Radar group home page Who's who in the radar group About Chilbolton Research projects Field experiments Quicklooks of radar data Real-time cloud radar and lidar images Radar Group publications Analysis software and documentation Radar links Back to Meteorology Department Back to University of Reading

Last update
30 Apr 2003

This page is maintained by
Mark Fielding

All content Copyright © The University of Reading unless otherwise stated.

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 14549151299185
Total number of RHIs 11340931258154
RHIs with KDP < -0.5°/km 91140033
RHIs with LDR > -30 dB due to propagation 5726505564
RHIs with direct lightning hits in Z 419072
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