Reprint 1104

Application of Lightning Characteristics Identification

in Nowcasting of Rainstorms

S.M. Olivia Lee, H.Y David Lam

& K.M. Kenny Wong

28th Guangdong-Hong Kong-Macao Seminar on

Meteorological Science and Technology,

Hong Kong, 13-15 January 2014

Application of Lightning Characteristics Identification in

Nowcasting of Rainstorms

Olivia S.M. Lee, David H.Y. Lam and Kenny K.M. Wong

Hong Kong Observatory

Abstract

Previous studies indicated that there is a close correlation between lightning characteristics (including lightning density, lightning peak current and electric charges) and rainfall amount of thunderstorm cells. In this study, thunderstorm cases in Hong Kong from 2011 to 2013 were examined. Lightning data from the lightning location network jointly set up by the weather services of Guangdong, Hong Kong and Macao in the Pearl River Delta was analyzed. Specific features in lightning density, lightning peak current and electric charges of thunderstorms were found to be associated with further development of some thunderstorms into rainstorm situations affecting Hong Kong. This paper will also introduce a newly developed prototype nowcasting application product which may help weather forecasters identify and track those features.

1. Introduction

The Hong Kong Observatory (HKO), in collaboration with Guangdong Meteorological Bureau (GMB) and Macao Meteorological and Geophysical Bureau (SMG), set up a lightning location network in 2005 to monitor lightning activities over the Pearl River Delta and neighbouring regions round-the-clock. Currently, the network comprises seven lightning sensors (see Figure 1). Six of them are the Vaisala IMPACT ESP model [1], which are located at Chung Hom Kok, Tsim Bei Tsui and Sha Tau Kok in Hong Kong; Taipa in Macao; Sanshui and Huidong in Guangdong. The remaining one is the LS7001 model located at Yangjiang in Guangdong. Sensors at Chung Hom Kok, Tsim Bei Tsui, Sha Tau Kok, Taipa and Sanshui were in place since 2005, while sensors at Huidong and Yangjiang were installed in 2008 and 2012 respectively.

A good spatial coverage of the stations in the network provides sufficient

redundancy for detecting cloud-to-ground lightning with high efficiency (see Figure 1 again). Since the lightning sensors and processors are designed to detect and analyse mainly cloud-to-ground lightning, the efficiency of cloud-to-cloud lightning detection is not high and is estimated to range from 10% to 50%. Lightning information, including lightning stroke timing, location (latitude and longitude), stroke type (cloud-to-cloud or cloud-to-ground lightning), polarity and peak current, waveform rise and fall time are updated every minute for the reference of the weather forecasters. 2. Data and Methodology

In this study, thunderstorm cases occurring in Hong Kong from 2011 to

2013 are examined. A thunderstorm case is regarded in the study as an episode when lightning is detected within the territory of Hong Kong by the lightning location network. When there is a lapse time of more than 30 minutes within an episode when no more lightning signals are detected within Hong Kong, that episode is considered as ended. There were a total of 347 thunderstorm cases in the period from 2011 to 2013. Among them 52 cases are localized thunderstorms brought by unorganized isolated rain echoes. Out of the remaining 295 thunderstorm cases that were associated with organized rain echoes, 53 cases eventually developed into situations necessitating the

issuance of Amber/Red/Black Rainstorm Warning Signals1. The aim of this study is to develop operational guidelines to help weather

forecasters in assessing the need of rainstorm warnings. In particular, we would like to find out whether there are any characteristics in the lightning data in the 295 thunderstorm cases which lead to the eventual development into a rainstorm situation.

3. Results and Discussions

Previous studies [3,4] indicated that heavy rain is associated with an

increase in lightning density and electric charges of total lightning data (i.e. cloud-to-cloud and cloud-to-ground lightning). In the current study, three total lightning characteristics, namely lightning density, lightning peak current and discharge rate (i.e. Peak Current/waveform rise time) of the 295 thunderstorm cases are plotted, examined and compared with Observatory’s weather radar reflectivity images.

To facilitate comparison with weather radar reflectivity images, total

lightning characteristics within a 6-minute interval (time of a PPI volume scan of Tai Mo Shan and Tate’s Cairn Weather Radar) are plotted on the same basemap as weather radar reflectivity images.

It is observed that before rainstorms set in, approaching rain echoes

coming within 100 km from Hong Kong are found to exhibit certain features in the afore-mentioned three total lightning characteristics.

