Synoptic Scale Baroclinity
*Keep in mind that the northern hemisphere polar front is
the one that I am using as an example in the following.
Cold air advection from the poles moves southward and adiabatically warmed sinking air advecting northward, helps create a substantial temperature gradient along the polar front. Albeit there is a strong temperature gradient along the polar front and the ITCZ, the temperature gradient anatomy is quite different. At the ITCZ, the temperature is lower at both sides and is the highest at the ITCZ itself, causing air from both sides to converge into it and rise evenly. At the polar front however, the temperature to the south of the baroclinic boundary is higher than the one to the north. As a result, the colder air from the north slides beneath the warmer air, forcing the more buoyant warmer air aloft. This induces thermodynamic fluxes incessantly hither and thither at the mid latitude regions. That is the primary reason why the majority of mid latitude cyclones form in close proximity to the polar front. And as many of you know, a significant synoptic scale cyclone is often associated with anomalously high temperatures at its right side and cooler than temperatures at the left side due to the counterclockwise rotation.
Part II B) The Polar front and the Jet stream
The polar front also affects the thickness of the atmosphere—the side with the higher temperatures have higher thickness values, while the side with the lower temperatures has smaller thickness values. The thickness is the pressure difference between the surface and the higher altitudes. For example, the pressure near the surface is 1010mb and 200mb at about 70,000 feet. That would be a thickness of about 810mb. The rule of thumb is that the temperature is the only factor that affects the thickness and the higher the temperature, the higher the thickness. Since the thickness is higher at the southern side of the polar front, it would mean that the pressure fluctuates less at increasing altitudes. Therefore height for height, the pressure would be higher at the southern side of the polar front. And as many of you know, air flows from areas of higher pressure to lower pressure, so the pressure gradient force (PGF) will be facing northward. Indeed this is one of the fundamental laws of atmospheric sciences, air flows from an area of high pressure to low pressure. Due to the Coriolis effect, the air would then be deflected toward the east, causing the wind to blow from west to east. And as most of you know, the upper level jet blows from west to east. Now, the jet stream does not always blow zonally or completely straight from west to east. The flow often becomes jagged or meridional, resembling a series of waves. The cooler air tends to advect southward forming troughts and the warmer air moving northward tends to form ridges, and baroclinic boundaries are flanked by the ridges and troughs. Cyclogenesis often occur along these convergence zones. Due to the fact these synoptic scale cyclones form along baroclinic convergent boundaries, frontogenesis often occur shortly after cyclogenesis.
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