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Lemon and Doswell (1979). Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm evolution and mesoscyclone structure as related to tornadogenesis. Mon. Wea. Rev ., 107 , 1184-1197. - PowerPoint PPT Presentation
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Lemon and Doswell (1979)
Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm evolution and mesoscyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107, 1184-1197
Lemon and Doswell attempt to synthesize the observations and limited modeling studies available through 1979 into a conceptual model of the supercell and mesocyclone structure.
Their paper establishes the importance of the rear flank downdraft in tornadogenesis in supercells.
Their conceptual models are still valid today with some modifications They also evaluate the likely source of the strong vorticity associated with tornadoes
.
SW NE
Observations suggest that the following sequence occurs in some supercells:
Initially, a rotating updraft is present (not shown…would be panel before (a))
Dry (low e) impinges on the rotating updraft aloft (7-10 km) creating a downdraft. The downdraft/updraft interface becomes a second center of rotation: the incipient TVS
The new rotation center begins to dominate the flow until a single rotation center (coincident with the TVS) is present along the updraft/downdraft interface within the rotating updraft.
The TVS migrates under the rotating updraft
Rotating updraftTornado vortex signature
Stippled = updraftShaded = downdraft
Dual-Doppler analysis of the 8 June 1974 Storm over Oklahoma City that produced 3 tornadoes
Storm relative winds (1 grid length = 10 m/s)
Initial mesocyclone corresponds to central updraft region where echoes are weak (called the Bounded Weak
Echo Region BWER)
Mesocyclone centered on strongest vertical velocity gradient later in storm when tornadoes occurred
8 June 7424 May 73
Union City, OK Manhattan, KS
“Clear slot” actually wrap around curtain of precip in rear flank downdraft
Wall cloud and updraft
Evidence of the rear flank downdraft is the clear slot or curtain of precipitation that appears SW of the wall cloud/tornado
Radar data from tornadic supercell (from Barnes 1978)
Shaded = 1.5 km reflectivity 32 dBZ or greaterContoured = 7.5 km reflectivity Heavy line = 15 dBZ Light lines = 32, 45, 52 dBZBlack dot = storm topWinds = storm relative at 6.75 km 1 full barb = 10 m/s
Hook echo
Flanking line of clouds along rear flank gust front
Weak echo along rear flank of storm marking location of rear flank downdraft
Heavy precipitation and forward flank downdraft
Radar data used to develop conceptual model of supercell structure
Schematic plan view of a tornadic supercell at the surface.
T
Red line = radar echo. Gust fronts and “occluded” region depicted with blue frontal symbols. UD = UpdraftFFD = Forward flank downdraftRFD = Rear flank downdraft (RFD)T = Tornado Streamlines are relative to the ground
Three dimensional view of the development of the mesocyclone and
tornado within a supercell
Storm relative flow at mid-levels between the time of the top left panel and the top right panel on the 3-D depiction
Strong temperature gradient
Original mesocyclone
Transformed mesocyclone(i.e. TVS location)
Cyclonic rotation
Anticyclonic rotation
y
p
xz
v
x
wD
x
vfD
y
fv
x
v
dt
d
Simplified vorticity equation for coordinate system where x axis is perpendicular to the gradient of temperature (baroclinic zone) aloft in the mesocyclone
What is the source of rotation in the mesocyclone?
xy
Time rate of change of vorticity following a parcel
Coriolis effects
D = divergence= specific volumeOthers = usual meaning
Vortex stretching
Vortex tilting
Pressure/densitysolenoidal term
Lemon and Doswell estimated the terms and concluded that
Tilting of the ambient shear dominated vorticity production, although stretching of the vortexand solenoidal effects were also important