Using WSR-88D Reflectivity for the Prediction of Cloud-to-ground Lightning: A Central North Carolina Study A collaborative research project between NCSU and NWS – Raleigh Brandon Vincent and Larry Carey North Carolina State University Doug Schneider, Kermit Keeter, Rod Gonski NWS - Raleigh

How Is WSR-88D Reflectivity Able to Predict the Onset of Cloud to Ground Lightning?

- + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Idealized Thunderstorm Tripole + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Charge separation most likely occurs during rebounding collisions between ice crystals and large ice hydrometeors such as graupel and hail that remain suspended in the mixed phase zone by the updraft of a growing thunderstorm. + + + + + + - - - - - - - -20° C - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -10° C + + + + + + + + + + + + + + - - + WSR-88D reflectivity can be used to indirectly identify this electrification process within a growing thunderstorm because graupel and hail return large reflectivity echoes. Non-inductive Charging (NIC) Theory

Methodology of Research 50 Cases Covering 13 Lightning Days. For 24 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 26 post analysis cases at NCSU, WATADS (WSR-88D Algorithm Testing and Display System) was used to display and analyze the collected level II radar data. Individual convective cells were identified and tracked using the SCIT (Storm Cell Identification and Tracking) algorithm in WATADS. A combination of PPI displays and vertical cross-sections were used in WATADS to analyze the reflectivity values at the –10 and -15 C isotherm heights within a convective cell.

Data Analysis Criteria Criterion # Volume Scans Z-Threshold Environ. Temp. 1 35 -10C 2 -15C 3 40 4 5 6 7 8 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 -10C or -15C isotherm

Best Lightning Detection Criteria (Based on CSI and lead-time) is 1 Vol /40 dBZ/-10C or 1 Vol /40 dBZ/-15C

Best Lightning Detection Criteria (Based on CSI and lead-time) is either 1 Vol /40 dBZ/-10C or 1 Vol /40 dBZ/-15C

False Alarm Examples Reflectivity of 40 dBZ barely reached the -10°C height, and did not reach the -15°C height.

Detection Examples Reflectivity of 40 dBZ and greater easily exceeded the -10°C and -15°C heights

Nowcasting a Storm’s First Strike Since the onset of CG lightning can be predicted, what are some circumstances that the NWS might prioritize for trying to do so? Situations where you can consider nowcasting storm's first strike: - First developing storms of the day - Unexpected isolated storms - Widely scattered storms - Storms developing in haze - Developing storms threatening - Highly populated areas   - Outdoor recreational areas (e.g., lakes)   - Large outdoor gatherings

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 (easiest to use the procedure “Lightning detection” under “Radar Procedures” in the AWIPSUSR account). 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 nowcast issued for a storm’s first strike “A DEVELOPING STORM IS EXPECTED TO PRODUCE CLOUD-TO-GROUND LIGHTNING STRIKES IN MOORE COUNTY NEAR SEVEN LAKES. ANOTHER DEVELOPING STORM NEAR HIGH POINT MAY PRODUCE LIGHTNING IN SOUTHERN GUILFORD AND NORTHERN RANDOLPH COUNTIES. DON'T WAIT UNTIL YOU SEE THE FIRST LIGHTNING FLASH BEFORE HEADING TO SAFETY. INSTEAD...MOVE INDOORS AT THE FIRST SIGN OF THREATENING SKIES OR THE FIRST SOUND OF THUNDER. MAKE SURE THAT LIGHTNING IS WELL AWAY FROM YOUR LOCATION BEFORE RESUMING OUTDOOR ACTIVITY.”

Summary The best predictor of cloud-to-ground lightning strikes was a reflectivity of 40 dBZ at the height of the -10°C level. Nowcasting a storm’s first cloud-to-ground lightning strike would no longer be appropriate once deep convection is initiated and/or there is a threat of severe weather. 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.