Storms and plumes

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  • The set up on Thursday 28 June, with warm, moist energetic air becoming destabilized by a trough and a cold front approaching from the west. Severe thunderstorms ensued.

    Altocumulus castellanus clouds, indicators of unstable conditions in the middle layers of the troposphere.

    The tornado that touched down near Sleaford, Leicestershire, on Thursday 28 June. Photo courtesy of @suepowell31.

    Two of the large hailstones that fell on Leicestershire. Photo courtesy of @AlexMilesUK.

    Flooding in Whitley Bay, North Tyneside. Photo: Owen Humphreys/PA Wire.

  • Storms and plumes
    29.06.2012 07:00

     

    Take a line from approximately the Bristol Channel to The Wash.

    South of that on Thursday there were a few showers here and there but for many people lots of warm sunshine, and strawberries were consumed serenely at
    Wimbledon in temperatures approaching 28 degrees Celsius.

    To the north of this line, though, was some of the most severe weather imaginable in this country, especially across southeast Wales, the Midlands and the east and northeast of England. There was torrential rain, flooding, landslips, frequent lightning and lengthy thunderstorms, huge hailstones, some maybe 7cm in diameter, and reports of damage from squalls and tornadoes.

    So large were some of the hailstones that cars were dented and greenhouses smashed. In fact, although it is hard to gauge because of sketchy records, these were likely to have been some of the largest hailstones experienced in these isles. Unconfirmed reports put one at 9cm across, a phenomenal size and one more usually associated with storms over the Plains in the USA.

    Radar images on Thursday afternoon showed in the East Midlands the sort of ‘hook echo’ which is a typical signature of a supercell storm – the sorts of storms that can indeed produce tornadoes.

    Later in the day a ‘rope’ of intense storms also snaked across north Wales, northern England and parts of the Borders, while most of the rest of Scotland was wet and locally thundery.

    Why did the weather suddenly turn so violent?

    A lot of energy is clearly required, and this was provided by a phenomenon called a Spanish Plume. This occurs when hot air rises from the Spanish Plateau just ahead of a cold front and upper trough approaching Europe from the west. Ahead of that there is very warm, moist air moving towards west and northwest from the Mediterranean.

    This is forced to rise over the high ground of France and the Spanish Plateau and by convection caused by the heat of the Spanish interior. Note that temperatures have recently been in excess of the mid 40s Celsius in central and southern Spain.

    The atmosphere becomes unstable in its mid-levels, and the cold front adds to the destabilization as cold air overruns at higher levels in the atmosphere. Sometimes we can see castellated clouds at medium levels as a result – a type called altocumulus castellanus.

    Deep convection in the plume causes thunderstorms to develop along a rapidly narrowing band, and this takes on the nature of a cold front, while the original front atrophies, withered by subsidence immediately behind the plume.

    It’s complex, though, and thunderstorms don’t necessarily crack off immediately. Instigated by the heat from the Spanish Plateau, there is an inversion - an atmospheric layer where temperature steadies or rises for a while rather than falls with height, and usually in this case where there is an intrusion of drier air.

    This can put a cap on the growth of cumulonimbus clouds – that is, thunder clouds - until convection becomes so strong that it bursts through this layer and explosive development of thunderstorms can occur.

    If there is also strong wind shear – in other words a notable change in wind direction or speed (or both) with height - then supercells can develop, maintaining a strong rotating updraft. And this, like Thursday, is when we need to be wary of large hailstones and tornadoes.

    By: Stephen Davenport