alioth wrote:
caused by the vertical movement (and of course a large parcel of upward moving air has quite a bit of momentum, so won’t immediately start moving horizontally at the speed of the airmass it’s pushing up into) pushing up into the smoother stable airmass above pretty much like mountains will do to a stable airmass.
This. If air is very stable, wind directions can shift rapidly with altitude, there is next to no “friction” between layers. If the air is mixing vertically (both up and down), the preservation of the horizontal momentum of the air “smoothes out” the change in wind direction with altitude. And as we all know from the wind charts, wind tends to veer to the right and become stronger with altitude until you get halfway through the troposphere.
More vertical mixing → less wind shear.
Think of layers of air as sheets of paper being dragged across each other. Not much friction. Now make the paper very rough – more friction. Now put some pins vertically in the paper – everything moves together.
Now back to the original question: why the abrupt windshear at the lowest cloud layer?
The lowest layer of the troposphere is called the “atmospheric boundary layer”. In this layer, you get vertical mixing of the air near the ground throughout. It is then typically capped by a strong inversion which decouples it from the next layer up, the “free toposphere”. Outside summer, there often is stratocumulus cloud at the top of that boundary layer. Not much below – any mist, fog or low cloud disspiates as the ground warms in the morning, and not much above as rising air is blocked by the inversion.
And since the wind shifts at that inversion, you get the wind shear exactly where the lowest cloud layer is, exactly as in your picture.
Peter wrote
Whatever drives lateral airflow at one level should be driving lateral airflow at a different level. It is a pressure gradient which drives it. Why would the horizontal pressure gradient be so dramatically different?
Except the horizontal pressure gradient is different. An unstable atmosphere with condensing cloud releases heat. Which effects pressure, if I paid attention that should increase pressure.
A strong inversion in hot conditions can generate a very strong seabreeze. Upto 25 knots with sufficient pressure changes. Without the inversion the wind never reaches anywhere near that velocity.
You don’t need a big pressure difference to get significant wind. The pressure gradient in the vertical is much steeper than in the horizontal. A pressure difference enough to move the altimeter 60ft over a distance of 60Nm is enough to generate 25knots of wind at the latitude of Shoreham. i.e. 2 isobars per degree @ 33degree latitude approx 25knots.
