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Importance of latent heat release in ascending air streams for atmospheric blocking

Nature Geoscience volume 8, pages 610–614 (2015)Cite this article

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Abstract

Atmospheric blocking is a key component of extratropical weather variability1 and can contribute to various types of extreme weather events2,3,4,5. Changes in blocking frequencies due to Arctic amplification and sea ice loss may enhance extreme events6,7, but the mechanisms potentially involved in such changes are under discussion8,9,10,11. Current theories for blocking are essentially based on dry dynamics and do not directly take moist processes into account12,13,14,15,16,17. Here we analyse a 21-year climatology of blocking from reanalysis data with a Lagrangian approach, to quantify the release of latent heat in clouds along the trajectories that enter the blocking systems. We show that 30 to 45% of the air masses involved in Northern Hemisphere blocking are heated by more than 2 K—with a median heating of more than 7 K—in the three days before their arrival in the blocking system. This number increases to 60 to 70% when considering a seven-day period. Our analysis reveals that, in addition to quasi-horizontal advection of air with low potential vorticity12,13,14,15, ascent from lower levels associated with latent heating in clouds is of first-order importance for the formation and maintenance of blocking. We suggest that this process should be accounted for when investigating future changes in atmospheric blocking.

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Figure 1: Diabatic heating and PV anomalies along trajectories computed backwards from the blocking region.
Figure 2: Diabatic heating during blocking life cycle.
Figure 3: Properties of blocking trajectories.
Figure 4: Spatial distributions of blocking trajectories.

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Acknowledgements

We thank MeteoSwiss for providing access to ECMWF analysis data. Discussions with H. Davies have been very helpful in designing this study. C.M.G. acknowledges support from the Swiss National Science Foundation (Grant PZ00P2_148177/1).

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Authors and Affiliations

  1. Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland

    S. Pfahl, C. M. Grams & H. Wernli

  2. Statistik Stadt Zürich, 8022 Zurich, Switzerland

    C. Schwierz

  3. Federal Office of Meteorology and Climatology MeteoSwiss, 8058 Zurich, Switzerland

    M. Croci-Maspoli

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  1. S. Pfahl
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All authors designed the study and discussed the results and manuscript. S.P. analysed the data. S.P. and H.W. wrote the manuscript.

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Correspondence to S. Pfahl.

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Pfahl, S., Schwierz, C., Croci-Maspoli, M. et al. Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geosci 8, 610–614 (2015). https://doi.org/10.1038/ngeo2487

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