# Presentation: 076853

## Severe Hail Detection Using Reflectivity Elevations and Freezing Levels

**Rod Donavon**
**National Weather Service**
**Grand Forks, ND**

- The views expressed are those of the author and do not necessarily represent those of the National Weather Service.

## Where is the Grand Forks (FGF) CWA?

## Severe Hail Detection

- During the 2001 and 2002 Convective Seasons, the staff at FGF have had great success issuing Severe Thunderstorm Warnings based on 50-55 dBZ Reflectivity Max Heights (AGL)  and Freezing Level Heights (MSL).
- Average Lead Time during this period was 20.7 minutes, an increase of 6 minutes over 1996-2000
- POD increased from 0.854 to 0.919
- FAR lowered from 0.299 to 0.177

## Reflectivity All Tilts Product

- Allows Radar Operator to view all available elevation slices within the  current Volume Scan in Real Time.
- Radar Operator can view storm structure and make decisions before derived products become available.
- Can give idea of updraft strength and hydrometeor potential based on reflectivity heights.

## VCP 11: Radar Elevation Angles

## Data Used For Study

- Storm Data to help identify events
- Model Soundings and NCDC Freezing Level Charts
- Archived Radar Data
- Correlated 50-55 dBZ reflectivity max heights and associated hail reports with FZLs
- Max Reflectivity Heights obtained using elevation scans and not with cross sections.

## Quality Control of Data

- Core Dumps were not used. Looked for a core maintained for previous 2 volume scans.
- Elevated Convection excluded (Future)
- Hail report discarded if report likely not largest hail with storm at reported time.
- Hail report discarded if unable to associate with a storm.

## Results For All Hail Sizes

## Results for 0.75-1.00 Inch Hail

## The Numbers Game  Establishing a Warning Criteria (TEST)

- _Freezing Level Heights_		          _Warning Criteria_
- 8000-11500 ft MSL                                Multiply FZL by 2
- 12000 ft MSL			          Multiply FZL by 2.1
-                                                                 = 26000 ft AGL
- 12500 ft MSL			          Multiply FZL by 2.25
- 				           = 28500 ft AGL
- 13000 ft MSL			          Multiply FZL by 2.4    					           = 31000 ft AGL
- 13500 ft MSL			          Multiply FZL by 2.6
-                                                                = 35000 ft AGL
- 14000 ft MSL                                         Multiply FZL by 2.7
-                                                                = 38000 ft AGL

##  Central Iowa April 18, 2002

## What is the Criteria for 4/18/02?

- Freezing Level on 4/18/02 = 12500 ft MSL +-200 ft

- Warning Criteria = 28500 ft

## Results For Central Iowa 4/18/02

- 21 Hail events using 10 mi/15 min Rule
- 50-55 dBZ Reflectivity Max Heights using All Tilts Product
-      Non-Severe = < 30000 ft AGL
-      0.75-1.00 in Hail  =  30000-36800 ft AGL
-      1.25-2.00 in Hail  =  37000-41000 ft AGL

## Stats for Using TEST Severe Criteria

- POD = 1.00
- FAR =  0.087 2 Counties **Severe Hail after crossing adjacent
-                                          county border**
- Missed Events = 0
- Mean Lead time = 16.5 min                             **Lead times only used for developing storms**

## Issue The Warning Now?

## Using System During ExplosiveDevelopment

- Freezing Level:                            = 12000 ft
- Warning Criteria
- =  26000 ft

## Cross Section at 2045Z

**2.00 inch hail reported at 2050Z.  Later produces 4.50 inch hail.**

## Can Hail Size Be Estimated?

## Adding 1.25-2.00 Inch Hail

## Adding >2.00 Inch Hail

## Can Hail Size Be Estimated Cont.Central Iowa 5/16/99:  FZL = 12700 ft MSL

- 50-55 to 41000 ft
- 2.50 inch hail

- 50-55 to 38500 ft
- 1.75 inch hail

## Hail Size Estimation

**50-55 dBZ to 24000 ft                   Pea sized Hail**** **

## Hail Size Estimation15 Min Later:  Second Storm

**50-55 dBZ to 41000 ft                        1.75 inch hail**

##  Does Horizontal Storm Size Matter?

**No!  It is Updraft Strength and the Height of the 50-55 dBZ Core that counts!   ****50-55 dBZ to 41500 ft:  2.75 inch hail**

##        How Does The Iowa Data Compare to FGF Data?

-        **How Does The Iowa Data Compare to FGF Data?**

## Hail Size Potential Related to FZL?

## Potential Errors

- Height differences between Elevation Angles at greater distances from the RDA
-    New VCP gamma/77 to help (Fall 2003)
- Cone of Silence
- _Superrefraction_ of radar beam
-        -  overestimation of target height
- _Subrefraction_ of radar beam
-        -  underestimation of target height

## Subrefraction vs Superrefracton

- Subrefraction Scenario – inverted V sounding with moist layer aloft
- Superrefraction Scenarios – Temp Inversions, thunderstorm outflows and Cold Frontal Passages

## Most Hail Reports are Estimated

## Conclusions

-  Useful tool in the warning process.  Severe Potential detected 0.5-1.5 volume scans before derived products.

-  Updraft Strength at FZLs below 10000 ft rarely can support hail greater than 1.00 inch.

-  Rare to have >1.00 inch hail when FZLs are >14000 ft.  Too much melting occurs.

-  Hail size potential increases as the 50-55 dBZ reflectivity max height increases.

-  False Alarm Rates (FAR) are low

## Conclusions Cont.

- System works very well for Explosive Development and Pulse Severe
- Criteria likely to work well for much of the Northern Plains.  Southern Plains remains to be determined

## Future Additions

- Will continue to add to database
- Convert FZL from MSL to AGL to test usefulness
-     for higher elevations
- Elevated Convection Comparison.  Will severe criteria to be lower?
- Compare data from other Regions. Currently NWS San Angelo, TX working on a comparison.
- Possibility of Detecting Downburst Winds with 50-55 dBZ core dump

## Questions or Comments?

**Email:  rodney.donavon@noaa.gov**