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OWL, 16, pp. 85-101

A probable microburst at Weston on the Green on Sunday 24 July 1994

D. J. Naylor Meteorological Office, RAF Brize Norton

Weston on the Green is a small airfield situated 25 km to the north-east of KAF Brize Norton. Its importance as far as forecasting at Brize Norton is concerned is that it is used by the military for parachute training. This is done by means of a large barrage-type balloon under which is sus- pended a basket from which the trainees jump. Obviously, strong winds and the risk of thun- derstorms and low cloud are important to this sort of operation. The balloon is moored to the ground by means of very strong wires and can generally withstand gusts of up to about 17ms 1. On Sunday 24 July the balloon was flying but no training was in progress due to the forecast risk of thunderstorms.

A microburst is a concentrated severe down- draught that induces an outward burst of damaging winds at the surface with a maximum horizontal extent of 4km (Fujita 1985). The wind shears involved (both speed and direc- tion) with this phenomenon have been well documented from research in the USA, where aircraft crashes have been directly attributed to microbursts. Observational and forecasting guidelines are presented in Caracena et (11. (1989) and comparisons will be made for data

in this case. During the late afternoon of Sun- day 24 July the balloon at Weston on the Green was destroyed by a phenomenon which may have been such an event.

Synoptic situation and weather

There were three distinct zones to the day's weather. They were well marked at 0900GMT (see Fig. 1) and can be described as follows:

Zone A: Light westerly or south-westerly winds with dew points generally around 13 to 15°C. Paqchy low cloud affected some coasts and nearby hills but inland there was cumulus/ stratocumulus topped by altocumulus. Maxi- mum temperatures reached 18 to 21°C during the afternoon.

Zone B: The convergence zone in which the winds were light, blowing from between south- west and north-west in the western half, and from between south and north-east in the east- ern half. North of 51"N there were frequent outbreaks of rain with thunderstorms, while further south where the air was drier no rain or thunder was reported. Dew points were of the order of 16 to 18°C and there was extensive

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Fig. 1 The three distinct zones of weather over the British Isles at 0900GMT on 24 1994; see text for a description of the weather in each zone (A, B, C). Places mentioned in the text are shown by the letters B - Brize Norton, H - Herstmonceux and W - Weston on the Green.

altocumulus castellanus and embedded cumulonimbus cloud, the tops of which were as high as 12000m. The upper-air sounding for Aughton (Liverpool) at 1200GMT (see Fig. 2) was typical of this zone north of 51"N, with very moist air above 2200m.

Zone C: Light easterly winds with dew points between 16 and 20°C. Maximum tempera- tures reached 28 to 32°C in this zone. Air with a surface temperature of 31°C would be suffi- ciently buoyant such that, if lifted, it would form first cumulus clouds and then deep cumulonimbus. (The upper-air sounding for Herstmonceux at 1200GMT (see Fig. 3) was typical of this air.) Early low cloud cleared rapidly during the morning but there was ex- tensive cirrus and patchy altocumulus. Some of this altocumulus was castellanus in nature (which produced thundery rain at Brize Norton at 042OGMT and another pulse of thundery rain to the east of Brize Norton at 07OOGMT). Winds aloft were light and veered with altitude.

Zone B moved eastwards at about 7 m s-1 and thunderstorms which had formed over west Wales during the morning tended to develop further east as the day went on. At Brize Norton the temperature only just reached 30°C but it appears that local convergence

Fig. 2 1200GMT on 24 July 1994

Upper-air ascent at Aughton (Liverpool) at

induced the extra lifting required to produce the storms.

At 1500GMT (see Fig. 4) winds became generally westerly west of a line (i.e. the broken line in the figure) from Lyneham to Portland Bill but were southerly and south-easterly to the east of this. In the vicinity of Brize Norton dew points in the south-easterly flow were around 15°C but in the westerly flow they were nearer to 18°C. A cumulonimbus cell rapidly developed to the south-west of Brize Norton airfield at 1445GMT, producing the first rum- bles of thunder at 1516GMT. Thereafter, num- erous other cumulonimbus cells developed quickly to the north and east of the airfield.

The thunderstorm at Brize Norton ceased at 173OGMT, giving only 0.6mm of rain in total and a gust of 8ms-1 but a colleague driving in

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Fig. 3 Upper-air ascent at Herstmonceux at 1 2 0 0 ~ ~ 1 1 on 24 July 1994. The solid line denotes the temperature and the dashed line the dew point of the environnient us recorded by the radiosonde. The dotted lines indicate the temperature and dew point of a parcel of air that is lifted from the surjace (assuming a temperature there of 31°C

~ see text) dry-adiabatically to cloud base, and then following a saturated adiabat until the parcel temperature again matches that of the environment. The shaded area is known as the convective available potential energy. Also shown is the calculation of the downdraught tem- perature (17"C), assuming a starting wet-bulb tem- perature of 0°C at around 675mbar.

from the east reported heavy rain (with roads awash) and some large hailstones under a night-like sky. There was a report of a severe thunderstorm in Oxford between 1550 and 1715GMT accompanied by hailstones with a diameter of 0.75cm. The Radcliffe Observa- tory recorded 18.4mm of rain in 15 minutes and 35.5mm in total. Reports received by the