3.1 High Lightning Density Previous studies have demonstrated that the change in total lightning density is related to the development of thunderstorm cell and severe weather [5]. It is believed that increased lightning activities are closely related to the development of intense thunderstorm cells which bear heavy rain. In the current study, total lightning density is the number of cloud-to-cloud and cloud-to-ground lightning strokes per unit area over a time

                                                       1  A description of the Rainstorm Warning System can be found from the Observatory website: http://www.weather.gov.hk/wservice/warning/rainstor.htm 

period of 6 minutes. Out of the 53 rainstorm cases in this study, total lightning density of approaching rain echoes in 39 cases are over 0.2 counts/km2, making a Probability of Detection (POD) of 39 ‘hits’/53 rainstorms=0.74 for the condition ‘Total Lightning Density > 0.2 counts/km2’. Among these 39 ‘hits’, in 29 cases (i.e. 74% of the time) the first occurrence of the condition were more than 30 minutes before the issuance of Rainstorm Warning Signal, indicating the occurrence of the condition does provide a lead time for assessing the potential of the thunderstorm case leading to rainstorm situations. As to the False Alarm Rate (FAR) of the aforesaid condition, since 41 non-rainstorm cases are also found to display the said condition, the FAR amounts to 41 ‘false alarms’/(39 ‘hits’ + 41 ‘false alarms’)=0.51.

Figure 2 shows a rainstorm case illustrating the occurrence of high lightning density preceding the onset of rainstorm. 3.2 Lightning Peak Current of Large Magnitude Lightning Peak Current of a lightning stroke is an indication of electrical charges and energy of lightning activity. It is believed that thunderstorm cells with large electrical energy have higher potential for intense convective development leading to rainstorm situations. In the current study, out of the 53 rainstorm cases, total lightning peak current of approaching rain echoes in 42 cases are negative current with magnitude over 60 kA and 2 cases are positive current with magnitude over 80 kA, making the POD of the condition ‘Total Lightning Negative Peak Current Magnitude > 60 kA or Positive Peak Current Magnitude > 80 kA’ equal to (42+2) ‘hits’/53 rainstorms = 0.83. Among these 44 ‘hits’, in 39 cases (i.e. 89% of the time) the first occurrence of the aforesaid condition were more than 30 minutes before the issuance of Rainstorm Warning Signal. As to the FAR of the condition, since 28 non-rainstorm cases are also found to display the said condition, the FAR amounts to 28 ‘false alarms’/(44 ‘hits’ + 28 ‘false alarms’) = 0.39.

Figure 3 is the same rainstorm case as shown in Figure 2. It also illustrates the occurrence of large magnitude of total lightning peak current preceding the onset of rainstorm.

3.3 High Discharge Rate In the current study, it is observed that out of the 53 rainstorm cases, the Total Lightning Discharge Rate (i.e. Peak current/Waveform Rise Time) of approaching rain echoes in 34 cases are over 20 kA/microsec, making a POD of 34 ‘hits’/53 rainstorms = 0.64 for the condition ‘Total Lightning Discharge Rate Magnitude > 20 kA/microsec’. Among these 34 ‘hit’ cases, in 26 cases (i.e. 76% of the time) the first occurrence of the aforesaid condition were more than 30 minutes before the issuance of Rainstorm Warning Signal. As to the FAR of the condition, since 22 non-rainstorm cases are also found to display the said condition, the FAR amounts to 22 ‘false alarms’/(34 ‘hits’ + 22 ‘false alarms’) = 0.39.

It is believed that high Total Lightning Discharge Rates of thunderstorm

cells are indicating a great surge of electrical energy which accounts for explosive convective development that is very likely bringing rainstorm situations.

Figure 4 is a rainstorm case displaying such explosive convective

development. It illustrates the occurrence of a high total lightning discharge rate preceding the onset of rainstorm. 4. Application in Rainstorm Nowcasting Based on the above discussions, the following empirical criteria have been devised for operational trial use: Criteria

POD FAR

Total Lightning Density > 0.2 counts/km2

0.74 0.51

Total Lightning Negative Peak Current Magnitude > 60 kA or Total Lightning Positive Peak Current Magnitude > 80 kA

0.83 0.39

Total Lightning Discharge Rate Magnitude > 20 kA/microsec

0.64 0.39

Moreover, a prototype lightning characteristics webpage (see Figure 5)

has been developed to help forecasters to track the conditions identified in Section 3. Total lightning characteristics, namely Lightning Density, Lightning Peak Current and Discharge Rate at 256 and 128 km ranges are displayed. Animation and zoom-in features allow forecasters to look at and track the three characteristics. 5. Conclusions and Way Forward

With the availability of a web-based tool, forecasters can easily monitor

the evolution of total lightning characteristics of approaching rain echoes and assess the potential of rainstorm development.

