Lamb Weather Types

Professor H.H. Lamb subjectively classified each day’s weather over the British Isles from 1861 to February 1997. The Lamb classification splits the prevailing synoptic conditions into one of ten categories; eight cardinal directional categories, and three other categories, cyclonic, anticyclonic or zonal. Since his death in 1997, an objective scheme to classify the daily circulation according to the Lamb weather typing scheme was developed by Jenkinson and Collison.



Aileen nul points

Figure 1 – Data courtesy of the CRU/UEA

A bit of a slow day weather-wise, so I thought that I’d just download the latest Objective LWT data from the Climate Research Unit and look at the Gale Index of the named storms since the project started in September 2015, and noticed that storm Aileen had scored the lowest gale index of any of the previous named storms, with a GI of just 15.5 on the 12th and 22.9 on the 13th (fig 1). Because the CRU only calculate the Objective LWT from the daily 12 UTC chart, Aileen must have slipped through the net somehow, although the two previous days (the 10th and 11th) did score considerably higher, with values of 37.4 and 38.7,

I calculate my own 6 hourly GI from NCEP reanalysis data (fig 2), and my 12 UTC values are almost identical to the ones from the CRU, but Aileen does register a GI of 30.9 and 33.4 at 00 & 06 UTC on the 13th. Not particularly interesting and all very nerdy stuff I’m afraid…

Figure 2 – Data courtesy of NCEP reanalysis

So why has September 2017 been so cool?

So why have temperatures in the first half of September 2017 been so cool across most of northwest Europe? One glance at the mean pressure chart for the first 16 days will give you the answer (fig 1). The Azores high has been +5 hPa stronger than average, and the Icelandic low has become elongated eastward, with pressure 10 hPa lower than average across the northern Isles. This has resulted in a strong west northwesterly flow across the central Atlantic across much of central Europe. I’m sure that the fast-moving ribbon of air which some people call the jet stream has something to do with it, I find that it usually does.

Figure 1 – Data courtesy of NCEP reanalysis

Here are the fine details of how the circulation has been behaving over the last couple of years with the help of some values from the objective LWT analysis (fig 2). I’ve highlighted September to show how strong and persistent the combined SW-W-NW theme has been this month.

Figure 2

It’s quite noticeable, that from the from the third week in July, maximum temperatures in the CET series have generally been rather flat and slightly below average (fig 3). If you look at the spells bar chart (the fifth chart down) there have been few if any prolonged warm or cold spells longer than 3 days or more with anomalies 2°C either above or below the long-term average, compared with previous summers.

Figure 3 – Courtesy of NCEP reanalysis

Looking over a much larger area with the 12 UTC mean temperature anomalies for the first 16 days of September, you can see that the increased westerly flow has resulted in a large -3°C temperature anomalies across central Europe, and even higher +5°C warm anomalies over northeast Turkey (fig 4).

Figure 4

What about the coming Autumn and Winter?

What does all this portend for the coming Autumn and Winter? I have got absolutely no idea. But if this kind of strong anomalous west or northwesterly flow continue like it’s been doing, I would guess that it looks likely to be cold, windy and rather wintry at times.

The cold September of 1952

Figure 1

The coldest September in the UK on record, well in the abridged 1910 temperature series from the Met Office that is, was that of 1952, with a mean anomaly of -2.79°C (fig 1), in fact the headline in the Monthly Weather Report for the month read – Unusually cold.

Figure 2

It was also the fourth coldest September in the CET series that started in 1659, and still holds three extreme minimum daily CET records from 1878, and also four extreme low maximum records as well (fig 2).

Figure 3 – September 1952

September 1952 was generally anticyclonic from the 7th to the 16th, which did allow some night frosts to occur, before turning very cyclonic from the 25th (fig 3). You may have noticed that September 1986 was the joint third coldest in the UK since 1910 (fig 1), but unlike 1952 this was mainly due to the anticyclonic nature of the weather (fig 4), in fact 1986 is the joint 10th most anticyclonic on record since 1871 in the Objective LWT series. So September 1952 was cold both by night and day, whilst 1986 was colder at night than day. In contrast to either of those two years, 2017 has so far turned out so far to be generally colder by day than by night.

