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30 Apr 2003
This page is maintained by Mark Fielding
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Lightning study
Robin Hogan and Anthony
Illingworth
General overview of the
storms observed on 28 July 2000
Introduction
This NASA-funded project is to investigate the potential of
polarimetric radar to detect lightning danger for aircraft and
spacecraft launch vehicles. It is known that thick anvil cirrus can
be highly charged, but it is very difficult to detect this from the
ground. Ice crystals normally fall with their long axis in the
horizontal, but in the presence of strong electric fields they tend to
be aligned along the electric field lines. This phenomenon may be
detected by two polarimetric radar parameters:
- KDP: This parameter may be regarded as a measure of the
difference in the speed of the horizontally and vertically polarised
radar waves. In the normal situation of horizontally aligned crystals,
KDP is positive, indicating that the horizontal wave is travelling
more slowly than the vertical. Negative KDP indicates crystals
aligned with their long axis mostly in the vertical, something that
can only occur in the presence of an electric field. The phenomenon is
more easily seen in the differential phase shift PHIDP
(simply the integral of KDP in range) as a `searchlight' of
negative values behind the region of vertically aligned crystals.
- LDR: When the electric field lines and the crystals are aligned
diagonally to the incident radiation, the radar beam may become
depolarised. The Linear Depolarisation Ratio then becomes increasingly
positive behind the aligned crystals, in a searchlight effect similar
to that for PHIDP. From the inferred orientation of
the field lines using LDR and KDP, it may be possible to
determine the location of the charge centres in the cloud.
It should be noted that the direct returns from the cloud are
dominated by the quasi-spherical or tumbling graupel particles, so
ZDR tends to be close to 0 dB. Thus it is only really in
the propagation characteristics that the smaller alighned crystals may
be detected.
Summary of observations
This table lists the number of scans taken on each of five days on
which radar observations were made, and in how many of them each of
the different phenomena were observed. Click on the dates to see
quicklooks of all the radar scans taken on that day. Direct detection
of lightning and negative KDP and events have been indicated, but high
LDR is so common that it has not been highlighted.
|
4 July 2000 |
28 July 2000 |
1 Sept 2000 |
15 June 2001 |
4 July 2001 |
Total number of scans |
145 | 491 | 51 | 299 | 185 |
Total number of RHIs |
113 | 409 | 31 | 258 | 154 |
RHIs with KDP < -0.5°/km |
9 | 114 | 0 | 0 | 33 |
RHIs with LDR > -30 dB due to propagation |
57 | 265 | 0 | 55 | 64 |
RHIs with direct lightning hits in Z |
4 | 19 | 0 | 7 | 2 |
Cloud top height / temperature |
9 km / -45°C |
10 km / -50°C |
6 km / -25°C |
8 km / -40°C |
12 km / -55°C |
Direct detection of lightning
Another phenomenon we investigate is the direct backscatter from
the lightning channels themselves. Columns of ionised gas represent a
substantial target for the radar, and are seen as high returns in
isolated range-gates that persist for around 0.5 seconds (2
rays). This phenomenon was observed by Ligda in the 1950s, but here we
are able to infer information about the orientation of the lightning
channels using ZDR. It is also found that LDR is much more
sensitive to the strikes than Z (which is usually dominated by the
echo from the hydrometeors).
There were a total of 127 hits on lightning (from 29 scans) that
had a reflectivity factor at either horizontal or vertical
polarisation was more than a factor of 5 (7 dB) greater than the value
in an adjacent ray where lightning was not observed. This criterion
was imposed in order to ensure that the polarisation parameters were
dominated by lightning rather than the hydrometeors. A histogram of the reflectivity values of the
lightning shows a peak between 40 and 45 dBZ, but with some
strikes larger than 55 dBZ. A plot of ZDR versus LDR shows a tendency
for ZDR to be more often negative than positive, indicating
the channels to be more vertically than horizontal aligned, as would
be expected. Randomly oriented channels would have an LDR of on
average -9 dB. The observed average LDR of -12 dB is consistent with
the greater tendency for vertical alignment.
