Mesovortex Evolution and Dual Polarization Debris Signatures Associated with the 7 August 2013 Tornadic QLCS

Ashley WolfNWS Green Bay

Introduction

Brief Radar Overview Focus will be on the tornadic MV phase

Compare Tornadic Mesovortex and TDS Evolution

Application and utility of TDS criteria for tornadic QLCS events

Event Overview

Two phases of mesovortex development. Focus will be on the second phase which produced 6 known tornados (5 EF1 / 1EF2) within 45 minute period.

Little or no damage was observed between

the tornadic MVs.

Tornadic MV development close to GRB radar (<40 nm). Allowed for good data set for further analysis post-event.

Two Episodes of Mesovortex Formation Focus on Episode 2

Episode 10400-0515

Episode 20515-0700

Radar Overview

apex

Mesovortices developed as line segment surges and pivots to N-S orientation. Becomes favorably aligned to 0-3 km shear vectors (Schaumann & Przybylinski (2012).

This segment of the QLCS most likely to be balanced. Tornadic MVs form generally in 30 mile corridor bounded by bow apex to south and thunderstorm outflow boundary to north.

MVs intensify rapidly & become tornadic. Line segment accelerates to 65 KTS.

Mesovortex Evolution and Tornadic Debris Signatures

TDS criteria…

TDS Criteria Used In This Study

Reflectivity > 30dBZ CC ≤ 0.80 ZDR ≈ 0 Associated with “Strong” Velocity

Signature

WDTB recommended criteria CC < 0.80 Schultz et al criteria CC < 0.70

9

0522 0527

*

* Time of initial MV Genesis

Time from MV genesis to TDS ~ 2 Vol Scans

MV1 – Max initial TDS Height ~ 2 KMMV1 – Max TDS Height ~ 3.5 KMMV2 – Max initial TDS Height ~ 0.5 KMMV2 – Max TDS Height ~ 2.25 KM

MV1 & MV2 eventually merge.

MV1

MV2

MV1

0522 0527

Max TDS Height

*Elapsed time following MV Genesis >>>>>

He

igh

t (k

m)

He

igh

t (k

m)

MV1

MV2

CC 0.9 CC 0.9

SRM 0.9 SRM 0.9

MV2

MV1

10

More rapid evolution associated with MV4

Time from MV genesis to TDS ~ 1 Vol ScanMax initial TDS Height = Max TDS Height ~ 1 KM

0538 UTC

MV4

Elapsed time following MV Genesis >>>>>

CC 0.5

SRM 0.9

MV4

MV4

11

MV5 appears to develop on or just south of boundary.Time from MV genesis to TDS ~ 1 Vol Scan. Very rapid evolution!!! Classic ND characteristics. Appeared to be the strongest MV.

Max initial TDS height = Max TDS Height ~ 2.25 KM

0542

Elapsed time following MV Genesis >>>>> CC 0.5

BV 0.9

0542

0542Z 0.5MV5

12

Time from MV genesis to TDS ~ 1 Vol Scan. MV6 forms on boundary. Classic ND Vr characteristics once again.

Max initial TDS Height ~ 0.5 KMMax TDS Height ~ 1.5 KM

0604

CC 0.5

0604

0604

SRM 0.9

Z 0.9

??

Vr (m/s)

MV6

13

Max rotational velocity (Vr) generally AOB 1.5 kmNon-descending MV evolution Mean maximum Vr ~ 26 ms-1

Mean maximum Vr at initial TDS detection ~ 19 ms-1

Average TDS depth ~ 2 km. Greatest TDS depth ~ 3.5 km (MV5)Max TDS depth observed in same volume scan in which TDS

first identified in half the cases.

Time from MV genesis to first observed TDS ~ 1 volume scanInitial TDS observed before maximum Vr

Summary of Observed MV Evolution and TDS Characteristics

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From Schultz et al (2012) – Their Figure 10.

TDS Distance vs MAX TDS Height vs EF RatingIncludes Various Convective Modes (Alabama)

August 7, 2013

Max TDS heights observed in this event were comparable or slightly higher than EF1/EF2 tornadoes examined by Schultz et al (2012).

Still lots to learn about characteristics of QLCS debris signatures!

TDS Distance vs MAX TDS Height vs EF Rating (Various Convective Modes)

Applying TDS Criteria for Tornadic QLCS Events

Use caution when applying TDS criteria in tornadic QLCS events! Tornadic circulations are smaller-scale, develop very

rapidly, are short-lived and typically shallow compared to classic supercell-type storms.

Associated TDS will be more difficult to identify▪ Range dependent▪ Transient▪ May be embedded in clutter/noise near the radar▪ Smaller in size (diameter) and shallower in depth

May require less stringent dual-pol TDS criteria

Utility of the Dual-Pol Data

During an Event Identifying TDS signatures aids in warning decision process.

Confirms existence of short-lived QLCS tornadoes. Very difficult to identify visually as typically shrouded in rain.

Post Event: Damage Survey Despite apparent MV merger based on the SRM data, two TDS

signatures were identified in close proximity. ▪ Two distinct damage paths able to be identified during damage survey.

No damage reports received for last tornado (MV6). After examining dual-pol data, decided to investigate for damage. ▪ Discovered EF-1 tornado damage path just east of Green Bay.

This event stresses the importance of a thorough damage survey following suspected tornadic QLCS events. Prior to dual-pol and TDS applications, straight line wind damage may have been assumed (with perhaps no survey conducted at all). Helps the science!

Further Research

Compare supercell TDS to QLCS TDS Characteristics

Better understand relationship between QLCS MV evolution and associated TDS signatures

Acknowledgements

Thank You!

Gene Brusky Ed Townsend Ron Przybylinski Jason Schaumann

Questions???