The April 13th Landspout in Calgary

I’m not here to persuade anyone of my findings, but to show the process I went through that ultimately led me to adjusting my thinking to what meteorologists already knew. After having recently spoken to a meteorologist from Environment Canada regarding the event, I decided to go back and have a look at as much available data as possible for myself.

On April 13, 2016, a probable landspout tornado occurred near the interchange of Stoney Trail  and the Trans-Canada Highway in northeast Calgary at approximately 3:50pm MDT. Due to its proximity to a major population center, the tall, dusty vortex was witnessed and recorded by many people.  

I had initially doubted that the vortex was a tornado due to what turned out to be a few faulty assumptions about non-supercell tornado environments. My skepticism was based on the following:

1) The tornado was somewhat of a statistical outlier, being the earliest recorded since at least 1980;
2) The tornado occurred in a low CAPE environment (though not non-zero), in a very dry boundary layer; and
3) I was unable to find any convincing evidence that the vortex was linked to a parent updraft – which is necessary for the designation of “tornado” - along with my personal observation of the high-based, shallow depth, glaciated appearing and virga producing convective clouds, which I believed almost precluded entirely the possibility of a tornado occurring.

I would like to address each of my own objections, and share my findings with those who are interested. Keep in mind this will be a very informal treatment of this topic, especially given the forum in which it is being presented (my storm chase blog!). I primarily used the SPC’s Mesoanalysis Archive to retrieve graphics of relevant data about the meteorological conditions near Calgary on April 13 - and while it may not be as good as having some real time data (that I failed to collect around the time of the event), it is the best I will be able to work with.

Here is the link to the SPC’s mesoanalysis archive.

A glossary of the acronyms used in this post is listed below as well.

Conceptual Models of the Formation of Dust Devils and Non-supercell Tornadoes

Tornadoes can be associated both with supercell thunderstorms, as well as with non-supercell thunderstorms. Environments favourable for the development of mesocyclonic tornadoes are typically synoptically energetic and require specific atmospheric conditions, including usually significant instability, strong low level wind shear, and rich boundary layer moisture that leads to low LCL heights. And while nonmesocyclonic tornadoes may also form in such environments, their formation is not limited to them. In fact, they often form in synoptically benign environments, associated with higher based convective clouds and weak vertical wind shear. Supercellular tornadogenesis appears to be strongly dependent on the presence of downdrafts, while non-supercell tornado formation is often hampered by downdrafts – being instead more often associated with developing updrafts beneath rapidly growing TCU.

The classic case for the development of non-supercell tornadoes, often called “landspouts”, occurs when shearing instabilities along axes of converging winds trigger the development of columns of low level vertical vorticity, that can “spin up” into tornadoes if they become situated beneath rapidly developing updrafts (Davies, 2006). Vortex stretching can be further enhanced by the presence of steep low level lapse rates – a condition that also favours the development of dust devils (Brady & Szoke, 1989). Low level superadiabatic lapse rates (lapse rates greater than that of dry adiabatic, or more than 10C/km – a condition of “absolute instability”) lead to low level convective instability, which can result in the formation of vigorous thermals near the Earth’s surface (Onishchenko, Pokhotelov, Horton, & Fedun, 2015). These thermals often form on hot days with light winds over dry surfaces, and can spin up into dust devils when they become collocated with a local source of vorticity (resulting from eddies of wind interacting with buildings or differential heating of surfaces, for instance). Thus, under certain circumstances, we should not be surprised to see the two occurring in the same environment.

Schematic of the development of a non-supercell tornado, as a result of existing low level vorticity being stretched by the updrafts of rapidly growing TCU above (Wakimoto & Wilson, 1988).

Synoptic Background Environment on April 13, 2016:

Around the time of the event in the late afternoon of April 13, a 500mb shortwave trough was traversing SE Alberta, with an attendant area of positive vorticity advection and associated upper divergence over SW Saskatchewan that was supporting a surface low there. A weak cold front associated with the low extended back SW toward the Rocky Mountain foothills and swung through southern Alberta, with Calgary International Airport reporting the associated wind shift from westerly to northerly by 4PM MDT.  

Temperatures on April 13 were seasonally a few degrees above average, with a surface dewpoint depression of approximately 24C prior to fropa. As supporting mesoanalyses graphics from the SPC will indicate, a deep, very dry boundary layer with steep low level lapse rates was in place when the event occurred. Indeed, as insolation increases in intensity in springtime, the development of superadiabatic lapse rates in the low levels above a dry surface is not uncommon.

