Bicycle chase: interesting supercell near Toulouse, France

[Oscar van der Velde]

Hi all,

Last Tuesday I had the pleasure of issuing an ESTOFEX forecast, with this time widespread severe weather to be expected in western France. I work in Toulouse, which I thought would be at the eastern edge of the area affected by storms. forecast for July 4th

In the early evening I saw a storm develop southwest of Toulouse, on radar, with quite a classic V-notch feature briefly. There were a couple of other interesting V-notch storms, a big one near Agen. As my little storm seemed to drift into my direction, I decided to head out again to the hill, 15 minutes on bicycle (I don't have a car). The week before I did the same but the radar appeared to trick me, but let's not talk about that case...

As I got on top of the hill with my camera and 28mm wide angle lens, a sweaty experience with 86/66 F, I got rewarded by the view of the approaching cell. A very intense rain shaft, with small 'feet'. Some mammatus overhead with sometimes lightning in the anvil. A laminar cloud band lingered in front of the rain shaft.

First study the sequence of the 4th of July and get your own ideas...
link to the photo series of the storm

Interesting things happening....
An upward bulge developed in this laminar band, below the high rain-free base. This developed into a laminar upward cone and started going into the updraft base. Meanwhile, the northward extension of the cloud band became transparent and eventually disappeared. In just 12 minutes, a whole tower had formed that now looked fuzzy but somewhat turbulent.

I figured that the parcels from the cloud band had gone through the LFC, or perhaps the reverse, the LFC has somehow lowered locally to allow a full-fledged updraft tower now to begin at lower altitudes.
The tower growth seemed to be the cause for the disappearance of the cloud band downstream, as perhaps air would be going upwards instead of forward to wherever it was flowing to....

Anyway, I was very satisfied to see such a great play of nature during my own forecast day - what more can a meteorologist wish for! Well worth working some extra sweat for. This day/night yielded many reports of large hail and severe winds blowing off rooftops, trees blocking roads, widespread crop damage (I read that such a big event occurs about every three years), floodings, and power outages.

Besides this photo series, I also posted the June 25th anvil crawler lightning photos tonight. I have yet to bring all my Plains chasing into the galleries, starting tomorrow, so bear with me (thank god the season was so bad... -kidding)

cheers,
Oscar

 

[joel ewing]

Oscar, thanks for taking the time to post those really fascinating photos.
What a great storm-spotting vantage point you seemed to be on at the time. It makes me wonder how often tornadic storms actually happen in France, and if Toulouse (spelling?) experiences naders too.

 

[Oscar van der Velde]

It makes me wonder how often tornadic storms actually happen in France, and if Toulouse (spelling?) experiences naders too.
[/b]

Hi Joel,

There actually was a "Eurotornado" conference here in Toulouse in February 2000, which I did not attend unfortunately. It addressed the occurrence of tornadoes and other severe weather phenomena in Europe, mostly statistics per country. The follow-up in Prague, 2002, was named European Conference on Severe Storms, and I have visited the last one in León, Spain, in November 2004.

(on a side note, I found that severe storms research in Europe is generally in a quite infantile stage.... like presenting a radar loop and a sounding from a hail event and listing 1000 indices. I was very happy to see Harald Brooks' presentation about a world climatology based on shear/instability environments known to be conducive to severe weather)

There is an article about tornadoes in France: an article from 1989
Maybe there is something more recent but couldn't find it so quickly.
I do see quite a number of classic looking supercells on radar in France every year. Same in especially Germany, Switzerland, northern Italy, very often confirmed by reports of large hail, photos of wallclouds, not so often by tornadoes. It isn't so often that strong low-level shear happens together with good instability. Maybe today in Finland...

Oscar

 

[Andrew Khan]

Hello, Oscar. I have an inquiry for you, would it be reasonable to suggest that the "laminar upward cone", could have been some sort of vortex, stretching upward, rather than downward, perhaps? It seems possible given the ESTOFEX forecast, is saying extremely strong shear, which is likely why is capable of retaining it's solidified edges, of vorticity and not breaking up, by the other winds in the upper levels.

 

[Oscar van der Velde]

Hello, Oscar. I have an inquiry for you, would it be reasonable to suggest that the "laminar upward cone", could have been some sort of vortex, stretching upward, rather than downward, perhaps? It seems possible given the ESTOFEX forecast, is saying extremely strong shear, which is likely why is capable of retaining it's solidified edges, of vorticity and not breaking up, by the other winds in the upper levels.
[/b]

Hi Andrew,

this had come to my mind as well. I still assume the air would be moving upward inside of it (I mean, not that it is an upside-down tornado-like cone). The sharp point lets me think it is not a surge initiated from below by convergence for example... in my mind that would create a big bulge that would not grow upwards into such a narrow shape. It suggests a force from above that picks it up. However, if it really rotated I am surprised that not the whole band took on a spiral shape. It might be that winds at the rear side of the band were of opposite direction of that in front of the band.
Initially I thought a stable stratification in the cloud band causes its inertial character.... unless the whole band was rotating around its horizontal axis.....? This might be supported by its tube-like appearance when it dissolved at the north side. This would then be a very neat example of tilting of horizontal vorticity into the vertical by the updraft! But I never really saw such horizontal rotation in that band. Too bad I didn't bring my tripod and do time lapse shots.
Shear was strong, but not extreme, and low-level shear generally not strong enough for tornadoes (together with high LCL).

Here are some soundings from 12Z/00Z (storm was at 17Z... no 18Z available):

Santander, northern Spain
Zaragosa, northeastern Spain
Bordeaux, southwestern France

The Santander and Bordeaux environments produced a bow-echo storm, that continued all night ahead of the 500 hPa vort max. The Zaragosa profile must be quite applicable to Toulouse. In Toulouse reached T/Td 31/18 degrees Celsius.

Oscar