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Identifying Large Midlevel Updrafts with Spectrum Width

Identifying Large Midlevel Updrafts with Spectrum Width. Matthew J. Bunkers NOAA/NWS, Rapid City, SD Leslie R. Lemon OU/CIMMS & NOAA/NWS/WDTB, Norman, OK. Motivation and outline. Hypothesis : SW can be used to indirectly infer potential for “very large” hail

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Identifying Large Midlevel Updrafts with Spectrum Width

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  1. Identifying Large Midlevel Updrafts with Spectrum Width Matthew J. Bunkers NOAA/NWS, Rapid City, SD Leslie R. Lemon OU/CIMMS & NOAA/NWS/WDTB, Norman, OK NWA 32nd Annual Meeting, Reno, NV, 13-18 October 2007

  2. Motivation and outline • Hypothesis: SW can be used to indirectly infer potential for “very large” hail • Very large hail linked with broad & strong UDs • Broad & strong UDs relatively “smooth”/laminar • SW related to turbulence; used to infer smooth UDs • “Large areas” of low SW in UD region implies potential for very large hail • SW largely ignoredand underutilized

  3. Large UDs and large ( 2”) hail • Stronger UDs  larger hail • Hailstone VT ~25-50 m s-1 for ≥ 2” hail • However, intense*/narrow UDs canbe detrimental (Browning 1977) • Embryos “wasted” or hailstones rise too fast • Large/broad UDs appear most critical • Optimally long (single) hailstone trajectories • Updraft-relative flow very important (Nelson 1983, 87) * Maximum observed/estimated UD speeds around 50 m s-1 based on several studies.

  4. Smooth UD observations • Aircraft penetrations of UDs: 1960s80s • Below/near cloud base & within midlevels • U. of WY, John Marwitz and collaborators • SDSM&T, T-28 storm-penetrating aircraft • Strong UD cores are unequivocally smooth • Weaker/smaller UDs sometimes turbulent

  5. T-28 T-28 path •Fig. 9 from Musil et al. (1986, JCAM)  • ~50 m s-1 UD ~23 kft; UD core spans 7-8 km • Adiabatic UD core; 6 g m-3 liquid; minimal ice • Minimal turbulence in UD core; no mixing • 3.5” diameter hail West to east

  6. SW and turbulence • SW: Measure of velocity dispersion in sample • (i) data quality • (ii) turbulence intensity • (iii) mean wind shear across beam • Gust fronts, mesocyclones, and broad/intense UDs • Assumes high signal-to-noise ratio (SNR); VCP dependent • FMH #11, Part C… • Low SW values within UDs indicate unmixed UDs, characterized by high helicity

  7. SW complications • Three-body scatter spikes (TBSSs) may distort storm patterns, producing large SW • Lemon (1998a, 1999) • Smallcomb (2006) • Also large SW with*: • Areas of low SNR • UDmeso coincidence • All are fairly common,but you can look higher * The usual range limits for velocity also apply for SW.

  8. Procedure for examining UDs • Start with Z/V & note the following: • BWER location, high reflectivity core aloft, storm-top divergence, and max echo top • Evaluate SW in conjunction with above • Heights 15-35 kft (4.6-10.7 km); higher better • Find max breadth* of SW values <4 m s-1 • Look just prior to hail occurrence • Only 1-4 min for hail to reach ground based on VT * Updrafts can be horseshoe-shaped or oblong, typically oriented  to motion (not often circular).

  9. Radar analysis procedure • Used GR Analyst • Increasing use in media and NWS • Smoothing turned off (mostly) • Easier to compare bins • Looked for vertical/temporal continuity • Some cases indeterminable

  10. Example 1: 18 Jun 1992, 2253z 14.6° 9.8° 0.4° 1.4° 2.4° 4.3° 6.0° 9.8° • KTLX, VCP21 • 2.75” hail 2300-2303z • Width: 3.8 nm or 7.0 km at 4.3° • Distance: 35 nm • Height: 18 kft agl 3.3° x

  11. Example 1: cross-section

  12. Example 2: 29 Jun 2000, 2300z 2.5° 3.5° • KLNX, VCP11 • 4.5” hail 2307z • Width: 8 nm or 14.8 km at 2.5° • Distance: 93 nm(near limit) • Height: 30 kft agl 0.6° x