22Z 500mb height and temperature

The shortwave trough is evident over SE AB and SW SK. Also, when viewed in series, WSW flow aloft over southern AB can be seen to be advecting colder temperatures from the west into the region, aiding in destabilization of the middle troposphere.

22Z 700-400mb differential vorticity advection

The vort max associated with the 500mb shortwave trough is indicated over SW SK, with associated divergence aloft as a result of PVA.

23Z MSLP and winds

The 23Z image was selected since it shows the contours of the pressure trough associated with the surface cold front better than the 22Z image. The wind shift line, now just south of the Calgary area, is clearly evident.

CYYC METAR at 21 and 22Z.

Of note: a wind shift from WNW to NNE, and towering cumuli with base heights of 9300-9500ft AGL (approximately just under 3000m).

Hourly data from Calgary International Airport, between 12PM and 8PM MDT

The time span was selected to paint a picture of the trends in the times both pre and post-event. Of note, slight pressure falls prior to fropa with rises afterward, as expected. The quality of the air mass being advected in post-wind shift is not markedly different, with only a slight drop in temperature and a marginal increase in RH (it was still very dry). Dropping dew points with rising temperatures in the early afternoon likely indicate mixing of drier air aloft in the boundary layer.

Historical METAR data was retrieved from here

Historical hourly data was retrieved from here

The Event:

Early in the evening of April 11, a very impressive dust devil was observed in Airdrie:

Airdrie Dust Devil

This was the first of several very impressive dust devils, as well as one landspout that occurred that week. It bears mentioning that numerous other vortices could have occurred around southern Alberta due to the favourable environment, but were not observed due to their transient nature, distance from population centers or human activity, and often translucent appearance.

Two separate video clips of the event in NE Calgary can be seen at this link:

Calgary Landspout Video

Also, here’s a photo from Tim Hall posted on the Alberta Storm Chasers Facebook page:

“Not sure which dusty this is but it was massive, shot from the corner of 567 and 772 west of Airdrie.”
– Tim Hall

The photo and videos of the landspout were very impressive - and while a link to a parent updraft is not conclusive in this photo alone, one of the video clips (the one shot while driving west) seems to very clearly reveal a line of dark, flat-based TCU directly above the circulation.

As the METAR indicated, several TCU were present in the Calgary area from mid-afternoon through early evening. And while I was unable to obtain close-up reflectivity or velocity radar scans around the time of the event, images from EC’s public radar are archived at this link:

http://climate.weather.gc.ca/

XSM radar at 350PM MDT (time of the event)

Very weak echoes can be seen INVOF north Calgary, where TCU were being observed.

XSM radar at 410PM MDT

Now, some precipitation echoes are visible along the NE side of Calgary, albeit still quite weak. Moreover, given the deep, dry boundary layer within which these echoes exist, it is possible that little or no precipitation associated with these returns is making it to the ground – falling instead in the form of virga. Nonetheless, the difference between the two frames reveals that updrafts existed in the area where the landspout was observed

The full loop of images surrounding the event can be viewed here.

Here’s a copy/pasted version of the AWCN from EC found in an article about Alberta’s first tornado of 2016 on a NewsI880 page:

The Favourability of the Environment for Non-Supercellular Tornadogenesis:

It should be noted that the SPC does have a composite index called the “Non-Supercell Tornado parameter”, however it is not available in the mesoanalysis archive. Nonetheless, it can help us in understanding the quantitative variables that constitute non-supercell tornado environments.

Screen grab of the product from the SPC Mesoanalysis website (http://www.spc.noaa.gov/exper/mesoanalysis/help/help_nstp.html)

We have already determined that a wind shift line was present, resulting from a weak cold front tracking through the Calgary area in the late afternoon. This boundary would not only serve as a source of low level vorticity, but also of convergence that could lead to the development of TCU – given ample instability in the environment. 

While there wasn’t a tremendous amount of instability, a decent amount relative to the time of year was indicated on the mesoanalyses, with SBCAPE values of over 250J/kg, and MLCAPE values of up to 250J/kg during the late afternoon. I personally found these values to be surprising given the very low moisture content of the boundary layer, but very steep lapse rates and cold 500mb temperatures likely permitted a “sliver” of CAPE to develop. The presence of CAPE (even in a relatively shallow layer) and lack of CIN would enable the development of convective cloud given ample lift, in the form of CU and TCU (that were indeed observed in the late afternoon).

