Tornadoes and Tornadic Storms Robert Davies-Jones, R. Jeffrey Trapp, and Howard B. Bluestein Presentation by Christopher Medjber Severe Convective Storms, Meteorological Monographs, 28, no. 50, American Meteorological Society, pg

Introduction  Definitions  Climatological distribution of Tornadoes Midlevel and Near-Ground Mesocyclogenesis  Mesocyclone observations

Definitions  Supercell  Long-lived (> 1 hr)  Has a single, quasi-steady rotating updraft (mesocyclone)  Has a high degree of spatial correlation between its mesocyclone and updraft

Supercell Characterization Low precipitation (LP) supercell Low precipitation (LP) supercell Moderate precipitation (“Classic”) supercell Moderate precipitation (“Classic”) supercell High precipitation (HP) supercell High precipitation (HP) supercell

Definitions  Tornado  A violently rotating, narrow column of air  Average diameter of ~100 m  Extends to the ground from the interior of a cumulonimbus or cumulus congestus  Appears as a condensation funnel pendant from cloud base and/or as a swirling cloud of dust and debris rising from the ground

Tornadoes Type I and Type II

Type I Tornadoes  Forms within a mesocyclone, with the parent storm being either an isolated supercell, a supercell in a line of thunderstorms, a miniature supercell containing a small mesocyclone, or a miniature supercell within the rainbands of landfalling tropical cyclones

Type II Tornadoes  Is not associated with a mesocirculation (i.e. mesocyclone)  Generally form from small and weak vortices along a stationary or slowly moving windshift

Climatological Distribution of Tornadoes Tornadoes occur worldwide! Tornadoes occur worldwide! Tornadoes are most prevalent in the “Great Plains” of the United States and in north-east India-Bangladesh Tornadoes are most prevalent in the “Great Plains” of the United States and in north-east India-Bangladesh “Tornado Ally” (area extending from north Texas northward to western Iowa “Tornado Ally” (area extending from north Texas northward to western Iowa

Climatological Distribution of Tornadoes Over past 50 years, annual tornado reports have increased from 200 to 1200 Over past 50 years, annual tornado reports have increased from 200 to 1200 Likely due to population density, improved reporting procedures, etc. Likely due to population density, improved reporting procedures, etc. Not due to global climate change! Not due to global climate change!

Tornado Deaths  Death tolls have decreased  Mean tornado-warning lead time of 13 minutes  Improvements in public awareness, education, forecasting, issuances of watches and warnings, and the operational use of WSR-88D radar

Midlevel and Near-ground Mesocyclogenesis At midlevels, mesocyclone develops from the tilting of horizontal vorticity associated with the vertical shear of the winds At midlevels, mesocyclone develops from the tilting of horizontal vorticity associated with the vertical shear of the winds Near the ground, mesocyclone develops form due to tilting of horizontal vorticity from low-level density gradients Near the ground, mesocyclone develops form due to tilting of horizontal vorticity from low-level density gradients

Mesocyclone Observations  Time-lapse photography  Doppler Radar - Use of inbound and outbound Doppler velocities  Only about 50% of objectively defined mesocyclones produce tornadoes

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