Met Forecast Of Summer Downpours Ignores Evidence

  • Date: 03/06/14
  • Paul Homewood, Not A Lot Of People Know That

There is no evidence of any trend to heavy rainfall days becoming either more common or severe.

The BBC report on the latest study by the Met Office & Newcastle University:

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Global warming will lead to a significant increase in extreme summer downpours in the UK, a study suggests.

The Met Office and Newcastle University researchers say there could be five times the number of “extreme rainfall events” exceeding 28mm per hour, under extreme warming projections.

This would cause “really severe” flash flooding in many parts of the UK, according to the scientists.

However, they caution that this result is based on only one computer model.

Flash flooding in Britain has had devastating impacts on communities in recent years.

In Boscastle, north Cornwall, about 200mm (8in) of rain fell in four hours in August 2004 causing a 3m wall of water to sweep through the village.

In the summer of 2012, in Newcastle, the equivalent of a month’s rain fell in just two hours, causing widespread flooding in the city. [...]

As stated, the study is based purely on one model, so how does it compare with recent trends and observations?

There is virtually no hourly rainfall data available, to use for long term comparisons. However, daily data is available, which should give us a good idea whether summer downpours are getting worse. It is, after all, reasonable to assume that, if “extreme rainfall events exceeding 28mm/hour” are becoming more common, then we should also expect to see a like increase in “extreme rainfall days”.

We can test this by having a look at a few sites, starting with Oxford, pretty representative of the area the study is looking at. Figure 1 plots the days with 30mm and over for summer months, available from Met Office data since 1930.

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Figure 1

There is no evidence of any trend to heavy rainfall days becoming either more common or severe. The record for the wettest day was set in 1968, and extreme rainfall days were most common between 1950 and 1980, as Figure 2 illustrates.

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Figure 2

We can repeat the exercise for Shawbury, in the West Midlands, and Heathrow.

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Figure 3

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Figure 4

Both show the same pattern, with the most extreme rainfall occurring between 1950 and 1980, suggesting that there is a pattern here. This is significant, because for most of that period, with the exception of the heatwaves of 1975/6, these decades experienced largely colder than normal summers.

There is therefore an implication here that extreme rainfall in summer is associated with cold, and not hot, conditions.

There is one more test we can do. Figure 5 shows the average rainfall/rainday, for the summer months in England South.

The Met Office only started keeping rainday data in 1961, so we have no figures prior to then. Nevertheless, the 1960’s and early 70’s show up as giving the wettest days. The purple trend line gives no evidence of any increase.

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Figure 5

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