James G. LaDue, WDTB – FMI 2005

Severe storm radar signatures

Jim LaDue

Warning Decision Training Branch,

NWS

Norman, OK

James.G.LaDue@noaa.gov

Topics

•

Estimating updraft strength

•

Mesocyclones

•

Assessing the potential for

•

Severe winds

•

Large hail

•

Tornadoes

•

Heavy rainfall

James G. LaDue, WDTB – FMI 2005

Updraft strength estimation

•

Upper level reflectivity core

•

Storm reflectivity and velocity structure

•

Low-level convergence


Elevated reflectivity core

•

Where does the precipitation form relative to the freezing level?

•

How high does the precipitation extend?

Link to loop


Case 1: July 10, 2003 very severe reflectivity profile

• Let’s examine the reflectivity profile of storm K0 and compare it to the

Severe Hail Index as determined by the previous page



•

0109 UTC

•

Deep, high reflectivity well

VIL = 83 kg/m 2

VIL density = 4.94 g/m 3

MEHS = 3” above/below

–20 

C level

-20° C

0° C

Z=55dBZ

Result: baseball hail



•

0109Z

•

Values are integrated upward

•

The Severe Hail

Index integrates the vertical reflectivity profile

•

Hail Detection

Algorithm

(HDA) converts this to estimated hail size

-20° C

0° C

Note how rapidly the SHI increases above the 0 °C level

POSH=90%


Case 2: July 10, 2003 Nonsevere updraft

• Let’s examine the reflectivity profile of storm F6 and compare it to the Severe Hail

Index as determined by the HDA worksheet


Case 1: July 10, 2003 nonsevere

•

0109 Z

•

Bottomreflectivity profile

Storm parameters for cell F6 heavy and weak

VIL = 42 kg/m 2


MEHS=.75” reflectivity

-20° C storm F6

0° C

Z=55dBZ


Case 1: July 10, 2004 nonsevere reflectivity profile

•

July 10, 2003

– Wichita

Storm parameters for cell F6

•

Note that SHI

VIL = 42 kg/m 2 responds


MEHS=.75” exponentially less given 10-

15 dBZ lower reflectivities in this storm

-20° C

0° C

POSH=90%

Result: no severe reports


Interim summary: updraft intensity through upper-level reflectivity

•

Use the higher slices to detect the towering cumulus above the boundary layer echoes

•

Look for the first strong core to develop above the freezing layer.

•

More severe storms have

• higher elevated reflectivities

• and more top heavy vertical reflectivity profile

•

Slightly smaller reflectivity aloft leads to large changes in expected weather


Severe updraft structural signatures

•

Objective : Understand how the following signatures are an indication of, or contribute to severe updrafts in convection

•

WERs

•

BWERs

•

Stormscale velocity


Nonsevere storm structure

•

Upper level storm top lies over lowlevel reflectivity maximum

•

Lower reflectivities overall

•

Low height of reflectivity thresholds


Severe sheared storm structure

•

Upper level storm top lies over lowlevel reflectivity gradient on the side of low-level storm relative inflow

•

Strong echo overhanging a

Weak Echo

Region (WER)


Trailing mesocyclone supercell updraft storm structure

•

Upper level storm top lies over lowlevel WER

•

Sometimes a

BWER (Bounded

Weak Echo

Region)

•

Sustained intense elevated reflectivity core

•

A mesocyclone


Severe sheared updraft intensity –

BWER detection

•

BWER (Bounded Weak Echo Region)

Needs a connection to the lowlevel WER

BWERs difficult to detect this far out

-20° C

Typical

BWER detection tops off near the

-20° C level


3.4°

2.4°

1.5°

0.5°

Classic severe updraft signature case

•

Trailing updraft

•

Normal width

•

Produced a few record sized hailstones

BWER = 2mi max size

-20° C


Severe updraft in a squall line segment

•

Squall line moving 52 kts

•

Storm top 26 kft AGL

•

Is there a

WER?

