Glidemet

Quite a complex situation this weekend. For those in central England, Booker’s blog gives a good low-down on what happend. To explain a little of what went on, here’s a visualisation of noon Saturday using the IDV. The base satellite image is the 12z Meteosat standard res visible copyright and courtesy EUMETSat. Overlaid are various parameters from the 12z GFS.

The green isosurface of 100 knots windspeed marks the jet stream and the colour wash shows relative vorticity. Red is positive vorticity and blue is negative. Vorticity is a measure of spin, positive meaning the air is spinning more and negative less. How does this affect the weather? Well, think of a spinning ice skater. To speed up her spin she pulls her arms in (conserving her angular momentum). The air behaves the same way: if its spin is increasing, air is being drawn in—-it is converging. Air can’t go down into the ground, so it goes up. Rising air cools and the widespread uplift lowers lapse rates destabilising the atmosphere allowing tall clouds to grow. Here you can see the PV is clearly feeding the taller clouds from Northern Ireland across to the North Sea, and also inside the trough of the jetstream at the bottom of the picture.

Early forecasts were a bit more aggresive in developing the trough, initially showing quite heavy rain in the west and stronger winds. The forecasts changed fairly rapidly reducing both the cloud and rain side and the windspeed.

Sunday’s forecast wasn’t perfect either, seeing a layer of fairly broken medium cloud instead of sunshine. The model output on Saturday did predict that, but I could not post that day, sorry.

Early outlook forecast
Monday will be a mild cloudy day with a light southwesterly wind. Tuesday will be equally cloudy and damp at times too, with little wind at all. Wednesday sees the wind come round to the east—-a brisk wind too—-and daytime temperatures drop to only a few degrees, and no doubt again cloudy. A fair chance of easterly wave in Wales mind you.

Thursday keeps the easterly wind, still fairly strong due to a low pressure system taking an unusually southern track into the Bay of Biscay. A fair chance of some rain in the south on Thursday. Friday is currently shown as having us under something of a col, with a chance of some better weather. The weekend sees the return of westerlies.

Easterlies, with plenty of the usual stratus, have been dominant for the last week and continue to be so. It’s cold, and either it’s overcast with stratus - or it’s not. It’s hard to forecast the stratus accurately, though by Thursday it did look like Sunday would be brighter than Saturday, which was largely correct.

On Saturday, the cloud cleared much more slowly than had been forecast by the likes of the Met Office and the BBC forecasters on Friday. I had doubted those forecasts on Friday evening and my feelings proved largely correct - there was a clearance, but it was much slower than had been forecast, not reaching central areas until dusk.

So why did the cloud clear on Saturday, but not Friday? Let’s look.

This first image is to orientate yourselves. It shows the position of a cross-section through the atmosphere across the country and into Europe.

This next image is a close-up side-on to the same cross section at noon on Friday. It shows lines of constant theta-e called isentropes - all that matters with this is that it could be thought of as being a bit like density. Air moving in the wind follows these lines: either up, down, or staying level. In this case, the lines slope down from east to west. Any air that was blown to the west along these lines would sink as it moves westwards.

If we look at a more north-south transect for the same time, you can see that the lines this way neither rise nor fall but stay level:

This image is the same roughly east-west transect as the first one, but for noon Saturday:

As you can see, the lines have the same downwards slope. So what was different between the two days? Well, on Friday the wind at the height of the cloud was barely blowing along these lines:

But on Saturday it was:

So on Friday the air (and thus the cloud) stayed at the same level, but on Saturday the wind was blowing along the sinking lines, so the air itself was sinking, warming, and evaporating the cloud.

Sunday was clear in many more areas than Saturday. This time the GFS didn’t give a very useful view of the isentropes (lines of equal theta-e), but Meteoblue did:

(Sorry this image in particular is so wide if you’re using less than 1600x1200! I was borrowing a fancy laptop when I made it).

This transect runs from Cheshire on the left to the English Channel off the Kent coast at the right - again, parallel to the wind, which was coming from the right in this picture. The blob in the lower right is a bit of cloud, but as you can see the isentropes were sloping down and the cloud dissolved.

