abbreviated lecture on supercells from comap 2002rowdysites.msudenver.edu/~sng1/mtr4500/what is a...
TRANSCRIPT
What is a Supercell?
• A storm with a persistent and deep mesocyclone (Doswell and Burgess 1993). – How persistant? – How deep? – How do we define a “mesocyclone”?
Persistence
• In comparison to a convective time scale, defined by the time it takes for air parcels to rise from within the inflow layer of the updraft to the anvil outflow. – Many hours? – No – could be just tens of minutes!
• Steady State? – Mostly not – constantly evolving
Deep
• A significant fraction of the depth of the cumulonimbus cloud in which the circulation is embedded. – Large and tall (10-15 km)? – No – might only be several kilometers!
Mesocyclone
• The vorticity magnitude, where a “mesocyclonic vorticity unit” is 10-2 s-1. – But does this mean that a storm with 0.99
mesocyclone vorticity units is much different than a storm with 1.01 mesocyclone vorticity units? Is one a supercell and the other not?
– What about the horizontal scale of the rotation? Large? Small? Medium?
Supercell: A Single Cell?
• As observed with high-resolution observations, multicellular structure can even be imposed upon supercells.
Spectrum
• It is clear that supercells exist with a variety of magnitudes and dimensions, as well as morphologies.
• There are no discrete categories of supercell types.
• There are even no discrete categories of storm types (can have hybrids)!
Union City Oklahoma • The first tornadic
supercell well-sampled by a research Doppler radar
• Does this one supercell become the archetype for all supercells?
• Almost (for a while – while database was limited to big storms in Oklahoma)
Our Paradigm Was Modified
• Not all supercell storms are like those few that were sampled by the Doppler radars in Central Oklahoma!
• There are some commonalities… • There are also some differences… • The key is to be able to recognize these
features for effective warnings.
In Common…
• Persistence • Depth • Rotation
– During the mature phase, it is the updraft-downdraft pair that is rotating.
• High shear. Not necessarily high CAPE (but needs to be adequate). – When low shear but very high CAPE, storm can augment local
shear. • Distribution of precipitation sets up stage for
tornadogenesis. • Many move to right of mean wind (but not all).
Differences…
• Precipitation distribution: – Classic (CL), Low-precipitation (LP), high
precipitation (HP) – Embedded within lines, regions, etc.
• Dimensions: – Vertical (high-, medium-, low-topped)
• Function of EL height – Horizontal (large, medium, miniature)
• Function of ???
Classic Supercells
Classic Supercells
Classic (CL) Supercells
• The real “value” of a CL supercell is that it appears to be the most efficient of the three types to produce significant tornadoes.
• Can occur nearly anywhere in U.S. when NSE supports them.
Classic (CL) Supercells
Classic (CL) Supercells
Reflectivity Characteristics
• Hook, Bounded Weak-Echo Region (BWER), V-notch.
CL Supercell Example (3 May 99 OKC)
BWER and Hook
LP Supercell
Low-Precipitation (LP) Supercells
• Most, if not all, precip falls well downwind of updraft.
• There is little or no rear-flank downdraft, and thus almost no chance for tornadoes.
• Virtually never observed east of the Mississippi River.
• Typically associated with high LFCs and strong anvil-relative winds.
Low-Precipitation (LP) Supercells
HP Supercell
High-Precipitation (HP) Supercells
• Lower mid-level and anvil-relative flow. • Interactions with other storms – “seeding”, more
storms can occur with weak caps. • Typically associated with weaker tornadoes, but
can produce significant tornadoes (Plainfield IL). • More of a severe wind, hail, and flash flooding
threat. • Are the more-common supercell type east of the
Mississippi owing to NSE conditions there (weaker caps, etc.), and may be the most common type everywhere in the U.S..
High-Precipitation (HP) Supercells
HP, CL, and LP all on one day (4 Oct 98 – OK)
CL LP HP
Hybrids
Hybrids
• Sometimes the updraft base region of a classic supercell exhibits “little or nor visible precipitation”, and an exposed “barberpole” updraft. – Many would confuse these features with an LP
supercell! – CIN layer – laminar front face, horseshoe shape
Mesocyclone Core Evolution
F1 Phoenix Mini-MINI-supercells
All at same Zoom factor!
Tornado location
Supercells of many dimensions
F1 Colorado Mini-supercell
F4 Oklahoma Supercell
Cone of Silence
Falcon Co Hurr. Opal
“mini” “large” “mini”
“high-topped” “low-topped” “low-topped”
Verti
cal
Dim
ensi
on
Hor
izon
tal
Dim
ensi
on
Pond Bank PA
Supercells of many dimensions
Low-Topped Supercells
• Height of convection a function of EL, and not the region or season that storm occurs. – Are typical with cool season environments (low EL)
but can also occur in warm-season environments where lack of instability can also cause a low EL (below tropopause height).
• Low-topped storms have been observed nationwide.
Low-Topped Supercells
• Common with cold core upper lows: – Are the leading cause of “cold-air funnel”
reports. • Low CAPE (sometimes < 1000 J/kg), but
strong low-level lapse rates, mid-level drying (for insolation), and significant low-level wind shear. – These conditions can also be met with
landfalling tropical systems (“TC-Mesos”).
KLWX Sterling VA 30 April 1994
Low-Topped Mini-Supercell example
KPUX Pueblo 22 June 1995
Not all Mini-Supercells are low-topped
Cone of Silence
KMLB Melbourne FL 11 Nov 94 T.C. Josephine
TC-Meso (low-topped and mini)
KEVX Eglin FL 4 Oct 1995 Hurricane Opal
TC-Meso (low-topped but NOT mini)
Severely-sheared low-topped supercells
KMPX Minneapolis MN 26 Oct 1996
Severely-sheared low-topped supercells
KMPX Minneapolis MN 26 Oct 1996c
Very mini-supercells (in Oklahoma!)
Very mini-supercells (in Oklahoma!)
Very mini-supercells (in Oklahoma!)
Tall Supercell Example (31 May 98 New York)
Top ~ 13.4 km
Cross-section
Anticyclonic Tornadoes
• From cyclonic supercells
AC
C
Anticyclonic Tornadoes
• From anticyclonic supercells
Anticyclonic Tornadoes
Anticyclonic Tornadoes
• From anticyclonic supercell?
Supercell/Non-Supercell Tornado Hybrids