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Extreme Daily Rainfall At Oxford

March 27, 2014

By Paul Homewood

 

 

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http://www.metoffice.gov.uk/media/pdf/4/8/Drivers_and_impacts_of_seasonal_weather_in_the_UK.pdf

 

In their recent report, “Drivers & Impacts of Seasonal Weather in the UK”, the Met Office show the above graph, and comment:

 

There are suggestions, however, that the character of daily UK precipitation has changed over the last 50 years. There is evidence that heavy rainfall events may have become more frequent over time: What in the 1960s and 1970s might have been a 1 in 125 day event is now more likely a 1 in 85 day event (see figure 4.1)

 

This is a claim that they have made frequently before, and I have been critical of it, as the 1960’s and 70’s are well recognised as unusually dry decades. For instance, Professor Stuart Lane, of Durham University states:

 

The period since the early 1960s and until the late 1990s appears to be relatively flood free, especially when compared with some periods in the late 19th century and early 20th Century. “

 

Consequently the evidence for “an increase in heavy rainfall events” may be no more than part of a natural cycle.

We can test this by looking over a longer period. The Met Office have daily rainfall data for Oxford going back to 1930, and from that we can replicate the Met Office’s graph.

Their figures are based on roughly 1 in 100 day events. At Oxford, over the 83 years, there have been 283 days with 19mm or more of rainfall, an average of 3.4 per year, so we are roughly at the 1 in 100 figure.

The chart below shows the number of such days each year, along with the 5-Year running average.

 

 

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  • The dry period in the 1960’s, and again in the 1970’s is evident, but the longer view shows that there is nothing unusual about recent years.
  • There is certainly no evidence that the trend is increasing.
  • The two years with most extreme days were 1971 and 1992.
  • Even during the very wet year of 2012, there were only 5 days of extreme rainfall, a number equalled or exceeded on 21 other occasions.

 

Professor Lane points out that the UK is entering a flood rich period that we haven’t seen for a number of decades, adding:

 

“We entered a generally flood-poor period in the 1960s, earlier in some parts of the country, later in others. This does not mean there was no flooding, just that there was much less than before the 1960s and what we are seeing now. This has lowered our own awareness of flood risk in the UK. This has made it easier to go on building on floodplains. It has also helped us to believe that we can manage flooding without too much cost, simply because there was not that much flooding to manage.

We have also not been good at recognising just how flood-prone we can be. More than three-quarters of our flood records start in the flood-poor period that begins in the 1960s. This matters because we set our flood protection in terms of return periods – the average number of years between floods of a given size. We have probably under-estimated the frequency of flooding, which is now happening, as it did before the 1960s, much more often that we are used to. “

 

Surely the Met Office know all of this? They certainly have all the data they need to extend their graph back to 1930, and earlier. Indeed, I suspect if they went back to 1910, they would find extreme events being even more common than now.

In which case, why do they continue to publish misleading information?

 

 

References

Daily rainfall data is not available online at the Met Office, (one of my bugbears, as this sort of data is easily accessible at NOAA). The information was emailed to me by them at my request.

If anyone needs it, I can easily forward.

I might also point out that, in the last few months, the Met Office have tightened up their policy regarding giving out such information. Under a “fair usage” policy, they are only prepared to send out 5 years worth of this sort of data. Hardly what you would expect from a publically funded, supposedly transparent organisation! One wonders what they have got to hide.

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6 Comments
  1. March 27, 2014 12:50 pm

    I have written to the MO asking why they excluded data prior to 1960 from the graph.

  2. March 27, 2014 1:02 pm

    There appears to be a cyclical element in your graph; with peaks roughly every 10 years.

  3. filbertcobb permalink
    March 27, 2014 1:59 pm

    I think Professor Lane has moved on – http://mesoscaphe.unil.ch/stuart.lane/en/presentation/

  4. Paul permalink
    March 27, 2014 7:37 pm

    I too noticed the apparent periodicity in this data set. If I weren’t so lazy, and was as well motivated as our admirable host, I would investigate this further.
    There is a longer, 200 year, data set for the lake district here:http://www.tandfonline.com/doi/abs/10.1623/hysj.49.5.769.55131
    Which indicates a fairly benign long term sequence but with reference to the NAIO as a strong forcing agent. Brian Fagin, The little ice age, has a similar view.

  5. March 28, 2014 6:32 am

    I guess they count on you (us) not knowing or checking.

  6. May 8, 2014 5:12 pm

    Extreme Daily Rainfall At Oxford

    Not so at Cambridge just over 60 miles away
    http://www.vukcevic.talktalk.net/Oxbridge.htm

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