Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 171

Figure 4.4. Illustration of the ''Magnus effect'' as a clockwise-spinning baseball experiences a
force that deflects it to the right. In the figure, the baseball is shown in its reference frame, as it
is thrown from left to right (relative streamlines are shown). Air approaches the baseball and is
deflected about either side. Air is diverted to the right but is slowed down by an adverse
pressure gradient as a result of spinning, so there is relative high pressure on the left side,
while air is diverted to the left but is accelerated by a pressure gradient acting in the direction of
the flow as a result of spinning: the net result is a force acting to the right of the baseball's path
(red arrow).
vertical shear of the wind in the environment of the storm plays an important role
in their longevity.
During a hail project in Alberta conducted in the late 1960s and another in
northeast Colorado in 1970 conducted by the University of Wyoming and by
NCAR, John Marwitz found that supercell storms form in an environment of
much stronger vertical shear than that of ordinary cells and in an environment of
stronger shear than that of multicell storms, whose existence requires at least
low-level shear. Early theories correctly pointed out that in supercells, owing to
vertical shear, precipitation falls out away from the main updraft, allowing the
updraft not to weaken and dissipate as precipitation falls back into it. Since then,
there have been many observational studies that have demonstrated the
relationship between a high-shear environment and supercells.
In 1963, Yoshi Ogura, writing in a monograph while at MIT, and Keith
Browning and C. R. Landry, writing in a radar conference proceeding, were
the first to suggest that the tilting of low-level horizontal vorticity might be re-
sponsible for creating cyclonic vortices (such as tornadoes) in convective storms.
Stan Barnes at NSSL in 1968 in an NSSL Technical Memorandum on a study of
a number of storms and in 1970 in the analysis of a specific storm was the first to
present detailed observational evidence that the source of storm-scale rotation in
them was in fact due to the tilting of horizontal vorticity in the environment
( Figure 4.5 ). This horizontal vorticity is associated with the vertical shear of the
environmental wind. Col. Robert Miller, in his classic severe storm forecasting
manual revised in the early 1970s, implicitly recognized observationally the
importance of vertical shear in that it was noted that when a low-level jet
intersects an upper-level jet at a substantial angle this synoptic situation was a
necessary condition for severe weather outbreaks.