STORM TRACK: March 31, 1983 (Volume 6 Issue 3)

Several letters were received on the gustnado article in the last ST, and in response I now propose other (but more detailed) formative mechanisms than were shown in the last issue. Actually, that model (Illustration 4 of the Jan. 31 ST) was an amalgam of sketchy ideas from both Tim Marshall and Chuck Doswell, and not just from Tim as was incorrectly stated. The following are two of Tim Marshall's conceptual models and discussion of the driving mechanism of a gustnado, after his thoughtful review of the January issue.

"I ... tried to approach a rational explanation of why some form on the gust front and others form ahead of it," as the Editor observed last May 27, with laminar gustnadoes occurring well ahead of the outflow gust to the west. "I think that a general uplift is occurring on a large scale. As the warm air is lifted over the boundary, it may accelerate. Thus, it "stretches" the air column and produces vorticity. ... The ground friction retards the outflow movement. This causes air aloft, say 100-300 feet, to overtake and accelerate out ahead of the surface outflow boundary. I believe that this "nose" of the outflow has been documented by Wakimoto (1975). Therefore, whirls which develop along the surface boundary can't extend upward very far, whereas, if the whirls develop out ahead of the surface outflow boundary, they may extend up to cloud base." (Fig. 1)

Figure 1

Moreover, "in an article by Wakimoto (Monthly Weather Review, Aug. 1982), he showed some interesting Doppler velocities of outflow cross-sections. The velocities clearly show that there is "stretching" along the outflow boundary. ... The stretching may be illustrated by taking a hypothetical cylinder and drawing in the velocities. More air is being removed from the top of the column, so that the air column is actually stretching (Fig. 2). This, of course, is only one possible method for vorticity and may not explain your cases. I tend to think that gustnado development is a below boundary layer phenomenon (700mb), and so upper level winds, shear or jet, are not as significant,."

Figure 2

However, "I didn't realize in my earlier correspondence that there was new convection overhead" on May 27, ahead of the gust front. "In that case. I accept all of your points. Convection can produce the same effect as the rising air along the gust front. Buoyant air will also accelerate upward by its own accord. My comments were intended to represent a general discussion of gustnado developnent, since I am very interested in this phenomenon.

I've also seen gustnadoes develop from what appeared to be an acceleration of the outflow boundary or perturbation. Once, I saw half a dozen occurring simultaneously from what appeared to be a similar situation. In fact, one of the gustnadoes crossed the road, and I drove directly through it. It jerked my car and dust flew into my eye (windows were open). I found out from experience that this is a No-No!" Now, let's try to construct a three quarter view of a gustnado.

"Thunderstorm scale depiction of outflow boundaries tend to be simulated as smooth. However, fine lines from radar PPI actually show the cross section of the outflow boundary to have numerous perturbations along it (Fig. 3).

Figure 3

These perturbations along the outflow are attributed to variable pressure fluctuations and especially ground friction. In regions where the outflow boundary is accelerating, vorticity develops on the periphery.

Figure 4A and 4B

The "angle of attack" of the inflow wind or convergent wind is primarily responsible in locating where the vorticity center will be concentrated. Figure 4A shows no gustnado development, while Figure 4B shows multiple gustnado development Speed shear in the inflow wind is important. It could be that small regions of acceleration in the inflow wind can also enhance gustnado development. Since the acceleration of outflow can also be present at cloud base, gustnadoes may be attached." The Editor notes that the stretching in Figure 1 may also occur in the Figure 5A, model. Weak, non-tornadic rotation can also be explained by a brief wrap-up of surface outflow, spiralling in response to the "variable pressure fluctuations" or "ground friction" mentioned by Tim. This is shown in Figure 5B, contributed by Randy Zipser.

Figure 5A and 5B

Tim Marshall also sent some additional notes on the May 19, 1982 Pampa, Texas tornado and some striking illustrations of the apparent, cloud base/ground dynamics.

"I've enclosed a few schematics which illustrate a plan view of meso-scale features which occurred in the Pampa storm. ... The fourth tornado," occurring east of Pampa and which destroyed the Halliburton Industrial Park, "was different from the others, forming on the western periphery of the updraft. I could see two lower cloud bands on either side of me, moving towards the tornado. Eric Rasmussen and others looking NE saw "incredible" west to east movement of the cloud base in back of the tornado, with the clouds wrapping up tightly around the vortex shown in Figures 6A and 6B. Also of interest was our view of how low subvortices formed with two tornadoes -- Figures 6C and 6D.

Figure 6A, 6B, 6C, and 6D

Figure 7

Another interesting feature, which I discovered from the Pampa tornado #4 (Fig. 8B next page and 6A/6B above) is the presence of a sort of inverted geometry to the wall cloud. I'm still thinking about this one.

Figure 8A and 8B

Figure 8A -- Tornado #2, 1823-1845 LST, 5 miles west of Pampa. Tornado occurred on north-center side of updraft. West winds were encountered at film site, while tornado was still several miles west-northwest. Some light precip occurred after the windshift. Narrow "slice" of rain occluded the tornado at the end. Periodic anti-cyclonic funnels were on the eastern perimeter of the updraft.

Figure 8B -- Tornado #4, 1853-1915 LST, 3 miles east of Pampa. Tornado occurred on west edge of updraft with extensive updraft region continuing northwest. No windshift, was encountered at Tim marshall's filming location (right-most one in Figure below).

(Note that, for tornado #2, "the outflow showed no appreciable temperature change from the inflow (since we were too busy filming the tornado, we didn't actually measure this). However, it appeared to me that, when precipitation was falling, it didn't seem any colder.")