Hailstorms Everything you wanted to know about hail (and more) ATMO 352 Spring 2013 Background: Photorealistic rendering of a WRF simulated hailstorm Hail Climatology First-order surface observation stations with five-year average number of hail days during 1896-1995, from Changnon and Changnon (2000) Severe Hail Climatology • Hailstorms responsible for nearly $2.4 billion of damage to crops and property annually in the USA (Changnon 1999) – Little documented on longterm or seasonal forecasting techniques • Large hailstones (D > 5 cm) often associated with supercell thunderstorms – Often accompanied by intense wind gusts, violent tornadoes, and extreme precipitation Softball-sized hail r=5 cm, 50,000 microns Vivian, SD Hailstone Holds record for size (8” diameter) and mass (1.94 lbs) In Short, Large Hail Needs… (1) Supercooled water (mass to be collected by the hailstone) (2) An Embryo (some initial particle to collect supercooled water, usually a frozen raindrop or snow aggregate) (3) An updraft (must be sustained aloft for sufficient time to allow growth) (4) Cold Temperatures Freezing – Just the facts • At T < 0°C, water molecules join together to form a crystalline structure • If the drop is free of impurities, the thermal agitation will prevent freezing • Freezing of liquid water in the atmosphere largely depends upon the presence of foreign particles called ice nuclei – The nuclei do not have to be inside the drop; water can freeze on contact (freezing rain, aircraft icing) – Typical ice nuclei: desert dust, clay minerals, decaying plant leaf material, ice crystals themselves, etc. – Most IN “activate” at temperatures cooler than -10 °C Supercooled Water • If ice nuclei are not present, liquid water can be super-cooled to temperatures as low as -40 °C • We normally deal with bulk amounts of water. The presence of one ice nucleus will freeze the entire amount of liquid water. • In a cloud, the liquid water is divided upon millions of droplets; each must contain or interact with an ice nucleus to freeze – There are substantially fewer ice nuclei in the atmosphere than cloud condensation nuclei (CCN) – Ice multiplication and/or shattering can help generate new ice crystals “Warm Rain” • You’ll learn more about the warm rain process in physical meteorology • Important in tropics, mid-latitude summer, and possibly mid-latitude winter • In short, “warm rain” is a 4 step process: (1) Nucleation of cloud drops on aerosol particles (2) Additional Condensation due to supersaturation (3) Drop Growth by Collision and Coalescence (4) Drop Breakup Drop D > 45 m coalescence SS>0 condensation RH~100% SS=0 Drops nucleated III II IV Drop breakup I Air cools, RH increases “Cold Rain” Process Vapor diffuses towards the crystals, growing by deposition and depleting water vapor in the air Further growth by accretion or aggregation can also occur Accretion or riming: growth by collision with supercooled drops which freeze on contact Aggregation: growth by collision of ice crystals Cloud Particle Imager Data from AIRS II Flights (2004) “Hole Punch” Clouds Areas of mid- or high-level, liquid clouds. Ice grows at the expense of evaporating liquid drops (Bergeron process) and creates a cloud free region Conical Graupel (Knight and Knight, 1973) Preferential collection along crystal edge Embryos Summary • Growth in cloud may form frozen drops or conical graupel as initial embryo • Takes 20–30 minutes for either process • Where do they form? – Feeder cells upwind (10 – 20 km) – Upwind flanks of main updraft – Secondary growth from shedding/melting – Can NOT be in main updraft: not enough time to grow Graupel/Hail Growth • Primary graupel growth by freezing of raindrops, riming of ice crystals, aggregation of snow, etc. • Large hailstones acquire most mass by accretion of supercooled drops (e.g., Knight and Knight 2005) • Secondary generation possible during “wet growth” (shedding) • 40 – 60 minutes of growth required to form large hail Knight and Knight (2001) Further Growth • Latent heat is released when a supercooled water droplet freezes on a hailstone surface • Dry Growth: rate of supercooled drop collection is low, hailstone surface remains below 0 °C, drops freeze immediately upon impact – Opaque ice (air bubbles), brittle, small crystals • Wet Growth: Hailstone surface warms to 0 °C, freezing does NOT immediately occur, water drops “spread out” across hailstone and some shed – Clear ice, larger crystals, drops may fill pores of hailstone and lead to densification – Wet surface makes stone “sticky” for collection with ice Hailstone Thin Sections (Knight and Knight 2005) Wet growth Dry growth Growth Trajectories “Recycling” trajectories not as common as once thought Most trajectories up-and-down once around main updraft Embryos may be ingested from other sources Hail in Multicells May produce its own embryos, but hail may not grow to very large sizes Graupel/Hail Size Distributions • Early observational work confirmed an exponential size distribution for graupel/hail particles at the ground – (Waldvogel 1974; Federer and Waldvogel 1975; Knight et al. 1982; Cheng and English 1983; Chen et al. 1985; among others) • Other observations reveal a better match to a gamma distribution – (e.g., Matson and Huggins 1980; Ziegler et al. 1983) Matson and Huggins (1980) Hailstone Terminal Velocity • Hailstones are assumed to fall at their terminal velocity, Vtg p = 500 hPa – Balance between drag and gravitational forces • Larger/more dense particles have greater fall speeds • Faster graupel/hail fall speeds for lower air density Sea level 4 g g Dg Vtg 3 C D Knight and Knight (2001) 0.5 • May have significant impacts upon precipitation estimates – Large, faster-falling particles are less prone to horizontal advection; more intense precipitation over a local region (e.g., McCumber et al. 1991; Gilmore et al. 2004a; Gilmore et al. 2004b; van den Heever and Cotton 2004; etc.) Faster falling graupel/hail Slower falling graupel/hail Precipitation Mass Updraft ( +2.5 m s-1) Downdraft ( -2.5 m s-1) Hailswath Mechanics N Light Rain Large Hail Heavy Rain & Small Hail Gust Front Hook echo Anvil Edge Nautical miles 0 5 10 Hook echo WSR-88D Radar Image The mesocyclone wraps some of the heaviest precipitation around the updraft creating a “hook echo” on radar. The largest hail falls in a narrow swath located near the updraft core. Sounding Investigation • Identify convective mode • Total CAPE • CAPE in -30 to -10 °C • Rotation/turning hodographs • Height of the Freezing Level • Wet Bulb Zero height No one single parameter has been shown to have significant skill when considered alone! Impact of CAPE • Supercells usually occur with significant CAPE (values 10002000 J kg-1 or more) • Bulk Richardson Number (ratio of instability to shear) can be used to predict storm type – Values between 10-50 generally associated with supercell storms CAPE Shape Wet Bulb Temperature LCL Edwards and Thompson (1998) Thermodynamic Summary • Generally, you’d like to see: – Wet Bulb Zero (WBZ) Heights of 2.2 – 2.8 km • Too high, too much melting • Too low, low-level air too negatively buoyant – Freezing Level heights < 4 km • Need deep cloud layer for hail to grow within – CAPE values > 2000 J kg-1 • To first order, wmax = (CAPE)0.5 – 850–500, 500–300 hPa lapse rate > 7 K km-1 Don’t forget about “dynamic factors”: Fronts, shortwaves, outflow boundaries, rotation, etc. Nowcasting Hail • Radar imagery can be used to determine the relative strength of an updraft (ability to grow large hail) and diagnose the presence of hail in clouds – – – – – (Bounded) Weak Echo Regions (WERs/BWERs) “V-notches” “Hail spikes” Dual polarization Vertically Integrated Liquid Water (VIL) Weak Echo Regions (WERs) • Strong updrafts will suspend precipitation particles aloft creating an overhang/WER when observed on weather radar • WERs are good indicators of potentially severe storms Bounded Weak Echo Regions Bounded WERs can be seen on vertical cross sections or as “doughnuts” of weak reflectivity on horizontal sections V-Notch • Strong supercells may have a slot of weak reflectivity along the downshear edge • Why? Vertically Integrated Liquid Water Content (VIL) Computes total water mass in a vertical column Three Body Scattering Hail Spikes Where is the radar in each case? Hail Spikes in 3D Differential Reflectivity ZDR 4 mm zHH ZDR [dB] = 10 log( ) zVV 2.7 mm 3.7 mm 1.8 mm 2.9 mm 1.4 mm (Pruppacher and Klett, 1997) – Depends on axis ratio oblate: ZDR > 0 prolate: ZDR < 0 – For drops: ZDR ~ drop size (0 - 4 dB) – Hail: ZDR ~ 0 – 1 dB Dual Polarization - Horizontal ZDR minima of near zero co-located with highest Z High reflectivity core in purple ZDR “Hail Hole”: Large Zh and near zero or negative ZDR Z Dual Polarization - Vertical NSSL Cimarron Polarimetric Radar viewpoint of 9 June 1993 Squall line Adapted from Zrnic and Ryzhkov (1999) CASE STUDIES Working in groups, review the individual cases and answer the following questions: (1) Is there hail in this storm? (2) Is the hail reaching the ground? (3) Would you warn on this cell?