Bill Taylor’s Talks

Resume on the Jetstream talk  (Given 23rd January 2018)

Jet streams are rivers of fast moving air in the upper part of the atmosphere and are created when warm sub-tropical air moves north and meets cold polar air moving south. This usually happens around the latitude of the UK.

They are several hundred miles across, several thousand miles long and 3 or more miles up in the atmosphere. They are typically around the 300mb (about 5½ miles up) to 250mb levels and so the 300mb upper air chart is a good one on which to see them. Where the contour lines on the 300mb chart are close together, the wind is strong, this is the same rule as a surface chart. Also when looking down stream in the jet, with the wind blowing from behind, the polar air is to the left and the tropical air is to the right. This is analogous to Buys-Ballot’s law that says that in the northern hemisphere, on a surface chart, in the same orientation to the wind, low pressure is on the left.

Jet streams are significant because as parcels of air are carried along the upper flow into them, these parcels are accelerated and so they cross the contour lines at the entrances and exits. This leads to an excess or a deficit of air at 300mb in the entrance and exit regions and there is therefore a compensating change of pressure at the surface. Falling surface pressure where there is a deficit aloft and rising surface pressure where there is an excess. It turns out the falling pressure at the left exit of the jet is usually stronger than at the right entrance so this is where the largest pressure falls are and where the lows deepen fastest resulting in the deepest low pressures.

Typical speeds in the jet stream across the Atlantic in Summer are around 150kt, with about 180kt in winter and 200kt in a named winter storm. In the Pacific however, 200kt is more usual.

Although jets are mainly associated with air masses of differing temperatures coming together, the same air mass temperature difference can be achieved by the underlying geography. In particular in the NW Pacific, where in winter, the very cold continent of China is close to the very warm water of the tropical Pacific Ocean. This very large temperature difference generate a strong jet stream, usually over 200kt in winter and 250kt at times.

Normally, the jet stream waves as it moves east so that it might go down to 45N then further east curve up to, say, 55N. This wave has a shallow amplitude. In some situations the amplitude increase from about 45N perhaps to, say, 70N. The longitude difference between successive troughs or successive ridges is the wavelength and as the amplitude increases, the eastwards movement slows and at about a longitude difference of 70degrees, the whole eastwards pattern can stop.

If the UK is in the polar air, so with the jet stream to the south, a prolonged period of cold weather is likely. If the pattern stops with the UK in the tropical air, so with the jet to the north, an extended warm spell can develop leading to a heatwave in Summer.

Finally, there is some evidence that global warming is leading to the loss of sea ice in the northern hemisphere in Summer and Autumn which in turn could lead to a more meandering jet stream and the threat of a slower west to east progression and the implied threat of more extended cold or warm spells. In other words the threat of more extreme UK weather.

Bill Taylor

9/2/18


Hello Mike,

Here are some notes on my talk…. (February 2017 on ensembles)

It aimed to explain why, in the era of computers, there were still uncertainties in weather forecasts. It turns out that the evolution of the forecast depends on the initial conditions given to the model and that on some occasions the evolution is incorrect. The answer is to compare successive runs of the same model, so for one model, each run for a few days  and compare models so the GFS v. the ECMWF v. the Met Office. Looking for agreement all the time at the particulat date and time that you are interested in.

If (when) there is agreement between the GFS and Met Office for example, then the wind guru and Met Office detailed forecasts for say, Calshot, ought to be very similar and therefore credible. You could try to find a wind guru or met Office forecast spot near Poole.

The page  http://old.wetterzentrale.de/pics/avnpanel1.html shows a set of 8 charts from the American GFS model and allows you to see the model’s evolution on one screen.

I have looked at the Surface Pressure Charts link which goes to netweather.tv. The comparison charts where the GFS and ECMWF forecasts are laid side by side are very useful as they can be compared directly and it can be seen when the two models diverge and therefore can’t be trusted. I haven’t seen this website before with the comparison charts. They look very useful. Thanks for pointing them out.

The ensemble page I was using is in the http://old.wetterzentrale.de/topkarten/fsavneur.html link. Go to the top left hand corner and slide down to “diagrams” and click. Then click on “GFS 1 (7d) + GFS ENS…..(Europa)… then select “London” and “Ensemble t850 und Nds”. The chart shows the temperature over London at 850 mb or around 5000ft. Look for the point at which the ensemble members diverge from each other and the operational run (the thicker green line). The bottom scale is the date.  You can’t place any reliance after they have diverged.

Hope all this is helpful and not too confusing,

Good luck

Bill

75 Years of PYRA

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