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Atmospheric flows are often decomposed into balanced (low frequency) and unbalanced (high frequency) components. For a dry atmosphere, it is known that a single mode, the potential vorticity (PV), is enough to describe the balanced flow and determine its evolution. For a moist atmosphere with phase changes, on the other hand, balanced–unbalanced decompositions involve additional complexity. In this paper, we illustrate that additional balanced modes, beyond PV, arise from the moisture. To support and motivate the discussion, we consider balanced–unbalanced decompositions arising from a simplified Boussinesq numerical simulation and a hemispheric-sized channel simulation using the Weather Research and Forecasting (WRF) Model. One important role of the balanced moist modes is in the inversion principle that is used to recover the moist balanced flow: rather than traditional PV inversion that involves only the PV variable, it is PV-and- M inversion that is needed, involving M variables that describe the moist balanced modes. In examples of PV-and- M inversion, we show that one can decompose all significant atmospheric variables, including total water or water vapor, into balanced (vortical mode) and unbalanced (inertio-gravity wave) components. The moist inversion, thus, extends the traditional dry PV inversion to allow for moisture and phase changes. In addition, we illustrate that the moist balanced modes are essentially conserved quantities of the flow, and they act qualitatively as additional PV-like modes of the system that track balanced moisture.
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Oceanic moisture sources contributing to wintertime Euro-Atlantic blocking
Abstract. Although conventionally attributed to dry dynamics, increasing evidence points to a key role of moist dynamics in the formation and maintenance of blocking events. The source of moisture crucial for these processes, however, remains elusive. In this study, we identify the moisture sources responsible for latent heating associated with the wintertime Euro-Atlantic blocking events detected over 31 years (1979–2010). To this end, we track atmospheric particles backward in time from the blocking centres for a period of 10 d using an offline Lagrangian dispersion model applied to atmospheric reanalysis data. The analysis reveals that 28 %–55 % of particles gain heat and moisture from the ocean over the course of 10 d, with higher percentages for the lower altitudes from which particles are released. Via large-scale ascent, these moist particles transport low-potential-vorticity (PV) air of low-altitude, low-latitude origins into the upper troposphere, where the amplitude of blocking is the most prominent, in agreement with previous studies. The PV of these moist particles remains significantly lower compared to their dry counterparts throughout the course of 10 d, preferentially constituting blocking cores. Further analysis reveals that approximately two-thirds of the moist particles source their moisture locally from the Atlantic, while the remaining one-third of moist particles source it from the Pacific. There is also a small fraction of moist particles that take up moisture from both the Pacific and Atlantic basins, which undergo a large-scale uplift over the Atlantic using moisture picked up over both basins. The Gulf Stream and Kuroshio and their extensions as well as the eastern Pacific northeast of Hawaii not only provide heat and moisture to moist particles but also act as “springboards” for their large-scale, cross-isentropic ascent, where its extent strongly depends on the humidity content at the time of the ascent. While the particles of Atlantic origin swiftly ascend just before their arrival at blocking, those of Pacific origin begin their ascent a few days earlier, after which they carry low-PV air in the upper troposphere while undergoing radiative cooling just as dry particles. A previous study identified a blocking maintenance mechanism, whereby low-PV air is selectively absorbed into blocking systems to prolong blocking lifetime. As they used an isentropic trajectory analysis, this mechanism was regarded as a dry process. We found that these moist particles that are fuelled over the Pacific can also act to maintain blocks in the same manner, revealing that what appears to be a blocking maintenance mechanism governed by dry dynamics alone can, in fact, be of moist origin.
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- Award ID(s):
- 2040073
- PAR ID:
- 10313877
- Date Published:
- Journal Name:
- Weather and Climate Dynamics
- Volume:
- 2
- Issue:
- 3
- ISSN:
- 2698-4016
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/wcd-2-819-2021