  13. Results (37 cases 2”+ hail) • Based on SW, UD widths 5-15 km (3-8 nm) • Agrees very well with previous obs. studies • SW indeterminate at times • Data can be very noisy; hard to locate signature • Many BWERs have high SW (SNR, TBSS, meso) • Function of VCP and viewing angle • Correlation only 0.35 • Disregarding the two 7” hailstones,  = 0.54

  14. Plot with all data

  15. Plot without the 7” stones

  16. Summary • Corroborates prior studies of “smooth” UDs • SW has only limited potential for inferring ≥ 2” hail • SW can be rather “messy” • SW cannot be used alone* • BWER, STD, 50/65-dBZ cores, meso, TBSS* • Can this signature be used operationally? • F.A.R. unknown, pending further study… • SW resolution in AWIPS? VCP12 sampling? • Will dual-pol radar trump this signature?

  17. Thanks for your attention! PowerPoint available here: http://weather.gov/unr/?n=scm

  18. References • Browning, K. A., 1977: The structure and mechanisms of hailstorms. Hail: A Review of Hail Science and Hail Suppression, Meteor. Monogr., No. 38, Amer. Meteor. Soc., 1–43. • Browning, K. A., and R. J. Donaldson Jr., 1963: Airflow and structure of a tornadic storm. J. Atmos. Sci., 20, 533–545. • Browning, K. A., and G. B. Foote, 1976: Airflow and hail growth in supercell storms and some implications for hail suppression. Quart. J. Roy. Meteor. Soc., 102, 499–533. • Crum, T. D., and R. L. Alberty, 1993: The WSR-88D and the WSR-88D operational support facility. Bull. Amer. Meteor. Soc., 74, 1669–1687. • Donavon, R. A., and K. A. Jungbluth, 2007: Evaluation of a technique for radar identification of large hail across the upper Midwest and central plains of the United States. Wea. Forecasting,22, 244–254. • Foote, G. B., 1984: A study of hail growth utilizing observed storm conditions. J. Climate Appl. Meteor.,23, 84101. • Klazura, G. E., and D. A. Imy, 1993: A description of the initial set of analysis products available from the NEXRAD WSR-88D system. Bull. Amer. Meteor. Soc., 74, 1293–1311. • Knight, C. A., and N. C. Knight, 2001: Hailstorms. Severe Convective Storms. Meteor. Monogr., No. 50, Amer. Meteor. Soc., 223–254. • Krauss, T. W., and J. D. Marwitz, 1984: Precipitation processes within an Alberta supercell hailstorm. J. Atmos. Sci.,41, 1025–1034. • Lemon, L. R., 1998a: The radar “three-body scatter spike”: An operational large-hail signature. Wea. Forecasting, 13, 327–340. • Lemon, L. R., 1998b: Updraft identification with radar. Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., 709–712. • Lemon, L. R., 1999: Operational uses of velocity spectrum width data. Preprints, 29th Int. Conf. on Radar Meteor., Montreal, Canada, Amer. Meteor. Soc., 776–779. • Lemon, L. R., and D. W. Burgess, 1993: Supercell associated deep convergence zone revealed by a WSR-88D. Preprints, 26th Conf. on Radar Meteor., Norman, OK, Amer. Meteor. Soc., 206–208. • Lemon, L. R., and S. Parker, 1996: The Lahoma storm deep convergence zone: Its characteristics and role in storm dynamics and severity. Preprints, 18th Conf. on Severe Local Storms, San Francisco, CA, Amer. Meteor. Soc., 70–75. • Marwitz, J. D., 1972: The structure and motion of severe hailstorms. Part I: Supercell storms. J. Appl. Meteor., 11, 166–179. • Marwitz, J. D., 1973: Trajectories within the weak echo region of hailstorms. J. Appl. Meteor.,12, 1174–1182. • Musil, D. J., A. J. Heymsfield, and P. L. Smith, 1986: Microphysical characteristics of a well-developed weak echo region in a high plains supercell thunderstorm. J. Climate Appl. Meteor., 25, 1037–1051. • Musil, D. J., S. A. Christopher, R. A. Deola, and P. L. Smith, 1991: Some interior observations of southeastern Montana hailstorms. J. Appl. Meteor.,30, 1596–1612. • Nelson, S. P., 1983: The influence of storm flow structure on hail growth. J. Atmos. Sci., 40, 1965–1983. • Nelson, S. P., 1987: The hybrid multicellularsupercellular storm—an efficient hail producer. Part II: General characteristics and implications for hail growth. J. Atmos. Sci., 44, 2060–2073. • Smallcomb, C., 2006: Hail spike impacts on Doppler radial velocity data during several recent lower Ohio Valley convective events. Preprints, 23d Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., CD-ROM, P9.2. • WDTB, 2005, Hail storms. http://www.wdtb.noaa.gov/courses/awoc/ICSvr1/lesson2/player.html • WDTB, 2005, Storm interrogation. http://www.wdtb.noaa.gov/courses/awoc/ICSvr3/lesson23/player.html • WDTB, 2005, Updraft location in a sheared convective cell. http://www.wdtb.noaa.gov/courses/awoc/ICSvr3/lesson2/player.html • Witt, A., and S. P. Nelson, 1991: The use of single-Doppler radar for estimating maximum hailstone size. J. Appl. Meteor., 30, 425–431.

