Vorticity Maximum of 27 November 2000

Here is an animated gif of hourly water vapor imagery with RUC initial tropopause pressure (2 PVU surface, contoured in whites and reds, every 25 mb) and lightning activity (cyan contours).

These images are hourly composite base reflectivity and lightning activity (white pluses=positive cloud-to-ground strike, yellow pluses=negative cloud-to-ground strike) for the same hour.
2100 UTC
2200 UTC
2300 UTC

Note that the positive lightning forms right near the rest of the Appalachians, in relatively shallow clouds (echo tops between 10-20 kft, not shown). There are no soundings in the vicinity of the vort max. The upper-air charts show little moisture in the lower troposphere.

I presume that low-level forcing by the terrain, coupled with the mid- and upper-level forcing by the vort max aided in the brief (2 h) period of thundersnow associated with this vort max. It is also significant that when the vort max reached the warm moist Gulf Stream waters, deeper convection broke out with negative cloud-to-ground strikes.

To examine the fluid-trapping properties of this vortex, we plot the resultant deformation (E) squared minus the relative vorticity squared, a diagnostic shown to be related to the fluid trapping in a nondivergent framework (Cohen and Schultz, in preparation). When this diagnostic is negative, parcels are trapped (i.e., vorticity dominates over deformation). As you can see from this animation of the RUC2 forecast initialized at 1200 UTC 27 November, the vort max shows remarkable consistency in the scale and location of fluid trapping.

In this animation, we see the vertical-motion field, consistent with that from quasigeostrophic reasoning: ascent (negative omega) in the region of positive vorticity advection and descent (positive omega) in the region of negative vorticity advection.

While this vorticity maximum (coherent tropopause disturbance, or CTD, see Hakim 2000, MWR, Feb. issue) is nothing particularly unusual, what makes this case interesting is (1) the trapped dry air in the center of the circulation observed from the water vapor imagery shows the fluid trapping in this CTD very nicely, (2) the unforecasted lightning as it passed over the Appalachians, and (3) the small-scale nature of the potential vorticity depression.

What helped keep this feature coherent was: (1) no chance to interact with other neighboring CTDs (there were none), (2) no strong deformation in the background flow to disrupt the CTD, (3) no strong diabatic processes, which would have been associated with the lack of cyclogenesis with this feature (it was moving over a cool, dry anticyclone). (See Mary Bedrick's master's thesis at SUNY Albany.)

These maps reminded me of a case from April 1999. The animation is on the web at: http://uiatma.atmos.uiuc.edu/~weinand/eddies.html. Look for the complete paper in MWR to appear in the next few months (probably the December 2000 issue).

The big difference between these two events was that the 1999 case seemed to be an instability of an existing shear line whereas the 2000 case seemed to arise from a preexisting vort max that got sheared out. Similar looking feature, similar background flow, different processes appeared to be relevant. :-)

Greg Carbin has the following interactive animation of the same features at: http://www.spc.noaa.gov/staff/carbin/wavetrain/.

In addition, here's a link to the 12z UA maps for that particular day...17 Apr. 1999, courtesy of Greg, as well.

Thanks to Phil Bothwell of the SPC for helping generate the graphics.

If anyone is interested in taking this project further, please contact me (david.schultz@noaa.gov).

Last update: 29 November 2000