deep convection: classification

Mesoscale M. D. Eastin

Deep Convection: Classification



Types of Convective Storms:

• Single Cell Storms• Multicell Storms• Supercell Storms


The Convective Cell:

• Ordinary deep convective cumulonimbus (Cb) clouds• Have been studied and documented since the late 1800s

Single Cell Storms



• First detailed documentation of “thunderstorms” was by Horace Byers and Roscoe Braham in the late 1940s

• Documented all convection that occurred during a 3-month period in a ~100 square mile area near Orlando, FL

• Data collection included 50 surface stations, 6 balloon launch sites, radar, and aircraft simultaneously flying at 5 altitudes

• Results described the evolution of an ordinary convective cell in three stages

Single Cell Storms



Cumulus Stage: Developing cumulus cloud dominated by an updraft > 10 m/sMinimal updraft tiltNo downdraftsPrecipitation develops aloft and is suspended by updraft

From Byers andBraham (1949)

Single Cell Storms



Mature Stage: Cloud extends through depth of the atmosphere Anvil cloud begins to spread out near tropopause

Downdraft develops due to precipitation loading and evaporational coolingPrecipitation reaches the groundLeading edge of downdraft produces a gust front

Single Cell Storms




Dissipating Stage: Precipitation core and downdraft wipe out the updraftCell becomes dominated by a weak downdraftLight precipitation at the ground

Single Cell Storms




• Basic building block of all convective systems

• Lifespan is 30-60 minutes

Occur in environments with: weak vertical shear (< 10 m/s), variable CAPE (500-2000 J/kg), and small CIN ( > -50 J/kg)

• Motion is roughly the speed and direction of the mean flow in the 0-6 km AGL layer

• Gust front spreads out equally in all directions and rarely initiates new convective cells

• Can produce rain, hail, strong winds, but rarely tornadoes

Single Cell Storms


Single Cell StormsThe Convective Cell:

• Basic building block of all convective systems

• Lifespan is 30-60 minutes

Occur in environments with: weak vertical shear (< 10 m/s), variable CAPE (500-2000 J/kg), and small CIN ( > -50 J/kg)

• Motion is roughly the speed and direction of the mean flow in the 0-6 km AGL layer

• Gust front spreads out equally in all directions and rarely initiates new convective cells

• Can produce rain, hail, strong winds, but rarely tornadoes


The Multicell Storm:

• A collection of single-cell storms at various stages in their lifecycle

• New cell development regularly occurs on gust front flanks

Multicell Storms

Note: These images are qualitatively consistent with one another

Cell 5

Cell 4

Cell 3Cell 2

Cell 1



• New cell development occurs on the flanks of the gust front where convergence with the ambient storm-relative low-level flow is maximized

• Individual cell motion (Vc) may be different than the overall storm motion (Vs)

• Individual cells continue to move at the speed and direction of the mean flow in the 0-6 km AGL layer

• The storm may move at a speed slower or faster than the mean wind (and in a different direction) depending on which flank has the maximum convergence

Multicell Storms



• Main inflow approaches the storm and is lifted by the spreading gust front

• By the time the updraft has reached the tropopause (anvil cloud), it is often well behind the leading edge of the gust front

• Downdraft air originates at mid-levels from precipitation loading and evaporational cooling

Updraft and downdraft are well separated, allows the system to live for a much long time than a single cell

Multicell Storms



• Common features include a shelf cloud, overshooting tops, and an anvil cloud

• Lifespan 2-12 hours

Occur in environments with:• moderate vertical shear (10-20 m/s)• variable CAPE (500-3000 J/kg)• small CIN (> -50 J/kg)

• Can produce copious rainfall, hail, high winds and some tornadoes along the gust front

Shelf Cloud Example

Multicell Storms



• Often observed in a wide variety of overall system structures

Examples include:

• Squall Lines (all varieties)• Bow Echoes• Mesoscale Convective Complexes

Examples of Multicell Storms on Radar

From Houze (1993)

Multicell Storms


The Supercell Storm:

• Single-cell storm that develops in isolation or splits from a multicell storm

Defining characteristic is a single, quasi-steady, rotating updraft – often observed by radar as a strong “mesocyclone” and with a “hook” echo

• Most rare, but most dangerous, storm type - can produce large hail and strong, long-lived tornadoes

Supercell Storms

From Houze (1993)



• Life span up to 8 hours

Motion is often slower than and to the right of the mean flow in the 0-6 km layer

Occur in environments with:• strong vertical shear (> 20 m/s)• large CAPE (1000-4000 J/kg)• small CIN ( > -50 J/kg)

Supercell Storms



Early radar observations help identify many common structural characteristics during the mature stage of a supercell

Forward Flank Downdraft (FFD)

• Strongest and largest of the downdrafts• Located below the primary anvil cloud and separated from primary updraft • Associated with the most intense precipitation and gust front

Rear Flank Downdraft (RFD)

• Located adjacent to the primary updraft• Associated with mid-level mesocyclone • Collocated with the “hook” appendage

Both downdrafts are driven by water loading and evaporational cooling

Note how their two gust fronts create “meso-fronts” similar to an occluded low From Lemon and Doswell (1979)

StormMotion

Supercell Storms



Early radar observations help identify many common structural characteristics during the mature stage of a supercell

Primary Updraft (UD)

• Helical in structure• Updraft speeds can reach 40-50 m/s• Located at the occlusion point of the two intersecting gust fronts• Located within the “hook” structure

Hook Echo (see thick black contour)

• Distinct notch in the radar reflectivity• Location of maximum inflow• Location of primary updraft• Location of any tornado• Caused by the mid-level mesocyclone advecting precipitation around itself• Good evidence of a mesocyclone• Also called an “inflow notch”

