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Improving Understanding of Atmospheric River Water Vapor Transport using a Three-Dimensional Straightened Composite Analysis
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  • Guangzhi Xu,
  • Lin Wang,
  • Ping Chang,
  • Xiahui Ma,
  • Shuyu Wang
Guangzhi Xu
Beijing Normal University

Corresponding Author:xugzhi1987@gmail.com

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Lin Wang
Institute of Atmospheric Physics, Chinese Academy of Sciences
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Ping Chang
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Xiahui Ma
Ocean University of China
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Shuyu Wang
Key Laboratory of Physical Oceanography and Frontiers Science Center for Deep Ocean Multispheres and Earth System
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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.