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© 2000 Roger Edwards
Investigating the Potential of Using Radar to Nowcast
Cloud-to-Ground Lightning Initiation over Southern Ontario
18th Annual GLOMW
Toronto, Ontario
Y. Helen Yang / Patrick King
Ontario Storm Prediction Centre
Environment Canada
23.03.2010
page 2 – 23.03.2010© 2000 Roger Edwards
Motivation
Lightning is a high-impact weather phenomenon!
timely & accurately forecast lightning
future lightning watch/warning products in Canada?
9-10 deaths & 92-164 injuries in Canada each year!(Mills et al. 2008)
page 3 – 23.03.2010© 2000 Roger Edwards
Purpose
• To investigate the potential use of radar echoes in nowcasting cloud-to-ground (CG) lightning initiation– To determine a reflectivity threshold value that best
predicts the onset of CG lightning
– To find any correlation between radar echo tops and CG lightning initiation
– To study differences (if any) between negative and positive CG lightning initiation in terms of radar characteristics
page 4 – 23.03.2010© 2000 Roger Edwards
Data
• Lightning data from CLDN– Detection efficiency ≥ 90%; location accuracy within 0.5 km
• Radar data from URP– Horizontal resolution ~ 1 km– Temporal resolution ~ 10 min– Data from the King City
Radar (WKR) only▪ Domain of study
page 5 – 23.03.2010© 2000 Roger Edwards
Data
• ‘Airmass’ thunderstorms
CG lightning No CG lightning Total
negative 1st flash
positive 1st flash
both
1st flash
63 6 8 66 143
• 143 cases of airmass thunderstorms from Jun-Aug 2008
• A ‘case’ consists of one cell or a cluster of cells on a radar display that may or may not eventually produce lightning
page 6 – 23.03.2010© 2000 Roger Edwards
Methods: the premise
• Graupel-ice mechanism for cloud electrification– larger riming graupel and smaller ice crystals collide,
and consequently electric charges are exchanged by these hydrometeors
– rebounding ice crystals tend to become positively charged, while graupel particles become negatively charged
– occur in the mixed-phase region in or near a storm updraft
page 7 – 23.03.2010© 2000 Roger Edwards
Methods: the premise
• cloud electrification within a storm updraft
(MacGorman and Rust 1998)
ice crystals
graupel particles
• in mixed-phase layer
main negative charge region
• constant altitudes• • • where CG lightning is often initiated
-20°C
-10°C
page 8 – 23.03.2010© 2000 Roger Edwards
Methods
• What reflectivity threshold value at which temperature level can best predict the onset of CG lightning?
• Recall one of the objectives from earlier…
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 X X X X X n/a
-15 X X X X X n/a
-10 n/a n/a X X X X
altitude?? temperatureupper air sounding data
page 9 – 23.03.2010© 2000 Roger Edwards
Methods: x-section of a case
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 n/a
-15 X n/a
-10 n/a n/a
2120Z 18 Aug 2008
2130Z 18 Aug 2008
2140Z 18 Aug 2008
-10°C-15°C-20°C
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 X X n/a
-15 X X n/a
-10 n/a n/a X
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 X X X n/a
-15 X X X X n/a
-10 n/a n/a X X X
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 X X X X n/a
-15 X X X X n/a
-10 n/a n/a X X X
Hit (H)Miss (M)False (FA)AlarmLead time[min]
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 H H H H n/a
-15 H H H H n/a
-10 n/a n/a H H H
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 H H H H M n/a
-15 H H H H M n/a
-10 n/a n/a H H H M
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 H H H H M n/a
-15 H H H H M n/a
-10 n/a n/a H H H M
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 30 30 20 0 M n/a
-15 40 30 20 20 M n/a
-10 n/a n/a 30 20 20 M
2200Z 18 Aug 2008
page 10 – 23.03.2010© 2000 Roger Edwards
Findings: reflectivity threshold
POD [%]
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 99 96 90 70 55 n/a
-15 100 100 96 91 73 n/a
-10 n/a n/a 100 100 88 64
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 36 31 27 10 7 n/a
-15 44 39 31 23 5 n/a
-10 n/a n/a 41 31 16 8
FAR [%]
page 11 – 23.03.2010© 2000 Roger Edwards
Findings: reflectivity threshold
CSI [%]
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 64 67 68 65 52 n/a
-15 56 61 67 71 70 n/a
-10 n/a n/a 59 69 76 60
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 19 17 13 10 6 n/a
-15 23 22 19 14 10 n/a
-10 n/a n/a 23 19 17 13
Averageleadtime [min](±5 min)
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 64 67 68 65 52 n/a
-15 56 61 67 71 70 n/a
-10 n/a n/a 59 69 76 60
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 19 17 13 10 6 n/a
