Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 239

Figure 4.51. Ensemble mean of the rate of baroclinic generation at 750m AGL of storm-
relative streamwise horizontal vorticity (10 4 s 2 ) (color coded) and storm-relative ensemble
mean wind (plotted every 2 km) from mobile Doppler radar data from the U. Mass. X-Pol
radar and data from the WSR-88D radar at Dodge City, KS on May 4, 2007 in south central
Kansas, assimilated into a numerical cloud model. Tornado paths are indicated by thin purple
lines and radar reflectivity factor (dBZ) is contoured at 35 and 55 dBZ in gray. Vertical
vorticity (black contours) shown at intervals of 10 2 s 1 beginning at 10 2 s 1 . Note the region
of positive (red) generation of streamwise vorticity along the forward-flank region and the
region of negative (purple) generation of streamwise vorticity (or the generation of positive
anti-streamwise vorticity) along the RFD region and the leading edge of the rear-flank gust
front. The upward tilting of streamwise vorticity or the downward tilting of anti-streamwise
vorticity could produce a mesocyclone (courtesy of Robin Tanamachi).

the FFD from ambient air and thus have acquired streamwise horizontal vorticity
( Figure 4.51 ) . Air entering the low-level mesocyclone from the southwest or west
may also encounter horizontal gradients in temperature and, therefore, horizontal
vorticity can also be generated there ( Figure 4.51 ). In addition, environmental
low-level horizontal vorticity may be advected towards and tilted upward into the
low-level mesocyclone from the southeast ( Figure 4.52 ). In summary, a number of
vastly different types of trajectories along which changes in vorticity may be quite
different are likely.
An alternative and perhaps better way to analyze low-level mesocyclogenesis
is to use circulation analysis. Rich Rotunno and Joe Klemp in the early 1980s
pioneered this approach for the analysis of low-level mesocyclones in supercells.
Consider a material curve surrounding and centered about a low-level meso-
cyclone ( Figure 4.53 ). This curve lies in the horizontal plane and cuts across
isotherms or, more precisely, across lines of constant equivalent potential tempera-
ture

e . If potential vorticity was initially (before the storm's updraft deformed the