Transcript Identifying Large Midlevel Updrafts with Spectrum Width Matthew J. Bunkers Leslie R. Lemon

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
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 ignored
and 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 can
be 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.
3
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
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T-28
T-28 path
• Fig. 9 from Musil et al.
(1986, JCAM) 
West to east
• ~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
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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
6
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.
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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
14.6°
6.0°
4.3°
2.4°
1.4°
0.4°
9.8°
3.3°
• 2.75” hail 23002303z
• Width: 3.8 nm or
7.0 km at 4.3°
• Distance: 35 nm
x
• Height: 18 kft agl
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Example 1: cross-section
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Example 2: 29 Jun 2000, 2300z
• KLNX, VCP11
3.5°
2.5°
0.6°
• 4.5” hail 2307z
• Width: 8 nm or
14.8 km at 2.5°
• Distance: 93 nm
(near limit)
x
• Height: 30 kft agl
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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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Plot with all data
Hail Size vs. "Updraft" Width
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y = 0.34x + 1.73
Hail Diameter (in)
6
2
R = 0.12
5
4
3
2
1
0.0
2.0
4.0
6.0
8.0
10.0
"Updraft" Width (nm)
14
Plot without the 7” stones
Hail Size vs. "Updraft" Width
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y = 0.37x + 1.38
Hail Diameter (in)
6
R2 = 0.29
5
4
3
2
1
0.0
2.0
4.0
6.0
8.0
10.0
"Updraft" Width (nm)
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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?
16
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, 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.
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
4.3°
3.3°
6.0°
2.4°
• 3” hail 19451955z
• Width: 7.3 nm or
13.5 km at 3.3°
• Distance: 70 nm
x
• Height: 28 kft agl
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Example 3: cross-section
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Example 4: 2 Sep 1995, 1255z
• KFSD, VCP11
(tough case)
12.0°
4.3°
0.4°
• 4.5” hail 13001316z
• Width: 4.8 nm or
8.9 km at 4.3°
x
• Distance: 31 nm
• Height: 15 kft agl
(up to 53.5 kft)
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Counter-example: Aurora, NE
• KUEX, VCP11
(6/22/03, 2354z)
12.0°
5.3°
0.5°
• 7” hail 0004z; no
TBSS; “tall” core
• Width: 3.8 nm or
7 km at 5.3°
x
• Distance: 36 nm
• Height: 21 kft agl
(up to 56 kft)
* Only one hail report ≥ 2” (i.e., the 7” record); likely a special combination of microphysics & kinematics. 23
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)
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