-15 23 22 19 14 10 n/a
-10 n/a n/a 23 19 17 13
FAR
ave.leadtime
page 12 – 23.03.2010© 2000 Roger Edwards
Findings: echo top threshold
• Things to keep in mind:– Echo tops in relation to only warm season lightning– Echo tops of convections on warmer days higher
than those during cooler days
– Higher echo tops stronger updrafts
– Weak updrafts cannot produce intense electrification needed to generate lightning
page 13 – 23.03.2010© 2000 Roger Edwards
Findings: echo top threshold
• Maximum echo top prior to or at the start of CG lightning activity
altitude of 7 km ~ -13 to -29°C levels
page 14 – 23.03.2010© 2000 Roger Edwards
Findings: vs
• Things to keep in mind:
– small sample size
Number of lightning-producing cases
‘-’ first lightning flash
‘+’ first lightning flash
Both
‘-’ & ‘+’
total
63 6 8 77
– Cases with both polarities were counted towards both ‘-’ and ‘+’ cases
776
page 15 – 23.03.2010© 2000 Roger Edwards
Findings: vs
• Initial lightning flash location to storm location of maximum reflectivity on MAXR [km]
x
page 16 – 23.03.2010© 2000 Roger Edwards
Findings: vs
• Initial lightning flash location to storm location of maximum reflectivity on MAXR [km]
mean 2.2 3.5
median 1.9 2.5
longest 7.5 11.2
shortest 0.0 0.0
storm location of max. reflectivity
highly-reflective graupel concentrated
main negative charge cloud region
(± 0.5)
page 17 – 23.03.2010© 2000 Roger Edwards
Findings: vs
• Reflectivity threshold predictors
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 100 93 97 93 90 93 70 79 54 79 n/a
-15 100 100 100 100 97 93 92 93 72 93 n/a
-10 n/a n/a 100 100 100 100 87 100 68 64
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 37 76 33 72 28 66 11 35 7 21 n/a
-15 46 81 41 78 33 72 24 62 6 19 n/a
-10 n/a n/a 43 79 33 71 17 48 8 31
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 63 23 66 27 67 33 65 55 51 65 n/a
-15 54 19 59 22 66 27 71 37 69 76 n/a
-10 n/a n/a 57 21 67 29 74 52 64 50
P O D
F A R
C S I
?
page 18 – 23.03.2010© 2000 Roger Edwards
Findings: vs
• Reflectivity threshold predictors– Average lead time [min] (±5 min)
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 19 15 18 14 14 8 11 5 7 3 n/a
-15 24 15 23 15 20 15 14 10 10 3 n/a
-10 n/a n/a 24 16 20 13 18 9 14 10
green = ‘-’ first lightning flashesred = ‘+’ first lightning flashes
Temp level
[ºC]
Radar reflectivity threshold value
[dBZ]
20 25 30 35 40 45
-20 19 15 18 14 14 8 11 5 7 3 n/a
-15 24 15 23 15 20 15 14 10 10 3 n/a
-10 n/a n/a 24 16 20 13 18 9 14 10
page 19 – 23.03.2010© 2000 Roger Edwards
Findings: summary
• -10ºC / 40 dBZ could best predict the onset of CG lightning
– POD=88% FAR=16% CSI=76%– Lead time ~ 17±5 minutes
• Trade-off between the lead time and FAR• Echo tops ≥ 7 km
– used in conjunction with reflectivity threshold to improve accuracy
• Above results are supported by other studies– e.g., Krehbiel 1986; Gremillion and Orville 1999;
Vincent et al. 2004; Wolf 2007
page 20 – 23.03.2010© 2000 Roger Edwards
Findings: summary
• Negative vs. positive first lightning flashes– Negative flashes were located closer to the main
negative charge region in a storm cloud– No definitive difference in skills to forecast lightning
of different polarities– Positive-first-lightning-producing storm clouds
became strongly electrified faster than negative-lightning-producing storm clouds
page 21 – 23.03.2010© 2000 Roger Edwards
Conclusions
• Potential to use radar echo reflectivity to nowcast CG lightning initiation
• Much more work is needed in developing a lightning nowcast algorithm in a future nowcasting software application tool
page 22 – 23.03.2010© 2000 Roger Edwards
Thanks
• National Laboratory for Nowcasting & Remote Sensing Meteorology
• Ontario Storm Prediction Centre
• Ed Becker, Glenn Robinson, Paul Joe, Norman Donaldson, Dave Hudak, and Syd Peel
page 23 – 23.03.2010© 2000 Roger Edwards
Outline
• Why…
• What purpose…
• How…
• What findings…
• What conclusions…• Thanks to…
page 24 – 23.03.2010© 2000 Roger Edwards
What findings… vs
• Magnitude of electric current [kA]
mean 14.1 21.9
median 12.2 22.6
maximum 33.1 30.5
minimum 2.4 15.5
page 25 – 23.03.2010© 2000 Roger Edwards
Methods
• Average altitudes corresponding to different temperature levels for the time periods examined in the study
A l t i t u d e s o f - 1 0 C , - 1 5 C , a n d - 2 0 C I s o t h e r m s
0 . 0
1 . 0
2 . 0
3 . 0
4 . 0
5 . 0
6 . 0
7 . 0
8 . 0
9 . 0
M a y 2 9 J u n 0 8 J u n 1 8 J u n 2 8 J u l 0 8 J u l 1 8 J u l 2 8 A u g 0 7 A u g 1 7 A u g 2 7
D a t e 2 0 0 8
Altitude MSL [km]
- 1 0 C - 1 5 C - 2 0 C