Atmospheric river

An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture in the atmosphere. Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans. Pineapple Express storms are the most commonly represented and recognized type of atmospheric rivers; they are given the name due to the warm water vapor plumes originating over the Hawaiian tropics that follow a path towards California. An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture in the atmosphere. Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans. Pineapple Express storms are the most commonly represented and recognized type of atmospheric rivers; they are given the name due to the warm water vapor plumes originating over the Hawaiian tropics that follow a path towards California. The term was originally coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s, to reflect the narrowness of the moisture plumes involved. Atmospheric rivers are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon River. There are typically 3–5 of these narrow plumes present within a hemisphere at any given time. In the current research field of atmospheric rivers the length and width factors described above in conjunction with an integrated water vapor depth greater than 2.0 cm are used as standards to categorize atmospheric river events. A January 2019 article in Geophysical Research Letters described them as 'long, meandering plumes of water vapor often originating over the tropical oceans that bring sustained, heavy precipitation to the west coasts of North America and northern Europe' that cause rainfall throughout the winter months.' As data modeling techniques progress, integrated water vapor transport (IVT) is becoming a more common data type used to interpret atmospheric rivers. Its strength lies in its ability to show the transportation of water vapor over multiple time steps instead of a stagnant measurement of water vapor depth in a specific air column (IWV). In addition IVT is more directly attributed to orographic precipitation, a key factor in the production of intense rainfall and subsequent flooding. For instance the water vapor image to the left shows two rivers on 5 December 2015: the first, stretching from the Caribbean to the United Kingdom, caused by Storm Desmond, and the second originating from the Philippines is crossing Pacific Ocean to the west coast of North America. The Center for Western Weather and Water Extremes (CW3E) at the Scripps Institution of Oceanography released a five-level scale in February 2019 to categorize atmospheric rivers, ranging from 'weak' to 'exceptional' in strength, or 'beneficial' to 'hazardous' in impact. The scale was developed by F. Martin Ralph, director of CW3E, who collaborated with Jonathan Rutz from the National Weather Service and other experts. The scale considers both the amount of water vapor transported and the duration of the event. Atmospheric rivers receive a preliminary rank according to the 3-hour average maximum vertically integrated water vapor transport. Those lasting less than 24 hours are demoted by one rank, while those lasting longer than 48 hours are increased by one rank. Examples of different atmospheric river categories include the following historical storms: Typically, the Oregon coast averages one Cat 4 atmospheric river (AR) each year; Washington state averages one Cat 4 AR every two years; the Bay Area averages one Cat 4 AR every three years; and southern California, which typically experiences one Cat 2 or Cat 3 AR each year, averages one Cat 4 AR every ten years. Atmospheric rivers have a central role in the global water cycle. On any given day, atmospheric rivers account for over 90% of the global meridional (north-south) water vapor transport, yet they cover less than 10% of the Earth's circumference. Atmospheric rivers are also known to contribute to about 22% of total global runoff.