Thunderstorm Climatology part I

Summer is ending soon, here in Romania. For thunderstorms it was a rather good year with numerous thunderstorms, although most thunderstorms were of the airmass thunderstorm type. Well organized storms didn't occur often.

On average nearly 40 thunderstorms days do occur here in central Transylvania. To see how this is distributed over the year I did some statistical research. And this is the result:






Table 1. Number of thunderstorm days for LRTM (Tirgu Mures) for the last 20 years. Data extracted from www.tutiempo.net

Season starts rapidly in the month of May, with July as peak month. However, my experience is that most severe thunderstorms do occur in the month of June, when the jet-stream is stronger. In July and August the jetstream shifts to the north (on average of course). Rapid decline of the number of thunderstorm-days is from August to September when, again on average, dry arid conditions are the norm. Besides, the air in the upper levels is warmer by then, reducing the instability.

Because the continental airmass in winter is often cold and stable, winter thunderstorms are extremely rare in Transylvania. However they do happen in the period december till february: once in four or five years or so. Mainly in cases with an active coldfront or in a situation with wintry showers in northwesterly flow.

At last I will show my own observations of thunderdays from the village of Curteni (near Tirgu Mures) with data from the last three years:

* This number may change






Table 2. Number of thunderstorm days for Curteni in the period oktober 2007 - august 2010.

Hail and Lightning

Last weekend, in a moderate southerly upper flow regime, a lot of thunderstorms developed over Romania.
In the evening of Friday 6 august I had the opportunity to watch and photograph the first good night-time thunderstorms of 2010:























Saturday afternoon numerous storms developed in Transsylvania. Here is a satelite-image of 17:45 local time with many "small" anvils blowing north by the southerly upper flow.




In the early evening an interesting small storm developed just to the S-SE of my place.


Despite the moderate southerly flow (30-35 kt at 500 hPa) this developing storm was almost stationary for about 20-25 minutes.









Main cause for this was that surface winds where from a northwesterly direction, as can be seen in the GFS prognostic sounding for 15 UTC. However, surface winds were not 3 kt as indicated by the sounding but an estimated 10-15 kt. It is likely that this had impact on storm-motion, although it cannot explain the stationary character for 100%. Nonetheless, 0-6 km (vectorial) windshear was increased to 35-40 kt, which is sufficient for well-organized storms.




I observed thunder and when precipitation started, the storm started moving (slowly) north, because the storm grew to the tropopause, and was steered by the stronger mid-level and upper winds.
In the village where I live, rain turned into a rain/hail mix for a few minutes, with largest stones of 1,5 cm (0.6 inch). Not that big, but a few stones showed an interesting appearance: one stone with a double embryo and clear ice around it.


"Hailstones grow by collision with supercooled water drops. Supercooled drops are liquid drops surrounded by air that is below freezing. There are two methods by which the hailstone grows, wet growth and dry growth. In wet growth, the hailstone nucleus is in a region where the air temperature is below freezing, but not super cold. Upon colliding with a supercooled drop the water does not immediately freeze around the nucleus. Instead liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice."



Later in the evening active thunderstorms in the north and northeast caused some nice CG's.











One day later (Sunday 8th), this was repeated, but then most lightning was in a southerly direction:


What you see here occured in the whimp of an eye: all the lightning flashes stroke within one second.

Linear Pulse-like Storm: whalesmouth and blowing dust

On August 5th, halfway te afternoon, storms fired over the southern Carpathian mountains (near Brasov). With a southerly flow the storms were advected to the Transsylvanian basin where they tapped into a more moist and unstable airmass. There the storms rapidly arranged in a linear storm system.



The sounding of Bucurest was (in this case) representative for the airmass and windprofile of the area where the storms developed. Fairly large instability (CAPE over 1800 J/kg) and moderate vertical windshear. This explains the further development of the line of storms, which can be seen in the following radar-images:


17:20 lt


17:50 lt


18:20 lt













The storms were moving with a speed of approximately 60 km/hr to the north-northwest. In the middle radar-image perhaps a shortlived small bow is visible. From that time, the line rapidly desintegrated due to the imbalance between CAPE and SHEAR.
Although linear, this storm behaved like a so called pulse storm, which can briefly cause severe weather. The vertical windshear was too low to support a storm-system with well defined separate up- and downdrafts. Also due to the strong outflow, the gustfront moved away from the the line of storms. With stronger windshear the gustfront remains close to the storms. Only then deep convection can organize and develop into a self-sustained long-lived storm-system.

Nonetheless, when the storms reached the Tirgu Mures area, spectacular skies appeared in southeasterly direction:


Out of the hazy layers a sort of battleship appeared.


I was expecting a shelfcloud, instead a large whalesmouth rapidly moved over my position. Notice the large curved front-edge of the whalesmouth.







Perhaps here a small shelf is visible?
















The wind is getting stronger, also some small funnel-look-alikes are visible on the front-edge where inflow meets outflow. Sometimes they become longer and small scuds are moving upward beneath. They aren't rotating and after ten of twenty seconds they disappear.



Dust is kicked up by the strong winds. On the picture one can see tree-crowns are blown nearly horizontal.















Skies are changing with every second that passes. Also there are white holes in the turbulent sky: high contrast, so rather difficult to photograph.


Best skies are in the north now, so I go to our street where horses show up out of the blowing dust....




Hmmm, this one is already halfway the distance cloud-earth. The border between inflow and outflow is where things got interesting today.....


















At last a satelite image of 18:15 local time. For an animation of the images click HERE

fragmented shelfcloud

In the early evening of the first day of august 2010 I observed a nice fragmented shelfcloud.


A SE-NW moving thunderstorm complex with highest radar reflectivities at the front approached the Tirgu Mures area. Since winds at all levels of the atmosphere were low, rather slow moving thunderstorms were the result.


At first sight it seemed a solid shelfcloud was approaching, but when it came closer from the east, it was clearly fragmented. Nonetheless, the clouds I observed were nice to watch:





















This was a bit better than all the airmass thunderstorms from the last two weeks. In an environment of moderate instability and low vertical windshear this is the best you can get, I guess. Also I think I was just lucky to be in the right place, since the outflow moved away from the thunderstorm-complex, which couldn't organize well because of the petty shear.

July 2010 Reprise

A few photographs from last month of July which were not published in my blog:


Thunderstorm with strong updraft in the late afternoon of 13 July.

















Two pictures of a rather flat, blocked shelfcloud in the evening of 13 July.
















Anvil with sheet lightning in the evening of 13 July.












First decent lightning-shot of 2010 in the evening of 13 July.












Strange, extremely high towering cumulus in the early evening of 19 July.