"Waghorn" wrote in message
...

"Alastair McDonald"
"Waghorn" "......why split fronts appear to be more common in the
British Isles
than in the Pacific Northwest.........."

It sounds to me to be similar to the effect of a breaking wave on the sea
shore.
.......................................


How does that fit with the facts?

Cheers, Alastair.


Er,nice start, but...........no, if I read you right ;-).

I don't think you have, but it is probably my fault.

Over the ocean basins 'splitting ' can't be induced by orography except in
the sense of it's dependence on the lower stratospheric/upper troposphere
dynamics ultimately related to downstream effects tied to the Rockies (in
the Atlantic basin).However it may well relate to this 'breaking' of Rossby
waves and the effect on maturing baroclinic waves late in their lifecycle,
ie at the end of the stormtrack.

I am assuming that the splitting happens when the fronts reach Britain,
not in the Open Atlantic. Thus, just as sea waves increase and break
as they reach the shore due to the shelving sea bed, so too the air
on leaving a flat ocean and moving onto a shelving topography
will also form waves which break. In general they will be invisible,
since they are covered by more air rather than being an obvious
difference in phase which happens with liquid waves breaking in
a gaseous atmosphere.

But it may be possible to see them by observing the behaviour of
clouds, although I am not sure what they would look like. Presumably
the cloud would be carried higher, just as sea wave gets higher. But
this analogy may be unnecessary.

At what height does the split happen at? If is more than 3000 feet
and less than the average height of the Rockies? It may be that
the bottom of the front is stalled by the mountains whereas the
split, being above the British mountain tops, is unimpeded and
flows onwards.

The other scheme I suggested is that when the front hits the
wedge shaped topography of Britain side on, it is imparted
will a roll, which creates the split effect. What is really needed,
to see whether that roll is possible, is a wind tunnel with a
wedge across the direction of flow, but with its sharp end
of the wedge pointing slightly upwind.

I am actually providing two solutions to the problem here. They
can't both be wrong!

Cheers, Alastair.