-R Series is GeoXo (Geostationary Extended Observations), with a launch planned in the 2030s. The GeoXo Imager, GXI, will improve on capability of GOES-R’s Advanced Baseline Imager (ABI). (See this announcement for development of the instrument) One of the channels that might be on this new imager will take observations at/around 5.1 µm. Why was this band chosen? And which present-day instruments can give observations at/near that wavelength to demonstrate that band’s utility?

The Cross-Track Infrared Sounder (CrIS) on NOAA-20 and Suomi-NPP is a Fourier transform spectrometer that provides 2000+ observations in 3 different wavelength ranges: Shortwave infrared (SWIR, 3.92- 4.64 ?m), mid-wave infrared (MWIR, 5.71-8.26 µm) and longwave infrared (LWIR, 9.14-15.38 µm). The toggle above compares 5.71 µm and 7.3 µm observations (wavenumbers 1755 and 1370) from five CrIS granules on 15 August 2022. Note how much warmer the 5.71 µm brightness temperatures are compared to the 7.3 µm brightness temperatures over northeast Mississippi (for example). The shorter wavelength observation are viewing features closer to the boundary layer.

A toggle of GOES-16 imagery and derived products for the same time is shown below. Strong instability (via the Derived Stability Lifted Index) and moisture (from the TPW field) are indicated in a region where the upper troposphere is dry (the 1200 UTC sounding on 15 August at Jackson MS shows considerable mid-level instability, although the SPC Outlook for that day showed only General Thunder.)

GOES-16 Band 2 Visible (0.64 µm) imagery, Band 8 and Band 10 infrared (6.19 µm and 7.3 µm, respectively) imagery, and derived Total Precipitable Water and Lifted Index, 1856 UTC on 15 August 2022 (Click to enlarge)

This paper (from the AMS Annual Meeting in 2021) explored some of the issues in using 5.1 µm. A strength of observations at this wavelength is that it lies in a region where absorption (and therefore cooling in the observations) by CO2 is minimal, and where absorption by water vapor in increasing (absorption by water vapor is much stronger for wavelengths between 5.5 µm and 7.5 µm)

Cooling effect of CO2 (top) and Water Vapor (below) on Spectral Response Function for 5.1 µm band (Click to enlarge)

This poster from the Collective Madison Meeting of the American Meteorological Society (in 2022) includes weighting function figures for 5.1 µm in various atmospheres, as shown below. In all atmospheres, a significant portion of the signal is coming from the surface. Compare these weighting functions to, for example, the 7.3 µm (“Low-level Water Vapor”, Band 10) for Tropical and US Standard atmospheres: very little surface information is present in observations at 7.3 µm. (See also the CIMSS weighting function website for ABI channels) The conclusion: observations at 5.1 µm can give more information about the water vapor distribution in the boundary layer than present-day water vapor channels on ABI. (See also this paper by Miller et al.)

Weighting functions at 5.1 for various atmospheres (Click to enlarge)

The IASI instrument on MetopB/MetopC does take observations at 5.1 µm. Stay tuned for a blog post using those observations. Thanks to Mat Gunshor, CIMSS, and Tim Schmit, NOAA/STAR, for enlightening discussions about the 5.1 µm observations.


GeoXo is also slated to carry a hyperspectral sounder (GXS). Sounders give far more complete assessments of the state of the atmosphere than an imager, even an advanced one like GXI!