Showers in polar air

In the early evening of Sunday 25 July 2010, dark-based showers with a manacing look, moved slowly in north-northeasterly direction. For a short time I observed weak rotation in lowered ragged fractus clouds beneath the base. After that the shower came in from the northwest and moved slowly southeast. So direction had changed!


















I think that a convergence-line/surface trough was in my neighourhood and caused this change of movement of the shower. It is possible that a pre-existing vortice in the surface windfield became stretched in the updraft and caused the observed rotation. Perhaps the late day-time/weak instability prehibited the forming of a real funnel.











This was not the first time that showers/thunderstorms in a (modified) polar airmass showed some nice visual sights.





Here a bonus-image of the sunset of 23 July with dissipating convection (at that time the airmass was still warm and humid).

Airmass Thunderstorm

Till July 22nd, I already counted 15 thunderstorm-days this month, which is above normal. Most of these thunderstorms are of the type which meteorologists describe with the term airmass thunderstorms.




Airmass thunderstorms develop in the course of the day due to the heating of the surface, and rapidly dye out after sunset.
The life cycle of one cell takes typically 30 to 50 minutes. They usually are not strong enough to produce severe weather.


A true single cell storm is actually quite rare. Even with isolated storms in weak vertical wind shear, the gust front on one cell often triggers the growth of another cell some distance away.
Single cells in this way form the building blocks of larger convective systems.




Thunderstorm in the direction of Bistrita, Romania. 21 July 2010.









In the late afternoon/early evening of 21 july, such a single cell developed exactly over my location. Since there was hardly any wind on all levels of the atmosphere, it didn't move and dropped it's precipitation right on the village where I live.



Radar-image of 21 July 2010, 18:40 local time.



VIS satellite-image of 21 July 2010, 19:15 local time.












And here the result..... more than 20 mm of rain in about three quarter of an hour.
















One day later (22 July) another round of airmass thunderstorms produced some photogenic oportunities:



Growing thunderheads in easterly direction.



Nice downdraft from a cell in the north. It produced moderate gusts (estimated 25 kt) when reaching my location.

Gustfront / Outflow Boundary

In the afternoon of 11 July 2010 showers and thunderstorms developed over the eastern Carpathian mountains. With a weak northeasterly flow they were advected to the Transylvanian Basin.


The storm to the northwest of Tirgu Mures produced thunder, but it didn't reach my location. However, a band of clouds with a rather flat base moved in my direction. (red dot = my position)



When it passed, the wind increased and it became cooler. This passage was the so called gustfront.
I also observed that the cloudband was curved and extended to the far west.





A gust front is the leading edge of cool air rushing down and out from a thunderstorm. When the downdraft and rain-cooled air reach the ground, the rain-cooled air spreads out along the ground and forms a gust front. Usually the winds associated with the gust front are not severe, but in extreme cases, a downburst can develop and produce severe wind gusts.
There are two main reasons why the air flows out of some thunderstoms so rapidly. One reason is the presence of relatively dry (low humidity) air in the lower atmosphere. This dry air causes some of the rain falling through it to evaporate, which cools the air. Since cool air sinks, this causes a down-rush of air that spreads out at the ground. The edge of this rapidly spreading cool pool of air is the gust front. The second reason is that the falling precipitation produces a drag on the air, forcing it downward.


The gustfront moved further south and I saw that on top of it the convection became deeper.


It developed to a nice towering cumulus, while the other clouds of the band almost dissipated.



Finally it became a shower, which also produced a few rumbles of thunder, thus officially a thunderstorm.
However, this thundery shower died soon, because the gustfront moved south faster than the shower, thereby cutting off it's energy.


Here's a satellite-animation where one can see the rapid movement of the gustfront (or outflow-boundary). Behind the boundary clearings can be seen!
The red arrow marks the new developing thundery shower near Tirgu Mures.



If the animation doesn't work click
here

the Shelf Cloud of 18 June 2010

A few weeks ago (in the late afternoon of 18 June) a squall line passed my location. It developed from a confirmed supercell near the city of Turda (hook-echo on radar, strongly righmoving and hail with the size of chicken-eggs), but became outflow-dominated before reaching my location. It produced a nice shelfcloud.
In the (near) future I hope to write more about shelfclouds since these can quite often be observed and are spectacular phenomena. For now enjoy the photographs:

Whales Mouth

On Wednesday 7 July I knew that storms and showers could develop but I wasn't expecting too much. In the mid- to late morning hours storms developed southwest and west of Tirgu Mures. They already clustered a bit, and when the leading edge passed my location this phenomena appeared:



This cloud formation is called a whale's mouth. It marks the leading edge of cold air expanding from beneath a thunderstorm. With some imagination it looks like the inside part of whales' mouth.....




"A whale's mouth forms when a downdraft of cold air descends from a thunderstorm. Upon reaching the ground, the pool of cold air spreads out, much like a puddle of water expanding. At its leading edge, it forces the warm, moist air surrounding the thunderstorm to rise and cool. This cooling causes moisture in the warm air to condense, producing the whale's mouth cloud.
The boundary between the warm and cold air is very turbulent, which is what gives the cloud its gnarled appearance."









In the direction of the city of Tirgu Mures (that's looking SSW) I saw a rather conspicious lowering with several tuba-like clouds. However, rotation was not observed, so it wasn't a tornado......
















It became clear that the storms clustered together and a linear system was developing. This was confirmed by radar later:


Storms were moving from SW to NE while the line is oriented NNE-SSW, so storms are almost moving along the line.
This is quite common and often observed in squall lines. It is the repeatedly formation of new storms on the front of the line, what causes the W-E propagation of the whole system.


propagation = movement of the individual cells + development of new cells

multicell thunderstorms

On Friday 1 July I witnessed the approach of a multicell storm. The storm came in from the northeast and showed all characteristics that can be found in textbooks.
As far as I know, besides thunder and rain, it didn't produce severe weather, but it was just fun to watch.....
















The multicell storm consists of a group of cells moving as a single unit, with each cell in a different stage of the
thunderstorm life cycle. As the multicell cluster evolves, individual cells take turns at being the most dominant.











A multicell storm forms when new convective cells develop along the boundary of the cold pool originating from an older cell. Such a boundary is often called an outflow boundary or gust front.These new cells tend to form along the upshear side of the storm, with mature cells located at the center and dissipating cells found along the downshear portion of the storm. Downshear is the direction from which the low- level wind blows when moving with the average wind (the movement expected for individual storm cells).



The average cell undergoes a life cycle that typically spans an hour or less, though the parent storm can survive for many hours. The longevity of the multicell storm is largely due to its ability to continually generate new, replacement cells as older ones decay.