Being a simple chap, I( have often wondered.... "Why does convection form in lines?  How does nature take essentially a 2-dimensionally isotropic process (simple parcel theory" and turn it into something with a strong 2-dimensional structure.

 I HAD thought the switch to two-dimensions occurred AFTER the clouds formed.  Thus a Cb forms, it develops a downdraft.  The downdraft interacts with the large scale flow fields, which have a 2-dimensional structure; and so the development of the next set of Cb's is lined up because of the interaction between the downdraft outflow and the ambient
low-level current.

 This process becomes stronger when the set of downdrafts from the new cells is already in a line and so the evolving system becomes more and more linear (linearer and linearer?).

 The alternative view is  the large scale conditions that trigger convection through uplift, instability, moisture inflow etc have a linear structure before the convection even begins.  In this view, the linear structure has nothing to do with the properties of convection, but more with the properties of they large scale environment which has deformation zones etc which are lined up.  In this viewpoint, the convection will already be lined up, or 2-dimensional" before and as it develops.

 The point of all this>?

 Have a look at the current Melbourne radar 512 km loop (as I speak... 2.15 pm Wednesday".... see the cells developing in a line stretching from Swan Hill to Sale..... the line is there BEFORE the convection begins....

Theory number two seems to be operating.

 I'll get out of ya way

John mcB

Harald Richter
Dearest John,
 
I have seen both procesees at work.  Storms can initiate in a linear fashion straight away, or a bunch of separate storms can interact to form linear convection such as a squall line.  Linear initiation  occurs more reliably when a strongly forced boundary (e.g., a strong cold front) plows into a highly unstable gathering of boundary layer  parcels.  Along the entire length of the boundary the forcing is sufficient for initiation.  In the second scenario cold pools interact ('collide' hmm) to form a new, usually more linear gust
front.

BTW, the Melton-Sunbury boundary has become a player in today's convective scenario for Melbourne.  Its interaction with a young cell NNW of Melton  has produced a sizeable storm N of Melton.  The storm S of Melton (deepest into the cool air behind the boundary) seems to have weakened.  The southerly coming off the bay today is cool (sub 20 C) giving
all boundary-crossers a hard time.

Harald