STORM TRACK: January 31, 1983 (Volume 6 Issue 2)

Tim Marshall's documentation on the May 19, 1982 Pampa, Texas supercell, which produced six tornadoes, is extremely interesting regarding storm structure. This cell did not exhibit the classical "rear flank downdraft" as modeled by Leslie R. Lemon and Charles A. Doswell III in the Monthly Weather Review, Vol. 107, No. 9, September, 1979.

Actually, Doswell allows that there are some variations and subtleties to the 1979 model (Illustration 9A), which were known then but not addressed, in order to present a cleaner example of the most cocoon structure. Tim Marshall's account seems to present one of these variations.

(To clarify the following discussion, the Editor has made a creative distinction (his own) between the traditional "tornadic wall cloud" and a "meso-low wall cloud." The banded cloud base around some supercells exhibits a circular organization, with well defined (sculpted) edges and covering 3-5 miles in diameter (Tim describes it from the Pampa storm, and I have seen it in the supercells at 'WaKeeney and Ellis, Kansas on June 5, 1974 and at Garden City, Kansas on May 30, 1978). By contrast, the traditional tornadic wall cloud is only about 1 to 1 1/2 miles wide and is characterized by a condensation lowering below cloud base. The WaKeeney-Ellis storm exhibited both at the same time, with a lowering wall beneath the nominal cloud base and a banded cloud "wall" above, shaped like the bottom of a cereal bowl. Tim refers to driving in and out, beneath the 'wall', where surface inflow winds changed dramatically and abruptly from about 20 to 60 knots (higher when outside the wall). Other meteorologists have also referred to such a banded base as a wall cloud. However, I am convinced that it is a formation distinct from the traditional wall, shelf, roll or other cloud descriptors. Therefore, for the purpose of this discussion, I will distinguish between a meso-low and tornadic wall cloud.)

The unique characteristics of the Pampa supercell included:

(1) Absence of a single dominating outflow/gust front after inception of tornadic activity;

(2) Presence of at least one warm outflow/gust front (perhaps more than one occurred, but location of the chase team only allowed one to be measured);

(3) Failure of any outflow to clear out or open up the cloud deck, as would normally be expected to the west or southwest of the vortex (however, several apparent rear flank downdrafts were indicated by rain wrapping, close in, around the tornadoes); and

(4) Sharp demarcation of surface inflow winds as they passed beneath the outer edge of the meso-low wall cloud (dropping almost instantly from 60 to 20 knots).

In addition to these anomalies, examination of one of Jim Leonard's prints, taken in Pampa and looking east at tornado #3, suggests a downward component to the meso-low, well above cloud base. The preceding seems to indicate a model something like Illustration 9B. Tim has reviewed one that is similar to it and takes no exception. Other than this, it is purely the Editor's own speculation, not endorsed by anyone else.

Basically, it shows a core with a dominant updraft, while a lower, outer ring, or meso-low wall, surrounds it. Some of the updraft is drawn up around the lower pressure core, but so much air is lifted that the anvil can't ventilate it all, and some centrifuges out to the lower ring. Here, with higher pressure, a weak downdraft moves around to cloud base, only to be curled back up (not reaching the ground) by the stronger inflow wind. When a localized rear flank downdraft develops, it is smaller at the top than the Doswell-Lemon model and doesn't ventilate a large enough anvil area to overwhelm the surface inflow. Whatever cooler air enters the downdraft remains close to the vortex and wraps around it, but also, fails to overcome the strong inflow. The inflow rises along both the updraft core and inner side of the meso-low wall. However, it turns downward from the meso-low when adjacent to the rear flank downdraft, and returns, uncooled, to the surface as warm outflow. The short gust front lasts only until the rear flank downdraft is eroded by the pervasive circulation in the meso-low wall, permitting the supercell to continue in its steady state. How, otherwise, would you explain this storm in a conceptual model?