Hail diagnosis from radar + NSE

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.

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.

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

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

II. S-Band Three-Body Scatter Spike (TBSS) 28 Oct 1999, 0625 UTC Also known as “Hail Flare” or “Flare echoes” a 10-30 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)

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 10-30 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)

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

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

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

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

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

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

IV. WDSS Maximum Expected Hail Size (MEHS)

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”

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.