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- Title
Improving the Understanding of Atmospheric River Water Vapor Transport Using a Three‐Dimensional Straightened Composite Analysis.
- Authors
Xu, Guangzhi; Wang, Lin; Chang, Ping; Ma, Xiaohui; Wang, Shuyu
- Abstract
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. Plain Language Summary: Atmospheric rivers (ARs) are "river‐like" plumes of airborne horizontal moisture transport that play a crucial role in atmospheric water cycle. Unlike terrestrial rivers, ARs are highly transient and with constantly changing shapes, making it difficult to summarize their structural and dynamical features using a composite analysis. We attack this problem by straightening the irregular ARs using a two‐dimensional interpolation, thus allowing us to create composites from a large number of AR samples. The results reveal the typical structure of a North Pacific AR, its relationship with an accompanying cyclone, the manner in which AR collects moisture from its lateral boundaries, and the changes in different contributing factors following its lifetime evolution. Thanks to its faster propagation speed compared with low‐level winds, an AR collects downwind atmospheric moisture on its poleward‐facing half, a mechanism that is particularly important during its early development stage. The large‐scale circulation associated with the accompanying cyclone accounts for most of the vapor plumes toward the Equatorward‐facing half of the AR. Key Points: Develops a new method to composite atmospheric rivers with irregular shapes and orientationsReveals the meso‐scale secondary circulation across an atmospheric river and highlights the relationship with frontogenesis processesDepicts the 3D structure of moisture transport within an atmospheric river and different geostrophic and ageostrophic contributions
- Subjects
ATMOSPHERIC water vapor; HUMIDITY; WATER vapor transport; ATMOSPHERIC rivers; GEOSTROPHIC wind; ATMOSPHERIC circulation; CYCLONES
- Publication
Journal of Geophysical Research. Atmospheres, 2022, Vol 127, Issue 11, p1
- ISSN
2169-897X
- Publication type
Article
- DOI
10.1029/2021JD036159