The direct lightning observations were compared with sferics data
provided by the UK Met Office on 4 July 2000 and 28 July 2000. In general they
match up quite well, although there are occasions when the radar sees
lightning not detected by the sferics system. Of course the radar scan
strategy meant that it was only sampling a small area at a time so
will not have seen all the events observed by the sferics system.
Animations
Animations of the development of storms as viewed by the radar can
be quite instructive, especially when the polarimetric parameters are
used to observe changes in the electric field. The following four
cases were constructed from sequences of RHIs along the same
azimuth.
- 28 July 2000 15:48-16:52 UTC Z,ZDR,LDR|PHI_DP,V (mpeg
format: Z,ZDR,LDR PHI_DP,V Z,V)
An
hour-long sequence encompassing the periods of the three next
animations.
- 28 July 2000 15:48-16:02 UTC
Z|ZDR|LDR|V|PHI_DP
A high-LDR region moves upwards in a convective tower, and there is
evidence of negative KDP.
LDR detection of the lightning channel in scans 96/7, 97/5 and 97/18.
- 28 July 2000 16:06-19:19 UTC
Z|ZDR|LDR|V|PHI_DP
High values of LDR and negative KDP later in the same storm.
Direct Z and LDR detection of lighning in scans 102/8, 102/12, 102/16 and 102/18.
- 28 July 2000 16:23-16:52 UTC
Z|ZDR|LDR|V|PHI_DP
Rapid changes in LDR and KDP in a growing cumulonimbus.
- 4 July 2001 11:49-11:56 UTC
Z|ZDR|LDR|V|PHI_DP
Storm advected through the beam: rapid changes in LDR and
KDP.
Direct detection of the lightning channel in Z and LDR on scan 24/3.
Composite images
The images below show a selection of images combining Z,
PHIDP, LDR and v. The black contours superimposed on the Z
and PHIDP panels depict KDP (negative values
indicated by thick dashed contours), and the white contours
superimposed on the Z and LDR panels depict the derivative of LDR with
range. Some pairs of consecutive scans around 30 seconds apart from 28
July 2000 show the effect of a lightning strike:
- 13:13:24-13:13:57
KDP changes from -1 to
1°/km as the electric field dissipates and the crystals switch
from vertical to horizontal alighnment. In the second scan the
lightning channel is observed directly in Z and LDR. A profile of Z, ZDR and
LDR along the affected rays shows how the effect is much more
detectable in LDR than in the co-polar channels.
- 13:33:51-13:34:29
In addition to a change
in KDP, LDR also falls from an unusually high value after the strike.
- 15:02:39-15:03:08
Change in KDP and LDR
in an isolated thunderstorm. The outflow at the top of the
cumulonimbus shows a strong divergence.
