Michael Foley

Thanks, Jon -- what you say is interesting, and certainly seems to be consistent with what is seen in the squall line we've been discussing.  There does seem to be a discrepancy between this, and the information in the COMET training module on "MCSs and Squall Lines" http://www.comet.ucar.edu/modules/MCS.htm, from which I have attached the relevant page and diagram for squall line motion (JMcB may have to post this on his web site if the attachment is rejected).

To quote from there: "For very long squall lines (greater than 200 km [110 nm] in length), individual cells may move at an angle to the line, but the net motion of the line is constrained to be perpendicular to its initial orientation, independent of the direction of the mean wind or mean wind shear vector." Perhaps they're wrong.  I guess that if the gust front at all points along a squall line at time t moves out in the same direction at the same speed, then the line at t + delta t will remain parallel to the original line but be translated in the direction of maximum cold-pool outflow. If there are leftward or rightward
extensions of the line beyond this, it will be because cell growth is favoured on one end or other of the line (e.g. your theta-e argument for the mid-lats).

We often see lines "trying" to re-orient themselves with the mid-level flow direction.  Sometimes there seems to be competition between this tendency to reorient, and the larger-scale existing orientation of the line (perhaps imposed by
a propagating wave of some type?)  e.g. from Saturday night: first radar image shows E-W oriented line moving up on Darwin from the south (last Saturday night), and the second image an hour later shows sections of the line twisting around to a more SW-NE orientation, more consistent with the midlevel jet of around 120/30 knots--but the line on the larger scale is still E-W oriented.

Bye,

Michael.
At 18:28 15/11/2002 +1100, you wrote:
      Hi Michael,

      (putting my musty tropical hat on)

      movement of tropical squall lines in the direction of the mean middle
      level flow is pretty typical. In fact, it is intimately tied
      to the dynamics of how these these things work. As the initial cells
      develop and mature, middle-level momentum is entrained by the
      convective scale downdrafts and transported into the boundary layer cold
      pool outflow. New cell development occurs on the leading
      edge of the cold pool, where localised convergence is enhanced. The
      system evolves into a meridionally-oriented line that moves in
      the direction of the middle-level flow. In other words, the squall line
      propagates by preferential development on the cold pool
      outflow, in the direction dictated by the middle-level momentum that has
      been entrained into the cold pool.

      There are some useful papers around that show the airflow structure of
      tropical squall lines in vertical perpendicular
      cross-section, and the rear/midlevel to front/lowlevel flow-through is a
      feature. This flow acts to overturn the high thetae air
      ahead of the line, replacing it with low thetae behind, like a giant
  mixer.

      In the tropics, there is high theta-e everywhere ahead of the line, so
      the line can grow very long. By contrast, in the
      midlatitudes, the high theta-e air tends to be equatorward, so the
      individual cells develop preferentially on the equatorward end of
      the line, and the line as a whole seems to deviate from the middle level
      flow.

      To get the long-lived lines, there needs to be a careful balance between
      the strength of the rear-inflow and the strength/density of
      the cold pool outflow, i.e. the right balance between shear and
      buoyancy.

      Something like that.

      Cheers...Jon

      --
      Jon Gill

Jon Gill
aah...nice to finish the week with some meteorology...

The COMET stuff is rather midlatitudinal in focus (or dare I say, bias). The 'development
ahead of the line' model of tropical
squall lines, versus 'propagation down the line' in the midlatitude model is the key
difference, I think, brought about by where the
high thetae air is and the different low-level shear structure in the midlatitudes
compared to the tropics.

The statement from the COMET page about "New cells will be triggered downshear in the
direction of the low-level (0-3 km) vertical
wind shear vector, along the leading edge of the spreading cold pool." is true for all (a
squall line doesn't follow different
physics depending on its latitude). But in the tropics, downshear is 'usually' westward,
in the midlatitudes, it is 'usually'
southeastward (in the Southern Hemisphere).

Cheers...Jon

--
Jon Gill