abbreviated lecture on supercells from comap 2002rowdysites.msudenver.edu/~sng1/mtr4500/what is a...

Abbreviated Lecture on SUPERCELLS

from COMAP 2002

Greg Stumpf stumpf@nssl.noaa.gov

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