Tornado and Storm Research Organisation (TORRO) included a ball-lightning incident at South Leigh near Witney, a fall of 'very large' hailstones at Hampton Poyle (between Kidlington and Weston on the Green) and a whirlwind at Kidlington. The balloon crew chief at Weston on the Green reported a severe thunderstorm with hail and a sudden gust to at least 23ms-1 (see Fig. 5 ) . Unfortunately, the pen of the anemograph lifted off the chart so the true value is not known. This gust of wind destroyed the balloon and, although the hail- stones may have been a contributory factor, the balloon crew chief and members of his squad- ron are convinced that the wind was the main culprit. The Brize Norton anemograph trace (not shown) did not show any gusts greater than 7ms-1 at this time.

Fawbush and Miller method

The Fawbush and Miller (1954) forecasting technique was developed in the USA to esti- mate the peak wind speed possible from a non- frontal thunderstorm. This is done by obtain- ing the temperature just before the storm and also the temperature of the downdraught. The latter can be calculated by finding the 0°C wet-bulb temperature on a representative up- per-air sounding and from this point construct- ing a saturated adiabat down to the surface; the resulting temperature is the likely downdraught temperature. The value of 0°C is used for the wet-bulb temperature since this reflects the fact that the evaporation of snow and graupel plays a major r6le in the formation of the down- draught. The low bulk density of snow relative to that of rain means that it evaporates more readily than rain. The difference between these pre-storm and downdraught temperatures is then read off on a scale (see Fig. 6) to obtain the expected peak wind speed. Applying this procedure using the 1200 GMT upper-air sounding for Herstmonceux (the most appro- priate data available) see Fig. 3), the down- draught temperature obtained is 17°C and from the scale this gives a peak wind of around 70kn (35ms-1) using a maximum temperature before the storm of 30°C. No temperature data were available for Weston on the Green, although the fall in temperature of 8degC

Fig. 4 Surface observations at ~SOOGMT on 24 July 1994. The thick dashes indicate the low-level convergence zone.

Fig. 5 Part of the anemograph trace for Weston on the Green on 24July 1994 (lkn=0.5lms1; times are EST (= GMT + 1 h))

Fig. 6 Peak wind speed and temperature differences in thunderstorms in the USA. Solid line denotes regression curve, dashed lines denote standard error of estimate.

281

--mi G M 1 Sheet No R q h r . NiI Upper I O m m . low

observed at Brize Norton (see Fig. 7) gives a theoretical peak wind of around 50kn (25ms 1). Both these calculations confirm that the reported gust at Weston on the Green was well within the limit of what was possible in theory.

Although the Fawbush and Miller technique was developed for non-frontal thunderstorms in the USA, it has been used (and is still in use) in the United Kingdom. Results obtained from the use of this technique have been most satis- factory when applied to non-frontal thunder- storms (see, for example, McNair and Barthram 1966).

Comparison with forecasting guidance in the USA

Studies in the USA have found that when analysing an upper-air sounding the following, in combination, are considered to be indicators of microburst potential:

(i) A large positive area between the en- vironmental curve and the ascending

parcel temperature ( i e . a large convec- tive available potential energy) - see Fig. 3.

(ii) Little or no capping inversion. (iii) At least 1500m of unsaturated air be-

neath the convective cloud base. (iv) A moist mid-tropospheric layer be-

tween 1500 and 4500m above the ground.

(iv) An elevated dry layer above an altitude of 4500m.

These conditions are all satisfied on the upper-air sounding for Herstmonceux at 1200 GMT (Fig. 3) which would have been represent- ative of the air over the Brize Norton area between 1500 and 1600GMT. It therefore ap- pears that the gust of wind at Weston on the Green on the afternoon of Sunday 24 July was almost certainly a microburst of the type ex- perienced in the USA.

Acknowledgements

Flt. Lt. Andrew from the Balloon Operations Squadron, Hullavington, for access to the ane- mograph trace for Weston on the Green; Sen- ior Meteorological Officer, Brize Norton, Mr A. F. Graeme, for the anemograph (not repro- duced here) and thermograph traces for Brize Norton and his constructive comments/sug- gestions towards the preparation of this paper; Jonathan D. C. Webb, of TORRO, for the various reports of thunderstorms in Oxfordshire.

References

Caracena, F., Holle, R. L. and Doswell, C. A,, I11 (1989) Microbursts, A handbook for visual zdentajication. US Dept. Commerce, 35pp.

Fawbush, E. J. and Miller, R. C. (1954) A basis for forecasting peak wind gusts in non-frontal thunder- storms. Bull. Am. Meteorol. SOC., 35, pp. 14-19

Fujita, T. T. (1985) 7’he downburst, microbursr and macrobunt. SMRP Research Paper No. 210, (NTIS No. PB85-148880) University of Chicago, 122pp.

McNair, R. R. and Barthram, J. A. (1966) A mesoscale investigation of a squall line. MeremZ. Mag., 95, pp. 304-310

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