Questions on prefrontal confluence.

[Thomas Mattek]

What exactly IS prefrontal confluence? Why does the confluence occur ahead of fronts instead of primarily on them? I know it's considered important to supercell-genesis, is the confluence associated with lift? Can the confluence zone itself demarcate a boundary if the wind shift or potential temperature is drastic enough? Can other boundaries (e.g. an outflow boundary) interact with the confluence zone?

 

[Paul Knightley]

There a a number of reasons why a pressure trough might reside ahead of a frontal boundary. The paper linked to below is worth a read and details/reviews several mechanisms. I won't list them here as the paper is worth a read - but, for example, there may be an upper short-wave trough passing over, inducing mid-level lift which causes a pressure fall at the surface - this trough may well be passing over more quickly than the front is moving, and so can induce a trough out ahead of it. The paper - http://www.atmos.albany.edu/facstaff/rfovell/NWP/schultz-2005.pdf

Regarding why they can be important (not just to supercells, but thunderstorms per se) is that low-level confluence/mass convergence is one mechanism which can lift parcels to their level of free convection, and, in the case of a low-level capping inversion, help lift parcels through that (although some form of large-scale lift aloft tends to help with this too). In the case of a strong cold front, storms along the front may line up into a squall line, quashing the chances of isolated supercells...pre-frontal boundaries/wind shifts can help storms form in the warm sector.

And, yes, all surface boundaries can interact with one another, locally enhancing, and suppressing, convection.