Global Precipitation

Adapted from What is Precipitation?, The GPM Monitor, Lena Braatz, Ed., June 2002.

precipitation (n) water that falls from the clouds toward the ground, esp. as rain or snow
(Definition from the Cambridge Dictionary of American English)

The description above sounds simple enough. Precipitation, however, can take other forms in addition to rain and snow. Sleet, hail, drizzle, and graupel (an amalgamation of hail and snow that resembles round snow particles) are all forms of precipitation. Water in its various forms covers most of the surface of our planet, and this basic element has a profound impact on life on Earth. Water that falls to the ground from clouds is essential to all living organisms, but precipitation can also trigger catastrophes that can harm or even obliterate life.

For these reasons, scientists strive to understand precipitation when it falls, where it falls, and why it falls. Greater knowledge of precipitation mechanisms will allow researchers to increase their understanding of the global water cycle, which is intimately linked to changes in Earths climate system. Precipitation data are utilized heavily in the models that scientists use to predict our weather; more accurate precipitation data will lead to enhanced weather prediction.

Globe with Precipitation Data
A global view of precipitation, focusing on the Pacific Ocean. Note that the highest precipitation (yellow and red areas) occurs over the ocean.
Precipitation data with a high temporal and spatial sampling rate will enable scientists to improve life on Earth, and perhaps even help save lives. Scientists will be able to refine flood and storm forecasts, more accurately predict the availability of freshwater resources, formulate agricultural plans that take predicted climate changes into account, and more.
But why do we need satellite-based estimates of precipitation? Why cant scientists just measure precipitation on the ground, as it falls? Measurement of precipitation is deceivingly simple just put a can in the back yard. However, unlike meteorological parameters such as pressure and temperature, precipitation is driven by small-scale processes, meaning that it is highly variable and rapidly changing. You have probably witnessed evidence of this complexity yourself, perhaps as a thunderstorm passed by, with its varying rates of rainfall and even hail during a brief period of time. Or you may have heard about a storm that dropped lots of rain in one part of town and only a few drops in another. As a result, we need to sample the data relatively frequently (every three hours or better) across the entire Earth. California Storms 2010 Extensive cloud cover from heavy rain storms along the California coast in early 2010.
In the land areas of most developed countries rain gauges and radar provide satisfactory data on precipitation. However, sparsely populated or underdeveloped regions frequently lack the resources to successfully record and share precipitation data. Oceans, which cover most of the Earths surface, are almost entirely devoid of precipitation measurements.
The only viable means of collecting precipitation data on a global scale is to utilize satellite observations. A variety of instruments has flown in orbit since the early 1980s, with major advances coming in the late 1980s and in 1997, with the launch of the Tropical Rainfall Measuring Mission (TRMM).
Advancements continue, including the upcoming launch of the Global Precipitation Measurement (GPM) satellite. State-of-the-art global precipitation estimates use data from the entire international constellation of satellites carrying precipitation-related instruments. Current approaches for computing satellite estimates do not account for regional nuances in precipitation, so surface observations from as many parts of the globe as possible continue to be critical for creating the best estimates.

Thus, despite its simple-sounding definition, precipitation presents a multifaceted, intricate challenge to the scientific community. Continued work is necessary for researchers to create the data they need to more completely understand the complex workings of our planets water system, and to ultimately improve the quality of life on Earth.

Plot with Precipitation Data
Average global distribution of precipitation for the month of February, based on satellite and rain gauge measurements. The highest rainfall, averaging more than 15 mm every day, occurs in the southern rain forest regions.