22Z SBCAPE and SBCIN

250J/kg of SBCAPE or more, with little to no SBCIN was present in the Calgary area.

23Z MLCAPE and MLCIN

The 23Z analysis was selected since the 250J/kg contour was not present on the 22Z, however it shows that MLCAPE values near the front around the time of the event were appreciable.

22Z 3km CAPE and surface vorticity

While the mesoanalysis doesn’t indicate appreciable surface vorticity, its resolution would be unable to account for local scale vorticity generated along the wind shift line

Deep layer wind shear wasn’t weak, with 0-6km bulk shear of up to 30 knots in the area (due to 500mb SW winds), however it didn’t appear to be enough to shear apart the updrafts that developed in the area (I personally observed a couple of really nice slanted turkey towers that afternoon).

22Z 0-6km shear

Other images in the 6 hour series available surrounding this time indicate a west-southwesterly 0-6km shear vector in the 20-30 knot range.

Of particular note were the existence of steep lapse rates, both in the low and mid-levels of the troposphere. This, combined with a local source of vorticity associated with the wind shift, was likely the main factor in vortex stretching that led to the formation of the landspout.

22Z Low level lapse rates

Lapse rates of at least 10C/km are indicated by the contour analysis – with local values likely far exceeding that nearest the surface. Also, given the downslope flow in the “warm sector” ahead of the approaching cold front, dry adiabatic lapse rates would have already existed (especially in a well-mixed boundary layer) – becoming readily superadiabatic with insolation in the contact layer.

22Z Mid-level lapse rates

Lapse rates of at least 8.5C/km in the middle troposphere are indicated, which are considered “steep”. This would have contributed to the small amount of instability present (that would have been greater with increasing amounts of low level moisture). LIs were near -1 (not shown).

I was unable to collect a prog sounding representative of the environment, however I did plot the information that was available to me on a Skew-T to visualize the trajectory of a surface-based lifted parcel (based on measured temperature and dewpoint in the “pre-event” environment). The lifted parcel would become saturated just below 3000m, and its temperature was about -26C at 500mb (accounting for the LI of -1 indicated on the mesoanalysis). The LCL and LFC were likely in close proximity, accounting for the trace of CAPE values below 3km, and would extend for some vertical depth above 500mb.

The Frequency of the Occurrence of April Tornadoes in Alberta and Other Similar Events:

Thanks to the Canadian National Tornado Database becoming available earlier this year, we have been able to see a list of all confirmed tornadoes in the country during the 1980-2009 period. Here is a link to the database:

Canadian National Tornado Database

According to the 30 year climatology (1980-2009), a total of 456 confirmed tornadoes occurred in Alberta, resulting in an annual average of 15.2. During the period, a total of 7 those tornadoes were documented during the month of April. They are as follows:

April 27, 1984 at New Sarepta, F2 (possibly a supercellular tornado given this rating)
April 28, 1984 at Boyle, F1 (also possibly supercellular, but non-supercell tornadoes occasionally reach this strength)
April 30, 1997 at Bassano, F0
April 20, 2001 at Calmar, F0
April 24, 2003 at Stand Off, F0
April 18, 2004 at Raymond/Magrath, F0
April 20, 2004 at Calgary, F0

(Note that the “F” scale, or “Fujita” scale, was still in use prior to 2013, when the “EF”, or “Enhanced Fujita” scale was adopted in Canada).

It is possible that others occurred in the 2010-2015 time frame that I personally am not aware of, but a database of these events is not yet public. Of note, in 2004, another early season tornado was reported near Calgary. So while Alberta tornadoes in April are rare, they are not unheard of – occurring at a frequency of about 1 every 4 years (per the ‘80-‘09 climatology, which was a greater frequency than the occurrence of significant (F2+) tornadoes during the same period). It should also be mentioned that these are only tornadoes that were confirmed – however, this doesn’t mean others haven’t occurred. Other factors such as a lack of social media in the past, less population, and the possibility that similar landspouts may not be reported as tornadoes due to their “dusty appearance” or lack of a parent funnel, likely means that the actual number of occurrences is higher – especially given the short-lived nature of such phenomena.

