Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 182

which are most prominent in shorter wavelength radar systems. The longer the
path length through non-spherical scatterers such as large raindrops, the higher
the DP . To determine if there are non-spherical scatterers in a volume the

quantity

1

K DP ¼

2 @ DP =@

r ð 4 : 25 Þ

the ''specific differential phase'', which is proportional to the radial gradient of the
differential phase, is useful (the factor of

1

2 appears in (4.25) because phase shifts
occur in both outgoing and backscattered radiation). So, when radiation enters a
region of many raindrops, DP suddenly increases, so K DP increases and stays

relatively constant, while DP continues to increase. K DP generally increases with

raindrop diameter for Rayleigh scatterers (i.e., those whose diameter is much less
than that of the wavelength of the radar used). When the wavelength of the radar
is decreased so that the diameter of the raindrops is a larger fraction of the wave-
length, ''resonance'' effects make the dependence of K DP on raindrop diameter
more complicated, especially for shorter wave radars (5 cm and 3 cm wavelengths,
for example), and in fact non-monotonic with respect to small changes in raindrop
diameter. K DP is a useful quantity, however, especially when there is a lot of
attenuation or beam blockage, which affects the measurement of Z DR , but does
not affect phase measurements.
Columns of relatively high K DP in supercells are found to the left (with respect
to storm motion) of the updraft ( Figure 4.13 ). There is evidence that in ''K DP
columns'' there is a mixture of rain and wet hail or wet graupel and is indicative
of drops of water shed from hailstones.

4.3.2 Downdrafts: forward-flank downdraft and the rear-flank downdraft

Idealized conceptual models and radar imagery depict the relationship between the
main updraft, which assumes a U shape, and the two main downdrafts (forward-
flank downdraft or FFD and rear-flank downdraft or RFD), storm-relative wind
flow, and radar reflectivity structure ( Figures 4.14 -4.15 ). While isolating the two
different locations for the downdrafts is useful in terms of defining storm
structure, there may not be two separate downdrafts and the two may actually be
contiguous. The FFD, however, forms before the RFD does.
It is likely that water substance conversion processes such as evaporative
cooling and/or sublimation and/or melting cooling and/or precipitation loading
play a major role(s) in driving the FFD and the RFD. When the boundary layer
is relatively dry, evaporative cooling/sublimational cooling is enhanced, but is
limited when the boundary layer is relatively moist. Evidence for precipitation
loading in some instances is found from what has been named the ''descending
reflectivity core'' (DRC), a blob-like protuberance of precipitation above the
weak-echo region and underneath the echo overhang ( Figure 4.16 ) by Erik
Rasmussen and colleagues and has been hypothesized to play a role in tornado
formation. Evidence for the RFD is also found in photographs, movies, and
videos of the cloud base associated with a low-level mesocyclone, in which the