Chapter 5

Near-surface Plan View of a Tornadic Supercell
FFD = Forward Flank Downdraft
RFD =
Rear Flank
Downdraft
T = likely tornado
location.
Bluestein (1993)
Evolution of Updrafts/Downdrafts, Mesocyclone, and Tornado in a Supercell
Classical View #1 from Observations
RFD
formation
aloft
RFD splits
meso and
occludes
inflow;
tornado to
ground
RFD hits
ground-> RFD
gust front;
tornado forms
aloft
New
updraft
forms on
right flank
In Cotton and Anthes (1989) as adapted from Lemon and Doswell (1979)
Tornadic Supercell: Classical View #2 from Numerical Models
Intensification
of low level
rotation and
formation of
tornado.
Importance of
FFD baroclinic
vorticity
generation
Bluestein (1993)
Zoom in to low levels of
Fig. 3.42 on prior page
RFD generated in response
to low level low in circulation
RFD occludes inflow
New updraft circulation
forms on right flank; process
starts over again.
Bluestein (1993)
Plan view of mesocyclone
evolution and cyclical tornado
production.
Tornado paths
Picture of actual cyclical
tornado production.
Dissipating tornado on left
and mature tornado on right.
Bluestein (1993)
Conceptual Model of the Life-cycle of a Non-supercell Tornado
1. Cu clouds from over
convergence zone with
pre-existing low-level
vortices
2. Strong updrafts
form in and beneath
TCu.
3. Strong Updraft of Cb is “lucky”
and becomes lined up with preexisting vortex. Stretching causes
tornado spin-up.
• Process usually associated with weak (F0-F1) tornadoes.
• Occur in weak-moderate wind shear environments, typically
associated with ordinary and multi-cell storms.
Bluestein (1993)
Plan View at mid-levels
Airflow in a
Supercell
Updraft core
BWER ≈ weak echo vault
Vertical crosssection
Hail cascade
Bluestein (1993) Figure 3.61 as adapted from Browning (1977)
Embryo
Curtain
Conceptual Model of
Hailstone Trajectories
In a Supercell Storm
Plan View at mid-levels
•Trajectory 0: Unlucky Embryo
• vented to anvil as small
hydrometeor
• Trajectory: 1-3: Lucky Embryo
• Large Hailstone
Vertical crosssection
Bluestein (1993) Figure 3.62 as adapted from Browning (1977)
Plan View of Precipitation Type/Amount Distribution in a Supercell (Classic)
Hail is adjacent
to and wraps
cyclonically
around strongest
updraft core.
Bluestein (1993)
Conceptual model of hail production in a multicell storm
Bluestein (1993) Figure 3.65 as adapted from Browning et al. (1976)
Hailstone Properties – Size and Shape
•
•
Hailstones range in size from
Diameter = 5 mm to 55 mm and
larger
– Record is now 7” (17.78 cm)
over Aurora, NE on 22 June
2003
Hailstones tend to have oblate
spheroidal shapes but can also
have very irregular shapes (e.g.
lumps and protuberances)
depending on growth mode.
Hubbert et al. (1998)
Pruppacher and Klett (1997)
Differential Reflectivity (Zdr) - hail
Zdr – rain vs. hail
– Zdr = 10 LOG10(Zh/Zv)
– Zdr in rain is > 0 and
usually ranges from 0.5 to
4 dB
– Zdr is near zero in hail with
a tendency for larger hail to
have negative Zdr
• Small hail: -0.5 ≤ Zdr ≤
0.5 dB
• Large hail Zdr < 0 dB,
negative Zdr
– So, hail is characterized by
• High Zh
• Low Zdr
Wakimoto and Bringi (1988)
“Hail Hole”: Large Zh and near zero or negative Zdr
Zh
Zh
Zdr
Hubbert et al. (1998)
Zdr
Doviak and Zrnic (1993) adapted
From Bringi et al. (1986)
Zh
Zdr
Herzegh and Jameson (1992)
Evolution of
thunderstorm
outflow.
Conceptual model
of microburst
evolution, based on
• Surface damage
patterns.
• Doppler radar.
Houze (1993)
3-D Depiction of a Convective Downburst
Ring vortex at edge
of gust front.
Although convective
downburst is primarily
a divergent circulation
at the surface, some
rotation may be
present.
Note some
rotation in
convective
downdraft.
Horizontal scale.
Houze (1993)
Microburst Photographs by Bill Bunting
(MIC, Fort Worth NOAA NWS Forecast
Office) as described by Caracena, Holle,
and Doswell in a very useful Internet page
called “Microbursts: A handbook for visual
identification”: “A microburst descends
from the parent cloud to the ground and
begins to spread out, in this sequence of
four photos from central Oklahoma on 24
October 1991. The outward flow of winds
is apparent at the ground in the last view.”
http://www.cimms.ou.edu/~doswell/microb
ursts/Additions.html
Depiction of the Effect of a Microburst (or Strong Downdraft) on Aviation
Aircraft taking off
during microburst
at and just above
the runway.
Houze (1993)
Microburst-related Aircraft Accident: Example of Flight 426
(August 7, 1975) at Stapleton Airport, Denver Colorado
Bluestein (1993)
Vertical cross-section of radar
reflectivity (dashed, dBZ) and differential
reflectivity (solid, dB) in a microburst
producing thunderstorm.
Melting can contribute to
downdraft strength, in
addition to evaporation
and precipitation loading.
Microburst at surface.
Houze (1993)
Dependence of
downdraft strength (m s-1)
on the environmental
lapse rate (hence
evaporation) and the
amount of rain (hence
water loading).
Houze (1993)