NWA 32nd Annual Meeting, Reno, NV, 13-18 October 2007

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

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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

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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.

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Smooth UD observations

• Aircraft penetrations of UDs: 1960s80s– 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

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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

T-28

West to east

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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

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• 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– UDmeso coincidence

» All are fairly common,but you can look higher

SW complications

* The usual range limits for velocity also apply for SW.

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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).

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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

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Example 1: 18 Jun 1992, 2253z

• 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°9.8°0.4°1.4°2.4°4.3°6.0°9.8°14.6°

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Example 1: cross-section

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Example 2: 29 Jun 2000, 2300z

• 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°2.5°3.5°

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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

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Hail Size vs. "Updraft" Width

y = 0.34x + 1.73

R2 = 0.12

1

2

3

4

5

6

7

0.0 2.0 4.0 6.0 8.0 10.0"Updraft" Width (nm)

Hai

l D

iam

eter

(in

)

Plot with all data

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Hail Size vs. "Updraft" Width

y = 0.37x + 1.38

R2 = 0.29

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2

3

4

5

6

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0.0 2.0 4.0 6.0 8.0 10.0"Updraft" Width (nm)

Hai

l D

iam

eter

(in

)

Plot without the 7” stones

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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?

Thanks for your attention!

PowerPoint available here:

http://weather.gov/unr/?n=scm

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, 84101. • 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 multicellularsupercellular 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.

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Large UDs and large ( 2”) hail

-25°C

* Microphysics and kinematics can be complicating and/or limiting factors to hail growth.

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Example 3: 17 Aug 1994, 1950z

• KTLX, VCP21

• 3” hail 1945-1955z

• Width: 7.3 nm or 13.5 km at 3.3°

• Distance: 70 nm

• Height: 28 kft agl

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2.4°4.3°6.0°3.3°4.3°

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Example 3: cross-section

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Example 4: 2 Sep 1995, 1255z

• 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)

0.4°4.3°12.0°

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Counter-example: Aurora, NE

* Only one hail report ≥ 2” (i.e., the 7” record); likely a special combination of microphysics & kinematics.

• 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)

0.5°5.3°12.0°

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Aurora cross-section

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Just in case

• Figure from Wakimoto et al. (2004)

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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.“

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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

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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

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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

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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).

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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)