Precipitation



Is the Brown Willy effect on the increase?

Figure 1

Yesterday’s showers up through the spine of Cornwall and Devon were another classic example of showers caused by peninsular convergence. It also goes under the less prosaic name of the Brown Willy effect, it’s amazing what you can find courtesy of Wikipedia. The first thing you will notice that yesterdays string of showers seem to originate a little south of Brown Willy, if my estimates from the weather radar are correct (fig 1), and are aligned around 240-060°. Rainfall accumulations are not particularly high along its length for the period 08-21 UTC, generally between 8 to 16 mm with a few isolated light blue pixels indicating accumulations greater than 16 mm in Somerset. The first band of showers seems to fizzle out, as a second band forms a little further to the north for a short distance, before the southerly band re-intensifies somewhere over the Blackdown hills. The band of showers stretched as far as East Anglia before losing its coherence. We never got anything more than a few spots out of it here in Bradninch during the day, 14 km to the northeast of Exeter, and the cut off between wet and dry was quite sharp.

The recent flooding in Okehampton on the afternoon of the 30th of July was another good example of peninsula convergence, which the Met Office NWP model just didn’t seem to get a grip on. Has the occurrence of peninsular convergence increased in recent years, or is that just down to extra vigilance on my part?

Figure 2

For once there was an upper air ascent in the right place, and for the right time (fig 3). The wind direction aloft in the 11 UTC Camborne ascent are remarkably constant, between 240 and 260°, well up to 15,000 feet.

Figure 3

70 out of 72 wet days so far this summer in Norway

Spare a thought for the Norwegians…

Figure 1

If you think that it’s been wet here in the UK so far this August, then spare a thought for the Norwegians, because it’s been much wetter in the southwest of Norway than it has here. Take a look at the inset hyetograph for Bergen (fig 1), and you’ll see that in the first two weeks of August they have already had 190 mm of rain. The wettest place in the UK in comparison is Capel Curig in Snowdonia with a total of 127.6 mm. I didn’t realise this until now but Bergen is apparently the wettest city in the world.

 

Has climate change shifted the timing of floods in the UK?

First off I will admit I haven’t fully read the study “Changing climate shifts timing of European floods” by Günter Blöschl and numerous other contributors from across Europe. The study has been picked up on by the BBC in this news article (fig 1), because they obviously see that it’s just another byproduct of AGW, rather than just climate change, and yes I do think there is subtle distinction. As far as I can see, none of the many contributors are employed by the UKMO, which I find unusual.

Figure 1 – Courtesy of the BBC

I have highlighted in yellow the areas in the news article that I have misgivings about, the first one is from Matt McGrath

The scientists believe this is due to changes in the North Atlantic Oscillation (NAO), the weather phenomenon that pushes storms across the ocean into Europe.

The North Atlantic Oscillation is simply an index of the pressure difference between the Azores and Iceland, as far as I know this index doesn’t push anything across any ocean, you might say that the Azores high or Icelandic Low are weather phenomena, but surely the difference between the pressure at two points is just a number?

Günter is the quoted as saying:

In southern England, it has been raining more, longer and more intensely than in the past. This has created a rising groundwater table and higher soil moisture than usual and combined with intense rainfall this produces earlier river floods

I refute that, unless the changes that the study is talking about have suddenly started to occur in the last five years. Cue some evocative pictures of rivers in southern England that have dried up in recent years that I’ve found on the Internet (figs 2 & 3). (N.B. to the BBC, two can play at that game!). Yes, I know these images are from 2012, but what about the River Derwent in the Lake District in May 2017 (fig 4). So it’s a well-known fact that river levels and groundwater tables do fluctuate, and can do so very quickly, that’s what they naturally do.

Figure 2 – The River Lavant, West Sussex in February 2012 courtesy of Press Associates and the Daily Mail.

Figure 3 – The River Pang, Berkshire in February 2012 courtesy of Press Associates and the Daily Mail.

Figure 4 – The River Derwent, Cumbia in May 2017 courtesy of Paul Kingston and Twitter

I can’t get any daily rainfall climate data for anywhere in the UK without paying loads of money to the Met Office, so I am stuck with the free 1910 monthly rainfall gridded data that they produce, I wonder if they used this kind of data or if the Met Office felt pity on their research and gave them the ‘real’ rainfall station data that they guard so jealously on our behalf? From that data here are some graphs with a simple linear trend for the southeast and central southern England region for winter, spring and summer (figs 5-7). I can’t see any discernible upward trend in rainfall in any of those three seasons, although I will admit that the 10 year moving average for summer is on the rise, and won’t be any lower after this wet summer.

Figure 5

Figure 6

Figure 7

Günter goes on further to say:

Half the stations recorded floods at least 15 days earlier than previously. A quarter of the stations saw flooding more than 36 days earlier than in 1960.