The ‘forward jump’ in the reflectivity from 2.4 to 3.4° is more than just movement between elevation slices

-20° C


Conceptual model of a more severe linear system updraft

•

Key things to note are the

•

Relatively nondescending RIJ

•

Front end echo overhang with linear BWER ahead of the surface gust front

•

Deep convergence zone


Updraft strength – Low-level convergence

•

Convergence parameters to affect updraft magnitude

• magnitude

• depth

• residence time of storm over convergence


Updraft strength – Low-level convergence depth

Assuming a steady state convergence

Depth:



Z = 2 km

Boundary width = 1 km (one .54 nm range gate)

Mean convergence over 1 km:



V = 10ms -1 / 1000m = .01 s -1

Updraft strength at 2 km W = (



V)



Z = .01s

-1 *2000m = 20 m/s

20 m/s

Convergence



V is simplified to be



V/width or

10 m s -1 /1000 m

6500ft - 2 km 1 km

10 kts (5 m/s)


Updraft strength – Low-level convergence depth

Assuming a steady state convergence

Depth:



Z = 3 km

Boundary width = 1 km (one .54 nm range gate)

Mean convergence over 1 km:



V = 10ms -1 / 1000m = .01 s -1

Updraft strength at 3 km W = (



V)



Z = .01s

-1 *3000m = 30 m/s

30 m/s

10 kts (5 m/s) 10000ft - 3 km


1 km

A severe upright convective system with deep, strong convergence

•

Deep Convergence zone

•

What do you think the spotter’s seeing?

15 kft


Conceptual model of a less severe linear system updraft

•

Key things to note are the

•

Descending RIJ

•

No deep convergence zone

•

Shallow sloping updraft over top of cold pool with numerous discrete cells merging into a line


Visual and radar obs of strong vs. weaker convective wind events

•

Case: 11 August

2004 Cocoa Beach,

FL

•

How deep is this gust front?


Visual and radar obs of strong vs. weaker convective wind events

•

Case: 11 August

2004 Cocoa Beach,

FL

•

How deep is this gust front?


Summary: Severe updraft signatures

• severe updraft signatures common to all storms in order of most severe first

•

BWER

•

WER

•

Intense reflectivity core, and deep relative to the –20

°

C level

•

Storm top displaced over WER

•



Stormscale rotation

•

First, a review

•

Which is (cyclonic convergent, anticyclonic convergent, convergent, divergent)?


Mesocyclones

A Rankine Combined Vortex


Mesocyclone criteria

Core diameter from

–V max to +V max should not exceed 10 km

Rotational Velocity

Vr = (| –V max

| + | V max

|)/2 exceeds user thresholds

Persistence



10 min

Vertical continuity

V r -V max

+V max


mesocyclone

•

Rotational velocity = (|max outbound| + |max inbound|)/2

•

Use representative inbounds and outbounds, not the absolute maximum values

Meso diameter = 3.5 nm

Vmax = 50kt

Vmin = -22kt

Rotational V = 36 kt


Another classic supercell


J. LaDue

Another classic


J. LaDue

A diversity of mesocyclone sizes

•

All of these were tornadic.

•

Only the big one shows a meso hit

G. stumpf

Courtesy G. Stumpf


Summary: mesocyclones

•

Less than 5 nm, 10km in diameter

•

Persistent

•

Not shallow

•

Partly occupied by updraft and downdraft

•

Begins as mostly updraft then matures as downdraft forms


Severe weather threats

•

Wind

•

Hail

•

Tornado

•

Heavy rain


Severe wind threats

•

Isolated downbursts

•

Supercell wind threat

•

Organized multicell wind threat


Pulse storm downbursts

The idea is to look for clues for potential downbursts before it reaches the ground.

•

Rapid and severe growth in updraft

•

Descent of the reflectivity core

•

Midlevel velocity convergence



Is this the time a warning should be issued?



