What is a Mesocyclone?

<p>A defining feature of supercells is the <em>mesocyclone</em>, which has been implicated in facilitating updraft maintenance and strength, as well as tornado and hail production. But what, exactly, is a mesocyclone?</p> <p>Popular textbooks define the mesocyclone as a “region of vertical vorticity with a characteristic width of 3-8 km and magnitude O(10<sup>-2</sup> s<sup>-1</sup>)” (Markowski and Richardson 2010), or a “cyclonically rotating vertical vortex with typical diameters ~5 to 6 km” with tangential windspeeds of “several 10s of m s<sup>-1</sup>,” and typical vertical vorticity of 0.01 s<sup>-1</sup> (Trapp 2013). The American Meteorological Society’s <em>Glossary</em> defines the mesocyclone as a “cyclonically rotating vortex, around 2-10 km in diameter, in a convective storm,” with a vertical vorticity threshold of >0.01 s<sup>-1</sup>. These definitions also indicate persistence and depth through a significant fraction of the updraft as important factors (e.g., Doswell and Burgess 1993). Using these definitions, however, many high-resolution supercell simulations would fail to generate a “mesocyclone.” Often, only a “vortex segment” – and not a closed vortex – is present (e.g., Dahl 2017). Further, such simulations often reveal noisy vertical vorticity fields lacking broad, coherent regions meeting the thresholds above.</p> <p>In practice, mesocyclones are detected using Doppler radar. In particular, mesocyclones are identified when cyclonic azimuthal shear thresholds are met (often with temporal and depth criteria, too). However, such measurements are incapable of unambiguously determining the presence of a <em>vortex</em>, and provide only a proxy for vertical vorticity. Further, a single Doppler radar generally offers no information on the updraft’s boundaries. Thus, many of the defining characteristics mesocyclones are not actually detectable operationally.</p> <p>In this presentation, we seek discussion from the community of whether an improved, dynamical definition for a mesocyclone is needed, how to reconcile such a definition to make it applicable to both simulations and radar observations, and to solidify ambiguities in the terminology of "low-" and "midlevel" mesocyclones.</p> <p><strong>We also offer an interactive activity:</strong> go to the link below and identify the coordinates (x, y) of the mesocyclone center and its diameter for the two images provided on that webpage. We will report on the results at the conference. </p> <p>https://forms.gle/FEKNWZe6mkcLcWpS6</p> <p> </p>