Tom’s Tornado Two - Some Science

The previous post "Tom’s Tornado!" left us hanging with the roar of the jet plane taking off from the Algonquin forest in 1913. 

The supercell thunderstorm updraft was directly above the wall cloud which was Tom's painting. The wall cloud was moving left to right and the damaging winds which comprise the tornado had created a large area of swirling debris. The roar of the tornado would have been very loud indeed. 

CANWARN Supercell Conceptual Model with the Portion
of the storm Painted By Tom Thomson within the Black Box

The following graphic zooms in on Tom's composition (black box both above and below) within the Supercell Thunderstorm Conceptual Model from my days with CANWARN. I will improve on this Supercell Conceptual Model in a coming post. 

The next graphic highlights the characteristics of the supercell that Tom observed in 1913. The damaging winds of the tornado are in the very centre of the debris bush. 

The cloud debris observed in the centre of the painting (black outline) tells several stories. This "Scattered Cumulus Under Deck" is referred to as SCUD cloud. The "deck" refers to the average convective cloud base which is the lifted condensation level (LCL)  in that particular air mass. Any cloud forming below that LCL must have augmented moisture content from the rain of the forward flank downdraft. Those parcels of moistened air are lifted by the supercell updraft as they orbit the wall cloud. With time, the SCUD cloud will be drawn inward to become part of that grey wall cloud. The distorted parallelogram shape of the SCUD reveals that the wind is stronger in its lower portions. This shear tells us that the wind actually increased closer to the ground and that the tornado was increasing in intensity when Tom sketched in that SCUD cloud portion of his weather observation.

The wall cloud must be moving from the left to the right in order to fit the supercell conceptual model. The brightening on the left (light blue outline) is the rear flank downdraft of the supercell thunderstorm conceptual model. The bright area on the right (red outline) is the space between the updraft and the precipitation of the forward flank downdraft. The heavy rain and hail shaft where already passed and well downstream from the tornado - beyond the edge of the panel and thus not painted. 

The left brightening should be brighter than the right as that air is drier and descending. The cloud pieces on the right (orange lines) are “beaver tail clouds” being drawn up into the updraft and condensing at a lower level to form the wall cloud. Tom even included what looks to be secondary vortices orbiting the large central circulation. These smaller vortices circling around the main circulation (tornado) are very common and referred to as “sisters”.

Note the difference in the colours within the painting. The top half (grey outline) was actually the wall cloud or lowering beneath the lifted condensation level where moisture was lifted into the rotating updraft to form cloud. The colour of the wall cloud is decidedly grey.

The area underneath the wall cloud (dark blue outline) is the actual tornado churning with debris from the forest. This "debris bush" would be filled with pine needles, brush and trees. This area is referred to as the "bear's cage" under a rotating wall cloud. Sometimes blinding precipitation will wrap a tornado on some or all sides completing the "cage" of the bear. Entering that area under the rotating updraft is as deadly as entering any bear’s cage.

The rear flank downdraft actually critically influences the intensity of the damaging winds and thus the tornado. The angular momentum of the rotating updraft is a property of the supercell. Some supercells have more than others and that is related to the heat, humidity and rotational tendency (helicity) of the air mass - all forms of energy. Helicity is related to the veering of the atmospheric winds with height and thus the tendency of a supercell to develop and rotate – enough said. The rear flank downdraft controls the size of the rotation at the ground and focusses the total rotational angular momentum of the supercell into the tornadic winds.

Imagine a very large Sumo wrestler figure skater with an incredible amount of angular momentum. The figure skater can rotate faster by drawing their arms and legs inward. The angular momentum is unchanged but the speed of rotation increases as the arms and legs are pulled inward. For a supercell the rear flank downdraft controls the area over which that rotation occurs. The skater analogy with the supercell means that the tornadic winds must increase as the area of rotation decreases. The rear flank downdraft can pinch the skates of the Sumo wrestler inward. The specific characteristics of the rear flank downdraft can turn any supercell into a dangerous tornado.

Further investigation on the probable Fujita scale of the tornado can be surmised by noting that there was a lot of tree debris. Thus the tornado intensity was at least F1 (120-170 km/h) which causes "intermittent uprooting of trees and trees snapping off at the trunk". An F2 tornado (180-240 km/h) results in large areas of trees uprooted or snapped off. 

Further, if we note the trees at the shoreline and make an estimate of 10 metres in height for the tallest tree, the debris field and tornadic winds on the ground was roughly 4 tree heights wide or about 40 metres. The tornado however was inland some distance from the shoreline so the actual damage path on the ground was certainly wider than 40 metres. However as stated below from the study of thousands of tornadoes, a typical F1 tornado has a path width up to 50 metres while an F2 tornado damage path is typically 50 to 175 metres wide with a path length of 4 to 15 kilometres. I think it is safe to ascertain that the tornado that Tom painted was in the F2 category. It is very likely that the damage path was wider than 50 metres.

University of Chicago Professor 
Tetsuya Theodore Fujita (1920-1998)
The "F" in the tornado rating scale.

Tom's tornado was likely F2 in intensity and packing winds of at least 180 km/h but not more than 240 km/h. This being said, trees are not a useful  damage sensor for tornadoes above F2 intensity. An F2 or stronger tornado removes the trees in its path. If you want to measure more intense tornadoes, one needs vehicles, homes and man-made structures. This is a weakness in the original Fujita scale as an F5 tornado ripping through a forest would do the same damage as a F2 tornado. The path width and length would likely be very different but perhaps not. 

Since I initially wrote this material in the early 1980s, I was using the 1971 tornado research work of Professor T. Fujita. I actually met and briefly visited with Dr. Fujita once in an elevator in Atlanta, Georgia but that is another story. The new Enhanced Fujita scale was adopted in Canada in 2013 and it includes more damage indicators but the basic science remains the same. 

The vortex of the tornado is similar to a cream separator. The aerodynamic balance of forces on each object determines where it ends up in the tornadic circulation and debris field. The strength of the updraft carries light objects upward easily. Larger and heavier objects get quickly sorted outward by the centrifugal force. The science and stories of the weird distribution of tornado debris can fill several libraries let alone a few brief blogs. I would like to include more science but will save that for other posts.  

Tom must have been pleased with this sketch. He signed it and Tom didn't autograph many of his records. Tom's paintings were simply observations of his journeys - something that his relation Dr. William Brodie had suggested that he record and collect. Very few people have painted tornadoes en plein air. I have done a couple of wall clouds but no tornado on the gorund. Next week in "Tom’s Tornado Three", we will focus on the story behind the art of "Thunderhead". 

Warmest regards and keep your paddle in the water … 

Phil the Forecaster Chadwick

PS: For the Blog Version of my Tom Thomson catalogue raisonné, Google Search Naturally Curious "Tom Thomson Was A Weatherman - Summary As of Now" or follow this link “http://philtheforecaster.blogspot.com/2022/10/tom-thomson-was-weatherman-summary-as.html”