Atmospheric Temperature

Introduction to MSU and AMSU Data Products

Satellite measurements of the atmospheric temperature are used for long-term monitoring of Earth’s climate. Satellite-borne instruments called microwave sounders can measure the intensity of the microwaves emitted by the atmosphere. This allows us to measure the average temperature of thick layers of the atmosphere at different heights above the Earth. These satellites have polar orbits, allowing them to measure the temperature over nearly the entire globe every few days.

The first satellite instruments to make this kind of measurement were the Microwave Sounding Units (MSU), which operated on a series of 9 NOAA-operated polar-orbiting satellites beginning in late 1978 and continuing to 2005.

Channels and Datasets

The MSUs make measurements of microwave radiance in four different frequencies (or channels) ranging from 51.3 to 57.95 GHz, a frequency range where microwaves are strongly absorbed and emitted by Oxygen molecules. Because the strength of the absorption, and thus also transparency of the atmosphere, varies substantially with frequency near this band, the four channels measure the atmospheric temperature in four thick layers at different heights. Of these 4 channels, we use 3 to construct our datasets, MSU channels 2, 3, and 4.

TLS stands for the Temperature Lower Stratosphere, and is constructed using data from MSU channel 4. As we can see from the diagram, TLS corresponds to a layer of the atmosphere from about 12 km to about 25 km above Earth’s surface. This layer is so high that its temperature has little direct impact on life and human society. People only go this high in commercial airliners, which can fly as high as 12 km, and in military aircraft, which may fly as high as 25 km.

TTS stands for the Temperature Troposphere Stratosphere, and is constructed using data from MSU channel 3. TTS corresponds to a layer of the atmosphere from about 3 km to about 20 km above the surface, with most of the weight coming from about 10 km. Over the past decades, the stratosphere (the layer of the atmosphere above ~15 km) has been cooling, and the troposphere (the layer of the atmosphere from the surface to ~15 km) has been warming. Because TTS is a combination of both these layers, there has been little long-term change in this layer.

TMT stands for the Temperature Middle Troposphere, and is constructed using data from MSU channel 2. TMT corresponds to a layer of the atmosphere from the surface to about 15 km, with the peak weight at about 4 km. TMT is mostly a measurement of the troposphere, the layer near the surface where we live, but has some influence (about 10 -15 percent depending on location) from the stratosphere. Because the stratosphere has been cooling, the amount of warming in TMT is slightly less than the warming of the troposphere by itself.

TLT stands for the Temperature Lower Troposphere. This measurement is constructed from MSU channel 2 by subtracting measurements made at different angles from each other. Because of this subtraction (which is a form of extrapolation) these measurements are more uncertain than the TMT measurements constructed from the same basic data. TLT does have the advantage that there is no influence of the cooling in stratosphere in these measurements, and it is closer to the surface and has a more direct influence on life and human society.

AMSU

Beginning in 1998, a series of follow-on instruments to the MSUs, the Advanced Microwave Sounding Units (AMSUs), began operation. The AMSU instruments are similar to the MSUs, but they make measurements using a larger number of channels, thus sampling the atmosphere in a larger number of layers, including a number of layers higher in the stratosphere. By using the AMSU channels that most closely match the channels in the MSU instruments, we can extend the dataset beyond 2005. TLS is extended using AMSU channel 9, TTS is extended using AMSU channel 7, and TMT and TLS are extended using AMSU channel 5.

With the advent of the AMSU series of instruments, we are now able to use microwaves to monitor temperatures high in the stratosphere using data from AMSU channels 10 through 14.

The temperature at these high-altitude levels has little direct effect on people and life on Earth. However, atmospheric scientists can use these measurements to help understand the mechanisms behind the depletion and eventual recovery of the ozone layer, and how changes in the power output of the sun influence the entire atmosphere. The ozone layer is important for both human society and natural ecosystems because it helps block harmful ultraviolet radiation from reaching the surface.

Calibration

The MSU and AMSU instruments were intended for day-to-day use in weather forecasting and thus were not calibrated to the precision needed for climate studies. A climate quality dataset can be extracted from their measurements only by careful intercalibration of the different MSU and AMSU instruments and by performing precise adjustments to account for changes in the time of day that the measurements are made. A detailed description of the intercalibration and adjustment procedure is available. These adjustments, combined with errors in complete sampling of the Earth on a given day, lead to some uncertainty in the final measurements. We have analyzed this uncertainty in detail. Our datasets are updated once each month. Time series plots and maps from the most recent data are available.

Acknowledgement

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.

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