Using WSR-88D Reflectivity for the Prediction of Cloud-to-

Ground Lightning

Nowcasting a storm’s first strike

Brandon VincentNWS-Newport

Larry CareyTexas A&M University

(formerly North Carolina State University)

Doug Schneider Kermit KeeterRod GonskiNWS-Raleigh

Overview

• Introduction - Why should we try and predict lightning?

• Background - Quick Review of Lightning and Cloud Electrification

• Methodology - Data collection / Analysis

• Results - What did we find?• Operational Use – Nowcasting a

storm’s first strike• Summary - Main points to

remember

INTRODUCTION:Why should we even try and predict lightning?

“It’s an act of God”

“Lightning is unpredictable”

“We can’t do anything about it”

Lightning is the one remaining threat that we do not warn for.

Curran et al. (2000)

Lightning Casualties and Damages in the United States

Lightning was #1 source ofconvective weather deaths from 1992 – 94 (tornado closerunner-up).

Lightning was #2 source ofweather related deaths behindflash + river flooding (tornadoclose #3)

Note: suspect lightning deaths, injuries and damages are significantly underestimated.

Casualty Rank1959-94

DamageRank1959-94

For lightning (‘59-’94),North Carolina ranked

#2 – Fatalities#4 – Injuries#4 – Casualties#4 – Damage

In the United States.

Curran et al. (2000)

Rank ofCasualty Rate perPopulation1959-94

Rank ofDamage Rate perPopulation1959-94

Curran et al. (2000)

For lightning (‘59-’94),North Carolina ranked

#8 – Fatality rate#10 – Injury rate#10 – Casualty rate#15 – Damage rate

Per populationin the United States.

BACKGROUND:Brief Review of lightning and cloud electrification

Thunderstorm Electrification – Non-inductive Charging (NIC)

Charge ReversalLevel (temperature)• Function of Liquid Water Content and Drop Size Distribution

6 km (T=-15C)

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-

--

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Williams (1988)

IdealizedThunderstormTripole

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------- --- -- -- --- --- --- --- - - --- -

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++++ +

++ ++ ++ ++

- -- -++

-- - ---

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- --20° C

-10° C

-

+

Non-inductive Charging (NIC) Theory

TypicalThunderstorm

• 90% -CG lightning• 10% +CG lightning

How is WSR-88D Reflectivity able to predict the onset of Cloud-to-Ground Lightning?

• WSR-88D reflectivity can be used to indirectly identify this electrification process because graupel and hail return large reflectivity echoes.

Large reflectivity values near the charge reversal temperature can be a precursor to cloud-to-

ground lightning strikes

Past Studies

• Previous research has identified reflectivity values between 30 and 40 dBZ at the -10 to -20°C level can precede CG lightning strikes by 5 to 30 minutes.

• Buechler and Goodman (1990)• Michimoto (1991)• Hondl and Eilts (1994)• Zipser and Lutz (1994)• Gremillion and Orville (1999)

METHODOLOGY:Data Collection / Analysis

Data Collection

• 50 Cases Covering 13 Lightning Days

• 24 cases were recorded in real-time at the National Weather Service in Raleigh, NC.

• 26 cases were post-analyzed at NCSU using archived Level II radar data and NLDN data.

• Archived radar data obtained via NCDC for the following days in 2002:

March 26, April 14, April 15, April 19, and May 30

• NLDN lightning data obtained for same days

How did we record the cases?

• For cases recorded at the NWS, a 4-Panel PPI radar reflectivity display with 1 minute NLDN data overlaid was used to investigate convective cells.

• For post analysis at NCSU, WATADS (WSR-88D Algorithm Testing and Display System) was used to display and analyze the collected level II radar data.

• NCSU post-analysis used a combination of PPI displays and vertical cross-sections in WATADS to analyze the reflectivity values at the -10 and -15C isotherm heights within a convective cell.

Data Analysis

• Criteria• Eight sets of criteria were used in

analyzing the recorded convective cells

• The criteria were comprised of the following 3 variables: the number of radar volume scans, the minimum reflectivity and the height of the -10C or -15C isotherm

Criterion # Volume Scans

Z-Threshold Environ. Temp.

1 1 35 -10C

2 1 35 -15C

3 1 40 -10C

4 1 40 -15C

5 2 35 -10C

6 2 35 -15C

7 2 40 -10C

8 2 40 -15C

RESULTS:What did we find?

