Supercells
The supercell, first described by Browning
and Ludlam (1962), is perhaps the most researched mesoscale phenomenon
in Meteorology. Supercells are a rarity: Doswell (1985) estimates
that they account for only 5% of severe storms in the eastern U.S. with
slightly higher values in the Great Plains. Although supercells are
relatively infrequent, they account for a large percentage of weather related
destruction and deaths. Supercells often engender large hail, severe
wind gusts, flooding rains, and destructive tornadoes. Below is a
brief description of the supercell convective type.
The definition of a supercell is not clear
cut. Supercell distinctions include radar structure (both reflectivity
and velocity) and time restraints. Simply put, a supercell is a cumulonimbus
cloud with a rotating updraft. A supercell often has an intense
rotating updraft that is visible on radar velocity imagery - this is known
as a mesocyclone.
The radar structure of a supercell is markedly
different from its multicellular counterpart. A supercell in radar reflectivity
imagery often contains a ‘hook echo' or ‘pendant echo' on the storm's right-rear
flank (relative to storm motion) - please see the special section on supercell
radar signatures. Although, the pendant may not be noticeable
in high precipitation (HP) supercells where the rotation is rain wrapped
and no weak-echo notch (WEN) is visible in the image. HP supercells
are particularly dangerous because a tornado can be obscured by sheets
of rain and hail, making it nearly impossible for the public to see the
vortex.
A hook echo - a defining characteristic of a supercell.
The pendant echo is indicative of a region
of strong rotation at which a funnel cloud or tornado may develop.
A radar feature related to the WEN and pendant echo is the bounded weak
echo region (BEWR). A BEWR is a distinctive overhanging, high reflectivity
region that is visible in volume scans (or range height indicator in coherent
radars). BEWRs are exhibitive of intense updrafts. The BEWR is similar
to the pendant echo in structure, only vertical.
A supercell is often quasi-steady
in intensity; updrafts are often sustained over long periods of time.
This is a distinct feature of a supercell. Updrafts in airmass cells
collapse forming a downdraft. Multicellular systems contain individual
updrafts that continually reform at the gust front edge. Because
the updraft and downdraft are separated in supercells, the cells may retain
their intensity. Albeit, some pulsation may be noticed: weakening
thought to be a signature of an occlusion of the storm's surface boundaries
and strengthening when the low reforms the conjunction of the boundaries).
Another feature of supercells is splitting.
A supercell often splits into two separate mirroring cells. The right-turning
(equatorward) supercell often does not move with the mean relative wind
and is more likely to produce tornadoes due to the fact the cell is cyclonic.
The left (poleward) cell is often anticyclonic; it can produce anticyclonic
tornadoes, but, they are rare.
A mature supercell may be thought of
as a mesoscale baroclinic system. In comparison with the Norwegian
Cyclone Model a supercell contains: a flanking rear downdraft (cold front),
an overrunning area of precipitation (warm front/stationary front), and
an intense meso-low pressure center. The flanking rear downdraft
is caused by and intense subsidence of rain-cooled air and entrainment
of stratospheric air. A mesohigh is found behind the rear flank.
Overrunning air and intense updrafts at the edge of the WEN induce condensation
and precipitation formation.
Supercells are often associated with
the tornadoes that they engender. Yet, supercells can produce damaging
straight-line winds, too. Damaging outflows are derived from the
intense downdrafts that often characterize these storms.
Below are schematics of supercell types
and a list of supercell characteristics. A list of excellent literature
about supercells on the web is directly below. This module cannot even
begin to describe the complex dynamics and structure of supercells.
Thus, the viewer is expected to visit these excellent resources for more
information on supercells and tornadoes:
Supercells
by UIUC
What
is a tornado? by Doswell
Tornadoes
and Tornadic Storms: A Review of Conceptual Models by Doswell and Burgess
What
is a supercell? by Doswell
On the
Environments of Tornadic and Nontornadic Mesocyclones by Brooks, Doswell,
and Cooper
Tornado
Forecasting: A Review by Doswell, Weiss, and Johns
Schematics of Supercell Storms
- Classic, HP, and LP. (UIUC)
Schematics of a classic supercell.
Schematic of high-precipitation (HP) supercells.
Schematics of low-precicipitation (LP) supercells.
Characteristics of as supercell storm.
--Special Topic --