UHI at Heathrow Met Office site

Figure 1

Thermal signature at Heathrow April 2010.

Introduction

Warning this is a long introduction

There is a great deal of anecdotal and circumstantial evidence of aerodrome traffic producing UHI in meteorological data at co-location sites around the world but more direct evidence is lacking. The author is aware of none, moreover obtaining such data is likely to be very difficult.

A study opportunity was offered by the peculiar events during April 2010 at one of the major international freight and passenger airport in the world, Heathrow. An analysis exercise was carried out 2011 but the results were not published.

The UK Met Office is responsible for air traffic meteorological data. This involves a complex web of organisations which can be read about in the PDF published by the CAA (Civil Aviation Authority) CAP782 where it should be kept in mind the entire airspace is shared military/other with joint radar, controllers and so on. The Met Office is part of the Ministry of Defence, where it’s primary role is military, civilian and other services are a side issue.

The Met Office public web site plays down the military operations, only mentioning civilian

The Met Office is the world-leader in aviation meteorology, one of only two World area Forecast Centres (WAFC) and contracted by the CAA under Single European Skies (SES) as the sole provider of aviation weather in the UK.

The above is mentioned because the Met Office have not been contacted to do with the study, their prior behaviour always withholding high resolution data and generally being awkward making contact a waste of time.

There are two meteorological sites involved, both visually following the classic Met Office enclosure and instrument style, but neither absolutely admitted as the sites used. Circumstantially there is no rational reason why complex and expensive met sites would be present but not used to meet critical obligations: if other instruments exists which are better… I am sure the reader can follow all the arguments.

Both aerodrome sites produce METAR data. This is transmitted twice hourly for EGLL (Heathrow) and hourly for EGWU (Northolt). Given the Met Office are the legal authority for air traffic met data in the UK it is almost certain the instruments in somewhat expensive met enclosures are the source of the METAR data, enclosures which otherwise would be of minor importance.

The Met Office use RTD which are calibrated, however the electronics is a further source of drift and error. These are fitted to various standard enclosures akin to the Stevenson screen. These are assumed to be competently manufactured, commissioned and maintained, really isn’t a problem in the UK although site metadata is not available to the public. Site exposure, suitability and other issues are a different matter.

Both sites have a remarkably similar exposure.

It was found the two sites do indeed track very closely but the radio data is quantised to whole degrees only and the time resolution is marginal for this kind of exercise.

The intended signature was the air traffic disruption as a consequence of the eruption of Eyjafjallajökull volcano, Iceland, mid to late May 2010..

3. METAR Fr. message d’observation météorologique régulière pour l’aviation is an encoded aerodrome routine meteorological report. sourceCurrent for EGLL

Site, Heathrow

Heathrow has two runways, both referred to as L and R (Left and Right) dependent primarily on weather conditions. The Heathrow station is facing runway (27R / 09L), hemmed in by a concrete wall, next to a wide road and buildings.

The site has changed a little relatively recently, now with a gap in the wall and a small building has appeared close by.

The exact location is just east of the tunnel access to the main passenger terminals, on the north side. Video frames can be unearthed taken by passengers from the runway.

This location is liable to both takeoff blast and less likely reverse thrust on landing, degree unknown. It is also downwind of the prevailing south westerly, although tends to easterly during periods of high pressure blocking patterns over the near continent.

Heathrow traffic is primarily long haul by heavy aircraft close to MTOW.

Image the enclosure is in the middle here. A trawl of eg. YouTube will find video taken by passengers from the runway.

(I’ve switched to using Bing/Microsoft because Google are increasingly dreadfully behaved, unusable now as a link, http://maps.google.co.uk and find Heathrow)

The enclosure is typical older style Met Office.

The site is not one of the few UK international climatic stations but is is used within the UK.

Site, Northolt

This is a former WWII aerodrome which today is used by short to medium haul for government officials, royalty, surveillance, VIP etc.

It is about 8km north of Heathrow.

This too has a questionably located met site. It used to be standard RAF practice with the met enclosure located near the control tower but has been moved to next to the heavily used A40 London feeder road off the M40 and London Orbital (M25).

Image the enclosure is in the middle here. Those of you with Google street view can get a closer look from the A40.

Relative site locations

Figure 2

Map courtesy Google. Overall relationship of the two sites.

Data

METAR data is transmitted twice hourly for EGLL (Heathrow) and hourly for EGWU (Northolt). Historic records are rare past a week or so. In this case I wish to protect the source from excessive attention/traffic, hence is not stated. This data usually needs preprocessing, is not a matter of an easy to use data format.

The radio data is low resolution, integer degree and tends to missing records.

Given the situation the author decided the easiest initial course of action was drop the excess EGLL records, leaving hourly data for both sites.

Similarly no use was made of the other parameters such as cloud cover, wind, which could be brought into use if necessary.

Figure 3

Straightforward overlay plot of both datasets. Station data shows the progressing from winter to the start of summer can be seen in the rising absolute temperature and increase in the amplitude of the diurnal cycle. England tends to prolonged periods of overcast (>50% of the time), especially in winter.

Figure 4

The difference data does not look promising yet does contain useful information in spite of the quantisation to whole degrees. The underlying data has an unknown output sample rate, probably 10 minute data or better.

Target signature of abnormal traffic

The anticipated signature was the air traffic abnormality as a result of air space closure and restrictions from 15th April 2010 toward the end of the month.

A further abnormal event occurred early April and was not anticipated but ought to have been, Easter bank holiday when planned extreme air traffic was present.

Results

The results for this study are primarily visual and is a presentation problem which the assertion proven as self evidentially correct by weight of public record. No statistics are provided. In the authors opinion a much longer dataset could be put together and then a statistics metric of likelihood of random effect could be computed. Given the awkward nature of the underlying data source this would take some time and is not warranted.

Figure 5

This is a patch plot of the difference signal for the whole data period used in this study. Grey areas are missing data.

Figure 6

A spectrogram gives a useful picture, if not ideal given the data. The amplitude is diminished by windowing which acts as low pass filtering, and mean values, divide by 1/root(2) for peak to peak.

Figure 7

This is an annotated repeat of figure 1 and is a patch plot of April 2010 only which contains the signatures.

Easter Good Friday April 2, Easter Monday April 5 with the excess around the latter date.

“New York and Dubai are among the top destinations for the millions of passengers who will pass through Heathrow airport. Almost two million people are booked on flights from Heathrow over the Easter period, with 336,000 reservations making Thursday the busiest day.” Guardian, but not the hotspot.

Long list of statements about cancellations 15th April 2010 at business traveller

Wikipedia has a catalogue of side effects.

Note there were further volcano problems during May 2010.

Further work

This is an incomplete work where the author shows what can be done as a provisional study and invites others to investigate in more detail.

It’s perfectly possibly my assertion is wrong so caution is needed.

The technique of using METAR in the absence of direct data obviously works although METAR archives are rare and paired stations with accurate instrumentation is probably very rare.

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Appendix

Example METAR data after processing from raw source

Spreadsheet in XLS format

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Post by Tim Channon, co-moderator