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xmetman

Are extreme rainfall events more common in the UK?

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Hi
 
After such a wet month as January 2014, I decided to look at what evidence there was for an increase in incidence of extreme rainfall events across the United Kingdom. The best way to do this as far as I can see is to use the daily records that make up the UKP datasets and which are maintained and made freely available by the Met Office. They split the country up into nine sub-regions and provide daily values back to 1931 for each. On top of this they have 3 sets of composite regional values for Scotland, Northern Ireland and England Wales as well.
 
The report contains graphs with an annual count of days with more than 12.7 mm of rain, and days with more than 25.4 mm of rain from 1931-2014, with the obligatory moving average and trend for all the UKP sub-regions.
 
(1) Central England
(2) E Scotland
(3) SW England & S Wales
(4) NE England
(5) N Scotland
(6) NW England & N Wales
(7) SE England
(8) S Scotland
(9) England & Wales
(10)N Ireland
 
I'll let you decide if global warming is the cause of the increase over England and Wales, and if it is, why has there been a decrease over the north of Scotland since 1931?
 
 
 
 
 
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Good analysis there, it's not very convincing is it.I have a feeling if we could go back another 50-100 years it would be even less so.

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Just natural cycles and the weather doing what the weather does. We've had similar discussions with prolonged dry spells where rivers and streams would never flow again.

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I tend to think the 'global dimming period' flat-lines the plot and that 'recent' changes (post 2,000) flatten out? Maybe run a 1983 to 2013 plot and see what the slope ends up like then?

 

To me the changes start to occur when we see the 'stuck' weather patterns that leave an area with perlonged periods of flooding, that has only recently begun to manifest since the reduction of ice/snow across the north began impacting the circulation patterns.

 

Sadly change , over the coming decade, promises to be far faster than 'normal' statistical period needs. with the total loss of the Arctic ice pack any signals that are now 'debatable will become undeniable over a single season.

 

Luckily we have the folk already teasing out the data from an ice pack only 1/3rd gone so their studies will dovetail instantly into the 'change' once it occurs.

Edited by Gray-Wolf

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The geological record in the flood plains holds the key to your question.

The layers of silt are like tree rings.

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Just natural cycles and the weather doing what the weather does. We've had similar discussions with prolonged dry spells where rivers and streams would never flow again.

 

That's exactly what Julia Slingo said on the radio today with the proviso that these would be enhanced by the warming climate. Basically more WV equals more rainfall but not, obviously, equally divided around the globe. As she put it, basic science.

 

I must admit I find it a bit odd that pointing out equivalent events in the distant past seems to preclude enhancement of events in the present due to global warming. And the impact these days carries far more significance and we appear to be walking into it like headless chickens.

Edited by knocker
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Hi
 
After such a wet month as January 2014, I decided to look at what evidence there was for an increase in incidence of extreme rainfall events across the United Kingdom. The best way to do this as far as I can see is to use the daily records that make up the UKP datasets and which are maintained and made freely available by the Met Office. They split the country up into nine sub-regions and provide daily values back to 1931 for each. On top of this they have 3 sets of composite regional values for Scotland, Northern Ireland and England Wales as well.
 
The report contains graphs with an annual count of days with more than 12.7 mm of rain, and days with more than 25.4 mm of rain from 1931-2014, with the obligatory moving average and trend for all the UKP sub-regions.
 
(1) Central England
(2) E Scotland
(3) SW England & S Wales
(4) NE England
(5) N Scotland
(6) NW England & N Wales
(7) SE England
(Posted Image S Scotland
(9) England & Wales
(10)N Ireland
 
I'll let you decide if global warming is the cause of the increase over England and Wales, and if it is, why has there been a decrease over the north of Scotland since 1931?
 

 

 

How do the stats look when all regions are combined?

 

Given that the regions are so small, you'd expect the trends to show considerable variability in that short timeframe. I look forward to more of your analysis on this

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That's exactly what Julia Slingo said on the radio today with the codicil that these would be enhanced by the warming climate. Basically more WV equals more rainfall but not, obviously equally divided around the globe. As she put it, basic science.

