Mesocyclone Evolution and Tornadogenesis: Some Observations

Abstract

Updraft mesocyclones in tornado-producing thunderstorms form along convergent and cyclonically sheared boundaries that separate inflow air entering forward and rear storm quadrants. Maximum tangential flow resides in girding wind bands, averaging usually less than 3 km in radius (depending upon the development stage), that strengthen near ground during toradogenesis. Strong inflow, concentrated at or below cloud base, is maintained throughout the intensification period. During the tornadic phase the parent mesocirculation exhibits an apparent "breakdown", i.e., a transition from single-cell to two-cell axial flow structure. At this critical period small eddies may appear within an elongated mesocyclone core and some may become tornado and/or grow to large proportions.Tornado dissipation may result from cyclonic rotation of the major mesocyclone horizontal axis which chokes the supply of inflow air and detaches the tornado from the principal updraft. Another apparent mechanism for dissipation involves an advanced breakdown stage where downdrafts fill the parent circulation.