Looking ahead, the current study can be expanded to cover longer period

of time with more rainstorm cases. Moreover, the rainstorm cases can be more objectively defined based on actual rainfall analysis to fine tune the empirical criteria.

In future, after upgrade of the computer processor of the lightning

network, it would be able to process more total lightning data and better monitoring of total lightning characteristics are expected.

References

[1] Vaisala, 2004: IMPACT ESP Model 141-TESP User’s Guide, Version 1.3, Vaisala Oyj, Helsinki, Finland. [2] Vaisala, 2007: Vaisala Thunderstorm CG Enhanced Lightning Sensor LS7001 Data Sheet, Vaisala Oyj, Helsinki, Finland.

[3] 黃秋平、李立信、蘇卓文: 在珠三角一帶與大雨相關的閃電特徵分析。

第二十一屆粵港澳氣象科技研討會，中國，香港，2007 年 1 月 24-26 日.

[4] 曹治強，李萬彪: 兩個中尺度對流系統的降水結構和閃電特徵。 氣

象學報，2005,63(2):243-248.

[5] Demetriades N.W., D.E. Buechler, C.B. Darden, G.R. Patrick and A. Makela, 2008: VHF Total Lightning Mapping Data Use for Thunderstorm Nowcasting at Weather Forecast Office. Third Conference on Meteorological Application of Lightning Data, New Orleans, LA, January, 2008.

 

 

Figure 1. Locations of lightning sensor stations of the Lightning Location Network. The red line shows the coverage of the network with cloud-to-ground lightning detection efficiency exceeding 70%.

3 km CAPPI Rainfall Rate of HKO Weather Radar with Range 128 km

Total Lightning Density* with Range 128 km

Figure 2. Sequence of images of 3 km CAPPI Rainfall Rate and Total Lightning Density on 22 May 2013. Amber Rainstorm Warning Signal was issued at 0130 HKT. * Total Lightning Density in 128 km range = No. of total lightning strokes over a time period of 6 minutes interval and within a 3 km x 3 km grid.

Amber Rainstorm Warning Sginal was issued at 0130 HKT.

Total Lightning Density > 0.2 count/km2 Rain echoes approaching

from the west

Rain echoes approaching from the west

Total Lightning Density > 0.4 count/km2

Total Lightning Density > 1.0 count/km2

3 km CAPPI Rainfall Rate of HKO Weather Radar with Range 128 km

Total Lightning Peak Current with Range 128 km

Figure 3. Sequence of images of 3 km CAPPI Rainfall Rate and Total Lightning Peak Current on 22 May 2013. Amber, Red and Black Rainstorm Warning Signal was issued at 0130, 0320 and 0410 HKT respectively.

Rain echoes approaching from the west

Negative Peak Current

Magnitude > 80 kA

Rain echoes approaching from the west

Negative Peak Current

Magnitude > 60 kA

Negative Peak Current

Magnitude > 60 kA

Red Rainstorm Warning Signal was issued at 0320 HKT.

3 km CAPPI Rainfall Rate of HKO Weather Radar with Range 128 km

Total Lightning Discharge Rate#

with Range 128 km Figure 4. Sequence of images of 3 km CAPPI Rainfall Rate and Total Lightning Discharge Rate on 30 August 2013. Amber Rainstorm Warning Signal was issued at 07:55 HKT. # Total Lightning Discharge Rate = Peak Current / Waveform Rise Time.

Rain echoes developing over north of Hong Kong and

moving east-southeastwards

Negative Discharge Rate Magnitude > 100 kA/microsec

Convective development observed as echoes moved

across the territory

Negative Discharge Rate Magnitude >= 160 kA/microsec observed, indicating explosive convective development

Amber Rainstorm Warning Signal was issued at 0755 HKT.

Discharge Rate Magnitude fell below 20 kA/microsec when heavy rain falling

 

 

 

 

Figure 5. Prototype webpage showing Total Lightning Characteristics

Animation Tool

Displaying 256 km

range

Animation Tool

Zoom in to 128 km

range display