StormMotion

Tornado (T)

From Lemon and Doswell (1979)

Supercell Storms



Modern Doppler radar observations continue to show these common features as well as the strong rotation associated with the mid-level mesocyclone

Doppler Velocity

OutboundFlow

InboundFlow

Mesocyclone

Mesocyclone

Supercell Storms



Bounded Weak Echo Region (BWER)

• Distinct “gap” of low reflectivity in radar cross-sections• Location of the primary updraft• Caused by a very strong ascent lofting all precipitation and hail (that normally fall through the updraft) to the upper levels• Updraft speeds must be greater than 10 m/s

• Located within the hook echo• Also called an “echo free vault”

Presence of a BWER and a hook echo is good evidence of a very strong and rotating updraft (i.e. a supercell)

Supercell Storms


From Chisholm and Renick (1972)


• Strong updrafts can produce very large hailstones if the updraft velocity is greater than the fall velocity of the hailstone (up to 20-30 m/s)

• The “overhang” of a BWER, observed in radar reflectivity, is often composed of small hailstones that are initially ejected from the updraft at upper levels, but fall back into the strong updraft at lower levels

• This cycle can repeat itself several times, allowing the hailstone to grow larger

A hail trajectory example might follow 1 → 2 → 3

Supercell Storms


From Burgess (1974)


• Often “split” into two separate storms

• After the split, the motion of the storm on the right (left) is to the right (left) of the mean 0-6 km environmental flow

• Called “right-movers” and “left-movers”

The right-mover usually continues as a long-lived supercell (thanks in part to continued access to the warm, moist low-level inflow from the southeast), and often experiences a slower forward speed

The left-mover usually begins to dissipate (in part due to the right-mover blocking access to the inflow), and often experiences a faster forward speed

We will discuss the dynamical processes involved with such events later……

LeftMover(LM)

StormSplit

RightMover(RM)

Supercell Storms

RM

LM

MeanWind


Supercell VarietiesA Spectrum of Supercell Types:

1. Classic supercells2. High-precipitation (HP) supercells3. Low-precipitation (LP) supercells4. Shallow (miniature) supercells

Classic Supercells:

• Structure described on previous slides• Tend to occur in the Central Great Plains and Midwest (west of Mississippi River)

• Are capable of producing large hail, violent tornadoes, and strong winds.

Classic Supercell


High-Precipitation (HP) Supercells:

• Produce more rain than classic supercells• Strongest RFDs and FFDs• Tend to be less isolated – located at the southern end of squall lines• Often occur east of the Mississippi River

• Are capable of producing large hail, weak tornadoes (rain-wrapped), downbursts, and flash floods

Note the Elevation Angles

Supercell Varieties


Low-Precipitation (LP) Supercells:

• Produce less rain than classic supercells• Weakest RFDs and FFDs• Tend to be smaller in diameter• Most often occur in the High Plains along the dryline

• Still capable of producing large hail, but tornadoes are less common

Note the Elevation Angles

Supercell Varieties


Shallow (or Miniature) Supercells:

• Small diameter (<6 km) and shallow (<6 km) compared to classic supercells• Most often occur in tropical cyclones• Small CAPE (<1000 J/kg) confined to lower and middle levels• Strong shear (up to 30 m/s) in lower 3 km• Capable of producing weak tornadoes

Miniature Supercells in Hurriance Ivan

Supercell Varieties

From Eastin and Link (2009)


The following questions naturally arise….

Given observations of the environment, which convective storm structure should you anticipate?

• Single cells• Multicells• Supercells

What environmental parameters should you look at?

• Vertical Instability (CAPE and CIN…more in next lecture)• Vertical Shear (hodographs…more in next lecture)

What physical processes are responsible for the aforementioned storm structure and evolution? (more to come…)



Summary

• Single Cell Storms

• History• Three Stages (basic characteristics and structure)• Significance

• Multicell Storms

• Basic Characteristics and Structure• Motion and Propagation• Varieties

• Supercell Storms

• Basic characteristics• Defining structures• Motion and storm-splitting• Varieties (differences in structure and environment)



ReferencesAtkins, N.T., J.M. Arnott, R.W. Przybylinski, R.A. Wolf, and B.D. Ketcham, 2004: Vortex structure and evolution within

bow echoes. Part I: Single-Doppler and damage analysis of the 29 June 1998 derecho. Mon. Wea. Rev.,132, 2224-2242.

Byers, H. R., and R. R. Braham, Jr., 1949: The Thunderstorm. Supt. Of Documents, U.S. Government Printing Office, Washington, D.C., 287 pp.

Burgess, D. W., 1974: Study of a right-moving thunderstorm utilizing new single Doppler radar evidence. Masters Thesis,Dept. Meteorology, University of Oklahoma, 77 pp.

Chisholm, A. J. and J. H. Renick, 1972: The kinematics of multicell and supercell Alberta hailstorms. Alberta Hail Study, Research Council of Alberta hail Studies, Rep. 72-2, Edmonton, Canada, 24-31.

Houze, R. A. Jr., 1993: Cloud Dynamics, Academic Press, New York, 573 pp.

Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci., 40,359-377.

Lemon, L. R. , and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis., Mon. Wea. Rev., 107, 1184–1197.

Weisman, M. L. , and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and Forecasting, Ed: Peter S. Ray, American Meteorological Society, Boston, 331-358.

Wilhelmson, R. B., and J. B. Klemp, 1981: A three-dimensional numerical simulation of splitting severe storms on 3 April 1964. J. Atmos. Sci., 38, 1581-1600.

deep convection: classification

Documents

eastinthe convective

ordinary convective

tornadoessingle cell

stages single cell stormsm

lifecycle new cell development

individual cell motion

new convective cells

convective systems lifespan