Glidemet

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.

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