• Best Lightning Detection Criteria (Based on CSI) is 1 Vol /40 dBZ/-10C

• Best Lightning Detection Criteria (Based on lead-time) is 1 Vol /35 dBZ/-10C

• Using colder temperatures in the criteria (i.e., changing the examined isotherm from -10C to -15C) yielded:

- Lower FAR, but lower POD’s and less lead time

• Changing the reflectivity criteria from 35 to 40 dBZ had similar effects:

- Lower FAR, but shorter lead times

• Changing the number of volume scans in the criteria from 1 to 2 had the effect of:

- Lower FAR, but also lower lead times and POD.

Best combination of CSI and lead time was 40 dBZ at the -10°C level for 1 volume scan.

General Trends

How about compared to other studies?

Researcher(s) Criterion FAR POD CSI Lead-Time

Gremillion and Orville(Florida – rapidly developing

storms would account for lower FAR and shorter LT)

1 Volume Scan 40 dBZ at -10C

7% 84% 79% 7.5 minutes

This Study 1 Volume Scan 40 dBZ at -10C

37% 100% 63% 14.7 minutes

Researcher(s) Criterion FAR POD CSI Lead-Time

Buechler and Goodman(various locations)

1 Volume Scan 40 dBZ at -10C

20% 100% 80% 4-33 minutes

This Study 1 Volume Scan 40 dBZ at -10C

37% 100% 63% 14.7 minutes

What was the difference in storm structure between the detection events and the false

alarms?

Day / Case Type ΔdBZ/kft from 0 to -10C

ΔdBZ/kft from 0 to -15C

ΔdBZ/kft from 0 to -20C

4/15/02 Detection 0.00 0.00 -0.52

4/15/02 Detection -1.15 -1.70 -2.20

4/15/02 Detection 1.15 0.71 0.52

4/19/02 Detection -1.25 -0.75 -0.57

4/15/02 False Alarm -1.15 -1.42 -1.99

4/15/02 False Alarm -2.76 -2.27 -2.30

4/15/02 False Alarm -1.61 -2.69 -2.83

4/19/02 False Alarm -1.25 -1.37 -1.04

Mean Vertical Reflectivity Gradient (0 to -20°C) for a Detection: -0.69 dBZ/kft

Mean Vertical Reflectivity Gradient (0 to -20°C) for a False Alarm: -2.04 dBZ/kft

False Alarm Examples

Detection Examples

Operational Use:Nowcasting a Storm’s First Strike

• Since a storm’s first CG lightning strike can be predicted, what then are some circumstances that the NWS might prioritize for trying to do so?

Nowcasting a Storm’s First Strike

Situations where you can consider nowcasting storm's first strike: - First developing storms of the day

- Widely scattered or isolated storms - Storms developing in haze - Developing storms threatening - Highly populated areas  - Outdoor recreational areas (e.g., lakes)  - Large outdoor gatherings

• After widepsread deep convection is initiated and/or there is a threat of severe weather, nowcasting a storm's first strike would not be prioritized.

How to Predict the First Strike in Real-time Using AWIPS

• Read the -10°C height off the latest sounding (use the sampling tool or hold down the left mouse button).

• Display a 4 panel of reflectivity from the 1.5° to 4.3° slices (varies with distance of storm from radar).

• Read the reflectivity at the elevation angle closest to the -10°C height (use the sampling tool or hold down the left mouse button).

• If the reflectivity is 40 dBZ or greater at that level, a lightning strike is likely in the next 10 to 20 minutes.

Example of a storm that produced lightning

12Z GSO sounding showed the -10°C height was around 16,000 ft.

Example of a storm that produced lightning

A reflectivity of 40 to 45 dBZ above 17,000 feet (1.5° slice) was found using the sampling tool. The first strike was 15 minutes later.

Example of a storm that did not produce lightning

12Z GSO sounding showed the -10°C height was around 21,000 ft.

Example of a storm that did not produce lightning

A reflectivity of 25 to 30 dBZ near 21,000 feet (4.3° slice) was found using the sampling tool. No lightning was produced.

Limitations

• Radar beam may not reach the -10°C height close to the radar.

• The height of the -10°C level may fall between elevation angles, so the reflectivity value must be estimated (upcoming VCP 12 will reduce this problem with more elevation angles and faster sampling).

• Regional variability? More local studies needed.

• Time limitations to investigate several storms – future work will include developing an automated algorithm.

What should I remember?

• Best predictor of CG lightning (based on CSI and lead-time) was a 40 dBZ echo at the -10C isotherm height.

• Nowcasting a storm’s first cloud to ground lightning strike is appropriate for certain situations, such as:

• The first developing storms of the day

• Unexpected isolated storms• Storms threatening highly

populated and/or outdoor recreational areas