Read the very first sentence of the statement Knocker

Dame Julia Slingo said the very  first sentence variable UK climate meant there was "no definitive answer" to what caused the storms.Then she goes on to give her opinion.Which contradicts the 14yrs standstill in global temperatures which the met office released last year.http://t.co/I18cB7WCqM

Edited by keithlucky

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Read the very first sentence of the statement Knocker

Dame Julia Slingo said the very  first sentence variable UK climate meant there was "no definitive answer" to what caused the storms.Then she goes on to give her opinion.Which contradicts the 14yrs standstill in global temperatures which the met office released last year.

 

Read what I said Keith. It had nothing to do with "caused" but enhancing.

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I downloaded the monthly data for the UK tried my hand at some basic analysis.

 

I calculated the standard deviation (σ) for each month, based on 1910-2012,  and then counted the number of months that ended up more than 1 Ïƒ above average.

 

Here are the results for the each year.....And with the 10 year mean

 

Posted Image Posted Image

 

 

So a very slight upward trend, and a very quiet mid 50s to late 70s period.

 

This hides some quite different seasonal trends though.

 

Spring

Trend............... .............. ............ ........... ........ 10 Year Mean

Posted Image Posted Image

 

 

 

Summer ................ .................. ................... 10 Year Mean

Posted Image Posted Image

 

 

Autumn ............... ................ ................... 10 year Mean

Posted Image Posted Image

 

 

Winter................... .................... ................... 10 year Mean

Posted Image Posted Image

 

I'll have a look at months 1σ below average later, and maybe some 2σ months also.

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This is difficult when some of the rainfall events have been so 'localised' due to stalled weather? The second flood of 2012 was a 'flash flood' here and the clouds just ganged up on Calderdale and 8 miles away saw a 'normal' rainy day? How can we just look at extreme events other than looking at monthly totals? Maybe a look at 50mm+ 'events' ( or some such?) might give a better regional view?

 

Since 07' we have seen 5 " 1 in a hundred" years events in the valley ( and a few more over the years before 07') so from my personal perspective I have seen an increase in the events but would this show up in the 'West Yorkshire' records?

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Regarding rainfall I've always been more interested in intensity and short term distribution than monthly, seasonal or yearly stats. Mainly because I've considered that variations in the former could be 'lost' in the latter. That being the case I was quite interested in pages 22 and 23 of the report.

 

 

The persistence of the recent storminess is unusual, and although clustering of storms is quite common, the continued run of deep depressions, through December, January and on into February, is not. It is this continued run of storms that has created the exceptional flooding conditions experienced in the Somerset Levels, for example.

 

The persistence of the weather patterns affecting both the UK and also the US, where abnormally cold conditions have continued to affect the eastern and southern states through January, has raised questions about whether the jet stream is making greater excursions, north and south, and whether these waves in the jet stream are becoming more locked in one position13. This is a critical question because it raises the possibility that disruption of our usual weather patterns may be how climate change may manifest itself. The Met Office is now actively researching the best way to detect changes in the dynamics of the jet stream.

 

Beyond the clustering of storms there is also the question of whether there is a detectable change in the amount of rain that the storms are carrying. Again this is a very difficult area because UK rainfall is highly variable in space and time. Changes in monthly, seasonal or annual mean amounts are difficult to detect so far, as the time series in Figure 3 demonstrates.

 

However, there is now some emerging evidence that, over the UK, daily heavy rain events may be more frequent (Figure 22). What in the 1960s and 1970s might have been a 1 in 125 day event is now more likely to be a 1 in 85 day event. This supports other evidence that UK rainfall is increasing in intensity14. This increase in the frequency/intensity of extreme daily rainfall events, as the planet warms and the atmosphere can hold more water, has been discussed in the literature for a number of years15, and robust evidence for this is Furthermore, where there are sufficiently long records of hourly rainfall data, it has been shown that rain rates potentially increase with temperature at rates that exceed the simple thermodynamic Clausius-Clapeyron relationship (6-7% increase in humidity for 10C rise in temperature) between temperature and humidity17 (Figure 23). This can be understood through the dynamic amplification of rain-bearing systems, where the induced circulation drives greater convergence of moisture into the system and hence heavier rainfall. The example shown in Figure 23 is from a 99-year record of quality controlled hourly precipitation observations at De Bilt in the Netherlands, and is therefore potentially representative of the UK. A detailed study of UK stations with long hourly rainfall records is urgently needed to corroborate this evidence. 

