Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 194

adjacent to the ''hook echo'', on the upshear side of the main body of the storm,
creating a sharp wind shift or zone of confluence that bears resemblance to a
synoptic-scale cold front and its spatial relationship to a synoptic-scale extra-
tropical cyclone. Sometimes the hook echo is not apparent owing to a lack of
scatterers, but there is a narrow appendage of enhanced radar echo. The hook
echo may also be accompanied by a thin, bowed line of radar echo connected to
it ( Figure 4.14d, top), which marks the leading edge of the gust front associated
with the RFD; this feature is called the ''rear-flank gust front'' (RFGF). As noted
previously, the RFD may be caused, in part, by the same processes (negative
buoyancy) that produce downdrafts in ordinary cells; in addition, however, it may
be forced in part by dynamic effects, which will be discussed subsequently. The
contribution from the negatively buoyant component may be particularly
complex, since it involves not only the trajectory of unsaturated air underneath
the precipitation that is falling out, but also the precise nature of the precipitation
particles, which determines the rate and total amount of evaporational cooling and
melting cooling.
Polarimetric radars provide an estimate of the three-dimensional distribution
of the types of hydrometeors present in a storm. Only direct aircraft penetrations
can confirm the existence of the types of hydrometeors actually present, while
surface observations from instrumented vehicles or pods can quantify the degree
of cooling. For many years an armored aircraft, the T-28, flew into severe convec-
tive storms. At the time of this writing in 2012, there are plans for a new armored
aircraft, the A10, to do the same. When the RFD is driven mostly by dynamical
effects there may not be much if any temperature gradient across the rear-flank
gust front or, even in the presence of evaporative cooling and melting, the effects
of cooling may be counteracted by enough subsidence-induced warming that there
is little temperature gradient across the RFGF. The RFD, like ordinary down-
drafts, exhibits temporal variations and there can be a series of more than one
RFD surge, as has been noted in dual-Doppler analyses ( Figure 4.18 ).
There is anecdotal evidence (from storm-chasers) that when the difference
between the surface temperature and dew point temperature exceeds 25 F the
evaporation of raindrops is so intense that the RFD produced via evaporative
cooling is strong enough that the speed of the RFGF far exceeds the speed of the
parent storm and new convective growth is suppressed as warm, moist, ambient
air is cut off from the updraft. One may use RKW theory also to infer that new-
cell growth will not occur when the temperature difference between that of the
cold pool and that of the ambient air exceeds some threshold for a given amount
of low-level vertical wind shear normal to the leading edge of the cold pool.
However, detailed theoretical studies of gust front behavior in a supercell (i.e.,
when there is rotation and significant three dimensionality), at the time of this
writing, are lacking.
The FFD, unlike the RFD, is not usually associated with a well-defined wind
shift ( Figure 4.15, bottom panels; Figure 4.18 ). It is therefore not directly analo-
gous to the warm front in a synoptic-scale, extratropical cyclone (of course,
neither do all extratropical cyclones have warm fronts). However, as will be dis-