Bob Hartig
EF5
Over the last couple of years, my interest has gotten piqued by the role of gravity waves in tornadogenesis. This is a topic that surfaces from time to time in papers or discussions of storms such as the Jarrell tornado, but it's not something I hear about often. Probably that's because gravity waves are a mesoscale rather than a synoptic feature, and they seem serendipitous rather than predictable. More knowledgeable heads than mine, feel free to correct me if I'm wrong, but grav waves appear to be one of those things of which a forecaster says, "Oh, look, gravity waves," rather than, "We're expecting gravity waves to show up today."
So these things are a fascinating enigma for me. I haven't understood exactly what they are nor, until recently, the part they sometimes play in tornadogenesis.
Then one night last July, a severe thunderstorm moved across the south end of Grand Rapids and rolled an EF1 tornado across my old neighborhood, just one block away from the apartment I once lived in. Some of the damage was significant, with at least one wood/steel/cement-block commercial building leveled (seemed like more than EF1 damage to me, but I'm no expert on structural integrity).
KGRR did an excellent analysis of the event and concluded that gravity waves (which they called "reflectivity tags") had played a significant role. One of the mets gave a presentation at a meeting of the AMS, and I think I finally grasped the basics of how these things work. Here's the gist of it in a humble layman's terms:
Gravity waves enhance low-level vorticity. They don't produce it; if some appreciable degree of vorticity isn't already present, then gravity waves accomplish nothing. But if they've got something to latch onto, then gravity waves can take a preexisting circulation and amplify it.
They accomplish this progressively. A single gravity wave probably isn't going to get a nascent mesocyclone to spin up a tube. What it will do is stretch the circulation vertically, causing it to tighten. (You know: the old analogy of the twirling ballet dancer bringing her arms to her side.) As the wave passes, so does the stretching effect, and the circulation "relaxes." However, it retains some of the new energy, and the next wave in the series adds to it. The effect is cumulative, with each new wave further tightening and intensifying the circulation until it reaches a critical point.
Obviously there are other factors also contributing to the mix, but the July tornado has convinced me that gravity waves are something to take into serious account when they're present.
Here is a reflectivity grab of what I believe were gravity waves during a convective event last year in Nebraska.* The waves were also clearly depicted by velocity and spectrum width products. The spectrum width did a particularly good job of showing the waves interfacing with storms on their western edge. (First time I've found a use for spectrum width.)
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* At first I wondered whether these were OFBs--but a whole series of them so close together? I've never seen such a thing before, and it just doesn't sit right with me. So I'm assuming these are gravity waves.
So these things are a fascinating enigma for me. I haven't understood exactly what they are nor, until recently, the part they sometimes play in tornadogenesis.
Then one night last July, a severe thunderstorm moved across the south end of Grand Rapids and rolled an EF1 tornado across my old neighborhood, just one block away from the apartment I once lived in. Some of the damage was significant, with at least one wood/steel/cement-block commercial building leveled (seemed like more than EF1 damage to me, but I'm no expert on structural integrity).
KGRR did an excellent analysis of the event and concluded that gravity waves (which they called "reflectivity tags") had played a significant role. One of the mets gave a presentation at a meeting of the AMS, and I think I finally grasped the basics of how these things work. Here's the gist of it in a humble layman's terms:
Gravity waves enhance low-level vorticity. They don't produce it; if some appreciable degree of vorticity isn't already present, then gravity waves accomplish nothing. But if they've got something to latch onto, then gravity waves can take a preexisting circulation and amplify it.
They accomplish this progressively. A single gravity wave probably isn't going to get a nascent mesocyclone to spin up a tube. What it will do is stretch the circulation vertically, causing it to tighten. (You know: the old analogy of the twirling ballet dancer bringing her arms to her side.) As the wave passes, so does the stretching effect, and the circulation "relaxes." However, it retains some of the new energy, and the next wave in the series adds to it. The effect is cumulative, with each new wave further tightening and intensifying the circulation until it reaches a critical point.
Obviously there are other factors also contributing to the mix, but the July tornado has convinced me that gravity waves are something to take into serious account when they're present.
Here is a reflectivity grab of what I believe were gravity waves during a convective event last year in Nebraska.* The waves were also clearly depicted by velocity and spectrum width products. The spectrum width did a particularly good job of showing the waves interfacing with storms on their western edge. (First time I've found a use for spectrum width.)
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* At first I wondered whether these were OFBs--but a whole series of them so close together? I've never seen such a thing before, and it just doesn't sit right with me. So I'm assuming these are gravity waves.
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