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Kinematic, Microphysical, and Electrical Structure and Evolution of Thunderstorms during the Severe

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Title: Kinematic, Microphysical, and Electrical Structure and Evolution of Thunderstorms during the Severe


1
Kinematic, Microphysical, and Electrical
Structure and Evolution of Thunderstorms during
the Severe Thunderstorm Electrification and
Precipitation Study (STEPS)
  • Kyle C. Wiens
  • 21 January 2005

2
Outline
  • Background and motivation
  • Objectives of this research
  • Instrumentation and methodology
  • Results

3
BackgroundThunderstorm charge structure
Commonly observed tripole
ICintracloud CGcloud-to- ground
4
BackgroundThunderstorm charge structure
Saunders And Peck (1998)
Takahashi (1978)
5
BackgroundCG climatology
-CGs dominate! CGs?severe weather?
STEPS
6
Background/MotivationWhere are the CGs coming
from?
  • How do CG-dominated storms form, i.e., what is
    the parent charge structure?
  • Hypotheses for CG-dominated storms
  • Tilted dipole/tripole
  • Precipitation unshielding
  • Enhanced lower positive charge
  • Inverted dipole/tripole

7
Background/MotivationWhere are the CGs coming
from?
  • Previous studies of CG storms include only
    ground strike data with little or no information
    about parent charge structure.
  • Previous studies largely lack detailed
    kinematic/microphysical context for the lightning
    activity.

Urgently needed are observations that constrain
the location of the positive charge region
participating in the anomalous i.e.,
positive ground flashes. --Williams (2001)
8
Background/MotivationLightning?Severe storms
  • Williams et al. (1999) and others have noted
    jumps in total lightning flash rate which
    precede severe weather at the ground (hail and
    downbursts) by 10s of minutes.
  • Though lacking the updraft measurements to back
    this up, they posit that these jumps are
    coincident with surges in updraft and explosive
    vertical development of the storm.
  • Suggests that total lightning may have some
    utility as a predictor of severe weather. That
    is, lightning is a proxy for updraft strength and
    riming (hail growth).

9
Objectives
  • For a variety of storms
  • Determine thunderstorm charge structureshow they
    develop and evolve
  • Are charge structures clearly and consistently
    different in CG, -CG, no-CG storms?
  • Where are the CGs coming from?
  • How do these structures correspond with CG
    hypotheses and charging theories?
  • Investigate kinematic/microphysical influences
  • Updraft, shear, rotation, hail production?charge
    structure and lightning
  • Investigate utility of total lightning and CG
    polarity as indicators/predictors of storm
    severity.

10
STEPSInstrumentation
  • Instrumented balloons and aircraft
  • Three Doppler radars (two of which are
    polarimetric research radars)
  • National Lightning Detection Network (NLDN)
  • Lightning mapping array (LMA)

11
InstrumentationRadar
  • Three doppler radarsretrieval of 3D wind
  • Two polarimetric radars
  • Size, shape, phase of hydrometeors
  • Hydrometeor identification
  • Rain, graupel, hail, etc.
  • Useful summary polarimetric measurments
  • Echo volumes for time series and time/height
    contours

12
InstrumentationNMT Lightning Mapping Array (LMA)
  • Time and location of VHF (60-66 MHz) lightning
    radiation sources (100s-1000s per flash)
  • Maps all lightning activity
  • Gives total lightning flash rates
  • Allows inference of charge structure
  • Allows for determination of flash origin
    locations ?where the CGs are coming from

13
LMA
  • Examples of data and methodology

14
Color Coded By Time
Zoom in
15
Flashes
Color Coded By Time
16
LMA Total Flash Rates
NMT sorting algorithm groups LMA sources into
flashes based on their temporal and spatial
proximity.
29 June supercell
17
LMA Charge Structure
  • Interpret according to bi-directional discharge
    model.
  • Initiation in strong electric field between
    adjacent oppositely charge regions.
  • Simultaneous breakdown of positive and negative
    leaders away from initiation and through charge
    regions.
  • Negative breakdown is noisier than positive
    breakdown at VHF, so LMA is more sensitive to
    and sees more of the negative breakdown through
    positive charge.

