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Supercell storms. Storm splitting, environmental conditions, propagation. Ordinary/multicell storm. Browning et al. (1976). Fovell and Tan (1998). Supercell (rotating) storm. Plan view at surface UD = updraft FFD = forward flank downdraft RFD = rear flank downdraft Gust front Hook echo

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supercell storms

Supercell storms

Storm splitting, environmental conditions, propagation

ordinary multicell storm
Ordinary/multicell storm

Browning et al. (1976)

supercell rotating storm
Supercell (rotating) storm

Plan view at surface

UD = updraft

FFD = forward flank downdraft

RFD = rear flank downdraft

Gust front

Hook echo

Dot ~ tornado location

Lemon and Doswell (1979)

beneath a rotating supercell
Beneath a rotating supercell

Made using

ARPS and

Vis5D

storm splitting
Storm splitting

Wilhelmson and Klemp (1981)

storm splitting delcity str vprt qr mov
Storm splitting (delcity_str_Vprt_QR.MOV)

Two symmetric storms result from

initial impulse

Yields right & left movers

Note rotation

Domain is moving to

east; storm motion

subtracted

Made w/ ARPS

and Vis5D

slide10

Split

L mover weaker

R mover turns right

R mover hook

Purple = hail

slide11

Tornado report

Maverick, TX

dynamics of storm splitting
Dynamics of storm splitting

Klemp (1987);

See Houze p. 288-295

leads to splitting and rotation
…leads to splitting and rotation

Lifting of vortex tubes by updrafts create counter-rotating horizontal vortices… vertical vorticity

Low pressure in the vortices establish

new updrafts on original storm’s flanks

The original storm decays… and voila, the storm appears to have split,

yielding counter-rotating storms

Note now L pressure and updraft are colocated… shear has contributed

to storm stabilization

precipitation is not involved
Precipitation is NOT involved

Simulations without rain

still show storm splitting

close look at two papers
Close look at two papers
  • Weisman and Klemp (1982)
    • Environmental conditions favoring ordinary/multicell storms and supercell storms
  • Rotunno and Klemp (1982)
    • Why right-movers are favored in U.S.
    • Why right-movers move to right of mean winds
weisman and klemp 1982 wk
Weisman and Klemp (1982; “WK”)
  • “Dependence of Numerically Simulated Convective Storms on Vertical Wind Shear and Buoyancy”
  • 3D simulations, started with thermal
  • Varied CAPE by manipulating boundary layer moisture
  • Considered speed shear only

see Houze, p. 284-288

slide22
CAPE
  • Convective Available Potential Energy
    • “positive area” of a sounding
    • Integrated from LFC to EQL
    • WK and Houze (eq. 8.1, p. 283) expressions involve potential temperature
      • Should involve virtual potential temperature
wk s sounding
WK’s sounding

CAPE varied

by setting PBL

moisture

Standard case

has 14 g/kg

wk wind profiles
WK wind profiles

Us = wind speed change over 10 km

Note: no directional shear

wk wmax vs time
WK Wmax vs. time

q = 14 g/kg simulations

No shear case:

intensifies fastest

dies first

Larger shear - slower

development

By this metric, moderate

shear storm is

strongest

wk simulations have symmetry axis
WK simulations have symmetry axis

No initial flow in N-S direction;

Initial bubble is radially symmetric.

Therefore, storms have N-S symmetry

wk results
WK results

multicellular

supercellular

us 30 m s q 14 g kg
Us = 30 m/s, q = 14 g/kg

Surface rainwater (g/kg)

and surface positive

vorticity (1/s, dashed)

Low-level radar

reflectivity field

BWER =

Bounded weak

echo region

slide30

“Secondary storms” are

Subsequent ordinary cell

developments

(i.e., multicells)

“Split storms” are

supercells, resulting from

splitting of initial cell

wk analysis
WK analysis

“Bulk Richardson number”

∆u = avg. wind of lowest 6 km

minus avg. wind of lowest 500 m

“Storm strength”

Wmax = largest updraft

Denominator = theoretical updraft

max ~ ignores pressure

perturbation!

wk analysis32
WK analysis

Larger BRi = initial storm stronger

• more CAPE, or

• less shear

Largest is ~ 60% of theoretical max

Multicells favored for BRi > 30

or so…

Supercells

• smaller BRi

• relatively larger S

R = BRi

concerns w bri
Concerns w/ BRi
  • CAPE
    • As formulated in WK (and Houze’s eq. 8.1), moisture in environment neglected (apart from parcel definition)
    • Water loading, entrainment neglected
    • Convective inhibition (CIN) completely ignored
  • Shear
    • Relevant depth 6 km? Always?
    • Directional shear completely ignored
  • Uniqueness of BRi
    • Same value for low CAPE/low shear as high CAPE/high shear. Reasonable to expect same convective mode and/or intensity?
  • This has been a very valuable result
rotunno and klemp 1982 rk
Rotunno and Klemp (1982; “RK”)
  • Most tornadoes associated with supercells
    • tornadoes have same sense of rotation as parent storm
  • Further, vast majority of tornadoes are CCW rotating
    • after splitting, CW left movers tend to die (and/or be non-tornadic)
  • So why are left movers disfavored?
    • it’s directional vertical shear
straight vs curved hodograph
Straight vs. curved hodograph

Note mirror-image

storms; both have

hook-like echoes

Note only right-mover

has hook shape;

left-mover weaker

straight vs curved hodograph38
Straight vs. curved hodograph

W at 1.5 km

Without directional shear, symmetric pulse always remains symmetric

With directional shear, right mover soon favored

pressure decomposition
Pressure decomposition
  • Recall nondimensional pressure perturbation π’ subdivided into dynamic and buoyancy pressures
  • Dynamic pressure can be further partitioned into linear (L) and nonlinear (NL) pressures using perturbation analysis

NOTE the

overbars…

the base state winds

caused by vertical shear
π’ caused by vertical shear

π’ - heavy contours

 - thin contours

Vertical shear vector

shown

X-direction:

propagation to left right
Propagation to left & right

On flanks, both CW &

CCW cause L pressure

where vort is max, min

L

L

Net result…

L pressure on flanks +

L pressure ahead lead

to propagation to left

and right of mean winds

unidirectional shear case
Unidirectional shear case

• Vertical accel

largest on E

(forward) side…

storm moves E

• Nonlinear p’

encourages ascent

on flanks -- storm

splitting

• Result symmetric

(neither split

storm member

is favored)

Shear vector

shown

directional shear case
Directional shear case
  • RK’s analysis shows that pNL and buoy favor the left mover.
    • But pLfavors the right mover and dominates
slide44

• Hodograph with directional

shear. Thin shear vector

used for lower-level shear.

• Circle indicates updraft.

• Linear term encourages

low pressure downshear;

high pressure upshear

• For lower-level shear,

lowered pressure to NE

slide45

• Upper level shear produces

pressure perturbations in

different places

• On south side of updraft

there is lowered pressure

aloft overlying raised

pressure below. This flank

is favored for ascent.