Figure 4 – September 1986

End of July period third most cyclonic since 1871

I was trying to quantify just how cyclonic the end of July 2017 has been by means of the latest objective LWT data that I download from the CRU. I know that I said the other day that I don’t like the use of odd periods to find extremes from climate data, well apologies, I’m just as two-faced as the best of them, and I have had to resort to a 10 day period (19th July to the 28th July) to look at the series with. Here are the two charts I’ve come up with, the first is the percentage of LWT types in that period for each year since 1871 (fig 1), and as you can see in the first one, 2017 ranks joint 3rd most cyclonic since 1871 in this simple method.

Figure 1

The second chart is displaying the mean cyclonic (positive) shear vorticity for the same 10 day period, this time 2017 ranks 6th most cyclonic, with a value of 24.1 (fig 2). In both methods the years 1886, 1888 and 1915 figure prominently as being highly cyclonic as well.

Figure 2

The linear trend on both sets of results shows that the end of July has become less cyclonic  in the last 146 years, although the 10 year moving average in both is on the rise, so make of what you will of that.

When you look at charts for the period 19th to 28th of July 2017, you can see just how persistently mobile and cyclonic the weather patterns have been across the British Isles (fig 3).

Figure 3 – Charts courtesy of the Met Office

And just to verify that my LWT stats are playing ball, here’s a look back at the was probably the most cyclonic 19th to 28th of July, namely that of 1888 (fig 4).

Figure 4

 


The unusual warm and anticyclonic spring of 1893

Figure 1

I noticed that the warmest spring* in the daily CET record back to 1772 in Central England was 1893. I don’t make a habit of looking for exceptional warm springs in the Victorian era, it was just that the spring of 1893 was even warmer than the spring of 2017 which has just ended (fig 1). The other thing that caught my eye was how exceptionally high the mean maximum was (anomaly +3.82°C), and how comparatively normal the mean minimum (anomaly +0.37°). This obviously points to a very anticyclonic regime back in the spring of 1893 to produce very warm days and comparatively cold nights, the graph below (fig 2) shows the contrasting anomalies during that spring perfectly.

Figure 2

So just how anticyclonic was it? A quick scan of the reanalysis charts for that spring reveals it was very anticyclonic.

Figure 3 – Courtesy of Wetterzentrale

Here are the headlines from the monthly weather reports compiled by the Met Office back then:

  • March 1893 Exceedingly fine and dry in all but the extreme north and northwest where showers were more frequent.
  • April 1893 Remarkably fine, warm and dry, especially over southern England where the severe drought continued with scarcely any intermission.
  • May 1893 Mostly fine and dry, especially in the south and east till mid-month, then unsettled with rain and thunderstorms in places.
  • June 1893 Generally fine and dry first half with local thunderstorms, the second half saw frequent showers and thunderstorms.

As you can see from the LWT analysis (fig 4), spring 1893 is easily the most anticyclonic in the series that started in 1871, with 58% of LWT being either anticyclonic or anticyclonic hybrid. It wasn’t cold, because the predominant flow was generally southeasterly or southerly rather that east or northeasterly. Spring 2017 is currently in 22nd position with a couple more days of records to go.

Figure 4 – Data courtesy of the CRU/UEA

It was also the second driest spring [MAM] since 1766 in England and Wales (fig 5).

Figure 5 – Data courtesy of the Met Office

*  I’m old-fashioned, and because of this I prefer to use seasons that start and end (approximately) at the times of the various equinoxes and solstices, so most of these stats are based on so-called ‘astronomical’ rather than ‘meteorological’ seasons.

April 1938 – probably the most anticyclonic month on record

Figure 1 – Data and Images courtesy of The Met Office, CRU & Wetterzentrale

There’s no doubt about it April 1938 was quite an extraordinary month across the British Isles. Not only was it the most anticyclonic April on record, it was also the most anticyclonic* of any month in the objective LWT series that started in 1871 (fig 4). Mean anomalies for the month were in excess of +16 hPa above the 1918-1947 long-term average across northwest Ireland (fig 2 & 3), and according to the MWR for the month :

Mean pressure markedly exceeded the average throughout the British Isles, the excess at 7h. ranging from 10.6 mb. at Lerwick to 16.7 mb. at Malin Head. The mean pressure over Scotland as a whole was the highest recorded in the month of April for at least 80 years. At Oxford the mean pressure was the highest for April since 1881 and at Southport the mean pressure was the highest in April since record were first taken in 1871.