Sunday’s wave
What was particularly notable about Sunday was the wave soaring that was had. At Milfield 12,500’ was reached and Ray Staines commented below that at Brentor (Dartmoor Gliding Society) there were 11 wave flights to 5,000’ in the easterly wave off Dartmoor! The Aqua image for Sunday is stunning - well worth a look. Take some time to scroll around and look at all the features - the huge areas of herringbone pattern wave over and north of Ireland, the hundreds of ripples in the stratus sheet over England, the lines of cumulus in the clear area where the wave has lifted the air back up to its condensation level, the thousands of convection cells over the warm sea.

The classic sign of being under the curving isobars of an anticyclone were there, and Herstmonceux’s sounding shows the stable layer with a wind that’s steady in direction but increasing in speed with height:

Early outlook
Much the same - the high over the North Sea isn’t going anywhere fast! Tomorrow at least will be a mixed day with cloud or sun possible anywhere, and still cold with the constant easterly wind, though it will be lighter than it was today.

First a note about yesterday’s forecast, which went out last night as “sunshine and showers” for today. At the time the “voting” from the models was showers confined to the west on the UK meso, but widespread showers on the GFS and Meteoblue. Although the UK mesoscale has better physics and initialisation it’s not always right, so decided to side with the majority. In the event the UK meso proved right with the overnight runs of the GFS and Meteoblue converging on it.

Both the GFS and Meteoblue did successfully pick out a trough that crossed early in the morning, leaving East Anglia at around 9am. This was a notable feature bringing convective gusts of up to 50 knots to places like Boscome Down, Wittering and Luton as it crossed at dawn. The convective processes in the trough - what goes up must come down - brought “parcels” of the fast-moving upper air down to the surface, giving strong gusts up to 1.6 times mean wind speed this morning. As the ten-minute mean reported by METARs of course takes into account the gusts, the actual difference between gust and lull in actuality is even greater.

Moving on to forecasting, tomorrow (Saturday) will start dry in most places but rain will soon spread to all areas. By midday, the rain should look something like this:

That’s actually a plot of rainwater mixing ratio out of RASP. At the heart of RASP is just a common or garden weather forecasting model - the RASPy bits just take the output and make nice plots relevant to gliding - so it should be fairly adept at forecasting stuff like rain too, though here I’m using the IDV to plot the chart. Haven’t looked at the wind - look to see which way the rain drops are blowing. Later in the afternoon there’ll be a clearance to showers but too late to do anything really.

Sunday is an interesting situation - what was a very active low (bringing hurricane force winds to a large swathe of the North Atlantic on Saturday) will by then be slowly filling and sliding down right over Britain. Here’s a view for noon on Sunday:

Surface pressure is shown, and the 3D surface is a representation of the height of a certain level of pressure - in this case, 850 hPa. Nearest the viewpoint is a high pressure, shown by the surface isobars and the high red area of the 3D surface. The surface rapidly slopes down into the heart of the low over England, with a small secondary low off in the distance further north east over northern Norway. The wind barbs show the 925 hPa (~3,000’) winds swirling around the low. It’s going to be quite windy in the southwest, as warned by the Met Office. Outside of the southwest it still looks windy, and there’ll be a curl of rain or heavy showers around the centre of the low (the old occulsion).

Monday is still northerly, but right on the western limit for the South Downs. The Chilterns and Nympsfield ridges will work a treat though. As mentioned yesterday, after Monday there’s a quieter week with a light wind coming round to the south or south east after Tuesday.

Tomorrow (Friday) patchy fog first thing where skies are clear (soon clearing), then a cloudier day than of late but equally warm. Wind weakly from the west.

Saturday and Sunday much the same, though there’s indications that Sunday will be the brighter of the two days. Again, when and where it’s clear overnight fog can form, but it won’t persist into day time

Pretty Pictures from RASP
Very early days but I’ve managed to coax RASP’s raw output into the IDV for some 3D modelling. My dream is 3D visualisation of the inversion level and of mountain wave - can’t do that right now, but maybe in the future. For now, here’s some low-level (~5,000’) streamlines showing the air circulating around the high for 12z tomorrow:

And for something a bit fancier, a 3D cloud water isosurface over incoming short wave at the ground for the same time - you can see where the clouds are cutting the sun off from the ground, and the deeper cloud over Scotland. Yes I know this actually contradicts the forecast I’ve written above, but I’ve never tried this before so have no idea how it will actually relate to reality  .

Bit of a monster tonight, so I’ll start with the forecast and put the other stuff below, though I do want to first say thanks to everyone who’s mailed in with details of the Parham and Nympsfield ridges.

