Dear Fans of Elevated Mixing Layers,
I have several questions about the account of the heat burst (reproduced below). But first, let’s develop a bench language between us that will help us avoid confusion. Let’s call air that is warmer than average for its altitude “Warm” and air that is colder than average for its altitude, “Cold.” And let’s call “Moist”, air that has a high content of water vapor and distinguish it from air that is accompanied by lots of liquid or frozen water which we will call, “water burdened.”. The capital letters in each case will remind us that Warm air may not be cozy and Dry air may have a lot of moisture with it. Thus, air can have a temperature many degrees below zero and still be Warm and can be Dry, even though it is mixed with many tons of water.
Ok, so now for the problems:
A heat burst is caused when a shower or thunderstorm weakens over a layer of dry air. As the last of the precipitation from the weakening shower or thunderstorm falls through the layer of dry air NSTèNote that the explanation as written does not make use of the fact that this falling precipitation will impart downward momentum to any air if falls through. çNST, the precipitation begins evaporating thus causing the air to cool.
As this air cools it will become more dense,
NSTè Hold on, here. Evaporation will also cause the air to become less dense because it is becoming more Moist. I am not sure how trade off between these two variables works. I would love to see a table with temp on the x axis, water vapor on the y axis and density on the z axis. In fact, I would like to see a family of such tables for different levels of the atmosphere. çNST eventually more dense when compared to the surrounding warmer air and as a result, begins descending to the surface at a high rate of speed. Eventually, all of the precipitation within the descending air evaporates. NSTè So, now we have a Cool, Moist falling airmass. This sort of thing happens all the time in thunderstorms and is called a “downburst”. çNST At this point the air is completely dry NSTèNo. Wrong. The most that can be said is that all the water in it has evaporated. This does not make it Dry. In fact, it makes it Moist. çNST and because no more evaporation can occur, the air can no longer cool. The air however continues to descend toward the surface due to the momentum it has already acquired. As dry air descends through the atmosphere, compression due to increasing atmospheric pressure causes the air to warm. NSTè Well, I suppose. But we still have Moist air, don’t we? As it descends, it’s relative humidity will fall, but the amount of water vapor in the packet will not decrease because the packet is falling. çNST It is important to note that the density of this air is now going to begin decreasing because of the increasing temperature. However, because the descending air already has a great deal of momentum carrying it to the surface, the increase in temperature and resultant decrease in density does little to slow the descending air. So, the dry air continues to descend, all the while warming more and more due to the aforementioned compressional heating. Eventually, this descending air reaches the surface and the momentum, which was moving downward towards the surface, is now moving horizontally along the surface in all different directions, thus resulting in a strong wind! In addition, the intrusion of the very warm and very dry airmass from aloft, will cause the temperature at the surface to increase very quickly, and the dewpoint at the surface to decrease very quickly. Acquiring all the needed ingredients for a heat burst can be difficult, thus making the development and observance of a heat burst rare. NSTè We all know there was an elevated mixing level (layer of very Warm, Dry air) over running Moister Cooler air moving up from the Gulf. If we could find a way to get that layer down to the surface, then we would have explained the heatburst. The only think I can think of is that the falling mass of ice and water and the mass of falling air it took with it actually drives the EML through to the surface, but does not itself reach the ground. Ugh. More skyhooks. One feature of this explanation that puzzles me is the fact that the heat burst lasts as long as it does. A typical down burst last for a few minutes at most. Why does this warm air which (ex hypothesi) is less dense than the air it has penetrated not “bounce”.
Also, I am wondering if a falling mass of ice and water can reach the ground but set up a downward momentum in the column over it that will continue to drag air down to the surface for some time after the moisture is out of the picture.
These heat bursts seem a lot like Chinooks. A Chinook is also an exceptionally hot and dry wind. They occur when a Cool Wet airmass is driven over a high mountain range. The increase in altitude of the air squeezes out all the moisture and when the airmass comes down the other side of the mountain range it is hot and dry. What we need to explain heat bursts perhaps is to discover something out there on the flatlands to perform the function of the mountain range.
Nick
From: Friam [[hidden email]] On Behalf Of Roger Critchlow
Sent: Tuesday, June 11, 2013 2:26 PM
To: The Friday Morning Applied Complexity Coffee Group
Subject: [FRIAM] Atmospheric mechanics and thermodynamics
I was highly amused to read the description of how a heat burst happens here:
because it invokes the momentum of an atmospheric packet, something that I don't think any of our weather discussions has ever brought into our explanations.
Also note how the explanation proceeds as a logical-causal fait accompli, there is no physics or math involved in the explanation, just a narration of a sequence of physical causes.
-- rec --
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