TIROS Operations Vertical Sounder (TOVS)
NOAA's TIROS Operational Vertical Sounder(TOVS) is a
suite of three instruments: the Microwave Sounding Unit(MSU), the High resolution Infrared
Radiation Sounder(HIRS), and the Stratospheric Sounding Unit(SSU). Each instrument
measures radiation emmitted by the Earth at several different wavelengths. The HIRS
channel 9 measures Earth's emmitted infrared radiation at
9.7 microns
(10-6
meters). This is a "window channel" meaning that the radiation measured by the
HIRS instrument is emmited from the earth's surface (as opposed to radiation being
emmitted at other levels of the earth's atmosphere). The amount of radiation reaching the
HIRS instrument is dependant upon how much ozone is in the earth's atmosphere (less ozone
= more radiation). Therefore, the TOVS Total Ozone algorithm uses this channel (along with
information from other HIRS channels) to estimate the total amount of ozone in the earth's
atmosphere. The greatest contribution of the emmitted radiation
occurs in a region between 200 hPa and 30 hPa (13km to 27km). This
"lower stratosphere" region is below the levels where the
greatest contribution
to the total ozone amount
occurs(50hpa to 10hPa or 20km to 30km). Thus the ozone amount measured by the TOVS Total
Ozone algorithm is not a true measure of the "total" amount of ozone in the
earth's atmosphere. Rather it is a better measure of the ozone amount in the lower
stratosphere. To obtain a "total" ozone amount, the TOVS Total Ozone algorithm
adjusts the lower stratosphere ozone amount by a climatological amount that is variable
with season and latitude.
This is in contrast with satellite instruments which measure the amount of
backscattered radiation at various ultraviolet wavelengths. Backscattered radiation levels
at wavelengths where ozone absorbtion does and does not take place are compared with the
same wavelenghts measured directly from the sun to derive a "total ozone" amount
in the earth's atmosphere. This methodology is used by the
NASA TOMS and the NOAA SBUV/2 ozone
monitoring programs. This methodology provides a truer measure of the total ozone amount
in the earth's atmosphere. One drawback is that this method uses "backscattered"
sunlight. Which means that data cannot be retrieved in the earth's shadow or polar night
regions.
The TOVS Total Ozone algorithm can determine ozone amounts at all times since it is
derived from the Earth's emmitted infrared radiation. There are drawbacks to the TOVS
infrared methodolgy though. When the earth's surface is either too cold (e.g., the high
Antarctic Plateau) too hot (e.g., the Sahara desert) or too obscured (e.g., by heavy
tropical cirrus clouds) the accuracy of this methodolgy declines.
Information about the TOVS ozone algorithm was provided by:
Arthur C. Neuendorffer (retired)
Office of Research and Applications
Climate Research & Applications Division
Last Updated March 8, 1999 by Craig S. Long
July 21, 2021 by Jeannette D. Wild.
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