Tornado at Gosport

TORNADO AT GOSPORT IN SOUTHERN ENGLAND

9 AUGUST 2001

- author's eyewitness sighting and investigation report -


By ANTHONY GILBERT

51 Darwin Way, Gosport, Hampshire PO13 8AF


Abstract. The author observed the development of a tornado less than a mile away and immediately began a detailed site investigation. Personal injury and several thousand pounds in damages were sustained on a track of 2 km. As a representative of TORRO, the author offered technical advice to the public and journalists at the scene. Local synoptic conditions are analyzed in order to ascertain what initiated this damaging T2 tornado at this time and place on the coast of southern England.

SYNOPTIC OUTLOOK

The local forecast for the morning of Thursday 9 August 2001 had mentioned the possibility of thundery showers. A low of 992 mb was situated over Norway with a deepening trough moving east across the southern portion of the UK in a north-westerly air-stream. The 0000 UTC upper-air soundings suggested that severe weather was unlikely, so what I was soon to observe came as a surprise!

OBSERVATIONS

Early that morning the weather was calm but with an oppressive atmosphere. There were several cumulonimbus clouds scattered across the sky with notably low bases. Clear blue sky was visible between breaks in the clouds and there had been an exceptionally heavy burst of rain at approximately 0855.

While heading to work at approximately 0900 I was suddenly aware of a developing funnel cloud less than a mile ahead. Smaller ribbons of condensation appeared to branch downwards like talons, rapidly rotating around each other and converging at the base. These ribbons could be compared to several suction vortices, but I was unable to determine individual rotation (Figures 1 and 2).

This development lasted for only a few seconds before the vortex resembled the classic tornado shape (Fig 3) and made contact with the ground. No wall cloud was visible and the vortex emanated from a well developed solid cloud base. I had a clear view of the whole event with no rain obstructing the view. The tornado appeared to pass over the Kingfisher Caravan Park in Gosport, less than half-a-mile away. I pursued the tornado a little further until it started to lift from the ground and recede back into the cloud. It reformed momentarily but then dissipated altogether.

Almost immediately exceptionally heavy rain pounded the area for about 10 minutes causing flooding to many roads. This was accompanied by very loud thunder. The cloud base, beneath which there were signs of scud rotation, was no longer linear. Given the lengthy associated research I later undertook, this was more likely a direct result of the tornado's influence on the cloud base rather than the presence of a mesocyclone. The storm was now heading away toward the east. I was unable to use my cellular phone for much of this time, probably due to static interference. I eventually managed to contact Dr. Terence Meaden and Dr David Reynolds to report what I had just seen. I also contacted Dorcas Henry, a BBC weather presenter who 'in turn' informed me of damage that had just been reported within the caravan-park.

SITE INVESTIGATION

Upon arrival I found two badly-damaged caravans. One of them had been raised in the air to a height of approximately 15 ft and carried over a nearby car before landing upside down on top of another caravan (Fig. 4). The occupant, Mr. Ronald Brookes, was asleep at the time and was taken to hospital suffering from shock and minor injuries. Journalists were already arriving at the scene, and, after I had made a partial site investigation, I gave two live radio interviews and one televised interview for BBC TV. I was able to make reference to the likely maximum wind speeds and to classify the strength of the tornado using the TORRO tornado intensity scale. I spoke to Mr. Murrell, an eyewitness who said he saw what looked like a flock of birds and then realized it was debris being sucked upwards. A closed-circuit television camera showed the tornado passing through the main entrance and crossing the clubhouse, blowing out doors and levitating roof tiles, heavy wooden benches, tents etc., some of which were found 300 m farther on in a field.

The tornado made landfall on a section of beach called Browndown and passed through the Browndown Ministry of Defence camp, destroying a conservatory (at point 'a' in the map of Figure 5), and then on to the caravan site (point 'b'). Further damage was found on the Gomer Estate. It was here that I found nine garages damaged (point 'c'), some with roofs missing and doors buckled outward due to sudden pressure variation. The cars within were severely damaged or written off (Fig. 6).

TORNADO TRACK DETAILS

The tornado initially formed as a waterspout and at approximately 0900 made landfall on Browndown Beach (grid reference SU 570 994). It lifted two or three minutes later between houses 132-136 in Gale Moor Avenue (grid ref. SU 589 993) (see Fig 5). Although there is evidence of some veering amounting to several metres along its land track the general direction was along a line from W.S.W. to E.N.E. The track width was about 6 m, and the track length over land approximately 2 km. The damage suggests a rating of T2 (moderate tornado) on the TORRO intensity-scale, with maximum wind speeds up to 92 mph. Eyewitnesses confirm a slow forward momentum. Given the considered scale and velocity of the tornado, this may have increased its ability to lift debris vertically rather than scatter it. The storm's rotation was cyclonic (anti clockwise) as witnessed by myself.

