page 1© crown copyright 2004 progress in introducing new technology sensor sites for the met office...

© Crown copyright 2004

Progress in introducing new technology sensor sites for the Met Office long range

lightning detection system

May 2005


Introduction to Wind Profilers in the UK

John Nash, Nigel Atkinson*, Eric Hibbett, Greg Callaghan, Paul Taylor

Upper Air TeamObserving Methods Technology Centre,

Technology and Applied Science

*Satellite Applications, Numerical Weather Prediction

Met Office, Exeter


Introduction

The UK Met Office has run a long range lightning detection system since 1986, designated ATD (Arrival time difference system).

Rubidium oscillators at each detector station provide a time reference providing accurate time stamps on received SFERIC waveforms.

A new control computer was introduced in a project lasting from 1996 to 2000.

However, the operational detection efficiency of current ATD is still quite variable.

On some occasions detection efficiency around the British Isles is towards 90 per cent, but on other occasions the current detection efficiency over the British Isles is only about 10 per cent.


Access to ATD observations

Useful thunderstorm locations are obtained at even longer range although detection efficiency over Africa, for instance, may be relatively low.

Locations with high accuracy are marketed within Europe and are not circulated on the GTS

Location reports with degraded accuracy are made available on the GTS as SFLOC messages.

Access to accurate long range locations needs to be negotiated with a suitable Met Office contact.


The following slides indicate the average ATD reporting performance near the British Isles over 3 months compared to a second high detection efficiency lightning detection system. This is operated in the UK by EA Technology, with the electricity supply industry as its main customer.


Comparison of number of lightning reports by ATD and EATech in the area [14 W to 8 E, 48 N to 62 N ] for March to May 2003/4

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Assessment of detection efficiency by comparison with EA Technology over the British isles and surrounding sea areas [2]

March to MayATD has good detection efficiency over the UK


Comparison of number of lightning reports by ATD and EATech in the area [14 W to 8 E, 48 N to 62 N ] for September to November 2002/4

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September to NovemberATD detection efficiency is very poor at night


Reasons for ATD limitations near the UK

At 10 kHz, thunderstorm activity in the tropics often produces stronger SFERICS near the UK than the local flashes. Thunderstorms in south and Central America have the biggest impact on ATD local performance, with the worst effects from September to November at night

Current ATD outstations are only capable of reporting about 10, 000 flashes per hour. The dynamic range, sensitivity and

throughput of the current outstations are clearly insufficient to provide consistent 90 per cent detection efficiency around the British Isles.

When thunderstorm activity in Europe is high many of the outstations are rendered insensitive by local thunderstorm activity. A slightly wider spread of ATD outstations around Europe is required to sustain high detection efficiencies in summer


New outstation [NOS] was designed by Nigel Atkinson

NOS uses a standard PC containing two National Instruments cards ,

PCI-4451 for signal processing PCI-6602 for timing.

Control and processing software is implemented through the National Instruments graphical programming system, LABVIEW

Both input channels to the PCI-4451 are connected to the same ATD sensor [whip antenna]. One channel is operated with a higher gain than the other in order to optimise the NOS dynamic range.

Software usually selects data from the high gain channel, but if local thunderstorms cause overload it switches to the low gain.

Expected effective dynamic range of the A to D converters is expected to be around 80 dB.

Sferic waveforms are sampled at 12.8 µs


Reference signal for the counting card is provided by the Starloc II GPS system. This incorporates a rubidium oscillator allowing the timing reference of the NOS to be maintained even if there is a temporary problem with GPS reception.

Processing is set up to be similar to the existing ATD outstations in most respects.

However, only clean Sferic waveforms with a well defined threshhold peak substantially above noise level are accepted, so the NOS should not report system noise at high gain.

The NOS does not support the feedback loop that allows a central computer to request data for selected events. All detected waveforms are automatically forwarded to the central computer.

This is performed through an interface computer to the existing central computer [ATD Sferics interface computer].

