Improving the Understanding of Atmospheric River Water Vapor Transport Using a Three-Dimensional Straightened Composite Analysis

Irregular shapes of atmospheric rivers (ARs) hamper easy AR composite analyses and understandings of AR's moisture transport mechanisms. We develop a method to composite AR-related variables from a reanalysis data set. By averaging a large number of samples, the three-dimensional structure and some evolutionary features of a typical North Pacific AR are revealed. An AR is typically located along and in advance of the surface cold front of an extratropical cyclone. A mesoscale secondary circulation is observed in the cross sections of the AR corridor, where both geostrophic and ageostrophic winds make indispensable contributions to the moisture fluxes. Geostrophic moisture advection across the cold front within the Equatorward half of the AR is created by baroclinicity of the system and serves as the primary moisture source for the AR. Moisture fluxes from the warm sector of the cyclone are primarily due to boundary layer ageostrophic winds and are more important within the poleward half of the AR, particularly during the genesis stage. Faster AR movement compared with low-level winds enables the AR to collect downwind moisture. In an AR-relative view, this is represented as an easterly flow, consistent with the "feeder-airstream" concept, and emphasizes the role of local moisture recycling. Moisture transport within the Equatorward half is mostly due to geostrophic advection of the propagating AR-cyclone couple in an Earth-relative view. Driven by intensifying geostrophic winds, ARs tend to reach peak moisture transport intensity about 2 days after genesis. Then, reduced moisture level and influxes from lateral boundaries prevent further intensification.