Atmospheric Sounders

Atmospheric sounding measurements are made in columns with footprints typically between 5 - 50 km² in size. Sounders measure radiation emissions from molecules in the air between the satellite and Earth’s surface, where the intensity of radiation detected indicates the height within the atmosphere. Each spectral channel is sensitive to different absorption band ‘signatures’ of different molecules, allowing multiple features to be determined and at varying altitudes. 1)

The temperature of gas molecules is directly related to their vibration and movement, which causes them to absorb and emit radiation that can be detected by sounders. Thus, by measuring the radiation emitted by particular gases, we can measure their temperature. Oxygen and carbon dioxide emissions are commonly used to obtain temperature profiles due to their relatively uniform distribution throughout the atmosphere, whilst water vapour emissions are used for humidity profiles. 1) 2)

Sounders use either microwave or infrared (IR) wavelengths to make their measurements. Microwave sounders are superior to IR sounders in that they can sound through clouds, however, their spatial resolution is typically lower than IR sounders. 2) 3)

The term sounding originates from the practice of mariners using a weighted line to estimate the depth of water - a practice known as “sounding”. Space-based instruments “sound” the atmosphere at multiple depths for temperature, humidity, trace gases, and clouds. 4)

Example Products

Community Long-Term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS)

The Community Long-Term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS) is a sounding data archive hosted within the NASA Data Center. It facilitates several operational products that provide near real-time data, including the National Oceanic Atmospheric Administration’s (NOAA) Unique Combined Atmospheric Processing System (NUCAPS). All products use data collected from several satellite missions including JPSS-1, JPSS-2, MetOp, and Aqua.

The most common method used by meteorologists to display atmospheric pressure sounding data is through Skew-T Log-P diagrams. The y-axis displays altitude on an increasing linear scale and air pressure on a decreasing logarithmic scale. The x-axis displays temperature, which is constant following skew grid lines at a 45° angle. 4)

Tephigrams and Stüve diagrams are delineations similar to Skew-T Log-P diagrams, which use different grid systems and axis orientations.

Multi-panel sounder images

Sounding data can also be represented in tiled imagery of each spectral band (channel) acquisition. This allows users to interpret each channel individually to better understand the atmospheric properties. This data contains only the signals received by each channel, and has not yet been processed to derive the physical properties of the atmosphere.  

Related Missions

NOAA POES & MetOp

The POES (Polar Orbiting Environmental Satellites) program is a joint effort by NASA, NOAA, and international partners in Europe. This includes the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) as well as organisations in France, Canada, and the UK. Since the mission’s first launch in 1998, five satellites have continued to monitor the atmosphere, water masses, ozone, and the space environment. The satellites have collectively been equipped with four sounding instruments: the Advanced Microwave Sounding Unit A and B (AMSU-A & -B), the Microwave Humidity Sounder (MHS), and the High Resolution Infrared Sounder (HIRS).

These instruments are also carried by the European MetOp satellites, which first launched in 2006. In addition, MetOp also carries the Infrared Atmospheric Sounding Interferometer (IASI), which measures tropospheric moisture and temperature, column integrated contents of ozone, carbon monoxide, methane, nitrous oxide and other minor gases which affect tropospheric chemistry.

Read more: NOAA POES, MetOp

MetOP-SG

MetOp-SG (Meteorological Operational satellite - Second Generation) is a series of 6 meteorological satellites which will act as the successor to the MetOp series. The mission is developed by ESA and EUMETSAT, and is divided into two different series, MetOp-SG-A and -B. MetOp-SG-A will carry the New Generation Infrared Atmospheric Sounding Interferometer - (IASI-NG) and the Microwave Sounder (MWS).

Read more

JPSS Series and Suomi-NPP

The Cross-track Infrared Sounder (CrIS) is carried on all four of NOAA’s Joint Polar Satellite System (JPSS) missions, as well as the Suomi National Polar-orbiting Partnership (Suomi-NPP) mission - which is jointly operated jointly by NASA and NOAA. First launched in 2011 onboard Suomi-NPP, CrIS provides profiles of atmospheric temperatures, water vapor and trace gases at 14 km horizontal spatial resolution and 1 km vertical layer resolution. The instrument has 1300 spectral channels in the mid-wavelength infrared (MWIR) and thermal infrared (TIR) spectral regions.

