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Wave & Vortex Theory in Action - Kelvin-Hemholtz & Rayleigh-Taylor Instability

Posted by JAC on 9/29/2009, 7:08 am

http://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_instability

http://en.wikipedia.org/wiki/Rayleigh%E2%80%93Taylor_instability





AWIPS images of the GOES-12 6.5 m "water vapor channel" (above) revealed a pair of vorticies
immediately poleward of a well-defined jet stream axis
that was moving over the southeastern US  on 28 September 2009.
These vorticies likely represent either a type of  Kelvin-Helmholtz instability
(which can occur when there is sufficient velocity difference across the interface between two fluids)
or a type of Rayleigh-Taylor instability
(which can occur along an interface of two fluids of different densities).

Overlays of parameters from the 45-km resolution CRAS model at 12:00 UTC (below)
showed the presence of a 50-60 knot jet axis just south of the primary dry-to-moist gradient
on the water vapor image,
along with a ribbon of 500 hPa vorticity and a 500 hPa wind shear axis
over the region where the water vapor vorticies were forming.





A comparison of the 1-km resolution MODIS 6.7 m water vapor image
and the 4-km resolution GOES-12 6.5 m water vapor image (below)
show the advantage of improved spatial resolution for displaying the structure and gradients associated with the leading vortex around 18:15 UTC.





Examining the GOES-12 water vapor weighting function profiles at 00:00 UTC for Charleston SC
(located in the "dry" portion of the sharp water vapor image gradient) and Jacksonville FL
(located in the "moist" portion of the sharp water vapor image gradient)
shows that there was a pronounced downward shift in altitude of features displayed on the water vapor image for the dry air poleward of the jet stream axis.





A northwest-to-southeast oriented vertical cross section using GFS40 model fields
(below) displayed a minor intrusion of potential vorticity downward into the upper troposphere immediately poleward of the jet stream core
(which was located between the 200 and 250 hPa pressure levels).
The wind speed shear axis was located at a much lower altitude (between the 400 and 500 hPa pressure levels),
close to the altitude peak of the water vapor channel weighting function in the region of drier air.






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