For anyone interested, a nice meteorological breakdown of the April 18, 2004 landspout near Raymond exists at the following link:

http://www.umanitoba.ca/environment/envirogeog/weather/raymond/raymond.html

In this event, a similar thermodynamic environment existed as on April 13, 2016 – however, a thunderstorm occurred at Raymond while the Calgary event was not associated with thunder.

Two other landspout events occurred this year in the United States that had markedly similar visual characteristics to the Calgary landspout:

April 12 near Greeley, CO (a day before Calgary, and also Colorado’s first tornado of 2016):
http://denver.cbslocal.com/2016/04/12/colorado-first-tornado-season-greeley/

August 9 near Chicago’s Midway Airport:
https://weather.com/storms/tornado/news/chicago-tornado-landspout-9aug2016

And: http://www.weather.gov/lot/chicagolandspout_16Aug09

All were associated with high-based CU or TCU, and all developed along some sort of low level convergence boundary (a cold front in Calgary, a lake breeze front in Chicago, and local effects of the DCVZ near Greeley). In the case of the latter, Wakimoto and Wilson documented 27 non-supercell tornadoes in NE Colorado in 1987, with several interesting findings relevant to this discussion. The vortices occurred in a similar high-plains environment to southern Alberta, characterized by drier climate and generally higher-based storms. At least one vortex documented was only associated with CU, and several had CAPE values less than 500J/kg, suggesting that a more important factor in non-supercell tornadogenesis is likely the presence of steep low-level lapse rates.

Here is a link to the paper:
Non-Supercell Tornadoes

Conclusion:

While not 100% certain, it seems that given the conceptual model of non-supercell tornadoes and the meteorological environment near Calgary on April 13, 2016, that a landspout tornado is the most probable explanation of the vortex that was observed that afternoon. Having noted other similar occurrences this year, reviewed the scientific literature, and seen at least once convincing piece of video evidence, I was able to change my mind about my original conclusion. My pre-conceived meteorological notions, combined with a propensity to debate ideas, precluded me from seeing this earlier. Regrettably, despite a lack of any personally directed attacks made, my public doubting of the findings of Environment Canada’s meteorologists regarding the event led me to upset some folks. However, my keenness as a weather forecasting student has enabled me to realize my error, and learn from it. I had immense fun doing the research for this blog post, which if anything, will be fun and interesting to read for myself in the future.  

References:

Brady, R.H., Szoke, E.J. (1988). A Case Study of Nonmesocyclone Tornado Development in Northeast Colorado: Similarities to Waterspout Formation. American Meteorological Society, 117, 843-855.

Davies, J.M. (2005). Tornadoes in Environments With Small Helicity and/or High LCL Heights. American Meteorological Society, 21, 579-593.

Onishchenko, O., Pokhotelov, O., Horton, W., & Fedun, V. (2015). Dust devil vortex generation from convective cells. Annales Geophysicae, 33, 1343.

Wakimoto, R.M. & Wilson, J.W. (1988). Non-supercell Tornadoes. American Meteorological Society, 117, 1138. 

Glossary of acronyms:

CAPE – Convective Available Potential Energy; a measure of potential updraft strength. Sometimes seen with the prefix “SB” (surface-based) or “ML” (mixed layer)

CIN – Convective Inhibition; the amount of energy required for a parcel to reach its LFC (level of free convection – where it rises on its own due to buoyancy without being forcibly lifted)

CU - Cumulus

DCVZ – Denver Convergence Vorticity Zone; an area in northeastern Colorado where enhanced low level vorticity develops due to SE winds interacting with the Palmer Divide (a W-E ridge SE of Denver). The higher than normal amounts of low level vorticity in this area leads to its being one of the most tornado-prone regions in the world (due to an especially high number of non-supercell tornadoes).

EC – Environment Canada (now correctly ECCC - Environment and Climate Change Canada)

FROPA – Frontal passage

INVOF – In the vicinity of

LCL – Lifting Condensation Level; the level at which a parcel that is forcibly lifted becomes saturated

METAR – Meteorological Aerodrome Report

PVA – Positive Vorticity Advection; an area where higher values of vorticity are being advected toward – often associated with upper level divergence.

RH – Relative Humidity; a ratio expressed as a percent of how close the air is to saturation

SPC – Storm Prediction Center (United States)

TCU – Towering Cumulus