Geoscience Reference
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Figure 4.13. As in Figure 4.12, but for a K DP column. Specific differential phase, K DP
(deg km 1 ) is color-coded (from Kumjian and Ryzhkov, 2008).
disappearance of cloud material and what Al Moller, Les Lemon, and Chuck
Doswell, among others, in the mid-1970s named a ''clear slot'' 4 are seen.
Eventually, air from the rear-flank downdraft reaches the ground and curves
around the mesocyclone, effectively cutting off the supply of ambient, moist, rela-
tively warm, potentially buoyant air into the main updraft of the supercell. Such a
process has been referred to as an ''occlusion'' (not to be confused with the occlu-
sion downdraft to be described later), analogous to the occlusion process in
synoptic-scale, extratropical cyclones. The curved, bulging nature of the leading
edge of the RFD produces a curved band of convergence and rising motion along
it, which is shaped like a horseshoe, or letter ''U'' ( Figure 4.15 ) and sometimes a
bounded weak-echo region that is also U shaped ( Figure 4.11 , bottom panels).
The separation of the mesocyclone at low levels by the RFD into a region of
rising motion from one of sinking motion has been termed its ''divided structure''
( Figure 4.15 ).
Part of the main updraft is located within the deepest convective cloud, above
a lowered cloud base named the ''wall cloud'' ( Figure 4.17 ) by Ted Fujita, because
the side of the cloud feature is vertical like a wall. The wall cloud forms when
4 Like a ''dry slot'' in synoptic-scale cyclones.
 
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