AMOFSG/8-SN No. 41 5/1/10

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

EIGHTH MEETING

Melbourne, Australia, 15 to 18 February 2010

Agenda Item 5: Observing and forecasting at the aerodrome and in the terminal area 5.1: Aerodrome observations

THUNDERSTORM REPORTING WITH THE AID OF LIGHTNING AND RADAR DATA

(Presented by Hu Jiamei)

SUMMARY

This paper presents an algorithm for thunderstorm reporting at the aerodrome with the aid of observation data from the lightning detection system and the weather radar.

1. INTRODUCTION

1.1 In para. 4.4.2.5 in Appendix 3 of Annex 3 — Meteorological Service for International Air Navigation, it is recommended, in the reporting of thunderstorms in the local routine and special reports and in METAR and SPECI that “When thunder is heard or lightning is detected at the aerodrome during the 10-minute period preceding the time of observation but no precipitation is observed at the aerodrome, the abbreviation “TS” should be used without qualification”. The words “or lightning is detected at the aerodrome” were added in Amendment 73 to Annex 3. Currently, there is no guidance material on how to make use of the detected lightning in reporting thunderstorms, in particular, when both human weather observer and lightning detection equipment are in place.

1.2 Since 2005, the weather observers at the Hong Kong International Airport (HKIA) have access to the lightning location data provided by a lightning location information system (LLIS) installed by the Hong Kong Observatory (HKO). In order to harmonize among the observers on the practice of reporting thunderstorms based on the detected lightning locations, guidelines on the use of lightning information have been proposed. In view of the ICAO provision to report thunderstorm based on

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detected lightning in addition to thunder heard, the proposed guidelines take into account other available weather observations (e.g. radar echoes) to corroborate with the lightning data.

1.3 This paper presents the development of the guidelines.

2. MAJOR CONSIDERATIONS IN THE PROPOSED GUIDELINES

2.1 If thunder is heard, the weather observer should confirm and report thunderstorm with location assessed based on available information, including manual assessment by the weather observer (e.g. lightning observed), location of lightning provided by LLIS and/or location of weather radar echoes as appropriate.

2.2 If thunder is not heard but (a) lightning is observed by the weather observer; or (b) cloud- to-ground (CG) lightning is detected by LLIS, within 16 km of the Aerodrome Reference Point (ARP), the weather observer should consider reporting the thunderstorm after checking the presence of nearby weather radar echoes and consulting the aviation forecaster. Only CG lightning is considered in this paper. Cloud-to-cloud (CC) lightning is not considered because of its low detection efficiency by LLIS (30%, vs. 90% for CG lightning) and larger uncertainty in the detected location for CC lightning.

2.3 The major issues with the above approach include the determination of the thresholds of (i) the radar reflectivity for corroborating the presence a thunderstorm and (ii) the location of the lightning stroke relative to the radar echoes (exceeding a prescribed radar reflectivity) in order to confirm that CG lightning has indeed occurred. The development of these two thresholds is described below.

3. DEVELOPMENT OF RADAR REFLECTIVITY AND LOCATION THRESHOLDS

3.1 Based on the experience of other weather services (e.g. Meteo France, as presented in the paper entitled “Status of the automatic observation on aerodrome and ongoing improvements in France” in WMO CIMO TECO 2006), a reflectivity threshold of 34 dBZ may be adopted for radar echoes associated with thunderstorms. A study of the reflectivity threshold has also been conducted for the thunderstorms in Hong Kong in 2009. For a radar reflectivity boundary of 34 dBZ, about 99.70% of the CG lightning strokes given by the lightning location information system are situated within 15 km outside the boundary. This percentage value presents the hit rate or successful capturing rate of the CG lightning strokes. In order to determine the radar reflectivity threshold and the distance threshold away from the prescribed radar reflectivity, however, it is equally important to determine the false alarm in capturing the lightning strokes.

3.2 The false alarm cases are determined by considering the location of the CG lightning stroke relative to the 3-km radar reflectivity, the radar echo top, the number of lightning location sensors in determining that CG stroke, the major axis of the error ellipse of the stroke, the discharge current, the distance from the nearby lightning strokes, as well as any report by the human observer of thunderstorm/lightning. Relatively low electric field mill (EFM) readings (magnitude less than 4000 V/m) at the airport within 10 minutes of a CG lightning stroke and the absence of convective clouds over the CG lightning stroke area as shown on the satellite images are also considered in the identification of a false alarm case. The CG lightning strokes within 30 km from ARP of HKIA in 2009 are considered, viz. a total of 31,884 CG strokes in the period 5 March to 30 September 2009. Out of these strokes, 100 events of false detections (false alarms) are established. The probability of detections (PODs) and false - 3 - AMOFSG/8-SN No. 41

alarm rates (FARs) of the CG lightning strokes with different radar reflectivity thresholds and distances from the prescribed reflectivity threshold are shown in Figure 1. The distances (in %) of the various data points from the “perfect point” (POD=1, FAR=0 at the top-left corner of Figure 1) are shown in Table 1. Based on the 2009 dataset, it appears that we may choose a radar reflectivity threshold of 32 dBZ and a distance of 15 km in establishing whether a CG lightning stroke from LLIS is genuine or not. The threshold of 32 dBZ is comparable with that adopted in Meteo France.

3.3 While the event-based FAR of the optimum point in Table 1, namely, 0.23 per cent, appears to be rather small, it would however correspond to 19 false alarm cases in 2009 if we consider the 10-minute time intervals for which METAR/SPECI thunderstorm reports may be issued. This would represent some 8 per cent of the 236 reports in 2009, which is not a small percentage. The above consideration of the reflectivity and distance thresholds appears necessary.

