NASA Facts: TRMM Instruments
Visible and Infrared Scanner
The Visible and Infrared Scanner VIRS) is one of the primary instruments
aboard the Tropical Rainfall Measuring Mission (TRMM) observatory. VIRS is
one of the three instruments in the rain-measuring package and serves as a very
indirect indicator of rainfall. It also ties in TRMM measurements with other
measurements that are made routinely using the meteorological Polar Orbiting
Environmental Satellites POES) and those that are made using the Geostationary
Operational Environmental Satellites (GOES) operated by the United States.
VIRS, as its name implies, senses radiation coming up from the Earth in five spectral
regions, ranging from visible to infrared, or 0.63 to 12 micrometers. VIRS is included
in the primary instrument package for two reasons. First is its ability to deliniate
rainfall. The second, and even more important reason, is to serve as a transfer standard
to other measurements that are made routinely using POES and GOES satellites. The
intensity of the radiation in the various spectral regions (or bands) can be used to
determine the brightness (visible and near infrared) or temperature (infrared) of the
source.
If the sky is clear, the temperature will correspond to that of the surface of the
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 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 rule of thumb are 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 can be employed. Nonetheless, IR techniques usually have significant errors
for instantaneous rainfall estimates. The strength of the IR observations lies in the ability
to monitor the clouds continuously from geostationary altitude. By comparing the visible and
infrared observations on the Tropical Rainfall Measuring Mission with the rainfall estimates
of the TRMM Microwave Imager and Precipitation Radar, it is hoped that much more can be
learned about the relationship of the cloud tops as seen from geostationary orbit.
VIRS uses a rotating mirror to scan across the track of the TRMM observatory, thus
sweeping out a region 833 kilometers wide as the observatory proceeds along its orbit.
Looking straight down (nadir), VIRS can pick out individual cloud features as small as
2.4 kilometers.
The Tropical Rainfall Measuring Mission is NASA’s first mission dedicated to observing
and understanding the tropical rainfall and how this rainfall affects the global climate.
It is a joint mission with the National Space Development Agency of Japan. The primary
instruments for measuring precipitation are the Precipitation Radar, the TRMM Microwave
Imager, and VIRS. Additionally, TRMM carries the Lightning Imaging Sensor (LIS) and the
Clouds and the Earth’s Radiant Energy System (CERES). These instruments can all function
individually or in combination with one another.
TRMM is part of NASA’s Mission to Planet Earth, a long-term, coordinated research effort
to study the Earth as a global system.
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