Read more: JPSS-1/NOAA-20, JPSS-2/NOAA-21, Suomi NPP

MTG (Meteosat Third Generation)

With its first satellite launched in December 2022, MTG is a satellite constellation collaboratively led by ESA and the EUMETSAT to enhance weather forecasting capabilities. The constellation consists of satellites dedicated to imaging (MTG-I) and sounding (MTG-S) observations. The MTG-S satellites are equipped with an Infrared Sounder (IRS) and an Ultraviolet-Visible Near-infrared (UVN) Sounder. IRS observes over 10 spectral bands ranging from 12 - 45 MHz to observe carbon dioxide, ozone, water vapour, methane, nitrous oxide, and nitric oxide.

Read more 

GOES (Geostationary Operational Environmental Satellite)

The Geostationary Operational Environmental Satellite (GOES) constellation is the United States’ geostationary meteorological satellite programme, which has been operational since 1975. Sounders were included in the instrumentation suite for GOES-8 to GOES-15, launched between 1994 and 2010.

The GOES Sounder measures emitted radiation in 18 thermal infrared bands that are sensitive to temperature, moisture, ozone, and reflected solar radiation in one visible band. Its spatial resolution is 8 km with 13 bit data transmitted to the GOES receiving facilities.

Read more: GOES 8-12, GOES-13-15

GeoXO

Designed to replace the GOES-R series, GeoXO will provide enhanced weather, air quality, and ocean monitoring to address threats from extreme weather and climate change, benefiting public safety and environmental protection. The GeoXO Central satellites will carry a hyperspectral sounder (GXS) to provide continuous coverage for atmospheric data including temperature, moisture, and ozone. The instrument is the first IR hyperspectral sounder to provide continuous coverage for the United States. GXS utilises 1,550 spectral bands to gather detailed atmospheric data, enabling continuous spectra to be observed.

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Aqua

Launched in 2002, Aqua (formerly known EOS/PM-1) is operated by NASA as part of their Earth Observing System (EOS). It is jointly funded by NASA, Brazil’s National Institute for Space Research (NIPE), and the Japan Aerospace Exploration Agency (JAXA), with the mission of studying Earth’s water cycle. Aqua carries three sounding instruments: the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), and the Humidity Sounder for Brazil (HSB).

AIRS observes over 2378 spectral bands with a frequency range of 19 - 80 GHz. AMSU observes over 15 spectral bands (50 - 60 GHz) which are specific to oxygen’s absorption lines. HSB observes over four spectral bands with a frequency of 183.3 GHz corresponding with water vapour’s absorption line.

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Aura

Aura (formerly EOS/Chem-1) is a chemistry mission of NASA with the overall objective of studying the chemistry and dynamics of Earth’s atmosphere from the ground to the mesosphere. Launched in July 2004, Aura is NASA’ third EOS mission, following on from Terra and Aqua. Aura carries two sounding instruments: the High-Resolution Dynamics Limb Sounder (HIRDLS), and the Microwave Limb Sounder (MLS).

HIRDLS is a mid-infrared limb-scanning spectroradiometer designed to sound the upper troposphere, stratosphere, and mesosphere across 21 channels between 6 - 18 µm. MLS measures thermal emissions from the atmospheric limb in submillimeter and millimeter wavelength spectral bands.

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FY-3 (FengYun-3)

FY-3 is a series of five satellites operated by the Chinese Meteorological Administration (CMA) and the China National Space Administration (CNSA), with the aim to monitor temperature, clouds, biosphere anomalies, as well as meteorological and hydrological events. FY-3 first launched in 2008, each spacecraft carrying 12 instruments, four of which are atmospheric sounders. These sounders aim to collect global data on atmospheric temperature, humidity, ozone levels, and water vapour, across all weather conditions.

Read more

Other Missions

FY-4 (FengYun-4)
Meteor-M series: -1, -2, 2-2
INSAT-3D, EDS and -3DR
DMSP (Defense Meteorological Satellite Program) Block 5D

References  

1) "What is sounding?," NASA, November 17, 2022, URL: https://airs.jpl.nasa.gov/sounding-science/what-is-sounding

2) "How Atmospheric Sounding Transformed Weather Prediction," National Environmental Satellite, Data, and Information Service, May 2, 2019, URL: https://www.nesdis.noaa.gov/news/how-atmospheric-sounding-transformed-weather-prediction

3) "Atmospheric Temperature and Humidity Sounders," The Earth Observation Handbook 2012, CEOS, URL: https://eohandbook.com/eohb2012/sat_earth_obs_atmos_temp.html

4) “About Soundings,” NASA JPL AIRS, URL: https://airs.jpl.nasa.gov/overlay-soundings/ 

5) "Introduction to the SkewT Diagram," NASA, URL: https://airsnrt.jpl.nasa.gov/SkewT_info.html