Dust Devil or Landspout?

[Kyle Brittain]

Here in Alberta, spring has come early - and so has tornado season apparently.

Environment Canada confirmed an EF0 landspout tornado near Calgary on Wednesday, April 13, presumably based on photos/videos as well as synoptic background environment. However, this week we have had several large dust devils reported in similar conditions.

I have doubted the diagnosis of landspout on two accounts: based on my understanding of the conceptual model of environments that support landspouts (that I argue, was not met); and based on a lack of conclusive photo/video evidence that the vortex was attached to a parent TCU aloft. Moreover, the fact other large dust devils had been observed elsewhere in similar environments in recent days would make it seem more likely that the "landspout" was in fact another large dust devil, especially considering the environment.

At 22Z on April 13, the dusty vortex was observed under mixed skies (likely near the sun/shade boundary of nearby cloud). Cloud base height was 9300ft AGL (~2800m), T of 15C, Td of -7 (Tdd of 22C!), gusty westerly winds shifting to gusty northerly near the time of the vortex, and some weak instability (~250J/kg, evidenced by glaciated-looking high-based TCU with frequent virga). Also, 0-6km shear was 25-30kts...not your typical favourable shear for landspouts. My contention is that updrafts would be too weak to support the vertical stretching of near-ground vorticity all the way up to a cloud base nearly 3km up. Also, that virga precipitating into the sub-cloud layer would act to stabilize it, causing any surface-based parcels to encounter large CIN.

However, you be the judge. Based on the environment above that has a well-mixed, bone dry boundary layer, are landspouts possible?

The first video is the first large dust devil that was observed this week, on Monday evening:

Within the following news article are two clips of the alleged "landspout" - one appearing to have cloud immediately behind and the other not so much:

http://globalnews.ca/news/2637207/t...ncreasingly-common-twist-in-albertas-weather/

Finally, a picture someone got of the "landspout" from a skyscraper in downtown Calgary looking east:

https://twitter.com/yardcreative/status/720369562361856000

 

[JF Massicotte]

Here I uploaded a low level sounding showing the main ingredient driving this event: superadiabatic lapse rate over the first 2km AGL.

I think this was a landspout occuring below shallow TCU, the extreme lapse rate in the low levels probably made up for any weakness in the updraft when it came down to stretching. Very hard to call, and impossible to rule out any dustdevil, but radar was showing some convective showers over the area intensifying during the event, this would support the landspout theory.

 

[Jeff Duda]

At 22Z on April 13, the dusty vortex was observed under mixed skies (likely near the sun/shade boundary of nearby cloud). Cloud base height was 9300ft AGL (~2800m), T of 15C, Td of -7 (Tdd of 22C!), gusty westerly winds shifting to gusty northerly near the time of the vortex, and some weak instability (~250J/kg, evidenced by glaciated-looking high-based TCU with frequent virga). Also, 0-6km shear was 25-30kts...not your typical favourable shear for landspouts. My contention is that updrafts would be too weak to support the vertical stretching of near-ground vorticity all the way up to a cloud base nearly 3km up. Also, that virga precipitating into the sub-cloud layer would act to stabilize it, causing any surface-based parcels to encounter large CIN.

However, you be the judge. Based on the environment above that has a well-mixed, bone dry boundary layer, are landspouts possible?

This is an interesting case study. Thanks for bringing it up. The distinction between a landspout and a dust devil may not be as meteorologically as politically significant. In the US, dust devils are not counted as tornadoes while landspouts are.

Unfortunately, based on the information you and the second poster provided, I really can't say with certainty what it was. I also can't find evidence to rule either one out. I see from some pictures in the links you posted that there were cumulus clouds in the area, but 250 CAPE is pretty meager (although technically, even with 1 J/kg of CAPE, that makes a landspout possible). Despite being cool and dry, the PBL is very well mixed and deep with unstable lapse rates above, which is favorable for landspouts. The superadiabatic lapse rates in the surface layer is also conducive for dust devils. I think what is needed more than anything is a picture including what was directly above the vortex. A landspout should occur below a buoyant cloud base, and there should be some semblance of cloud base rotation or even a funnel above the dust column. I don't see either of those in any of the pictures I saw, but the POV of those pictures is not conclusive (i.e., bad angle or long distance, so it's possible such features could've been present and were either too small to be noticed or were off the edge of the image). The motion from the YT video (great quality, btw) makes me think dust devil, but I just can't rule out a landspout. Obviously, the folks at EC either found evidence that it was a landspout, or that's just the side of the fence they decided to hop on.