elevated thunderstorms and high-based thunderstorms
my friend matt powers from the college of dupage made an interesting point on email the other night, pointing out a mistake that's often made, is easy to make, but is a fairly significant one.
the gist of what he was saying was that elevated thunderstorms and high-based thunderstorms are rarely the same thing.
let me explain.
elevated thunderstorms are thunderstorms having their airmass source region somewhere above the surface. this usually happens when there's an inversion near the surface, for whatever reason--warm front, nighttime (aka nocturnal inversion), whatever the reason. the air being ingested by the thunderstorm isn't hugging the ground--it's above it. but usually (although not always), elevated thunderstorms feed on very moist air. that means that the cloud bases are fairly low to the top of the inversion. and unless you have a high inversion, this also means that the elevated thunderstorm has a relatively low (relative to the ground) base.
high-based thunderstorms are most often found in dry airmasses on hot days. the source region of the air being ingested into the storm is almost always at the surface. the reason the cloud base is so high is that the airmass is so dry that it takes a lot of cooling, the amount of cooling created when an air parcel is lifted to great heights, before saturation and cloud formation can occur. but like i said, the source of the air going into these storms is at the surface.
so to sum the theoretical part up, elevated thunderstorms generally have fairly low bases. but high-based thunderstorms are almost always surface-based.
confused yet? here's an example.
back in 2002, i was out chasing storms in southern manitoba. the airmass was ripe for big, bad storms to occur. 33/23, if my memory serves correctly. capes around 6000 j/kg. and the shear. oh, the shear! nice veering profile, strong mid-level winds, and enough backing at the surface to make things interesting. when storms finally did go, something was wrong. with the spread, i expected cloud bases to be at about 3000 to 4000 feet. and with the shear, the average wind throughout the column was about 15 knots--so that's how quickly i expected the storms to be moving.
well, was i wrong. the cloud bases were about 6000 feet (not in the high-based range), and the storms were moving at about 50 knots. (!!!!)
so what was going on?
the storms were ingesting air from slightly above the surface, but not the air right at the surface. that had 2 consequences. first, since the lowest moisture wasn't being tapped, the clouds were starting higher than expected. second, and more importantly, the storms also weren't tapping into the low-level backed winds, so the effective average wind through the storm layer was about 50 knots instead of about 15. in the end, i was able to relay this to the office and they held off on issuing warnings based on my reports, as told them they were elevated and thus a lot less likely to do any damage. and they didn't.
i was going to write a similar example about a high-based surface-based storm (now that i type it like that, it looks like a contradiction, but i'll get to that right away) but i can't really think of one. but you've all seen them: the storm that has its lowest cloud somewhere way up there, and you can just tell by looking at it that it's fairly benign.
to clarify from the last paragraph, and to sum up: when we talk about the storms, we talk about surface-based and high-based. don't let those names fool you. the "based" part refers to different things in each example. in "high-based", the "based" part refers to the lowest part of the cloud. the cloud base. in "surface-based" the "based" part refers to the source region of the air being ingested by the thunderstorm. the opposite pairs, then, are surface-based/elevated, and high-based/low-based. high-based thunderstorms, i would say, are those with cloud bases above 7500 feet.
wow, matt, thanks for making me type for about a year! ;)
posted by Dave "storm structure" Carlsen @ 11:39 AM
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