The ‘Oxfordshire Tornado’ of 7 May 2012

During the afternoon of Monday 7th May 2012, a tornado was reported in several areas of Oxfordshire, including Bicester, Eynsham, Kidlington and South Leigh. Trees, buildings, and vehicles in these areas were damaged by the storm which also produced large hail and lightning (BBC News). A number of eye-witnesses caught the tornado on film including one – Richard Glazer from Whitney, Oxfordshire – who drove through it as it was forming (or reforming) along the A34 near Kidlington (YouTube video). The photo below, one of the few to show a condensation funnel extending all the way to the surface, was taken from Shipton-under-Wychwood (source: Oxford Mail).

The map below shows the approximate track of the storm which spawned the Oxfordshire tornado – black markers show locations where a tornado was observed as stated in the BBC (and other) news coverage.

This storm was a special kind of thunderstorm, known as a supercell. These storms are characterised by the presence of a mesocyclone: a deep, rotating updraught. In addition to the basic ingredients for ordinary thunderstorms (instability, moisture, and an initial lifting mechanism), supercells require the presence of strong vertical wind shear in the lower troposphere in order to form. This shear results in the generation of horizontal vorticity which can then be tilted into the vertical plane by the storm updraught causing the latter to rotate. The 1200 UTC (1300 BST) sounding from Nottingham (340 km north of Bicester) shows a highly favourable wind profile for supercells, with significant turning of the wind in the lowest 3-km of the atmosphere. However, there is no mid-level instability at this location due to a strong temperature inversion at 850 hPa. This feature is associated with warm, moist air overrunning a warm front to the south. Soundings from the UKV model taken behind this front (in the area where the storm formed; not shown) reveal the presence strong instability below 450 hPa (~6 km) with several hundred J/kg of Convective Available Potential Energy (CAPE).

Observations from the UK radar network can be used to verify that the Oxfordshire storm was indeed a supercell. The left panel of the figure below shows a snapshot at 1530 UTC of the radar reflectivity (at 1° elevation) from the Chenies radar (courtesy of Chris Westbrook). This is compared on the right with a schematic picture of a “classic” supercell from the Oklahoma Climatological Survey. The similarity is striking. Perhaps most significant is the presence of the so-called “hook echo” (albeit less well defined than in the schematic) – this feature appears when rain and hail wrap around the mesocyclone at low levels.

More direct evidence of a mesocyclone is provided by images of Doppler velocity from the radars. These are created by measuring the phase shift between successive radar pulses which can be related to the movement of the targets (in this case rail and hail) in the direction parallel to the radar beam. The measured distances are then combined with the pulse repetition frequency (PRF) to determine the radial velocity of the targets. Images of this quantity at 1530 UTC from the Chenies (left) and Deanhill (right) radars at 1° elevation are shown below (again courtesy of Chris Westbrook). Positive values indicate flow towards the radar and negative values indicate flow away from the radar. Strong “velocity couplets” – adjacent areas of inbound and outbound flow – can be seen in both images indicating the presence of rotation. Similar scans from other elevation angles (not shown) confirm the existence of a deep, rotating updraught.

From a meteorological perspective, an obvious question regarding this case is “was it forecast?” Unfortunately, we were unable to obtain information regarding the Met Office forecasts for this day, although it does seem that the UKV model successfully produced some supercell-like storms in about the right location (Nigel Roberts, personal communication). Aside from the Met Office, there are two other main sources of severe weather forecasts for the UK: the European Storm Forecast Experiment (ESTOFEX) and the Tornado and storm Research Organisation (TORRO). ESTOFEX produces daily forecasts of severe convective weather (large hail, excessive rainfall, damaging straight line winds, and tornadoes) for the whole of Europe. On 7th May 2012, the forecasters issued a Level 1 (5–15 % probability of a severe convective storm within 40 km of a point) for much of Wales and the east-central Midlands (left panel below), i.e. slightly west of where the tornadic storm developed. In the associated discussion they noted the potential for “multicells and low-topped supercells with a chance of small hail, marginally severe wind gusts and maybe an isolated tornado”.  TORRO, which issues “watches” for severe weather in the UK and Ireland, also highlighted the potential for tornadoes – their risk area (shown in the right panel below) agrees very well with the observed track of the Oxfordshire supercell. However, it is not currently known at what time this forecast was issued or the details of the accompanying discussion. We will update with this information if/when it becomes available to us.

Tornadoes are not considered particularly rare in the UK – indeed, it is frequently said that the UK gets more tornadoes per square kilometre than the USA. However, the majority of these are associated with the passage of strong cold fronts during the cool-season (Bolton et al. 2003). In contrast, the 7 May 2012 tornado formed from a long-lived “classic” supercell, a rarity in this country due to the infrequent superposition of strong instability and vertical wind shear*. This event has therefore drawn significant interest in the severe weather community and will no doubt be the subject of future research articles. In our department, Chris Westbrook (in collaboration with Matthew Clark at the Met Office) is writing a paper for Weather focussing on the radar observations of the storm, while Humphrey Lean is carrying out very high resolution simulations of the event using the Met Office Unified Model. Expect to hear a lot more about this event in the future! * On the day this Blog contribution was being prepared (Thursday, 28 June 2012) a series of severe storms developed in the English midlands, producing torrential rain, very large hail, and at least one tornado. Based on examination of radar images we believe that several of these storms were supercells. This will be another exciting case to look into in detail.

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