Life Cycle of Atmospheric Rivers: Identification and Climatological Characteristics

摘要

An atmospheric river (AR) event is a strong poleward moisture transport that is composed of a series of spatiotemporally connected instantaneous AR objects. A new object-based tracking algorithm is developed in this study, which aims to identify an AR event and investigate its life cycle from origin to termination. The algorithm identifies duration, intensity, propagation speed and direction, and the traveled distance throughout the life cycle of the AR event. The tracking algorithm is applied to 6-hourly column-integrated water vapor flux from November to March during the period of 1979-2017, with a focus on the North Pacific. Most North Pacific AR events originate in the subtropics over the Northwest Pacific and terminate at higher latitudes over the Northeast Pacific including western North America. On average, long AR events that last more than 72 hrs travel 7 times longer in distance and have stronger intensity than short AR events that last less than 24 hrs. Finally, a new accumulated AR intensity index is developed, which summarizes the overall impact of AR events over a given domain during a certain period by incorporating number, lifetime, and intensities of AR events. Plain Language Summary Atmospheric rivers (ARs) are " rivers" in the sky that transport water vapor from the tropics to higher latitudes. ARs play an important role in water resources because they transport as much water as the largest rivers on the ground and can often induce extreme precipitation when making landfall. A technique is developed to numerically detect and track ARs as they propagate in time and space. Based on this technique, we study the climatological characteristics of the life cycle (i. e., from appearance to disappearance) of winter ARs over the North Pacific. Most of the ARs originate at about 25 degrees N over the Northwest Pacific and terminate between 50 degrees N and 60 degrees N over the Northeast Pacific. We also found that longer-lived ARs can travel 7 seven times longer distances compared to shorter-lived ARs. Finally, an index is developed to represent the overall activity of ARs of a given time period.