Tomorrow (Wednesday). East Anglia and Kent will see big showers coming down on the the northerly wind, but west of there - beyond the prime meridian, pretty much - it will be dry and mostly sunny. Wind will be straight out of the north at around 15 knots gusting 20 at times. Western areas will probably have cumulus up to about 3,000’ but the wind at that height will be rather brisk - a good 25 kts.

Thursday will hold on the northerly wind but it will likely be a little lighter. Probably more general cloud around but fewer showers. Friday looks a generally quite damp and drizzly kind of day, Saturday probably brighter away from the east coast. Sunday should be a bright pleasant day.

Long-range forecast
Today the Met Office updated their long-range forecast for October and November. It’s well worth going and having a read - it can be surprisingly accurate, but the newspapers tend to completely mangle it when they report it. Sounds like it will be a quiet couple of months.

Modelling yesterday’s cold front
A meteorologist has been modelling yesterday’s cold front in more detail. The following shows windspeed & flow vectors near the surface with contours showing the rain mixing ratio at 950 hPa (essentially the areas where the most rain is falling) at 7am BST yesterday, with an inset of the rainfall radar at the time:

Image modelled, produced and copyright David Smart. Modelling system was the WRF-EMS (very similar to RASP), visualised using Unidata’s IDV. Rainfall image copyright Met Office reproduced for educational purposes. You can clearly see the Line-Echo Wave Patterns both in the actual radar image and in the simulation, though the simulation brings the northern part of the rain across a bit fast.

House-keeping
I’ve decided that skipping Tuesday’s forecast was a bit lame, so will do it earlier instead. That means being without the ECMWF and Meteoblue models which come out at 8pm, but better something than nothing.

I’ve also added links to other forecast sites to sidebar - I do recommend checking a range of forecasts, then making your own mind up. Parham’s page, in particular, is very neat, showing if the field is open, if instruction is available, and what aircraft are in service. Every club should have one!

You’ll notice that I’m looking to forecast ridge days - this forecast does run year-round. I’m working on forecasting ridge and wave in Wales too, but for now it’s restricted to England.

Finally I want to again ask anyone who’s got a page with links to the old Bicester Airfield Forecast to update it to point to GlideMet and http://glidemet.co.uk - helps with the old Google juice . There’s no “www” in the address, and the site really doesn’t relate to Bicester more than anywhere else now.

In Other News
The BGA Competition Calendar has been updated - the Juniors will now run 2 - 8 August, while the 18s have been shifted back a day: 17 - 25 August. I must say I didn’t realise until recently what a tough time the competition committee have putting together the calendar each year - considering the challenges, they do a great job.

This example isn’t cyclogenesis, as the depression already exisits, but it is the same process. In this case, it’s simply enhancing what’s already there.

(The images are actually from forecast data for 12z today from the Monday 00z GFS run, so it won’t be exactly correct compared to reality!)

First, a plan view of the system, showing sea level pressure with surface wind as wind barbs and as colour shading (red areas being 40 kts). (In fact as I write, wind is already closing on 40 kts in the Channel.)

Now let’s look at the upper air features. The next picture is looking east and shows a 5% relative humidity isosurface - this is approximately stratospheric air, so illustrates the shape of the tropopause. Note the step-change in height - cooler, denser (and thus lower) air to the north, warmer, less dense (thus higher) air to the south. (Ignore the smaller scrappy bits below the main surface - they’re pockets of dry air being pulled downwards.)

This creates a pressure gradient, along which flows the jet. This image shows a green 50 m/s (~100 kts) wind isosurface (i.e., the jetstream) tucked into the tropopause step (viewpoint is still to the west but is actually looking up from below the earth’s surface!).

This next image is taken from slightly east looking down on the nose of the jet. The red surface shows strong ascent, and the blue areas weaker descent (it’s a bit like thermals - strong areas of lift surrounded by weaker sink). The reason why the jet left exit lifts up and the right exit sinks is quite complex - another time, maybe!

In the next image the view point is further east and the areas of rising and sinking air have been removed and a pink cloud-water surface put in. Note how the cloud is forming under the left exit, where the rising air is.

Finally, putting it all together: the pink cloud-water surface and the red rising air are shown together to illustrate their colocation. At the surface is six-hour accumulated rain - you can clearly see how the wettest area is below the rising air!

Normally in summer the jet is far to the north, beyond Scotland. This year it’s made a habit of coming and visiting us down here, thus several bouts of terrible weather.