New outstation [NOS] was designed by Nigel Atkinson


The new outstation has been designed and tested for more than a year. In the UK ,it reports more than 60,000 SFERIC locations per hour in summertime conditions and about 10,000 SFERIC locations per hour in wintertime conditions


Preoperational testing of NOS commenced in June 2004 at Exeter and Camborne

Software was modified in August 2004 to eliminate data gaps that were occurring.

Availability of Sferic waveforms for the events requested by the central computer was 97 per cent on average from both sites

There has been a significant increase in the number of lightning events reported overall by ATD since the testing of the two outstations commenced.

The number of ATD location reports doubled once, the two NOS were introduced with several of the existing outstations functioning poorly in August 2004.

Sferic reporting rates in summer were considered satisfactory, timing errors were within specification and waveforms were of acceptable quality.

The two NOS have subsequently proved essential for the continued operation of ATD since several old outstations have degraded in performance in the last year.


Number of flashes per month reported by M et Office ATD since late2001

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ATD reports very many more lightning locations than those purely over the British Isles

Green dashed line joins average of October/November

2 NOS introduced


New ATDNET system to be under evaluation during 2006

New ATDNET system is expected to have between 13 and 15 outstations,

10 within Europe, see next slide, and possibly five at longer range to reduce the rate of spurious locations in Europe and to improve the area of useful ATD detection efficiency across the whole of Africa and into central Asia.

ATD operating costs are currently dominated by communications costs , but the new ATDNET system is expected to use much cheaper communications, allowing a much larger area of coverage to be sustained with lower communication costs.

A new central computer has been purchased and installed at Exeter, and it is intended that the system should start testing of flash location software using data from five new outstations in September 2005.

A new interface computer [DMZ] required to meet Met Office security policy for Met Office network connections to external NOS locations will be ready by June 2005.


Possible locations of future ATDNET outstations

Agreements with countries hosting ATD outstations allow immediate access to observations from an agreed service area.

NOS ready with new communications, September 2005


Expected changes to long range coverage

The next slide shows a typical monthly summary of ATD lightning reports [ nearly all cloud to ground flashes]

This is followed by a monthly summary of lightning flashes detected by a satellite based lightning detection system, mostly observing cloud to cloud flashes.

ATD appears to detect a relatively high rate of cloud to ground flashes over the sea compared to the LIS satellite system

ATD is not very sensitive to thunderstorms in southern , central and eastern Africa.

It is hoped that the installation of new long range outstations will have the effect of eliminating this weakness.


2.57 million locations in month


Climatology can be compared against satellite systems - LIS


Possible long range NOS outstation locations to be ready by end of 2006, extending location coverage to south and east Africa.

??

Reunion

South Africa

Ascension Is

??


Conclusions

Met Office long range lightning detection will soon be upgraded to improve the stability of detection efficiency across Europe, with a completely renewed ATDNET system.

Long range detection will be expanded to cover the whole of Africa with a better location accuracy than is currently available

Location accuracy should also improve in South America.

Met Office is willing to collaborate with other meteorological services in extending coverage to other areas of the globe and providing access to the output of the current upgrade.


Assessment of detection efficiency by comparison with EA Technology over the British Isles and surrounding sea areas[1]

Comparison of number of lightning reports by ATD and EATech in the area [14 W to 8 E, 48 N to 62 N ] for December to February 2003/2004

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December to February, ATD has good detection efficiency in the daytime


Comparison of number of lightning reports by ATD and EATech in the area [14 W to 8 E, 48 N to 62 N ] for June to August 2002/4

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June to AugustATD detection efficiency is poorer than in earlier months


Prototype NOS testing- began in 2003

The requirements for satisfactory NOS operation were:-

No large gaps in waveform supply to the central computer

A 95 per cent availability of Sferic events requested by the central computer.

An increase in the number of lightning events reported by the existing system when using NOS data, especially when operating with only 5 of the existing outstations.

Time differences to be within 2 µs of those reported by collocated old outstations.

Noisiness of reported Sferic waveforms should not be high.

The number of Sferic events per hour in the UK summertime should not be in excess of 60,000.

page 1© crown copyright 2004 progress in introducing new technology sensor sites for the met office...

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