Improving Understanding of Atmospheric River Water Vapor Transport using
a Three-Dimensional Straightened Composite Analysis
Abstract
The irregular shapes of atmospheric rivers (ARs) and the scarcity of
sounding data have hampered easy AR composite analyses and
understandings about AR’s moisture transport mechanism. In this work we
develop a method to composite AR-related variables from a reanalysis
dataset. 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 front of the
surface cold front of an extratropical cyclone. A meso-scale secondary
circulation is observed in the cross-sections of the AR corridor, where
both geostrophic and ageostrophic winds make indispensable contributions
to the strong moisture transport. Geostrophic moisture advection across
the cold front within the Equatorward half of the AR is created by the
baroclinicity of the system, and serves as the primary moisture source
of the AR-resided atmosphere. Moisture fluxes from the warm sector of
the cyclone are primarily due to ageostrophic winds within the boundary
layer, and are more important within the poleward half the AR,
particularly during the genesis stage. The faster movement speed of the
AR compared with low level winds enables the ARs to collect downwind
moisture. While within the Equatorward half moisture transport is mostly
attributed to geostrophic advection carried along by the propagating
AR-cyclone couple. Driven by the intensifying geostrophic winds, ARs
tend to reach peak moisture transport intensity about two days after
genesis. Then reduced moisture and influxes from lateral boundaries
prevent further moisture flux intensification.