Microwave sounding units (MSU)
Microwave Sounding Units (MSUs) are sophisticated instruments designed to profile atmospheric temperature and moisture levels from space. First deployed on NOAA satellites in 1978, MSUs have undergone various upgrades, with the Advanced Microwave Sounding Units introduced in 1998 and the Advanced Technology Microwave Sounder (ATMS) being the latest iteration since 2017. These polar-orbiting satellites complete approximately sixteen orbits daily, capturing atmospheric data every eight seconds. The AMSU-A component measures thermal emissions from atmospheric oxygen across various channels to produce detailed temperature profiles across different atmospheric layers, while the AMSU-B instrument, later replaced by the Microwave Humidity Sounder (MHS), focused on water vapor concentration.
The data collected from MSUs are critical for climate research and weather prediction, providing insights into temperature trends, moisture levels, and other atmospheric properties. However, interpreting this data is complex, as different scientific methodologies yield varying temperature estimates. Notably, discrepancies in past data from weather balloons complicate assessments of long-term temperature changes. Despite these challenges, MSU data continues to play a vital role in enhancing the accuracy of weather forecasts and understanding climate dynamics.
Microwave sounding units (MSU)
Definition
A microwave sounding unit sounds (produces a profile of) the temperature and the moisture levels in theatmosphere. The first microwave sounding units were placed on National Oceanic and Atmospheric Administration (NOAA) satellites in 1978. In 1998, the Advanced Microwave Sounding Units replaced the older units. The satellites are polar-orbiting and synchronous with the sun. The satellites will make about sixteen orbits in twenty-four hours. The instruments make a scan every eight seconds, during which time the point on the surface below the satellite will have moved by 45 kilometers. During six of the eight seconds, the AMSU-A instrument is making thirty observations about 3° apart, from 48° on one side to 48° on the other. The next scan will be 45 kilometers farther along its orbit. After the thirty observations, the instrument makes an observation of a warm calibration target and an observation of cold space before returning to its original starting position. The AMSU-A instrument is determining temperature of the different layers of the atmosphere. Another instrument, AMSU-B, is scanning in a similar mode but makes three times as many observations and determines water vapor concentration. In 2005, the AMSU-B instrument was replaced by a Microwave Humidity Sounder (MHS). The MHS was replaced by the Advanced Technology Microwave Sounder (ATMS) in 2017.
The AMSU-A is detecting thermal emissions from atmospheric oxygen. Those emissions are in the microwave region of 23 to 89 gigahertz. The microwave radiation is low-energy and high-frequency. The AMSU-A instrument has several channels. Different channels record different wavelengths of radiation. Groups of channels emphasize different layers of the atmosphere. Combined together a profile of the temperature of the different layers of the atmosphere is produced. Channels 2 through 4 emphasize the surface, channels 5 through 8 emphasize the mid-troposphere, and channels 9 through 14 emphasize the lower stratosphere.
Significance for Climate Change
The data obtained from AMSU-A includes temperature and water vapor profiles of the atmosphere, snow and ice coverage, cloud water content, and rain rate. With data from other instruments, scientists can produce not only the temperature and water vapor profiles but also a measurement of the ozone, properties of clouds, and the amount of emitted by the Earth in areas not covered by clouds. The AMSU data are used in weather prediction. The more quickly the data get to weather prediction centers, the better the accuracy of the predictions.
It would appear that the data on temperature profiles that have been acquired since 1958 by balloons and since 1978 by MSU would allow for a determination of the increase or decrease of the temperature of the atmosphere. It has not proven to be so simple. Because of a change in equipment, the balloon data have a discontinuity that makes it much less valuable in determining temperature change. The data from MSU are not temperatures but thermal radiation from oxygen molecules. Different scientists use different methods to calculate the temperatures from the MSU data and consequently obtain different temperatures. Two of the groups doing calculations of this type are the Remote Sensing System (RSS) and the University of Alabama in Huntsville (UAH). The lower temperatures, in which stratospheric cooling has been eliminated, has been measured by RSS with an increase of 0.156° Celsius per decade and by UAH with an increase of 0.13° Celsius per decade. Other scientists have used the RSS data and the UAH data and have derived increases from 0.050° Celsius per decade to 0.20° Celsius per decade.
Bibliography
Chahine, Moustafa T. AIRS/AMSU/HSB—The Atmospheric Infrared Sounder, with Its Companion Advanced Microwave Sounding Unit and Humidity Sounder for Brazil: Providing New Insights into Earth’s Weather and Climate. Greenbelt, Md.: National Aeronautics and Space Administration, Goddard Space Flight Center, 2001.
Kegawa, Seiichiro, and Tsan Mo. An Algorithm for Correction of Lunar Contamination in AMSU-A Data. Washington, D.C.: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 2002.
Kegawa, Seiichiro, and Michael P. Weinreb. An Algorithm for Correction of Navigation Errors in AMSU-A Data. Washington, D.C.: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 2002.
Mo, Tsan. NOAA-L and NOAA-M AMSU-A Antenna Pattern Corrections. Washington, D.C.: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 2000.
"NOAA Fundamental Climate Data Record (CDR) of AMSU-B and MHS Brightness Temperature, Version 1." National Centers for Environmental Information, 2024, www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C00981. Accessed 17 Dec. 2024.