Glidemet

I thought it would be interesting to take a look at what happened last night, both down here in the south, and up in Scotland where there were severe gales.

All the images were produced using the IDV and GFS data for midnight.

This image shows some of what was happening. Up above is an orange isosurface of the jetstream. All the air within that surface is travelling at 90 knots or more. It shows the deep U-shape of an upper-level trough, and the speed shear on the outside of the bend is increasing the voriticity of the air. On the leading side it is adding to the planet’s rotation, and on the trailing side cancelling it out, so on the leading side it’s increasing the spin of the air (and thus drawing air in and up). The red isosurface is air that’s rising rapidly. It’s not possible to say how much this is due to the increased vorticity or actual convection within the front—-it’s the sum. The grey area is cloud, and on the surface is total rainfall over the last six hours (heaviest over the western hills) and surface pressure.

Zooming in and adding a cross-section through the front shows the drier colder air behind the front running into the warmer damper air ahead. This is the basic frontal process—-like a battle front between two armies. The jet stream above is enhancing the effect and making the weather along the front more severe.

That’s all well and good for the south but what’s all that in the background over Scotland? Lots of red rising air and grey cloud, plus those surface isobars are quite closely packed (i.e. it’s windy) and it’s obviously raining heavily. What’s causing the air to rise over Scotland? Once again it’s vorticity. The following is a plan view of relative vorticity with red showing where the air is being spun up and drawn in:

To be clear—-the vorticity is resulting in the general uplift of the air over a wide area, which cools as it rises. As it cools it becomes unstable, allowing convection. The red rising air in the 3D pictures is the sum of both processes. Looking at voriticity helps us to seperate the two.

Where’s the vorticity coming from? If you look back at the first 3D image you can see that some of this area is in the “left exit” of the jetstream—-always the point where uplift is strongest. On the left side of the exit the air diverges (pulling air up from below) and on the right side converges (forcing it down). However the jetstream is also doing something really weird to the tropopause, the boundary between the troposphere (where all our weather is) and the stratosphere (dry air above). The tropopause is shown in the next image by the magenta surface:

You can see that the tropopause has folded back on itself, and that the fold is colocated over the area of highest relative vorticity. The dry stratospheric air is intruding down into the troposphere, and air drawn down from the stratosphere really wants to spin. The reason why can only be explained mathematically (Jolien, if you’re reading this, you should be able to work it through ). Sometimes the dry cold (and thus dense) stratospheric air begins to move extremely quickly and a small but powerful “sting jet” of hurricane-speed air can develop in the heart of the storm. If this reaches the surface the winds can be very damaging. The 1987 Great Storm was actually a sting jet, and that was first time one had ever been observed. Sting jets are still an area of active research today.

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