Baroclinic Zone Question

[CZandbergen]

As of late I have have been doing a lot of reading and research regarding OFBs and their relationship to tornadogenesis and the production of violent tornadoes. One of the key ingredients we are all familiar with is the ingestion of streamwise vorticity into the updraft. I think I have my head wrapped around that concept and understand the implications. However, one of the attributes of OFBs and the streamwise vorticity they can generate is the baroclinic zone of the boundary. By definition, a baroclinic zone is a temperature gradient that exists across a constant pressure surface. This is where I am getting stuck and could use some help...

The question is how can a temperature gradient exist across a constant pressure surface? Is there just a temperature gradient? If so, does that mean it is constant pressure and constant moisture as well? I don't see how you can have a constant pressure and a temperature gradient unless there is a moisture gradient as well to account for the changes in density due to temperature and its direct correlation to temperature. Further, if there is a change in moisture to account for the change in temperature (to maintain a constant pressure), how is vorticity generated other than the interface (shear) of the wind on each side of the baroclinic zone? Or is that the answer?

Any help here is appreciated. I can't seem to grasp this just yet.

 

[Robert Edmonds]

The question is how can a temperature gradient exist across a constant pressure surface?

Go back to ideal gas law. Pressure= density*constant*temperature. If pressure is constant, but temperature is varying, density must be changing across the surface (assuming local thermodynamic equilibrium, we're not in a picobar atmosphere). OFBs are sources of typically denser and thus cooler air. As to how vorticity is generated, differing density can generate horizontal vorticity through buoyancy differences with the overall environment. Think of a spherical cool blob of air (heck a baseball falling from rest in air for that matter), horizontal vorticity pointing clockwise (looking down) will be generated at the interface with the cold air (or baseball and air interface). Also if the gradient of pressure is not aligned with the gradient in density then vorticity is also generated in the parcel (i.e. it takes longer to accelerate more dense air with the same pressure gradient).

As for:

how is vorticity generated other than the interface (shear)

The presence of shear is usually(I'll explain why I said usually if necessary) indicative of the presence of vorticity (at the interface), not necessarily the generation of vorticity (i.e. it's already present).

I'll let someone else explain the finer details, it's Friday night, and have been told I need to get a life.