4 July 2000
14:29:03
14:29:52
14:30:40
14:31:28
14:32:17
14:33:05
14:33:53
14:34:60
14:35:48
14:36:36
14:54:00
14:55:04
15:20:44
15:21:34
15:37:45
15:38:33
15:39:21
28 July 2000
12:26:51
12:27:25
12:45:25
12:54:50
12:55:25
12:55:60
12:56:35
12:57:10
12:57:60
12:58:34
12:59:07
12:59:41
13:00:14
13:02:19
13:02:53
13:03:26
13:04:00
13:04:34
13:05:38
13:06:12
13:06:45
13:07:19
13:07:52
13:12:50
13:13:24
13:13:57
13:14:31
13:15:04
13:23:49
13:24:24
13:24:59
13:25:34
13:26:09
13:26:44
13:27:19
13:27:54
13:28:29
13:29:03
13:29:38
13:30:31
13:31:36
13:32:10
13:32:44
13:33:17
13:33:51
13:34:29
13:35:03
13:35:36
13:38:44
13:39:13
13:39:42
13:40:11
13:40:40
13:41:10
13:41:39
13:42:08
13:42:37
13:43:06
13:43:35
13:44:38
13:48:45
13:49:16
13:50:20
13:50:49
13:51:18
13:51:47
13:52:16
13:52:45
13:53:15
13:53:44
14:45:36
14:46:13
14:46:51
14:47:28
14:48:06
14:48:59
14:49:37
14:50:14
14:50:52
14:51:29
14:52:23
14:53:01
14:53:38
14:54:15
14:54:53
14:59:14
14:59:52
15:00:31
15:01:10
15:01:49
15:02:39
15:03:08
15:03:36
15:04:05
15:04:33
15:05:18
15:05:47
15:06:16
15:06:44
15:07:13
15:07:48
15:09:42
15:10:11
15:10:39
15:11:08
15:11:37
15:12:05
15:12:34
15:13:02
15:13:31
15:13:59
15:14:28
15:14:56
15:15:25
15:15:54
15:16:22
15:16:51
15:17:19
15:19:16
15:19:41
15:20:06
15:20:31
15:20:55
15:21:20
15:21:45
15:22:10
15:22:35
15:22:59
15:23:24
15:23:49
15:24:14
15:24:38
15:25:03
15:25:28
15:25:53
15:26:17
15:26:42
15:27:07
15:27:35
15:28:00
15:28:25
15:28:50
15:29:14
15:29:39
15:30:04
15:30:29
15:30:54
15:31:18
15:31:43
15:32:08
15:32:33
15:32:57
15:36:06
15:36:37
15:37:42
15:38:22
15:39:03
15:39:43
15:40:24
15:41:04
15:43:60
15:45:04
15:45:45
15:46:34
15:47:30
15:48:16
15:48:46
15:49:17
15:49:47
15:50:18
15:50:48
15:51:19
15:51:49
15:52:20
15:52:50
15:53:32
15:54:03
15:54:33
15:55:04
15:55:34
15:56:05
15:56:35
15:57:06
15:57:36
15:58:07
15:58:37
15:59:08
15:59:39
16:00:09
16:00:40
16:01:10
16:01:41
16:02:11
16:06:05
16:06:36
16:07:06
16:07:37
16:08:07
16:08:38
16:09:08
16:09:39
16:10:09
16:10:40
16:11:10
16:11:41
16:12:11
16:12:42
16:13:13
16:13:43
16:14:14
16:14:44
16:15:15
16:15:45
16:17:57
16:18:31
16:19:06
16:19:40
16:20:41
16:21:31
16:22:33
16:23:33
16:24:17
16:25:01
16:25:45
16:26:29
16:27:13
16:27:57
16:28:41
16:29:25
16:30:09
16:30:53
16:31:37
16:32:20
16:33:04
16:33:48
16:34:32
16:35:16
16:36:00
16:36:44
16:37:28
16:38:12
16:38:56
16:39:40
16:41:45
16:42:29
16:43:13
16:43:57
16:44:41
16:45:25
16:46:09
16:46:53
16:47:37
16:48:21
16:49:04
16:49:48
16:50:32
16:51:16
16:52:00
16:52:44
16:55:47
4 July 2001
11:34:38
11:36:08
11:36:45
11:37:22
11:37:59
11:40:14
11:40:51
11:41:28
11:42:04
11:42:41
11:43:18
11:43:55
11:44:32
11:45:09
11:45:46
11:49:13
11:49:49
11:50:26
11:51:03
11:51:40
11:52:17
11:52:54
11:53:31
11:54:07
11:54:44
11:55:21
11:55:58
11:58:05
11:58:42
11:59:19
11:59:56
12:00:32
12:01:09
12:03:36
12:04:13
12:04:50
12:05:27
12:07:45
12:08:22
12:08:59
12:46:33
12:47:09
12:47:46
12:48:23
12:49:00
12:49:37
12:50:14
12:50:51
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