By: Suzanne Gray
The UK and the rest of western Europe experienced a heatwave in the middle of August 2020 with temperatures exceeding 30oC in Reading. Fortunately for us this was broken by a heavy downpour on the afternoon of Wednesday 12th August (Figure 1(a)) yielding over 4 mm of rainfall at the Reading University Atmospheric Observatory over a period of only about 20 minutes. While watching the rain cascading down the road towards my front door (we live at the bottom of a small, but very steep, hill) I naturally searched up the latest radar images and analysis charts to see the system causing the rainfall and to try to predict how long it might last.
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Figure 1: (a) Temperature observations from the Reading University Atmospheric Observatory for the 11th and 12th August 2020 and (b) cutout from a Met Office analysis with mean sea level pressure contours and marked fronts from 12 UTC on 12th August 2020 (copyright Met Office).
The Met Office had issued a yellow warning for thunderstorms and associated flooding for most of the UK but, as is usual with thunderstorm warnings, was not able to tell us exactly where and when the thunderstorms would occur. According to the Met Office’s synoptic analysis (Figure 1(b)), the mean sea level pressure gradient over the UK was very weak and this was associated with weak easterly winds (~2-3 ms-1 at our Observatory). A small-scale weak low-pressure system to the west of the UK was associated with an upper-level trough directly above, with an extension of the trough axis towards northern Spain. This synoptic situation transported the warm plume of air up from Africa, across Spain and into southern England giving rise to the heatwave. This is not an unusual occurrence and even has a name: “The Spanish plume”. More precisely, this situation compares well with the modified Spanish plume synoptic situation described in Lewis and Gray (2010).
Spanish plumes often lead to large thunderstorms over the UK, either initiated locally or imported from France. When thunderstorms are organised into a large single cloud system we call it a mesoscale convective system. The challenge is knowing when and where these storms will initiate because that initiation often depends on small-scale variations in the environment (such as due to local hills) that are not well captured by the numerical models used to generate weather forecasts. The storm that broke the heatwave in Reading was a mesoscale convective system that initiated over west London and tracked westwards as clearly shown by satellite imagery (Figure 2) and associated radar (Figure 3).
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Figure 2: Sequence of Meteosat Infrared Satellite imagery for (a) 14, (b) 15 and (c) 16 UTC on 12th August 2020 (copyright EUMETSAT).
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Figure 3: Radar imagery for 12th August 2020 at (a) 1425 UTC and (b) 1615 UTC captured from www.netweather.tv/live-weather/radar. Note that the times in the panels are in BST, so one hour ahead of UTC.
So, that got me wondering whether the existence of London as a major urban area could have had a role in initiating this event. Large urban areas are known to affect their local environment in many ways including generating locally enhanced temperatures known as urban heat islands. A bit of research led me published papers examining the relationship of urban heat islands to thunderstorms. For example, a review by Han et al. in 2013 found that updraughts produced by heat islands initiate clouds, and rainfall can be enhanced by high aerosol levels due to pollution; the enhanced surface roughness associated with cities doesn’t play a major role in thunderstorm initiation, though it may affect systems passing over them. Of course, more analysis would be required to tell whether London’s urban characteristics were important in initiating this storm, and so potentially provided some predictability, in this case or whether the storm initiated over London for some other reason. Whatever the cause though, I appreciated the consequent temperature crash and excuse to do some meteorological-based web surfing.