11/05/2017 (Sun) 09:45:15
We have a really nice example of a Genoa low developing.
When a cold front hits the Alps from the west, the temperature and pressure field stack so that they're perpendicular to each other. The lower pressure behind the cold front leads an east-west pressure gradient, while there is also temperature gradient from the warmer sea to the colder inlands. The misalignment factor (mathematically a cross product between the gradients of pressure and density fields, with density inversely correlated to temperautre) is called baroclinity. Typically, the two fields are relatively well aligned. but next to mountain barriers and warm seas (a great example being exactly the Ligurian sea)
So, what happens then? Pressure fields mean that in the south, the air is rising because of high temperature, while in the west, there's cold air rushing into the cyclone from elsewhere. Summed together, this means that the cold air is rushing in counter-clockwise from northwest, while cold air is retreating and rising counter-clockwise on the eastern side. This initiates the rotation, as well as deepens the cyclone, as warm air rises. The cold air that rushes in develops a secondary cold front, while the warm air moving away develops an area of convection along a secondary warm front. A small eddy cyclone develops to the south of Provence, in Ligurian sea, which usually drops huge amounts of rain the the following day or two.