Working with some recent UK rainfall records for our client gave us a chance to look at some change patterns across the UK. Here’s what we (briefly!) found.
We were commissioned to assess our client’s exposure to extreme rainfall events over the UK, which gave us a chance to play around with some climate data records for the country — namely ‘HadUK-GRID’ the flagship UK Government suite of climate observations spanning 1860 to the present day.
In the course of the commissioned work, we had some time to briefly evaluate changes in the country’s rainfall patterns – nothing near a publishable piece of work – but with some interesting features none-the-less.
How has the UK’s rainfall changed between the mid-20th century and the modern day?
Early climate predictions, in the late 1990s and early 2000s, simulating continued greenhouse modifications to the Earth’s atmosphere, projected wetter winters and drier summers for the UK. Looking at the data, these changes are evidently well underway.
Shown below are changes in UK rainfall between two refences periods — 1950-1969 and 2003-2022. Composing a 20-year average for each period goes some (but not all the) way to reducing the noise component i.e. minimising short-term natural variability – to leave any lower-frequency signal within the data.
To even the untrained eye, the deep blue patterns in winter months stand out: sizable increases in monthly rainfall totals – most obviously in western (and northern) UK.
Changes in monthly rainfall totals for winter months between mid 20th Century [1950-1969] and the modern era [2003-2022]. Units are mm/month. White regions in Scotland exceed +70mm.
This response fits well with a key property of the Earth’s climate system – the Clausius-Clapeyron equation: stating that a warmer atmosphere holds more water vapor – and that existing wet seasons and regions (to which winter in the UK is, due to storm genesis and position — another topic!) will get wetter.
Summer changes.
Eyeballing other months reveals some features of note – principally the general reduction in summer rainfall – with a particular peak in late summer – August into September. Again, these responses are perfectly in line with historical climate model predictions of how changes would unfold under 1.5°C global warming conditions – and beyond.
Observed reductions in summer rainfall might seem peculiar given the occurrence of summer flooding events – and the observed rise in these events (e.g. https://thefloodhub.co.uk/news/increase-in-floods-during-summer-months-analysis-reveals/). However, what is important in these situations is the timescale in which the rain volume – whatever it is – is delivered – e.g. short-sharp rainfall events surpassing the ground’s capacity to soak up surface water and lead to high-impact flash flooding events.
Looking at monthly-mean rainfall tells us nothing of changes in the daily – or even hourly – rainfall character, but projections suggest the number of UK days with ‘high-impact’ flooding days will rise by up to 8, per year if the projected global warming levels of 4C are realised.
Compounding the impact of these changes are projections of increasing long-term drought severity – driven of course as much by the absence of rainfall, as changes in extreme UK temperatures – notably over England and the south of the UK (e.g. https://link.springer.com/article/10.1007/s10584-021-03100-5 and https://www.metoffice.gov.uk/research/climate/understanding-climate/uk-and-global-extreme-events-heavy-rainfall-and-floods).
Why September stands out so starkly I’m not sure: changes in circulation patterns perhaps? More ‘Indian Summer’ type occurrences? Speculation of course.
In any case, we thought we’d share these plots — sometimes these quick and ready comparisons are useful for the public to see – in addition of course to the more in-depth, published analyses, some of which we’ve referenced above. You can see the pattern of changes for the remaining months below:
– Craig Wallace, 12/12/2024
Raw data used: v20240514 [v1.30.ceda] HadUK-GRID 1km monthly. NetCDF accessible here:
https://data.ceda.ac.uk/badc/ukmo-hadobs/data/insitu/MOHC/HadOBS/HadUK-Grid/v1.3.0.ceda/1km/rainfall/mon/v20240514
[Hollis et al., 2019 https://rmets.onlinelibrary.wiley.com/doi/10.1002/gdj3.78 ]
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