This one is a hard one to counter, especially without daily rainfall climate data and the dates of all fluvial flooding incidents since 1960, which I simply don’t have. But I don’t see that there is some kind of flood or monsoon season across the UK that starts at any precise date that you can readily identify, and if you can’t identify it, then how can you then go on to say that it’s starting 15 days earlier than it did in 1960? I do know from my interest in CET, that the spring is probably around 15 days earlier and it was in 1960, and that temperature is linked to increased convection and heavier rainfall, but I see little sign of it from the rainfall climate data that I can access.

Finally, here’s the last 12 months rainfall over southeast England (fig 8). It’s been a funny last 12 months as far as rainfall goes, up until mid May there was talk of an impending drought later in the year across southern and eastern areas, but the wet summer has put paid to that. There have definitely been some wet days in the last 12 months across the southeast of England, but they can occur in any month as far as I can see, and the accumulated rainfall is still only 84.7% of the annual average at the end of July 2017. What I really need is now is data from the environment agency, some kind of daily count on the number of alerts that they issue for rivers across the UK, a bit like the NAO, but not a phenomena, just a daily count.

Figure 8

PS I’ve just download the report to read – for a change it’s free to do so – and better still it’s only four pages long.

The cold wet August of 1912 and the Novarupta eruption

Figure 1 – Courtesy of Wetterzentrale

The coldest August since 1910 was that of 1912. It was both wet, cold and dull. At many coastal stations around the UK the sea temperature was higher than the air temperature. In the monthly weather report for 1912 the review of the month concludes by saying

Observers in various parts of the United Kingdom noticed in the rare intervals of fine weather the sky seldom assumed its ordinary blue tint, but appeared to be covered with a hazy film “producing grey whiteness of the unclouded sky, and extreme weakness of all sunshine”. A similar appearance was noted by several continental observers.

The mean temperature for the UK was just 11.7°C which is 3.26°C below the 1981-2010 long-term average. The mean maximum anomaly for the month was even lower at 3.94°C below average. Looking at the regional temperature anomalies (fig 2), the cold was across the board.

Figure 2 – August 1912

As well as being very cold, it was also very wet, especially in more southern regions, and particularly in East Anglia which saw over three times the monthly average rainfall (fig 3).

Figure 3 – August 1912

In fact August 1912 was and still is the wettest August in the entire EWP rainfall series that started in 1766 (fig 4).

Figure 4

As you probably noticed in the daily charts for the month (fig 1), August 1912 was a very cyclonic month as you can see in the Lamb Circulation types for the month (fig 5).

Figure 5

As regards the CET for the month, I can’t remember ever seeing a summer month as cold as this one (fig 6). August 1912 was, and still is the coldest August on record back to 1659, beating even the cold year of 1695 into second place.

Figure 6

Not only was it the coldest August on record, it was also ushered in the start of a three-month cold spell in central England, with a cold September (mean anomaly -2.5°C) and October (mean anomaly -2.4°C) to follow (fig 7).

Figure 7

At this point I would like to produce some statistics to show that August 1912 was also the dullest on record, but I can’t, the Met Office maintain that they only began measuring sunshine from 1929. The MWR comes to the rescue though, because it says about sunshine:

Sunshine was very deficient, a large number of stations situated in nearly all parts of the kingdom recording considerably less than half the average amount. In the Channel Isles and at a few places in the extreme southeast of England the mean daily duration ranged between 4 and 4½ hours, and was equal to about 30 percent of the possible. Over Central and Southern Scotland and at a few places in the northeast of England the daily duration was less than 2 hours; at Crathes, Glasgow and Eskdalemuir it amounted to only 1o percent of the possible.

What caused it?

Here’ a graph of 12 month rolling CET values for around that time, forget the date in the subtitle, another bug for the programmer to fix. I’ve overlaid the volcanic dust index events that were greater, or equal to 4, on the VEI on top of the line series, and as you can see the Novarupta event (VEI 6) looks like it may well have been responsible for global cooling that also affected our own CET series back in 1912 across, and fits well with the reports of a greyish white haze from the Monthly Weather Report for August 1912.

Figure 8

Novarupta

Figure 9 – Novarupta’s lava dome in July 1987

I had never heard of the Novarupta eruption until I started researching this article today. I never even realised it was the most powerful volcanic eruption of the 20th century, here’s what the Wikipedia article had to say about it:

The eruption of Novarupta in the Aleutian Range began on June 6, 1912, and culminated in a series of violent eruptions. Rated a 6 on the Volcanic Explosivity Index, the 60-hour-long eruption expelled 13 to 15 cubic kilometers (3.1 to 3.6 cu mi) of ash, 30 times as much as the 1980 eruption of Mount St. Helens. The erupted magma of Rhyolite, Dacite, and Andesite resulted in more than 17 cubic kilometers (4.1 cu mi) of air fall tuff and approximately 11 cubic kilometers (2.6 cu mi) of pyroclastic ash-flow tuff. During the 20th century, only the 1991 eruption of Mt. Pinatubo in the Philippines were of a similar magnitude; Pinatubo ejected 11 cubic kilometers (2.6 cu mi) of tephra. At least two larger eruptions occurred in the 19th century: the 1815 eruption of Tambora (150 km3 (36.0 cu mi) of tephra), and the 1883 eruption of Indonesia’s Krakatoa (20 km3 (4.8 cu mi) of tephra).

