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Precipitation, Global

Precipitation is the principal link between global water and energy cycles, weather and climate, and the activities of humankind. Natural and anthropogenic processes can alter the frequency, distribution, and magnitude of precipitating weather systems (e.g., the El Niño Southern Oscillation, urban heat islands, and deforestation). Changes in precipitation characteristics can affect Earth's energy balance through the redistribution of solar and terrestrial energy and latent heat fluxes. Planetary atmospheric circulation is largely driven by Earth's energy balance; thus, global precipitation patterns are a key element in climate variability. Therefore, the critical task is to continue to develop methods that allow for consistent, high-quality global observations of precipitation in order to gain better insight into the Earth system and weather and climate variability through improved forecasts. This entry discusses in situ measurements and remote sensing techniques used in the study of precipitation processes.

In Situ Measurements

Given that precipitation processes often evolve rapidly and are stochastic in nature, our understanding of their characteristics is limited in part by our ability to observe and accurately represent precipitation over a variety of time and space scales. This is especially true for developing countries and remote regions around the world, including equatorial (e.g., Amazonia, South America) and oceanic regions, where in situ observational (rain gauge) networks are severely limited. In addition to the insufficient global density of rain gauges, other drawbacks are that rain gauges are point measurements that often fail to effectively represent precipitation variability across relatively large areas. For example, it is not uncommon for a single thunderstorm to produce intense rainfall over an area that is not covered by rain gauges, systematically missing out on the occurrence of precipitation. Unfortunately, any sufficiently large rain gauge network would likely be highly expensive and impractical to implement. Other effects such as wind, evaporation, and water sticking to the side of the gauge can lead to underestimation of precipitation. Furthermore, there are often inconsistencies among rain gauge types and in calibration efforts.

Despite these pitfalls, one very important advantage of the use of rain gauges is their use of a metric that is based on the actual collection of precipitation. Another advantage includes excellent temporal sampling. Ultimately, rain gauges alone cannot adequately represent global precipitation due to the lack of coverage in the major part of the world. But since rain gauges physically measure precipitation at the surface, these instruments remain very important with respect to the verification and calibration of other instruments and approaches, including radars and satellites.

Remote sensing Techniques

In an attempt to compensate for the deficiencies of rain gauges, the use of active and passive remote sensing techniques has steadily increased since the mid to late 20th century. While the use of remote sensing often increases the spatial sampling of precipitation (when compared with in situ observations), this approach is not without limitations. The primary disadvantage is that the radiometric properties of precipitating systems are used to infer the occurrence and intensity of precipitation. These inferences are algorithmically derived from various cloud attributes, including cloud-top temperatures, cloud-type and height discrimination, and the emission and scattering properties of hydrometeors, among others.

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