Events Calendar

MISU Research seminar | Sonja Murto

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

by Sonja Murto
PhD student at the Department of Meteorology (MISU)

Time: 31 January 2019, 14h15–15h15
Venue: Rossbysalen C609, Arrhenius Laboratory, 6th floor

Authors: S. Pfahl, C. Schwierz, M. Croci-Maspoli, C. M. Grams and  H. Wernli

This is a presentation of a scientific paper, published in Nature geoscience in 2015. This paper highlights the importance of diabatic processes in the ascending air streams via its input of low PV air, to the maintenance and formation of atmospheric blockings, which are shown to contribute to the occurrence of extreme weather events in the Arctic.

Atmospheric blocking is a key component of extratropical weather variability and can contribute to various types of extreme weather events. Changes in blocking frequencies due to Arctic amplification and sea ice loss may enhance extreme events, but the mechanisms potentially involved in such changes are under discussion. Current theories for blocking are essentially based on dry dynamics and do not directly take moist processes into account. 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 vorticity, 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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