331

Weather – Decem

ber 2009, Vol. 64, No.12

A persistent cold frontA cold front reached Cornwall around 0600 UTC on 4 August 2009; 72 hours later, at 0600 UTC on 7 August, this front had still not cleared from Norfolk.

Rain spread slowly eastwards on the 4th and falls of over 25mm occurred across Devon, Dorset, Somerset and Wiltshire. Ahead of this front, increasingly warm air was advected northwards into eastern Eng-land. Isolated thundery showers broke out around The Wash on the 5th.

At 0000 UTC on the 6th, this trailing front was situated from the Humber to Portland Bill. The Larkhill ascent at 0600 UTC (just behind the front) showed pronounced verti-cal wind shear. An 8kn northnorthwesterly wind at 1000mbar height became a 32–35kn southerly at 700mbar and 500mbar, in advance of a major upper trough.

The 0259 UTC satellite image (Figure 1) shows a conspicuous thunderstorm area over Hampshire; this system tracked north-northeast from the Solent, where it was very active electrically from 0200 to 0300 UTC, reaching the Reading area at 0400 UTC after which electrical activity evidently ceased. At Portchester (Hants) 25mm of rain fell in 30min from 0245 UTC (Anon, 2009).

Further thunderstorm development occurred from late morning. The 1300 UTC UK Met Office radar image shows narrow bands of intense cells extending northnortheast

from Lyme Bay to the southeast Midlands (where two houses were damaged by light-ning at Wellingborough, Northamptonshire), and also a developing rain area moving up from France. Remarkable mid-afternoon temperature contrasts were evident across the frontal zone. Temperatures reached 29.8°C at Gestingthorpe (a Climatological Observer’s Link (COL) grade ‘A’ station) in Essex and 29.2°C at East Malling (Kent); meanwhile at 1500 UTC, Lyneham (Wiltshire) was reporting 14°C in continuous heavy rain and Brize Norton (Oxfordshire) 15°C in a thunderstorm.

After 1800 UTC, thundery activity (surpris-ingly) ceased but the rain areas merged and pivoted across eastern and central England as a frontal wave moved north. ‘Rainfall day’ accumulations of 40–60mm occurred in Wilt-shire, Berkshire, Greater London, Hertford-shire, Cambridgeshire and Lincolnshire.

Sincere thanks to Paul Brown for synoptics and the University of Wyoming for radio-sonde data .

Jonathan WebbTORRO, Oxford, UK

© Royal Meteorological Society, 2009DOI: 10.1002/wea.521

Figure 1. AVHRR Infra red Satellite image, 0259 UTC, 6 August 2009. (© University of Dundee, 2009.)

ReferenceAnon. 2009. Climatological Observers Link Bulletin 472: 6.

Supplement to Weather image

Figure 2. Rainfall for the 24 hours ending 0900 UTC on 16 September 2009. Isohyets are for 0, 5, 10, 20, 30, 40, 50 and 60mm. The narrow zones make clarity difficult – but the impression is clear!

An unusual occlusionOn 15 September 2009 a cool, cloudy north-easterly flow covered England and Wales, and an old occlusion lay from Essex, across London, to west Surrey and Hampshire. The occlusion was aligned with the flow and remained practically stationary during a period of reactivation between late morning and late evening - such that heavy rain fell almost continuously throughout an 11-hour period in a narrow corridor approximately 300km by just 15km.

Largest rainfall totals in the 24h to 0900 UTC on the 16th were 60.2mm at South Farn-borough (Hants), 53.2mm at Hampstead (London) and 50.4mm at Odiham (Hants). At Hampstead rain fell continously, apart from one short break around 1400, from 1135 to 2200 UTC, and it was heavy (over 4mmh–1) for a total of 5½ hours. The largest fall in one hour was 12.4mm between 1710 and 1810, and the greatest intensity (measured over two consecutive 5-min increments 1755-1805) was 21mmh–1.

Figure 2 was constructed the following day from cumulative radar data supplied by Meteogroup, modulated by available ground truth. Some of that ground ‘truth’ was manifestly false, highlighting problems with quality control of sites in the NCM (National Climate Message) collective; Wisley reported 0.8mm and Kew Gardens 1.0mm when their respective totals should have been approximately 15mm and 45mm respectively.

Philip EdenChilterns Observatory Trust,

Whipsnade,Bedfordshire, UK.

Email: Philip@weather-uk.com© Royal Metrological Society

DOI: 10.1002/wea.526