Figure 10 – Novarupta map: Approximate location of the June 6th, 1912 eruption. Ash fell on the town of Kodiak for three days, and although the town was about 100 miles from the volcano, it was covered with over one foot of ash which collapsed many buildings. Courtesy of Geology.com and MapResources.

There’s a interesting article on the Geology.com website about the eruption that you might find useful.

The latitude of the Novarupta eruption was just about perfect at 58° north for maximum effect across the northern hemisphere, as was the timing of the 6th of June to coincide with the cold months of August, September and October of 1912 in the CET series. I should imagine that the ash from the eruption would have taken at least a month or so to completely encircle the northern hemisphere and reduce the amount of sunlight. I am of course completely guessing that this was the cause of the cold August of 1912, I’ll have to spend some more time looking at NCEP reanalysis surface temperature data for 1912 to see just what affect it had on other countries across the northern hemisphere to completely be sure of my assertion.

Finally here are the daily CET values for the Summer of 1912 (fig 11), which I think says it all. This is one of the better articles that I’ve put together for my blog, I found little evidence of any link between the cold August of 1912 and Novarupta in any of my climate and weather books, and even though Philip Eden does mention the poor summer of 1912 in his book ‘Great British Weather Disasters’, he doesn’t make the link with the volcanic ash of Novarupta. August 1912 does get a short mention as the worst on record in the book ‘The Wrong Kind Of Snow’, but again no mention of why. So if you thought that August 2017 has been cool in its first ten 10 days, the mean temperature for the first 10 days of August 1912 was 2.5°C colder still.

Figure 11

August 9 – rainfall

Figure 1

If they had only fitted a rainfall gauge to the AWS at Broadness on the Thames at Gravesend, they probably would have measured over 50 mm of rainfall in the 24 hours ending 06 UTC this morning (fig 2). I still can’t understand why this heavy rainfall event didn’t cause any pluvial or fluvial flooding issues yesterday in this part of the world.

Figure 2

BBC News: Yorkshire and Lincolnshire flooding caused by heavy rain

Courtesy of the BBC

A news item from the BBC concerning flooding caused by heavy rain in Yorkshire and Lincolnshire – http://www.bbc.co.uk/news/uk-england-40875248  The BBC were a bit late on this one as most of the heavy rain occurred on Tuesday in that area.

Wet day in southeast

Figure 1

I estimate rainfall totals of between 50-75 mm since 06 UTC this morning from the weather radar imagery, in a swath that extends northeast from Sussex, across Essex to Suffolk. Quite something – there must be some local flooding going on in that area I should imagine.

August 8 – not a great day to be on holiday at Cleethorpes

Figure 1

A wet day on Humberside and certainly not a good day to be on holiday at Cleethorpes. My 06-06 total was a little low for Leconfield 47.3 mm estimated, 56.8 mm actual, but a single pixel at this coarse resolution could have been the reason for that (fig 1). There was a large arc of >50mm rainfall totals across South Yorkshire, Humberside and North Lincolnshire, which I’m sure is causing massive problems for farmers and holidaymakers alike at the moment. The yellow warning for heavy rain from showers over Wales and the Southwest was a wee bit over the top, apart from around Milford Haven where 34 mm of rain fell, and Bodmin, a good part of Devon remained mostly dry. Why we stayed dry in mid-Devon when there was so much vigorous cumulus congestus development yesterday morning I can’t fathom.

Figure 2

Plenty of heavy rain around

Figure 1

There’s plenty of heavy rain around today, either in the form of showers or longer spells of rain. In the latest 6 hourly totals, Wittering in Cambridgeshire, is the wettest place in the British Isles with 28 mm (fig 1 & 2).

Figure 2

Estimated accumulations from weather radar indicate totals in excess of 24 mm in a swathe across central and eastern counties, and in excess of 32 mm across parts of Pembrokeshire (fig 3). The Met Office, who already had warnings out for heavy rain across eastern England, have issued another one for heavy showers for Wales and the southwest of England, curiously they’ve not included the northwest of Scotland in this one.  Is it me, or are yellow warnings for heavy rain being issued much more frequently this summer?

Figure 3 – Weather radar estimates