  19. Large UDs and large ( 2”) hail -25°C * Microphysics and kinematics can be complicating and/or limiting factors to hail growth.

  20. Example 3: 17 Aug 1994, 1950z 4.3° 4.3° 6.0° 3.3° 2.4° • KTLX, VCP21 • 3” hail 1945-1955z • Width: 7.3 nm or 13.5 km at 3.3° • Distance: 70 nm • Height: 28 kft agl x

  21. Example 3: cross-section

  22. Example 4: 2 Sep 1995, 1255z 12.0° 0.4° 4.3° • KFSD, VCP11(tough case) • 4.5” hail 1300-1316z • Width: 4.8 nm or 8.9 km at 4.3° • Distance: 31 nm • Height: 15 kft agl(up to 53.5 kft) x

  23. Counter-example: Aurora, NE 12.0° 0.5° 5.3° • KUEX, VCP11(6/22/03, 2354z) • 7” hail 0004z; noTBSS; “tall” core • Width: 3.8 nm or 7 km at 5.3° • Distance: 36 nm • Height: 21 kft agl(up to 56 kft) x * Only one hail report ≥ 2” (i.e., the 7” record); likely a special combination of microphysics & kinematics.

  24. Aurora cross-section

  25. Just in case • Figure from Wakimoto et al. (2004)

  26. Just in case • Knight (1984) • “…the evidence shows that the echo vault itself was neither a sufficient nor a necessary feature for the hail production.“

  27. Just in case • Crum & Alberty (1993); Klazura & Imy (1993); Lemon (1999) • SW has contributions from: • Turbulence intensity • Mean wind shear across beam • Poor data quality (weak SNR) • Artifacts (e.g., TBSSs) • Beam broadening at far ranges • Particle fall speed dispersion • Antenna rotation, clutter, system noise

  28. Just in case • Abshayev (1982) • Detect hail with SW • Differences in fall velocities of hail and rain • Values >1.4 m s-1 indicate hail; larger values are associated with larger hail • Only works for zenith observations, thus not practical for hail detection

  29. Just in case • ~ 50 m s-1 UDs • Nelson (1983), dual-Doppler analysis • Musil et al. (1986), T-28 penetration • Bluestein et al. (1988), sounding ascent • Lehmiller et al. (2001), vertical radar beam • Wakimoto et al. (2003), radar from aircraft

  30. Smooth UD observations • Smoothness: accelerating flow*, condensation processes, helical nature of supercell UDs * Negative buoyancy below cloud base implies upward pressure gradient (e.g., Marwitz 1972, 1973).

  31. UD identification with radar • Lemon (1998b, 1999) • BWER/vault (Z) [Browning and Donaldson 1963] • If no BWER, use reflectivity core aloft • WER not location of deep, persistent UD • Horizontal momentum conservation (V) • Smooth and non-turbulent areas (SW) • WDTB, Witt & Nelson (1991), Lemon & Burgess (1993) • Max storm top and storm-top divergence • Inflow side of mesocyclone/mesoanticyclone • Deep convergence zone (DCZ)

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