(Courtesy of the Met Office © Crown Copyright)

I’m so pleased that the anomalies I generated for the month from the NCEP reanalysis data match the anomalies reported in the April 1938 MWR. NOAA doesn’t make things easy with their 6 hourly MSLP reanalysis data which is on a 2.5° x 2.5° grid back to 1948, but before then (from the 20th Century Reanalysis project) is on a finer 2° x 2° grid. This makes the file sizes much larger to download (~35 mb), and required changes to the code to handle both grid sizes, it also explains the strange LTA period of 1918-1947 that I’ve used in the anomaly chart (fig 3).

*I calculate a simple anticyclonicity index for the month by scoring the LWT for each day. Pure anticyclonic scores 1, while a hybrid anticyclonic type scores 0.5, add them and calculate a percentage of the maximum possible, and hey presto you have a simple anticyclonic index. You could of course have used the mean daily vorticity for the month that the objective LWT data series also produces.

Figure 2 – Data Courtesy of NCEP/NOAA Reanalysis

Figure 3 – Data Courtesy of NCEP/NOAA Reanalysis

I’m slightly concerned about the number of pure ‘A’ types in the objective LWT series from the CRU (fig 4). A few of the days look like they may have been better classified as more of a hybrid anticyclonic type rather than a pure anticyclonic type, take the 30th for example. Should that be an AE or ANE type perhaps rather than pure anticyclonic? Of course it’s impossible to be definitive about this though, because the objective LWT is derived from 12 UTC reanalysis data and the Wetterzentrale charts are generated using 00 UTC data. Going back to the original ‘subjective’ LWT data that Hubert Lamb developed, and who was the final arbiter on, and that scores an anticyclonicity index of 68.3. Who knows perhaps Lamb was being a bit hard on April 1938, especially in the first week. I will investigate this a little more.

Figure 4 – Data courtesy of CRU

It goes without saying that such an anticyclonic month was also very dry across the whole of the British Isles. Using the UK gridded rainfall data it was the driest April in the whole series that started in 1910 (fig 5).

Figure 5 – Data courtesy of the Met Office

Using the EWP series it was the driest April since at least 1766 when the series started (fig 6).

Figure 6 – Data courtesy of the Met Office

Sunshine was well above average in all western regions, but closer to average in eastern areas and the North of Scotland. The MWR says that Valentia Observatory had a total of 262 hours, the largest total for April in a record which started in 1880. It goes on to say that at Mallaranny, in County Mayo (notice how we didn’t exclude the Irish Republic back in 1938), they recorded 129.6 hours from the 8th-18th inclusive which is a daily mean of 11.8 hours for 11 consecutive days.

Temperatures were also above average in the west, but closer to average in eastern districts. Because I use the very warm 1981-2010 LTA to generate the anomalies for the April 1938 graphic (fig 1), all regions look rather cold, which just goes to show you just how misleading statistics can sometimes be.

Has the recent anticyclonic spell ended?

If you, like me, follow the atmospheric circulation across the British Isles by means of the objective LWT series maintained by the CRU, then you may have noticed that the mean pressure anomaly across the British isles has now been significantly negative since the middle of February, this is after a prolonged 5 month period of generally above average pressure anomalies, which started at the beginning of October. This recent spell of higher than average pressure can obviously be linked to the lower rainfall totals this Winter, and the fewer number of named storms. Surprisingly this March, which as a month is usually one of the more anticyclonic of the year has been cyclonic. The question is – and to which I have no answer to –  will the anticyclonicity return for the rest of the year?

Cold pool on Saturday looks interesting…

A change in weather type is under way, as an anticyclonic easterly makes its influence felt during the latter part of this week. A cold pool that the GFS model has steadily pushed westward, is forecast (T+120) to be centred over central England by midnight on Saturday night (fig 1). I should imagine that this has the potential to bring some frequent snow showers to Eastern Scotland and Northeast England, especially to coastal and hilly regions of the Grampians and Pennines. After that the easterly will warm up quite quickly from the east, although it might not feel like it on the surface, as thicknesses recover and the southeasterly airstream tightens. Interestingly the GFS keeps the mainly anticyclonic easterly theme going right until the 20th of February (T+288).