As an aside

300 m

C Doswell



As an aside



This time height trend of reflectivity shows the descending core hitting the ground just after

0006 UTC.

Updraft phase



Updraft begins to build a core aloft

2356 UTC



0002 UTC



Midlevel convergence signifies downdraft commencing

0008 UTC



0014 UTC



Downdraft impacts the ground

0019 UTC



0024 UTC



0029 UTC


Visual and radar obs of a strong

•

Case: 12 August

2004 Cocoa Beach,

FL

•

Downburst precursor wind event

Mid altitude convergence is strong:

A downburst precursor


Visual and radar obs of strong wind event

• 5 minutes later…



•

This event shows that VIL, height of Max reflectivity and storm top did not give lead time to downburst.

•

Tracking the descent of the core gave a better lead time

•

Monitoring updraft growth might give even better lead time

•

The stronger the elevated core, the stronger the initial updraft


Supercell severe wind threat

•

The most severe winds are in the Rear Flank

Downdraft (RFD) surrounding the mesocyclone.

•

Detected by deep convergence zone.

•

Pulse storm downburst mechanisms also occur


Supercell wind events

•

Can produce a large number of the most damaging wind events without tornadoes

•

Most common with

HP supercells


Rear flank downdraft contains the most intense winds but at this distance, 88D velocities overshoot highest winds.

Supercell wind events

3.4°

These high wind events often have a very deep convergence zone, extending 2 km or more.

Deep convergence zone

2.4°

15 kft


1.5°

0.5°

Organized multicell convective wind events

•

Squall lines

•

Bow echoes


Squall line heading toward radar

The worst winds are pointed directly at the radar. And the radar is close so that the low-level winds can be sampled.


Squall line not heading toward radar

Where do you think the strongest winds in this squall line will hit in the next hour?





• We must use other techniques to estimate wind severity

• Speed of motion

• Estimated strength of updraft

• Near storm environment

Radar sees only radial wind

True wind

Tangential wind component


Look for Mid Altitude Radial

Convergence Zone (MARC)

MARC signatures found within the strong cores

MARC signatures are small:

< 15 km long

> 25 m/s convergence

MARC signatures found here at

4 km AGL


Bow echoes

Narrow bow echoes are typically more severe than wide ones given everything else being equal.

Strongest winds


Bow echoes

•

Example of narrow bow echoes and very severe winds


Hail potential

•

Radar cannot directly detect hail

•

One big hailstone sends back the same energy as

1000s of regular raindrops

•

Either scenario could take place in a radar volume

•

Thus we have to infer the presence of hail from other clues


Favorable hail clues

•

Environmental

•

Dry air aloft, moist below, large instability

•

Enough wind shear for supercells

•

Fairly low freezing level (wet bulb) 7500-10000’

•

Storm structure

•

Intense reflectivity core (>55 dBZ) above the –20 C level

•

Strong updrafts with a WER or BWER

•

Storm rotation (supercells)

•

Updraft persistence



•

Intense elevated core

•

Know how high your elevation slices are to your

0° and –20° C heights at the storm location.

•

Look for high reflectivity (>55 dBZ)

LRM products at 24 –

33 kft and especially the 33-60 kft level.

-20 C

0 C



•

Bounded Weak Echo

Region (BWER)

•

Intense updraft forms a hole in the reflectivity core.

BWER

BWERs not typically seen this far out

Typical

BWER heights

3.4°

2.4°

1.5°

0.5°



•

Weak Echo Region (WER)

•

Intense updraft also levitates a large

3.4° region of core.

•

Look for high over low reflectivities on the inflow side of a storm

2.4°

WER

A B


Watch out for anvil WERs.

WER typically from sfc to

15-20 kft.

They are not updrafts.

