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How does the water cycle work? As every child learns, there
are four main parts to the water cycle – evaporation,
condensation, precipitation, and runoff. This enormous cycle
is constantly running and is a major player in local and global
climates. Understanding how the water cycle works in detail
is a big step in understanding how the planet itself works.
MODIS scientists are particularly interested in the gaseous
form of water within the atmosphere. Water vapor, in addition
to being a major part of the water cycle, can tell us a lot
about aerosols, aerosol-cloud interactions, the energy budget,
and the climate. To contribute to the study of water vapor,
MODIS collects data that are processed into a data product
named Near-Infrared Total Precipitable Water (MOD05).
The term ‘total precipitable water’ refers to
the total amount of water vapor in the atmosphere; if we were
to condense all of the water vapor in the atmosphere into
liquid form, we would get a layer of water covering the Earth’s
surface. The thickness of this layer (typically measured in
centimeters) is called TPW. Because water vapor is not easily
visible to the naked eye except in its condensed (cloud) form,
the MODIS makes use of its infrared (IR) channels, which can
see water vapor more easily. Even so, water vapor is hard
to observe, especially over Ocean surfaces, which are dark
and reflect very little light that MODIS can use to measure
TPW. MODIS gets around this difficulty by observing TPW over
sunglint, which is a phenomenon where sunlight reflects off
of the surface of the water at a certain angle and directly
back into the MODIS’ “eye.” Normally, sunglint
interferes with MODIS’ observations, but in the case
of TPW, MODIS turns it into an advantage.
Water vapor, in addition to being composed of one hydrogen
and two oxygen atoms, tends to attract a lot of other substances,
like dirt, dust, and chemicals, to form aerosols. Aerosols
reflect and absorb radiation in certain patterns. By looking
at the patterns of reflection, scientists are able to tell
what other substances the water vapor is carrying. This is
important for a number of reasons – a major one being
that when the water vapor condenses and falls back to the
Earth’s surface in the form of rain, whatever substances
it’s holding will fall with it. This can create phenomena
like acid rain and dust, which can be harmful to people, animals,
and plants, and do damage to cars, buildings, and other structures.
Studying water vapor can also help scientists learn about
how aerosols and clouds interact. Because clouds cover about
75% of the Earth at any given time, understanding how they
form, change, and disperse due to interaction with aerosols
is quite important. Clouds reflect and absorb a significant
portion of the radiation emitted by both the Sun and the Earth’s
surfaces, so it’s a given that clouds have a direct
and substantial impact on the world’s climate. Understanding
water vapor, and in turn clouds, is critical to accurate climate
modeling and weather forecasting. The Total Precipitable Water
product is designed to help atmospheric scientists achieve
these goals.
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