Figure 1 – Courtesy of OGIMET

Figure 2 – Courtesy of OGIMET

What are the Met Office saying?

Well basically they aren’t going for the GFS solution in any big way, although they do include a covering clause in their extended outlook forecast (fig 3) to cover themselves by saying, that there’s a “small chance” of the snow showers becoming heavier and affecting some central and western areas through the weekend. They also don’t hint at any warming after the weekend cold snap in the second part of their outlook, keeping it very noddy like, because people reading this kind of stuff will be very simple, and saying that it will remain on the “cold side” well into the start of March.

Figure 3 – Courtesy of the Met Office

Just to remind you just how very poor the Met Office are doing in their mid-range forecasting, have a look at last week’s forecast that they made for this coming week (fig 4). Yes, they do mention snow on high ground, but this in connection with a cyclonic westerly and not an anticyclonic easterly type. The classic bit is where they say that it will ‘remain mild but often wet and windy into the middle of February‘ which is starting to look very wrong indeed. They go on to add that ‘there are signs’ of a colder spell later in February, which may be coming a little earlier than they imagined.

Figure 4 – Courtesy of the Met Office

Front’s to the northwest, west, southwest and east in the T+120 chart (fig 5). The trough they’ve slapped across the North Sea, at a slightly jaunty angle, is the one that ties in with the cold pool, but apart from that you can’t really glean anything other than there’ll be snow showers down the east coast I would have thought. Why don’t the Met Office just simply publish all their NWP products out to T+120 and be done with it?

Figure 5 – Courtesy of the Met Office

 

Are January’s getting more anticyclonic?

Figure 1

The short answer to the question “are January’s getting anymore anticyclonic over the British Isles” appears to be no, they’re not. That’s according to the objective LWT series that extends back to 1871 at any rate (fig 1). But January 2017 did rate fairly highly as joint 10th most anticyclonic January in the series (fig 2), and does explain why January has been quite a dry month across the country, there must be good correlation between daily rainfall and MSLP. There also seems to be some kind of 4-6 year period to the January anticyclonicity looking at the figures for the last 30 years (2017, 2011, 2006, 2000 etc).

Figure 2

Currently third most anticyclonic Winter since 1871

Figure 1 – Data courtesy of the CRU at the UEA

Winter 2016/17 is shaping up to be one of the most Anticyclonic on record. At the moment it’s joint third most anticyclonic in all ‘meteorological’ Winters back to 1871 (fig 1), using the daily Objective LWT from the UEA. It’s no wonder it’s been such a relatively dry Winter so far, and why there has been so many foggy days and frosty mornings. So despite the anticyclonicity we have seen very little in the way of easterly types (AE, ANE or ASE) so far this Winter (fig 2), although the circulation may have become blocked at times, it appears that the block may well have sat over or just to the east of the British Isles rather than over Scandinavia.

Figure 2 – Data courtesy of CRU at UEA

Here’s what the Autumn of 2016 (fig 3) and December 2016 (fig 4) looked like across Europe with regard to precipitation.

Figure 3 – Courtesy of the CPC at NOAA

Figure 4 – Courtesy of the CPC at NOAA

Of course this article will probably put the kiss of death on what’s left of the Winter as far as high pressure is concerned, and we will see non-stop gales right through February, but hold on, high pressure may still play a part in February’s weather according to the Met Office latest extended outlook (fig 5).

Figure 5 – Courtesy of the Met Office

There are a lot of references to gales in the northwest in that forecast, the GFS seems to have other ideas about that for early next week though…

Figure 6 – Courtesy of OGIMET

This solution is not supported at all by the ECMWF (fig 7), who seem to have problems labelling highs and lows at the moment…

Figure 7 – Courtesy of the ECMWF

But not to be outdone, they do manage to run a ferocious looking low up the English channel in ten days time (fig 8). I’ll be watching eagerly to see just how the Met Office do, and if any gales we do get in the next 30 days are indeed limited to the northwest.

Figure 8 – Courtesy of the ECMWF