MEDIA COVERAGE

I was able to give live TV and radio interviews at the scene, one of which was broadcast on national news. TORRO's Information Sheet was again an invaluable source of information for eager journalists seeking basic facts. My presence as a TORRO representative helped avoid many of the incorrect statements and conclusions often made about tornadoes.

ANALYSIS AND CONCLUDING REMARKS

There are some interesting points to note with this event. Having personally witnessed the development and dissipation of the tornado, this provided an additional 'dimension' to the subsequent site investigation. That morning tornadic activity had seemed unlikely given the available vertical wind-shear data; nevertheless the day saw at least 14 likely tornadic events for central and south-east England. Although it is generally accepted that tornadoes and tornadic funnel clouds (especially of the weaker variety) are difficult to predict, it would appear that much research in forecasting them is still required.

The 1200 GMT soundings three hours after the event gave only marginal indication as to what had occurred earlier, as the atmosphere had been modified by the earlier thunderstorm (Fig. 7). Tidal conditions within the Solent can restrict ocean mixing with the English Channel and often result, especially during summer months, in the Solent's temperatures being slightly higher than the English Channel. This may have contributed to instability, especially when considering the exceptionally low temperature of -23 OC temp at 500 mb. The freezing level was also very low in altitude, at 808 mb or 1750 m. The Larkhill midday soundings showed relative humidity at 89 % and a 16 OC temperature at the surface. Earlier soundings had indicated some modest CAPE, viz. 789 J/kg. Whereas vertical wind-shear was relatively weak, there was significant wind veer from the surface up to 850 mb, indicating warm air advection at low levels which could be a main factor for the storm's development.

Available soundings tend to give a general synopsis for a large-scale area and cannot always be relied upon for predicting or analyzing small-scale weather events, especially where local convergence or topography may be influencing factors. These conditions may have allowed a parcel of warm moist air to rise rapidly and unhindered through the weak cap, strengthening the updraft as it pulled aloft. The developing storm cell may have converged with opposing winds moving from sea to land creating a swirl or eddy on a 'vertical axis' on the boundary zone between the two air masses (GRAZULIS. T. P. 2001. The Tornado. 4:69). This column could have been pulled upward or stretched into the cloud base as it encountered the strong updraft (Fig 8). This type of tornado formation has been analysed using computer models and is termed a 'landspout' in the U.S.A. but this does not make it any less a tornado or any less dangerous.

Soon after the tornado had dissipated, I had observed an area of scud below the cloud base which resembled a coil of condensation. This could easily have been mistaken for cloud rotation (mesocyclone), but in hindsight was likely to have been a direct result of the tornado's effect on the cloud base itself. Radar showed a fine line of low reflectivity or moisture convergence and a single isolated cell embedded within, which peaked and was visible for only a short duration. This area of convergence developed from west to east and in this instance is unlikely to have been a result of increased wind shear caused directly by the Isle of Wight, but more likely from the conditions surrounding the Solent water itself. The mid-day soundings show vertical wind-vectors as variable with minimal strength, particularly so above 500mb. Here again, we can see how little bearing the middle-to-upper level winds have in connection with the development of weaker, short-lived tornadoes. The opposite would likely be the case for the stronger and better-sustained tornadoes of the supercell type, which are rare within the UK.

This 'landspout' tornado differs from the usual concept of tornado formation associated with mesocyclones in that it initially forms from horizontal wind-shear (not vertical) from the ground upwards and relies much less on the forces associated with the cumulonimbus cloud. Whilst exact conditions necessary for it's formation may be critical it is nevertheless quite possible that a large portion of tornadoes within the U.K. do in fact develop in this manner and likewise would seemingly suggest 'storm radial helicity' an incorrect means of forecasting such an event, as is evident here.

Much reporting and research of a similar nature have been published over recent years in the Journal of Meteorology, some of which are indicated in the following list of references.

Acknowledgements. Many thanks to Patrick Burke (National Severe Storms Laboratory, Oklahoma, USA) for assisting with some of the technical detail in the report. Also to Dorcas Henry (weather presenter at BBC South Today) who initially passed on details of damage and location, and later that day informed me of some of the other tornadic events reported.