1. Hail diagnosis from radar + NSE

2. The formation of large hail requires long
hailstone residence times in regions of large
supercooled liquid water content in the
primary hail growth layer between -10°C to -30°C
followed by minimal melting of the fully grown
hailstones on the way to the ground.

3. Large Hail Diagnosis from Radar: 4 Techniques
determine thresholds for severe hail classes using the NSW 50 dBZ hail nomogram
recognise 3-body scattering (“TBSS” or “flare echo”)
recognise radar-based storm structure signatures indicative of large hail (e.g., WER, BWER, high ref. aloft,…)
interpret “MEHS” estimates from the WDSS Hail Detection Algorithm
I have dropped the storm top divergence criterion for hail diagnosis until
I am satisfied that this indicator can be of operational use.

4. Hail “technique” I: NSW 50 dBZ nomogram
UPDATE NOMOGRAMS WHEN NEW ONES ARE AVAILABLE
Note: Nomograms were developed using an S-band radar with 2° beamwidth.
Caution needed with C-band radar due to increased attenuation and finer beamwidth.
Nomogram Explanations in Victorian Storms Vignette

5. Hail “technique” I: NSW 50 dBZ nomogram
~6.3 km
T=0o C
Z = 3 km
UPDATE NOMOGRAMS WHEN NEW ONES ARE AVAILABLE

6. II. S-Band Three-Body Scatter Spike (TBSS)
28 Oct 1999, 0625 UTC
Also known as “Hail Flare” or “Flare echoes”
a km long low reflectivity (< 20 dBZ) mid-level echo “spike” aligned radially downrange from a high reflectivity (63 dBZ+) core
On C-band radars TBSS can be related to large raindrops rather than hail
TBSS is a sufficient (but NOT necessary) condition for large hail detection
8cm hail reported
Difficulty: delineate TBSS from a line of light showers
TBSS Examples: Sydney S-band radar (WSR74S, 2°beam width)

7. II. S-Band Three-Body Scatter Spike (TBSS)
14 Apr 1999, 0930 UTC
surface hail of at least 2.5 cm diameter should be expected when a TBSS is observed (on S-band)
TBSS provides warning lead time (for the largest surface hail) of minutes
References: Lemon (WAF 1998) ; Zrnic (Radio Sci. 1987); Wilson and Reum (J. Atmos. Ocean Tech. 1988)
Radar
radial
At least 9cm hail
Difficulty: delineate TBSS from a line of light showers
TBSS Examples: Sydney S-band radar (WSR74S, 2°beam width)

8. II. Three-Body Scatter Spike (TBSS)
radar beam strikes the intense hail core and energy is forward-scattered towards the ground
energy then scattered back from wet ground to the hail core where it is forward-scattered back to radar

9. III. Storm Structure Rather obvious examples are:
tall WER with plenty of ref. aloft
Tight low-level reflectivity gradient
BWER
Note: persistent BWER  high probability of updraft rotation  dynamic p’ gradients augment
Buoyancy  stronger updrafts allowing longer hailstone residence times aloft

10. Conceptual Model: Nonsevere Storm
updraft characterised by considerable slope
echo top, mid-level core and low-level reflectivity maxima are “vertically stacked”
main echo at
8 km (26,000 ft)
inflow
storm top
Storm has enough CAPE to survive in enhanced shear, but not enough
to set up decent updraft-shear interactions and become severe.
storm
motion
low-level
reflectivity
core
anvil edge

11. Conceptual Model: Severe Storm
updrafts become more vertical as they strengthen
Weak Echo Region (WER) on inflow side marks region of strongest updraft
echo top aligned over low-level reflectivity gradient on inflow side (“echo top displacement” with echo top located on top of updraft/downdraft interface)
inflow
main echo at
8 km (26,000 ft)
storm top
WER
storm
motion
low-level
reflectivity
core
anvil edge

12. Conceptual Model: Supercells
when a storm intensifies to supercell stage, the updraft becomes upright & the storm top shifts to a position located over a persistent Bounded Weak Echo Region (BWER)
largest hail falls in tight reflectivity gradient next to BWER
main echo at
8 km (26,000 ft)
low-level
reflectivity
core
storm top
anvil edge
BWER
storm
motion
inflow

13. Conceptual Model: Supercells
BWER that exists for >15 minutes is correlated with significant updraft rotation (mesocyclone)
Note: transient BWERs may also be found with severe non-supercell convection
main echo at
8 km (26,000 ft)
low-level
reflectivity
core
storm top
anvil edge
BWER
storm
motion
inflow
“classic” supercells may exhibit a low-level hook or pendant echo on the rear flank of their inflow side

14. IV. WDSS Maximum Expected Hail Size (MEHS)

15. WT emphasises cold temperatures + E emphasises high reflectivities
E(Z)
WT(H)
The individual components of MESH are (in reverse):
The Maximum Expected Hail Size: MESH = 2.54 sqrt(SHI)
The Severe Hail Index: SHI = 0.1 sum_over_all_tilts( W_T(H_I) x E_I dH_I )
The Hailfall Kinetic Energy: E(Z) = 5 x 10^{-6} x 10^{0.084 Z} x W_2(Z)
0oC and –20oC levels from 3-hourly Meso-LAPS or manual input
(editable by forecaster)
WT emphasises cold temperatures + E emphasises high reflectivities
 elevated high dBZ cores result in “gorilla MESH”

16. Review of Radar-Based Large Hail Estimates
50 dBZ nomogram show some skill in estimating hail size thresholds
TBSS/“Flare echoes” on S-band radar indicator of giant hail
3D storm reflectivity structure as indicator for large hail
WDSS Hail Detection Algorithm Maximum Hail Size Estimate (MEHS) as a confirmation tool
V. Storm top divergence.