1.5°

0.5°

A

B

Vertically Integrated Liquid

•

Integrates what the radar thinks is liquid water in the vertical

•

Not a reliable hail indicator, no set thresholds

• Does show location of the ‘biggest storm’


VIL

Hail is loosely associated with VIL but the threshold changes with season and location

70

60

50 45

50

55

65 65 65

55

50

45

40

40

30

20

35

10

0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month


40

VIL density

•

VIL is normalized by echo top height in meters and then multiplied by 1000 to yield a density of g/m 3

•

Attempts to reduce effects of different environments on a consistent large hail threshold


VIL = 47.5 kg/m 2

ET = 9.1 km

VIL = 70 kg/m 2

ET = 13.4 km

-20 C

0 C

VIL density

Warning performance statistics show a VIL density ~ 3.28 g/m 3 performs well as a large hail threshold in multiple

CWAs.

However…

Cerniglia and Snyder, 2002 – ER Tech memo


VIL density does not perform well in estimating severe hail size

Edwards and Thompson, 1998


VIL density

Other ordinary cell hail considerations

•

Reflectivities > 60 dBZ indicate a high likelihood of hail

•

Cannot discriminate hail size

•

The Hail Detection Algorithm tends to overestimate the Probability of Severe Hail

(POSH) in weakly sheared storms over low terrain

•

Hail potential increases as the freezing level approaches the ground or vice versa (i.e. topography)


Tornado potential

•

A tornado vortex signature

•

Strong gate-to-gate shear

•

Prefer to see this for at least two slices

•

The bottom should be on the lowest slice or within 600 m AGL

•

I prefer to see this persist for a couple scans

•

But some situations will not allow me to wait.

•

Due to beam spreading, my maximum TVS range is about 60 nm. After that, I’m only seeing mesocyclones.


Tornado vortex signature

Shear = |outbound + inbound| in adjacent gates

•Anywhere from 35 to more than 140 kts depending on range and severity


Occurrence of tornado with

LLDV

TVS low-level gate-to-gate velocity difference, LLDV (m/s)


LLDV m/s

FAR = green line

POD = red line

HSS = black line

Inset = POD vs FAR

Occurrence of tornado with MDV

TVS Maximum gate-to-gate velocity difference, MDV (m/s)


MDV m/s

FAR = green line

POD = red line

HSS = black line

Inset = POD vs FAR

A descending TVS

•

50% of are associated with supercells

•

(from Trapp et al., 1999)

•

Offers greater lead time

Trapp et al., 1999


Nondescending Tornado

Signatures

•

80% of squall lines

•

50% of supercells

(from Trapp et al., 1999)


What is the TVS seeing?



1. Flanking line

1.

1.



1. Flanking line

2. Dry slot and hook

1.

2.

1.

2.



1. Flanking line

2. Dry slot and hook

TVS

Tornado


1.

2.

TVS

1.

2.

TVS

Example of a TVS in polar vs. cartesian coordinates

NIDS velocity

1 km boxes

Full resolution

SRM 0.2 km gates

June 13, 1998 OKC


Squall line vortices: 29 June

1998

Adapted from Pryzbylinski (2002)


Time height comparisons

•

Core #2 was more intense V r

=30 m/s

(60 kts)

•

Nondescending V r with time indicates low-level vorticity becoming stretched upward

Time-height V r trace Core #2.


Mesocyclone strength: isolated vs linear convection

•

Comparison of circulation characteristics between

Przyblynski et al. 2001 data set and Burgess et al. (1982) data set.

•

Larger mesocyclone diameters in linear systems than with isolated cell mesocyclones

•

Weaker V r with linear systems

Rot Vel

(m/s)

19.0 Squall

Line

(Low)

Trad

Super-cell

(Low)

Squall

Line

Trad

Supercell

23.0

18.8

25.0

Diameter

(km)

7.2

Height

(km)

5.4

7.4

6.0

7.6

9.2

L = surface to 8200 ft.


