By Paul Homewood
Back in 2014, the Met Office’s Lizzie Kendon jointly authored a paper, “Heavier summer downpours with climate change revealed by weather forecast resolution model”.
The Met Office reported:
June 2014 – A recent research paper which appeared in Nature Climate Change has looked at how summer rainfall may change in future with a changing climate. Here, the lead author of the paper, Dr Lizzie Kendon from the Met Office, gives some additional insight into some aspects of the research.
This research aimed to understand how hourly rainfall may change in future with global warming. This involved running a very high resolution model, similar to the one we use for forecasting, to look at summer rainfall patterns over a 13-year period both now and at 2100.
Clearly potential conditions around 2100 in the UK are subject to variables such as the amount of human greenhouse gas emissions emitted, so we have to make some assumptions. We chose to use the IPCC RCP8.5 scenario – which is their highest end scenario which would see the most warming.
Why did we choose the highest IPCC (RCP8.5) scenario?
In this study we are trying to understand how hourly rainfall may change in the future with global warming. We chose the highest scenario in order to allow us to identify any signal of change above natural climate variability. The fact that we used the highest scenario, however, doesn’t mean we cannot translate our findings to lower emission scenarios or to time periods earlier this century. What we have confidence in is the direction of the change – namely in an increase in heavy summer downpours in future. That direction of change is not expected to vary between high and low emissions scenarios. Further research in the future could help narrow down how much heavier rainfall is likely to be under different IPCC scenarios and give more detail overall, but – given how computer intensive the study was – this is likely to take some time.
Why does our study only consider the southern UK?
The high resolution climate model used in this study, needed to allow us to examine changes in short-duration intense storms, is very computationally expensive. Even running for just the southern half of the UK, it took the Met Office’s supercomputer – one of the most powerful in the world – nine months to complete the simulations.
The southern UK was chosen for this initial study as convective storms, which are responsible for delivering intense summer downpours, are more common in that area. In addition we are interested in examining urban effects, and London is the largest urban area in the UK – so it made sense to include it in the study area. Similar climate change simulations for the northern half of the UK are currently being set up, and will be examined in a future study.
How robust are these results?
These results are based on one climate model and so we cannot assess modelling uncertainties. Although this model shows almost five times more events exceeding high thresholds indicative of serious flash flooding, we need to do more research before we can be confident of this figure. We have more confidence in the direction of the change – with increases in the intensity of heavy rain consistent with what we expect theoretically as the world warms. We need to wait for other centres to run similarly detailed simulations to see whether their results support these findings.
As Lizzie Kendon makes clear in the video (in the above report), summer downpours will become heavier in the future, according to her models.
She is looking specifically at hourly rainfall. As there is very little such data available, it is impossible to test the model retrospectively.
However, we can look at daily and monthly extremes. It is not unreasonable to assume that heavier downpours, of the sort modelled, would also translate into more extreme daily data.
In any event, as her paper begins:
The intensification of precipitation extremes with climate change1 is of key importance to society as a result of the large impact through flooding. Observations show that heavy rainfall is increasing on daily timescales in many regions.
it would seem sensible to look at daily extremes, which are likely to have a much greater impact on flooding than hourly ones.
Although Kendon specifically looked at southern England, let’s start by looking at the England & Wales series as a whole:
The standout was ex Hurricane Charlie in August 1986, which brought deaths and wide spread flooding to both Ireland and Britain, and must be dismissed as a one off.
Either way it is abundantly clear that daily rainfall is becoming more extreme.
Looking at the whole of England and Wales, of course, can be a bit misleading, as a high total may simply reflect a small amount spread over the whole country.
Equally, an extreme downpour over a small area would not be seen in the overall average.
So let’s also look at the South East, a much smaller area:
Again, we find no evidence of rainfall becoming heavier. There appears to be a period in the 1990s and early 2000s when extreme daily rainfall lessened. But over the full period, there seems to be little pattern.
Finally we can look at daily rainfall at Oxford, which is just about as central you can get in southern England:
Again there is no discernable pattern.
Of course, you would need to look at hundreds of “Oxfords” to get a fair picture of what are essentially local events.
Nevertheless, from the three sets of data shown above, there is no evidence that heavier summer downpours have been occurring since 1931, despite temperatures rising.
That does not necessarily invalidate the modelled projections for 2100, when Lizzie and the rest of us will all be dead. But it does strongly suggest that they cannot be relied on.
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February 23, 2018 at 01:15PM