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MODIS Data Product Non-Technical Description - MOD 05

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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