Dr. Norman Cheung was called on by the Sky News Sunrise show on 29th June 2012 to give commentary on hailstorms and flash floods affecting Northern Ireland, North England and Scotland in June 2012 and their disaster management.
With the impact of extreme weather in the UK recently, it has raised questions as to why we have experienced the number of hailstorms and flash floods to such an extent in June. It also raises the question whether we will have more intense rainstorms similar to those in summer of 2007 and 2009. What are the implications of these environmental hazards to our disaster management?
The air currents of high and low pressures flowing in wave-like patterns around the mid-latitudes are called Rossby waves. The UK is situated on these paths and therefore is under constant influence of the mid-latitude cyclones which normally occur along the frontal zones.
Warming temperatures of the Atlantic Ocean and the Rossby Waves
However, there are two environmental factors explaining the extremity of these storms in 2012. First is the relatively high sea-surface temperature in the Atlantic Ocean at the west of the UK. The Rossby waves weredisplaced at a more southerly location than average. When a family of these frontal storms along the Rossby waves turned north-eastward, they hit directly the north-western side of the UK, i.e. southwest England, Wales, north England and Scotland. Secondly, the persistence of the upper trough, i.e. the low pressure at the higher atmosphere, favoured the development of a strong convection within cumulonimbus clouds. The repeated updraft elements merging with the rotating updraft elements within these supercell storms caused the occurrence of tornadoes and hail.
A major hazard associated with hailstorms is flash floods, of which the characteristics are fast occurring, violent, small in scale and frequently associated with other hazards, e.g. mudflow which leaves little warning time. Hail the size of a base ball is not uncommon. Increased urbanization and a poor drainage network in the UK have already exacerbated the impact of flash floods. Furthermore, the current drought condition may increase the risk of flash floods, since dry and compacted soil absorbs water less easily.
Can we predict the weather outcome for the rest of the summer?
Whether we will have more intense rainstorms like those in summer 2007 and 2009 is still difficult to say. However, according to records, April – June 2012 were the wettest months in the UK for the last 100 years. Climatologically, if the atmospheric and oceanic conditions continue in the coming months, i.e. with the persistent southward location of the jetstream at the west of the UK and relatively higher North Atlantic sea surface temperature, we may have more storms to come this summer.