Satellite measurements of the Earth’s microwave emissions are a crucial element in the development of an accurate system for long-term monitoring of atmospheric temperature. Satellites provide global coverage at much higher densities than attainable with in situ observations. In situ observations also suffer from non-uniform temporal coverage and undocumented changes in the instrumentation used that can lead to local biases and increased uncertainty.
Microwave Temperature Sounding
Satellites can measure the temperature of the atmosphere by evaluating thermal emission from gases in the atmosphere. Molecular Oxygen has a complex of relatively strong absorption lines near 60 GHz. By choosing the different measurement frequencies, and thus different values of absorptivity, the emission from different layers of the atmosphere can be measured. RSS studies the measurements made by 3 series of satellite-borne microwave sounders in order to construct long-term, climate-quality atmospheric temperature datasets for use by the scientific community. The 3 instruments are listed below
The Microwave Sounding Units (MSU) operating on NOAA polar-orbiting satellite platforms were the principal sources of satellite temperature profiles from late 1978 to the early 2000's. The MSUs were cross-track scanners that made measurements of microwave radiance in four channels ranging from 50.3 to 57.95 GHz on the lower shoulder of the Oxygen absorption band. These four channels measured the atmospheric temperature in four thick layers spanning the surface through the stratosphere. The were 9 MSUs in total. The last MSU instrument, NOAA-14, ceased reliable operation in 2005.
A series of follow-on instruments, the Advanced Microwave Sounding Units (AMSUs), began operation in 1998. The AMSU instruments are composed of 2 sub units, AMSU-A and AMSU-B. AMSU-B is a humidity sounder (not discussed further), and AMSU-A is a 15-channel temperature sounder similar to MSU. Of the 15 channels, 11 (Channels 4 through 14) are located in the 60 GHz absorption complex and thus are most closely related to atmospheric temepratures at various heights above the surface. The increased number of channels relative to MSU means that AMSU-A samples the temperature of the atmosphere in a larger number of layers. The AMSU measurement footprints are also smaller than those for MSU, leading to higher spatial resolution. 3 AMSU channels (Channels 5,7,and 9) are closely matched to MSU channels 2,3 and 4. By using these channels,we have extended our climate-quality dataset to the present.
In addition, we have completed a preliminary analysis of AMSU channels 10-14, which measure temperatures higher in the stratosphere than the highest MSU channel (channel 4). These datasets began in mid 1998 with the launch of the first AMSU on the NOAA-15 satellite. These datasets, now 14 years long, are beginning to be long enough to become interesting for investigating long-term changes in the mid and upper stratosphere.
So far, there are 8 AMSU instruments, 5 on NOAA platforms (NOAA-15 through NOAA-19), 2 on EUMETSAT platforms (Metop-A and Metop-B), and one on NASA AQUA satellite.
Advanced Technology Microwave Sounder (ATMS)
In the future, the AMSU instruments will be phased out, and replaced with the Advanced Technology Microwave Sounder (ATMS). The first ATMS was launched October 28, 2011. Measurements made by the ATMS are not yet used in our products. We are working to cross-calibrate ATMS with AMSU so that ATMS measurements can be included in future products.
All microwave sounding instruments were developed for day-to-day operational weather forecasting use and therefore are typically not calibrated to the precision that is needed for climate studies. A climate quality dataset can be extracted from MSU and AMSU measurements only by careful intercalibration of the data from the MSU, AMSU and ATMS instruments, including the removal of spurious signals that are not due to changing atmospheric temperatures.
Period of Operation for MSU and AMSU
Figure. 1 Plot of which satellites are used for each month to construct the MSU/AMSU TLS dataset.
Microwave Sounding Data Products from RSS
We currently provide a number of data products constructed by merging the MSU and AMSU Data from different satellites together. These include
- 3 single-channel MSU/AMSU datasets (TMT, TTS, and TLS) that extend back to late 1978
- 5 single channel AMSU-only stratospheric datasets (C10, C11, C12, C13, and C14) that begin in mid 1998. These datasets are fairly early in their development process and should be considered preliminary (C10-C12) or experimental (C13 and C14).
- TLT is a more complex dataset constructed by calculating a weighted difference between measurements made at different Earth incidence angles to extrapolate MSU channel 2 and AMSU channel 5 measurements lower in the atmosphere.
- 2 multi-channel datasets, TTT and C25, that are constructed from weighted combinations of the single channel datasets. Since C25 is constructed from AMSU Channels C10-C13, it should be considered preliminary.
Each of these products is discussed in more detail in the Upper Air Temperature Measurements page.
MSU/AMSU data are produced by Remote Sensing Systems. Over the years, we have received support for the development of this dataset from a number of sources, including NOAA's Office of Global Programs, NOAA's Climate Program Office, and NOAA's Climate Data Record Program. Production of the current dataset (version 3.3) is supported by NOAA's Climate Data Record Program, while improvements to the methods used to produce the dataset are currently supported by NASA's Earth Science Division, which is part of the Science Mission Directorate