Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 181

Figure 4.12. Example of a Z DR column in a vertical cross section through a supercell (see solid
line in the inset in the upper right-hand corner of the radar reflectivity factor at low elevation)
on May 9, 2003 in central Oklahoma, as detected by the S-band KOUN radar. Differential
reflectivity, Z DR (dB) is color-coded; radar reflectivity factor (dBZ) is contoured by thin solid
lines (from Kumjian and Ryzhkov, 2008)

updraft strength. Therefore, the change of Z DR in a column ( Figure 4.12 ) is an
indicator of changes in updraft intensity, all other factors remaining unchanged.
When electromagnetic radiation propagates through rain, it slows down
(compared with how quickly it propagates through air). The ''propagation'' phase
delay (compared with the initial phase) upon being backscattered to the radar
when it is slowed down depends, in part, on the total amount of liquid water in
its path that is traversed. When the shape of raindrops is not spherical (i.e., as is
the case for large raindrops, which are oblate), then radiation in one plane
propagates more slowly than it does in the orthogonal plane, so there is a
difference in phase delay between horizontally and vertically polarized beams of
radiation: this phase difference is called DP or ''differential phase''. For most

radar systems receiving horizontally polarized radiation, the differential phase is
positive for raindrops, which are flattened (oblate spheroids), because horizontally
polarized radiation is slowed more than vertically polarized radiation, since more
water is encountered. The ''total differential phase'' is actually different, owing to
the added ''backscatter'' differential phase, which is a result of resonance effects,