18
LMA charge structure
Distribution of sources gives rough indication of
charge structure
19
Color Coded By Time
20
Color Coded By Density
21
Zoom in
Color Coded By Charge
22
Five-layer structure
Color Coded By Charge
23
Inverted IC flash
Color Coded By Time
24
Inverted IC flash
Color Coded By Charge
25
Five-layer structure
Color Coded By Charge
26
Inverted IC flash and Normal IC flash
Color Coded By Time
27
Inverted IC flash and Normal IC flash
Color Coded By Charge
28
Five-layer structure
Color Coded By Charge
29
Overlay onto radar
29 June supercell
30
Results from STEPS case studies
  • 29 June supercell (severe, CG-dominated)
  • 3 June (moderate, no CGs of either polarity)
  • 23 June (scattered multi-cellular, some
    CG-dominated, some CG-dominated)

31
29 June supercell
  • CG dominated storm, large hail, F1 tornado
  • Extreme flash rates (100s per minute)
  • Inverted charge structure throughout, but
    inverted is not a sufficient description of its
    complexity.
  • CG production accompanied dramatic increases in
    updraft, hail, total flash rates, and right turn
    of storm.
  • Detailed two-part study complete and submitted
    for publication (Tessendorf et al., Wiens et al.,
    JAS, 2004).

32
29 June LMA density animation
33
29 June supercell
  • Interesting correlation
  • between hail and CGs, but
  • not very robust.
  • Very strong correlation
  • between total flash rate and
  • graupel echo volume (and
  • also with updraft volume).
  • Some indication of lightning
  • jumps preceding severe
  • weather (i.e., hail).

34
Correlation analysis
29 June supercell
35
Correlation analysis
29 June supercell
Excellent correlation between (updraft/flash
rate) and especially (graupel/flash rate), even
when de-trended.
Physically reassuring that the best de-trended
correlations are when hail lags updraft and
graupel by 5-10 minutes.
Best correlation when hail lags CG flash rate
and total flash rate by 10 minutes (some utility
in lightning for severe wx nowcast?)
36
Graupel echo volumeNormalized 10 ms-1updraft
volume.
29 June supercell
Hail echo volume CG flash rate
LMA source density (log) CG source height
37
29 June supercell
CG source height histogram (0.5 km binsize)
CGs originate from mid-levels -CGs originate
from upper levels Consistent with
inverted structure
38
29 June supercell
Difference between CG start height and CG mean
start height
downward
upward
CGs initiate upward (neg. breakdown into
positive above) -CGs initiate downward (neg.
breakdown into positive below)
39
29 June supercell
  • Example illustration of early evolution

40
inverted dipole
5-layer
Color Coded By Charge
41

-
Early charge structure is well-defined gets
complex later... However, always has an
overall gross inverted structure.

-
-


inverted dipole
5-layer
5-layer
2 CGs
collapsed 5-layer
FrequentCGs
42
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43
Initially Inverted dipole OR Lowest two
layers of normal tripole Note positive charge
in stronger echo
44
Lower inverted dipole still there Starting to
form additional (independent?) inverted
dipole aloft.
45
Upper inverted dipole becomes dominant Additional
extreme upper positive charge
46
So much lightning cant see echo ..... so, resort
to a composite.
47
Positive charge regions are hot and easy to
spot.
48
29 June supercell
  • Examples of CG flashes and lightning
    holes
  • NMT folks have also documented lightning holes
    (e.g., Krehbiel et al., 2000 Hamlin, 2004) and
    guessed that theyre associated
  • with BWERs and strong updraft. Here Ill show
    that explicitly.

