Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 173

where v g is the geostrophic wind; f is the Coriolis parameter (2 O sin , where O

is the rotation rate of the Earth about its axis and is the latitude); and the

subscript h by the gradient operator denotes (horizontal) derivatives computed on
a surface of constant height. Thus, supercells should be found preferentially when
there are strong horizontal temperature gradients (e.g., near surface fronts and
baroclinic waves in the upper troposphere) and when there is the potential for
localized, strong, buoyant updrafts. On the synoptic scale, the horizontal

temperature gradient is 10K/1,000 km, so that in mid-latitudes

z j½ 10 m s 2 10 4 s 1 10 6 m 10 3 s 1 j@ v g =@ = ð 300 K Þ 10 K =

In the vicinity of fronts, where the horizontal temperature gradient is an order of
magnitude greater

z j 10 2 s 1 j@ v g =@

Moreover, vertical shear in the boundary layer is also a possible source of hori-
zontal vorticity that can be tilted onto the vertical in a convective storm.

4.3 OBSERVED SUPERCELL STRUCTURE: CLOUD FEATURES,
PRECIPITATION DISTRIBUTION, POLARIMETRIC
RADAR-OBSERVED PARAMETERS, AND WIND AND
TEMPERATURE FIELDS

A large leap in our understanding of supercells occurred during the 1970s thanks
to storm-chasers, who first systematically documented the visual cloud structure of
supercells ( Figure 4.6 ), the advent of the use of Doppler radar which led to
detailed depictions of the wind field in supercells in the late 1970s and early 1980s,
especially in dual-Doppler studies done by Gerry Heymsfield at OU (University of
Oklahoma), and Ed Brandes and Peter Ray at NSSL ( Figure 4.7 ), and the nearly
simultaneous advent of three-dimensional, non-hydrostatic ''cloud'' models, which
could be used to conduct controlled experiments. Not being able to control
nature, we must settle for whatever the atmosphere reveals to us when studying it
observationally. In the mid and late 1970s, Robert (Bob) Schlesinger at the
University of Wisconsin at Madison, Joe Klemp at NCAR, and Bob Wilhelmson
did pioneering experiments, in which the three-dimensional aspects of supercells
(rotation, deviant motion, etc.) were simulated for the first time ( Figure 4.8 ).
More recently, polarimetric radars have afforded a more complete look at the
spatial distribution of hydrometeors, etc. in supercells (and in multicells).

4.3.1 The main updraft in supercells

Supercells are prolific producers of large hail; it was hypothesized that large hail is
associated with a very strong updraft because large hailstones have a high terminal
fall velocity and because water substance must be recycled into and out of