4. GUIDELINES

4.1 Based on the considerations in Sections 2 and 3 above, the proposed guidelines for thunderstorm reporting are given in the Appendix. A concise summary of the guidelines are also presented in the form of a flow chart in Figure 2.

4.2 The above guidelines may also be used for SYNOP reporting purpose, e.g. when the METAR station is also the SYNOP station. According to WMO Manual On Codes: “For synoptic coding purposes, a thunderstorm shall be regarded as being at the station from the time thunder is first heard, whether or not lightning is seen or precipitation is occurring at the station”. It is proposed that, when thunder is not heard and where automatic lightning observations are utilized for SYNOP reporting, reference may also be made to the lightning location information and radar data in the reporting of thunderstorm following the proposed guidelines.

5. ACTION BY THE AMOFSG

5.1 The AMOFSG is requested to:

a) note the content of this paper; and

b) consider adopting in the AMOS Manual the proposed approach for reporting thunderstorms as given in the Appendix.

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AMOFSG/8-SN No. 41 Appendix

APPENDIX

Proposed guidelines for thunderstorm reporting

(a) If thunder is heard and lightning is seen by the weather observer, report the thunderstorm in the present weather of METAR when thunderstorms occur within the aerodrome or its vicinity. The aerodrome is taken to be 8 km from the Aerodrome Reference Point (ARP). Vicinity should be used to indicate the present weather phenomena which are not occurring at the aerodrome but between approximately 8 km and 16 km from the ARP.

(b) If thunder is heard but lightning is not seen by the weather observer, confirm if any lightning (cloud- to-ground (CG) and cloud-to-cloud (CC) inclusive) has been recorded by LLIS within the past minute and 16 km of ARP. If yes, report the thunderstorm based on location assessed from LLIS. If there is no lightning within range, check if any radar echoes with reflectivity above 32 dBZ were present within the past 6 minutes and 16 km of ARP. If yes, report the thunderstorm based on location assessed from the radar. Otherwise, report no thunderstorm neither at the aerodrome nor within its vicinity.

(c) If thunder is not heard but lightning is seen or CG lightning is detected by LLIS within the past minute and 16 km of ARP, check if any radar echoes with reflectivity above 32 dBZ were present within the past 6 minutes and within 15 km of the location of the lightning. Consider reporting the thunderstorm based on available information in consultation with the Aviation Forecaster. Otherwise, report no thunderstorm neither at the aerodrome nor within its vicinity.

(d) If thunder is not heard for 10 minutes after the time thunder was last heard or thunderstorm last reported, the cessation of thunderstorm is confirmed and thunderstorms shall be regarded as having ceased or being no longer at the aerodrome.

Note: 1. Procedures relevant to METAR only are highlighted in italics. The above procedures may also be used in the reporting of thunderstorm in SYNOP (e.g. when the airport station is also a SYNOP station). 2. ARP = Aerodrome Reference Point

AMOFSG/8-SN No. 41 Appendix A A-2

ROC Curve of POD vs FAR (2009)

100.0% 100.0%

20dbz 99.0% 99.0% 25dbz 30dbz 98.0% 98.0% 32dbz

POD 33dbz 97.0% 97.0% 34dbz 35dbz 96.0% 96.0% diagonal

95.0% 95.0% 0.0% 0.1% 0.2% 0.3% 0.4% 0.5% FAR

Figure 1 POD and FAR of the CG lightning strokes with different radar reflectivity thresholds and distances from the prescribed reflectivity threshold based on the lightning data in 2009. The diagonal line of this Relative Operating Characteristic (ROC) diagram is shown as a blue line. The point corresponding to (32 dBZ, 15 km) (see Table 1 below) is marked in red circle (POD = 99.73%, FAR = 0.23%).

km 0 5 10 15 20 25 30 Tabl dBZ e 1 25 9.609% 1.218% 0.429% 0.367% 0.371% 0.377% 0.376% Dist ance 30 13.038% 1.583% 0.519% 0.365% 0.370% 0.372% 0.376% s of 32 14.948% 1.803% 0.582% 0.357% 0.368% 0.372% 0.371% the 33 16.084% 1.882% 0.605% 0.365% 0.370% 0.373% 0.373% vari ous

34 17.402% 2.023% 0.641% 0.365% 0.367% 0.371% 0.375% data 35 18.752% 2.209% 0.675% 0.370% 0.367% 0.371% 0.372% poin ts in Figure 1 from the perfect forecast (top-left corner). The shortest distance is highlighted.

AMOFSG/8-SN No. 41 A-3 Appendix A

N Y Is thunder heard?

Is lightning seen Is lightning seen OR within 16 km of CG lightning detected ARP? by LLIS within the Y past minute and 16 N

Report the Y N thunderstorm based on location assessed by the Weather

Are radar echoes with Is lightning reflectivity above 32 dBZ present within the detected by LLIS past 6 minutes and 15 within the past kf thltif minute and 16 km Y N Report the thunderstorm based N on location assessed Do not report Y fLLIS thunderstorm at the aerodrome or within its vicinity. Consider reporting the thunderstorm based on available information in consultation with the Aviation Forecaster.

N Are radar echoes with reflectivity above 32 dBZ present within the past 6 minutes and 16

Y Report the thunderstorm based on location assessed from the radar.

Figure 2 Proposed flowchart in the reporting of thunderstorm.

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