 

Figures 3, 22 and 23

post-12275-0-75036100-1392138216_thumb.j

post-12275-0-53338800-1392138226_thumb.j

post-12275-0-97324200-1392138235_thumb.j

Edited by knocker

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I downloaded the monthly data for the UK tried my hand at some basic analysis.

 

I calculated the standard deviation (σ) for each month, based on 1910-2012,  and then counted the number of months that ended up more than 1 Ïƒ above average.

 

Here are the results for the each year.....And with the 10 year mean

 

Posted Image Posted Image

 

 

So a very slight upward trend, and a very quiet mid 50s to late 70s period.

 

This hides some quite different seasonal trends though.

 

Spring

Trend............... .............. ............ ........... ........ 10 Year Mean

Posted Image Posted Image

 

 

 

Summer ................ .................. ................... 10 Year Mean

Posted Image Posted Image

 

 

Autumn ............... ................ ................... 10 year Mean

Posted Image Posted Image

 

 

Winter................... .................... ................... 10 year Mean

Posted Image Posted Image

 

I'll have a look at months 1σ below average later, and maybe some 2σ months also.

Interesting that i remember the summers of the 1970s with the exception of 75 & 76 as being cool and wet..but it seems they lacked any extreme rainfall at all..so maybe they were relatively dry and cool?

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Having researched into this topic I don't think the evidence for UK rainfall having got more extreme, to an extent that goes beyond the bounds of natural variability, is strongly compelling yet, but if the pattern of frequent rainfall extremes that we've been seeing since 2007 continues for much longer, then that assessment will change. 

 

There is a lot of scientific evidence out there suggesting that in the long term, on average, rainfall across the globe will become more variable and extreme in response to a mean temperature rise.  That doesn't mean that rainfall will become more extreme everywhere- rather, the most likely scenario some places will become significantly more extreme, others only slightly more extreme, and others no more extreme.  Thus, what we're seeing in Britain now may well be a sign of things to come- but it isn't certain, as it depends on whether or not we end up in one of the areas of the globe that has more extreme rainfall.

 

Given that my research focused on the UK it would be interesting to see if global rainfall patterns have already trended in the way I describe above- I quite often read articles (including some in highly reputed journals) claiming that the number of extreme rainfall events is increasing globally.  However, since the global average temperature has warmed by less than a degree Celsius since pre-industrial levels, I wouldn't expect such a trend to be anywhere near as obvious as in a scenario with 2 to 3 degrees of warming.

Edited by Thundery wintry showers
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A lot has been said about how wet it is. The two areas of my primary interest are the SW and the SE. Here's the charts:

 

post-5986-0-55877300-1392373638_thumb.pnpost-5986-0-03507300-1392373647_thumb.pn

 

I think that this is much more curious than it first appears.

 

If you look at the SW chart the spike at the end, Jan 2014, peaks at mean + ~2.6sd which corresponds very roughly to about a 1 in 100 year event using 1931-2013 as the analysis period. If you look at the SE chart the spike at the end, again Jan 2014, peaks at mean + ~3.4sd which corresponds approximately to a 1 in 3000 year event.

 

It's worth mentioning about why the Environment Agency are responsible for the tragedy that ensued and continues to ensue. The Civil Contigencies Act 2004 specifically list the EA as a first responder, and, as such, they are to maintain plans for the purpose of ensuring that if an emergency occurs or is likely to occur the person or body is able to perform his or its functions so far as necessary or desirable for the purpose of (i) preventing the emergency, (ii) reducing, controlling or mitigating its effects, or (iii)taking other action in connection with it (CCA 2.1.d) They are also legally obliged from time to time assess the risk of an emergency occurring. (CCA 2.1.a)

 

In terms of the act an emergency is (a)an event or situation which threatens serious damage to human welfare in a place in the United Kingdom, (b)an event or situation which threatens serious damage to the environment of a place in the United Kingdom (CCA 1.1.a,1.1.b)

 

Furthermore an event or situation threatens damage to human welfare only if it involves, causes or may cause— (a)loss of human life,(b)human illness or injury,©homelessness,(d)damage to property,(e)disruption of a supply of money, food, water, energy or fuel,(f)disruption of a system of communication,(g)disruption of facilities for transport, or(h)disruption of services relating to health. (CCA 1.2.a-h)

 

 

 

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Similar to the graphs posted earlier, here's the annual graph of the number of months in each year with rainfall totals more than 1 standard deviation (σ) below average.

