Thursday, February 23, 2006

Column 15: thunderstorms, part 1

Spring is coming. That means that thunderstorm season is just around the corner.

I thought now would be as good a time as any to get back to basics and discuss thunderstorms. This column will be the first of an I-don't-know-how-many-part series on thunderstorms.

Today we'll concentrate on the different types of storms. The thunderstorm spectrum, if you will. And for our purposes here, I'm mainly going to talk about surface-based thunderstorms.

Let's start off with the single-cell thunderstorm.
A bubble of air (hereafter called a "parcel") rises because of a local enhancement of moisture or heat, and it keeps going as long as it's less dense (usually warmer, but this can also mean more moist) than the surrounding air. On its way up the parcel encounters very little wind. Eventually the air cools and condensation starts to occur. When this occurs, rain drops (or their frozen precursors) start to form, which then fall downwards. Still, because of little wind throughout the layer, the raindrops fall through the rising air (called the "updraft"), thereby negating and eventually erasing its effect. This process usually takes about an hour. This kind of storm is called may things: an air mass thunderstorm, a garden variety thunderstorm, a pop-up thunderstorm, a popcorn thunderstorm... They can occasionally cause brief (VERY brief!) severe weather, but usually it's just a little bit of rain and a wind gust to 40 km/h.

Then there's the multicell thunderstorm.
The same air parcel rises upwards, but this time it encounters a little bit of wind on its way up, and let's assume that the wind increases throughout the parcel's ascent. (Until it hits the tropopause or ends being warmer than its surroundings, that is.) The first effect of this is then to make sure that the rain doesn't squash the updraft. But still, what goes up must come down. So you get the rain and wind moving downward, separate from the updraft, in what's called a downdraft.
Where this downdraft falls, it's usually cooler than the air that's in place, so it acts as a wedge, lifting up more surface parcels. These newly-lifted parcels form new thunderstorms, which also don't rain themselves out. So rather than one storm, you get a cluster or line of storms.
Multicell thunderstorms can be severe. The 2 main types (or at least the main 2 I can think of right now) of severe multicell storms are bow echoes and mesoscale convective complexes (MCCs). These storms arise usually if the low-level wind shear is great enough (bow echo--a prolific damaging wind-maker), or if there's copious lift from a low-level jet lifting moist air over a warm front (MCC--a copious rainmaker).

Finally, we get to everyone's favourite, the supercell thunderstorm.
Supercell thunderstorms occur in environments where the wind speed (and direction) change, wind shear, is large enough to cause something very interesting to happen.
Picture an environment where the wind is from east to west at the surface at 20 km/h. Then let's say that it changes suddenly to being west to east, 20 km/h, at 1 km above the surface. Right at the change, if you put a giant windmill there, what would happen? The wind from west to east (above) and the wind from east to west (below) would combine to make the windmill spin. Well, this is what sets up when you have enough wind shear. Except instead of a windmill spinning, it's giant rolls of air spinning on their sides, sort of like a pencil rolling on a table.
Well, take that (pencil on a table) horizontal roll, and blast an upward-moving air parcel into it. The spin keeps going; it's just had its orientation changed, though, so that instead of rolling on a table, the pencil is now spinning like a figure skater. Keep this rotation going through the air parcel's upward motion, and you have a rotating thunderstorm: a supercell.
Supercells are special in that this rotation causes a bunch of really theoretical science-y stuff to happen. Because of this, the updraft and downdraft are not only separated, but they stay close together. This causes the downdraft air to be ingested back into the updraft, causing a steady-state system, one that can last for hours on end. As well, the science-y stuff actually enhances the speed of the updraft, making the storm stronger.
Supercells cause the majority of severe weather caused by thunderstorms. Anytime you see large hail (that's nickel-sized and larger), in all likelihood it has been caused by a supercell. If you see a strong tornado, it was definitely caused by a supercell.
(I'd go into how tornadoes form here, but it's a column unto itself. I'll try to remember to write it sooner rather than later.)

Every thunderstorm is different. You're likely to see not a supercell, but a multicell that acquired sufficient rotation and then the complex morphed into a supercell. Or a supercell that tapped copious low-level shear and changed to a bow echo.
Seeing the classic type of storm is a rare occasion unto itself.

Having a general understanding of what you're looking at gives you a better idea, then, of what type of weather to expect.


At 9:44 AM, Anonymous The Manitoba Storm Spotter said...

When thunderstorms last for more than an hour, are they usually multicell storms? I'm still trying to figure them out exactly, but I saw one illustration where they say multicell storms form one after another in the same line. Is this why multicell storms usually last a while? How long does the typical multicell cluster last for?

There was also another type of thunderstorms that puzzled me. I think their called elevated thunderstorms and their common in September and early fall? Why is that?

At 9:51 AM, Anonymous The Manitoba Storm Spotter said...

I just wanted to add one other thing in regards to multicell storms. If you had a thunderstorm that ended and it seemed like the storms were over only for another one to occur say an hour later, then another an hour later after that, would that be considered multicell storms or just separate thunderstorms occurring at different times all together? Or is multicell storms just those one big storms that last for a few hours with no breaks in between storms and no storms? This is something I have always wanted to know.

At 4:11 PM, Blogger Weather Central said...

usually if it lasts more than an hour, it's at the very least not completely a single cell. it might be a hybrid single/multicell cluster.
the reason you said there is pretty much exactly why they usually last a while. 2 to 4 hours, in my experience, is how long they typically last.

elevated thunderstorms are actually common anytime during thunderstorm season. they're storms that are initiated above the surface, usually with a low-level jet being lifted up to its level of free convection over a warm front. they're common whenever there's moisture there, so may till september is the season. but they're most common in june and july, when moisture is most plentiful.

as to your next question, if a storm was over, kicked out an outflow and then another storm started on that outflow an hour later, i'd call those separate single cells. multicells usually are many storms blobbed together. you can usually see them from a distance as 3 or 4 or however many updrafts going up.


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