Nonmesocyclonic tornadoes

•

Considerations

•

Favored with steep 0-3 km lapse rates

•

0-3 km CAPE

•

Boundary with strong vertical vorticity

•

Slow moving boundary

•

Difficult to see on radar


Tornadoes in weak shear environments

•

Start with strong boundary with developing

CU

•

Boundary shear starts to roll into misocyclones

C

B

A



•

CU updrafts grow

•

Misocyclones A and B grow and move to the right while C weakens

C B

A



•

TCU continue to grow. Elevated core may form

•

Misocyclone B phases with one updraft forming a tornado

•

Misocyclone

A remains unattached, only dust devils form

C

B

A


Pre-existing vertical vorticity: a case

•

Storms moving with cold front

•

Outflow boundary moving down front

•

Rapid updraft growth on intersection

This cell motion tracked

Onset of elevated core indicating significant updraft

-20° C

0° C


Heavy rain producing storms

•

Rain rate is dependent on

•

Updraft strength X moisture content X efficiency

•

Total rainfall is dependent on

•

Rain rate

•

Motion

•

Size of storm along the motion track

• Let’s talk about a certain type of storm with high efficiency

•

Warm Rain dominated storm


Low topped heavy rainfall convection

09 June 2004

•

Very humid airmass

•

Lots of shear and low-level

CAPE

•

Not a lot of upper level

CAPE


Low topped heavy rainfall convection

09 June 2004

Reflectivity dominated by numerous small drops at observers location


An earlier warm rain dominated supercell on 09 June 2004


Cross Section through Warm-

Rain Supercell

Notice the reflectivity drop off above the freezing level.


Why spotters are still needed

VCP 11 or 21?

Aspect Ratio

Radar Horizon


Viewing Angle

Summary

•

Updraft strength

•

Stronger updrafts have any one of these features

•

Higher reflectivity at higher altitudes relative to the equilibrium level

•

Strong echo overhang,

• a BWER

•

Very strong and deep convergence zone

•

Not all of these must be present for a severe storm but if more exist, the more confidence you have of identifying a severe storm updraft


Summary continued

•

Mesocyclones

•

Must be persistent

•

Extend through > 2 km depth

•

Have time continuity

•

< 10 km wide

•

No minimum rotational velocity threshold

•

Mature mesocyclones can be divided between updraft and downdraft


Severe hazards

•

Severe winds

•

The strongest Individual downdrafts often follow strongest updraft signatures

•

Accompanied by Mid Altitude Radial

Convergence (MARC)

•

Often follow a deep convergence zone

•

Occur with small vortices such as mesocyclones

•

Keep in mind the favorable environments

(DCAPE, shear)


Severe hazards

•

Large hail

•

Intense upper-level updraft (-10 to -30



C layer)

•

Deep, intense reflectivities

•

WER

•

BWER (especially large ones)

•

Updraft persistence (indicated by a mesocyclone)


Severe hazards

•

Tornadoes

•

Mesocyclonic

•

Strengthening rotational velocity with strong low-level updraft signatures

•

Onset of a tornado vortex signature (TVS)

•

Onset of a hook

•

Squall line

•

Front inflow notch

•

Vortex rotational velocity increasing in intensity

•


•

Nonmesocyclonic

•

Pre-existing source of vertical vorticity

•

Young but strong updraft

•

Favorable environment


Severe hazards

•

Heavy rain

•

Strong updraft is needed for inefficient storms

•

Weak updraft is enough if it is efficient

•

Large moisture

•

Slow motion

•

Large reflectivity core

•

Watch out for low topped warm rain events


resources

•

General radar interpretation - OKFIRST

• http://okfirst.ocs.ou.edu/train/materials/radar.html

•

NSSL mesocyclone and tornado case study page

• http://www.nssl.noaa.gov/wrd/swat/Cases/cases_pix.h

tml

•

NOAA radar page

• http://weather.noaa.gov/radar/

•

The Warning Decision Training Branch

• http://www.wdtb.noaa.gov/


James G. LaDue, WDTB – FMI 2005

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