49
CGs at 2239 (the first two)
Updraft, BWER, lightning hole
CGs clustered near precip core
50
CGs at 2239
normal IC
CG
CG source region
51
Elevated sources
Lightning hole
52
CGs at 2325
Updraft, BWER, lightning hole
CGs clustered near precip core
53
CGs at 2325
Roughly, an inverted tripole.
normal IC
CG
CG source region
54
Compare with EFM sounding
MacGorman et al. (2004)
55
Conclusions from 29 June supercell
  • Very strong correlation between total flash rate
    and graupel volume.
  • Updraft?BWERs? lightning holes
  • Inverted charge structure is reasonable
    description
  • CGs originate from the mid-level positive charge
    near the precip core
  • Strong, broad updrafts promoting hail formation
    may have also allowed for enhanced positive rimer
    charging via NIC mechanism.
  • Lower negative charge beneath inverted dipole
    developed prior to CG flashes, may have been a
    requirement for their production. Dont know how
    lower negative charge formed.

56
3 June storm
  • Isolated, LP-looking storm
  • Some small hail. No large hail
  • Moderate flash rates (10s per minute)
  • Clear inverted charge structure throughout
  • Upper positive for 30 minutes in the beginning
  • No lower negative charge
  • No CGs of either polarity.

57
3 June charge density animation
58
3 June
No lightning until strong reflectivity above
freezing level. Graupel/flash rate
correlation Lightning jump barely precedes
hail max.
Inverted charge structure throughout
59
Correlation between graupel echo volume and LMA
density/flashrate in both time and space.
60
3 June 2000

-

inverted dipole
Essentially all flashes initiate downward
into peak of LMA sources, i.e., from negative
down into positive. Somewhat more objective
illustration of inverted structure
-

inverted dipole
61
3 June
  • Examples of charge structure

62
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63
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64
23 June
  • Variety of storms and CG activity (see animation)
  • -CG dominated severe storm (nice constrast with
    29 June supercell)
  • Polarity switch from CG to CG-dominant
  • Merger process nearly coincident with polarity
    switch

65
23 June LMA density animation
66
23 June LMA charge density animation
67
23 June, storm 1
68
23 June, Storm 1
CG origin heights
CG (Mean start Start) height
69
23 June, storm 1


-
-


tripole
-CG max
CGs
70
23 June, Storm 1 Time 2015 Everything is
still simple tripole. Lower positive charge (and
CGs) concentrated in heaviest precip.
71
23 June, Storm 1 Time 2130 Northern cell has
collapsed leaving mid-level positive and
inverted tripole. CGs all come from this
collapsing cell Southern cell is more intense,
but still roughly a normal dipole, no CGs of
either polarity.
72
Conclusions
73
Questions?
74
AppendixMore details of hydrometeor ID
75
RadarPolarimetric Variables
  • ZH Size, concentration
  • ZDRShape, orientation, (liquid or solid)
  • KDPAmount of liquid water, size of drops
  • LDROrientation, canting, melting (wet hail)
  • ?HVCorrelation (mixture of types/melting)

76
RadarHydrometeor Identification
  • In a nutshell
  • We have inputs (radar variables)
  • We have some decision process
  • We get a result (hydrometeor type)

Basically, we want the hydrometeor type that best
fits the inputs.
77
RadarHydrometeor Identification
  • Define radar variable thresholds for various
    hydrometeor types.

Straka et al. (2000)
78
RadarHydrometeor Identification
  • Combine variables and define sub-ranges over
    which specific hydro types are expected

Straka et al. (2000)
79
Hydrometeor Identification(the fuzzy logic way)
  • Define functions for each input variable and each
    hydrometeor type.
  • These functions describe to what degree each
    variable is a member of the hydrometeor type
    family.
  • Another way to think of this is that each of
    these functions gives a score to each input
    variable. The higher the score, the greater the
    membership value of that variable to that
    hydrometeor type, i.e., the more likely it is
    that type.

80
Types of Membership functions
  • Membership Beta Function Liu and Chandrasekar
    (2000)

2-D MBF for rain
81
Combining the variables
  • So, each variable (x) gets a score (?) based on
    where it falls on the membership function for
    each hydrometeor type.
  • Score ?(x,m,a,b)

The score is like a probability that the radar
measurement is due to that specific hydro type.
82
Liu and Chandrasekar (2000)
83
Rain!
Hail!
84
LDR Cap
Zdr column
Big drops
85
Hail counts
Hail size
T28 Comparison
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