09/01/2022
ANATOMY OF AN ATMOSPHERIC RIVER (AR)
An atmospheric river is a narrow, elongated flow of moist air in the lower atmosphere. The flow corridor measures is based on satellite observations, an atmospheric river is greater than 2,000 km (1,245 miles) long, less than 1,000 km (620 miles) wide, and averages 3 km (1.8 miles) in depth.
Atmospheric rivers, like the Pineapple Express, form along the front edge of slow-moving, low-pressure weather systems related to the polar jet stream. The cyclone nature of these weather systems in the northern hemisphere causes winds to flow from southwest to northeast. Hence, the warm moist air from the tropics reaches Americas West Coast traveling as far north as Washington and Oregon. This moisture transport occurs under particular combinations of wind, temperature, and pressure conditions.
Atmospheric rivers are typically located within the low-level jet, an area of strong winds in the lower levels of the atmosphere, ahead of the cold front in an extratropical cyclone. Studies have found that typical atmospheric river conditions last around 20 hours over an area on the coastline. Strong land-falling atmospheric rivers interact with topography and can deposit significant amounts of precipitation in relatively short periods of time leading to flooding and mudslides. Atmospheric rivers also can have beneficial impacts by contributing to increases in snowpack, such as in the western United States.
View from outer space of AR from Asia to the US West Coast