 

Trend ................. ..................... ........................ 10 Year Mean

Posted Image Posted Image

 

 

So overall, there is a weak trend toward slightly less dry months for the UK, but the 10 year mean has been near or at record low levels since 2006.

And the seasonal trends

 

Winter trend ............... ..................... ................. Winter 10 year mean

Posted ImagePosted Image

 

 

Spring Trend.............. ................ ............. Spring 10 Year Mean

Posted Image Posted Image

 

 

Summer Trend ........... ............... ................ Summer 10 Year Mean

Posted ImagePosted Image

 

Autumn Trend............ ................. ............ Autumn 10 Year Mean

Posted ImagePosted Image

 

So, slights trends towards more dry months in Spring and Summer, and a slight trend toward less dry months in Autumn and Winter.

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Theres been so many wet records broken lately it is unprecedented it really started in 1998 I noticed.

All eyes on this summer,but a dry spring would a kind relief.

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I'm hoping that the Jet Pattern will remain similar to what we are just about to encounter with the Jet well to our north ( swinging South into Europe over Sweden) for long periods.

 

Another 76'/03'/06' would suit me fine! Let's see if we can have a drought called by Sept!!!! Maybe that would highlight the forecast 'extremeing' of our weather?

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The problem with much of this analysis is that it is focused upon extreme events which are rare by their very nature, so are unlikely to be much of an indicator unless they became dramatically more frequent. The scale of precipitation increase should be proportional to the increase in vapour holding capacity which varies with temperature by the Clausius-Clapeyron relationship. A rough rule of thumb is only approximately 7% per °C which means that any precipitation increases so far may easily be disguised within natural variability, be they seasonal, annual or decadal eg. jet stream positions etc. So for example warmer years due to mild winters may be wetter, but drier if due to a hot summer.

Anyway, the graph below compares scaled values annual EWP (divided by 10) and CET temperature(multiplied by 10) since 1931 as 20 year running means to smooth out some of the variability - there is a strong positive correlation of 0.78.

 

post-2779-0-91633200-1396356089_thumb.gi

 

The link appears to be stronger in more recent times and is certainly weaker if longer timescales are used, possibly as might be expected if AGW is prevalent later in the data. Taking end values gives an approximate temperature increase of 0.6°C and rainfall say from 910 to 960mm ~ 5.5% and maximum ranges of temperature and precipitation 0.89°C and 6.8% - thus roughly in line with what might be expected by Clausius-Clapeyron.

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The previous post assumes that the variability remains broadly similar i.e that the synoptic patterns of extreme events don't become more frequent and that increased rainfall is only due to increased temperature. This increase may allow a very small number of events to be classified higher, but it's hard to determine whether a 1000 year event is now 1 in 500 years or even if a 1 in 100 year is now a 1 in 50, and whether they were the direct results of warming.

Bearing that in mind, on reflection the previous post could still be seen as a bit of a cop-out by not examining extremes at all - surely some possible warming affect should be visible?

Using the EWP daily data since 1931 (noting that this covers a wide area rather than the more localised extreme events), there are 25202 days up to the arbitrary date of 1/1/2000 and 5173 since then. Creating simple probability distribution functions of the proportion of days of rainfall from the total days shows the higher rainfall experienced in recent years.

10mm raindays have been 14.76% more frequent since 2000 compared to prior.

12.7mm (half inch) 29.23%

15mm 38.42%

20mm 82.69%

25.4mm (one inch) 192.31% - only 10 days 1931-99 compared to 6 since 2000 they have occurred almost 3 times as often.

 

This seems a bit more dramatic than expected and it turns out that HadUKP data since the start of 2006 has been recalculated so the process was repeated with data from 1931-89 and 1990-2005

10mm increase 11.46%

12.7mm 19.03%

15mm 31.58%

20mm 50.23%

25.4mm 10.63%

Now the increases aren't as great, but follow the same trend apart from the dramatic increase for the highest totals. The difference in the results could be an issue with data homogeneity though we know we have had numerous extreme events since 2006, it could be natural variability particularly for small samples but also more generally - the 1990s coincided with a particularly positive phase of AO for example, or it could be that it has been generally warmer more recently.

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