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Instrument: VIRS : TRMM Visible Infrared Scanner
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Associated Platforms
TRMM

Spectral/Frequency Information
Wavelength Keyword: Visible
Spectral/Frequency Resolution: 0.63 micrometers

Wavelength Keyword: Infrared > Reflected
Spectral/Frequency Resolution: 1.6 micrometers

Wavelength Keyword: Infrared > Thermal
Spectral/Frequency Coverage/Range: 3.75 to 12 micrometers

Related Data Sets
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Description
VIRS is one of the three instruments in the rain-measuring package on TRMM and provides a very indirect indicator of rainfall. It also ties TRMM measurements to other measurements that are made routinely using the meteorological Polar-orbiting Operational Environmental Satellite System (POESS) and those that are made using the Geostationary Operational Environmental Satellites (GOES) operated by the United States.

VIRS senses radiation coming from Earth in five spectral regions, ranging from visible to infrared, at wavelengths of 0.63 to 12 micrometers. VIRS is included in the primary instrument package on TRMM because of its ability to delineate rainfall, but even more importantly, because it serves as a transfer standard to other measurements that are made routinely using the POESS and GOES satellites. The intensity of the radiation in the various spectral regions (or bands) can be used to determine the reflectance (visible and near infrared) or brightness temperature (infrared) of the scene.

If the sky is clear, the temperature will correspond to that of the surface of Earth, and, if there are clouds, the temperature will tend to be that of the cloud tops. Colder temperatures will produce greater intensities in the shorter wavelength bands, and warmer temperatures will produce greater intensities in the longer wavelength bands. Since colder clouds occur at higher altitudes the measured temperatures are useful as indicators of cloud heights, and the highest clouds can be associated with the presence of rain. A variety of techniques use the infrared (IR) images from VIRS to estimate precipitation. Higher cloud tops are positively correlated with precipitation for convective clouds (generally thunderstorms), which dominate tropical (and therefore global) precipitation accumulations. One notable exception to this correlation is with the high cirrus clouds that generally flow out of thunderstorms. These cirrus clouds are high and therefore cold in the infrared observations, but they do not rain. To differentiate these cirrus clouds from water clouds (cumulonimbus), a technique, which involves comparing the two infrared channels at 10.8 and 12.0 micrometers, is employed. However, IR techniques usually have significant errors for instantaneous rainfall estimates. The strength of infrared observations lies in the ability to monitor the clouds continuously from Geostationary altitude. By comparing the visible and infrared observations from TRMM with the rainfall estimates of TMI and PR, scientists hope to learn much more about the relationship of the cloud tops as seen from Geostationary orbit.

NASA Earth Science Reference Handbook [ Mission: TRMM ]

Online Resources
http://trmm.gsfc.nasa.gov/overview_dir/virs.html
http://daac.gsfc.nasa.gov/precipitation/TRMM_README/TRMM_1B01_readme.shtml
http://www.newmediastudio.org/DataDiscovery/Aero_Ed_Center/RS/TRMM_VIRS.html
http://eosweb.larc.nasa.gov/PRODOCS/ceres/SSF/Quality_Summaries/CER_SSF_TRMM_Edition2B.html

Instrument Logistics
Data Rate: 49.8 Kbps (day), 28.8 Kbps (night)
Instrument Start Date: 1997-12-20
Instrument Owner: USA/NASA