Atmospheric rivers actually mean rivers of water in our skies. But what is the science behind this phenomena that can become a very serious threat across regions of the globe. An atmospheric river (also known as AR) is a long and narrow flowing stream of water, actually condensed water vapour, in the atmosphere. It is capable of producing significant levels of rain and snow. When the atmospheric river moves across land and over mountainous terrain, the water vapour rises and cools to create very heavy precipitation or snowfall.
Many atmospheric rivers are of moderate size but some are exceptionally powerful and extremely large producing excessive rainfall and floods. These extreme occurrences are responsible for severe and almost instantaneous downpour of flood waters that disrupt normal lifestyle. The extreme rainfall results in extreme, serious flooding that is often associated with destructive mudflows, and mudslides down steep terrain causing extreme damage to property and severe loss of life.
A strong AR transports water vapour equivalent to 7.5-15 times the average flow in the Mississippi River in the United States. An atmospheric river system is part of the global water system and on occasions, its severity has devastating impact on the land terrain it traverses.. They make a serious contribution to the water supply but are also associated with very severe and extreme flooding.
The term, atmospheric rivers came into existence in the 1990’s when these narrow moisture plumes became more pronounced in our atmospheric systems. On average, an AR is between 250 – 375 miles (400-600km) wide. They are typically very long in comparison to their width. There are typically 3-5 such moisture plumes or ‘rivers’ present in the global moisture system at any one time.
The actual modelling of these moisture systems is based on the integrated water vapour transport (IVT) associated with these systems. The models predict the measure of water vapour over a series of multiple time steps (or sequences). This is more informative that the static value of water vapour depth in a column of air, known as the integrated water volume. The IVT is also more in keeping with the formation of orographic precipitation. This is when moist air is forced up over mountainous terrain producing rainfall.
There is a five-level scale introduced by the Script Institution of Oceanography in 2019. This scale is based on the 3-hour average of maximum vertically integrated water vapour transport (IVT). The atmospheric river is categorised from “weak” to “exceptional” in strength, and from “beneficial” to “hazardous” in impact. The amount of water vapour transported and the duration of the event are incorporated into the scale. The initial ranking is based on the 3-hour average IVT.
The severe flooding and devastation associated with these systems is capable of massive destruction across the area impacted, yet it can also provide significantly to the regional water supply and snow pack in mountainous areas. The more severe occurrences of these moisture plumes are those that reach the main news streams and weather forecast channels as they are predicted to result in extensive power outages, as well as massive destruction to natural environments and human infrastructures. This destruction is typically in the form of rapid flooding, mudslides and road slips, rock displacement and mudflows over powering waterflows. This is further exasperated by the almost instantaneous flooding that occurs.
A well documented occurrence of this phenomena is the “Pineapple Express” that originates over the waters of Hawaii. The tropical waters are transported across the Pacific to the west coast of the US.
