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LocoAko

"Dynamic Cooling"

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I know this is a very basic concept but I just want to confirm something here - I have a quick question about the term "dynamic cooling". I hear it used all the time on the board in a sense that it means that the precipitation, if heavy enough, will "drag down" cold air (can this actually happen?). However, the AMS glossary (and other technical papers) defined it as

Cooling that results from decreasing pressure. Therefore, dynamic heating results from increasing pressure. Because the pressure gradient is much stronger in the vertical than in the horizontal, 'dynamic' changes in temperature due to expansion or compression are more likely to occur from vertical motion than from horizontal motion.

To me, this just describes the process of the parcel cooling as it expands due to a drop in pressure as it moves upward. In this context, does this mean that strong upward vertical motion evacuates air from the atmospheric column, effectively lowering the pressure and therefore cooling the column, and this then translates to the surface? Are these two explanations the same and I'm just misunderstanding something, or is there a colloquial use of the term that is different from the technical explanation?

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I know this is a very basic concept but I just want to confirm something here - I have a quick question about the term "dynamic cooling". I hear it used all the time on the board in a sense that it means that the precipitation, if heavy enough, will "drag down" cold air (can this actually happen?). However, the AMS glossary (and other technical papers) defined it as

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To me, this just describes the process of the parcel cooling as it expands due to a drop in pressure as it moves upward. In this context, does this mean that strong upward vertical motion evacuates air from the atmospheric column, effectively lowering the pressure and therefore cooling the column, and this then translates to the surface? Are these two explanations the same and I'm just misunderstanding something, or is there a colloquial use of the term that is different from the technical explanation?

It does not really drag down cold air. However, if the precipitation is falling very heavily, two things are happening. One you have very strong upward motion and get adiabatic expansion as you go to lower pressures. As the air expands, less molecules bump into each other and therefore you get cooling. At least I think that's what happens. If you compress air it heats. You can test that by using an air pump. The biggest part of dynamic cooling I think is the expansion due to lifting. I guess you might also have some smaller cooler if you have a storm bomb out and have really rapid pressure falls and height falls. However, wherever you have lifting you usually have mass divergence above it so you are in a sense evacuating air.

The other thing that happens if you have precipitation falling heavily and at the higher levels it's still in frozen form so it ends up cooling as it melts (that's not as efficient as evaporation) but it still will provide cooling. As the atmosphere cools from the melting the freezing level lowers and the melting shifts to a new place in the column. The biggest part of dynamic cooling I think is the expansion due to lifting. I guess you might also have some smaller cooler if you have a storm bomb out and have really rapid pressure falls and height falls. Anyway, that's my poor attempt to explain it.

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It does not really drag down cold air. However, if the precipitation is falling very heavily, two things are happening. One you have very strong upward motion and get adiabatic expansion as you go to lower pressures. As the air expands, less molecules bump into each other and therefore you get cooling. At least I think that's what happens. If you compress air it heats. You can test that by using an air pump. The biggest part of dynamic cooling I think is the expansion due to lifting. I guess you might also have some smaller cooler if you have a storm bomb out and have really rapid pressure falls and height falls. However, wherever you have lifting you usually have mass divergence above it so you are in a sense evacuating air.

The other thing that happens if you have precipitation falling heavily and at the higher levels it's still in frozen form so it ends up cooling as it melts (that's not as efficient as evaporation) but it still will provide cooling. As the atmosphere cools from the melting the freezing level lowers and the melting shifts to a new place in the column. The biggest part of dynamic cooling I think is the expansion due to lifting. I guess you might also have some smaller cooler if you have a storm bomb out and have really rapid pressure falls and height falls. Anyway, that's my poor attempt to explain it.

Thanks. I did understand this as an adiabatic (or not so much, but I digress) process, but the whole "dragging air down" thing really confused me as it hardly jived with the AMS glossary definition and didn't make sense. It is just thrown around so freely on here that I was curious what the actual explanation was.

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Quck question. When 850mb temperatures start to fall below zero is that the upper level front coming through or is it CAA taking place? I'm thinking it might be CAA with the dynamic cooling as the heavier precip starts to fall.

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Quck question. When 850mb temperatures start to fall below zero is that the upper level front coming through or is it CAA taking place? I'm thinking it might be CAA with the dynamic cooling as the heavier precip starts to fall.

Depends. If the wind at 850 mb is blowing from cold to warm, it is cold advection. Other processes could be going on at the same time, adding to the cooling.

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Thanks. I did understand this as an adiabatic (or not so much, but I digress) process, but the whole "dragging air down" thing really confused me as it hardly jived with the AMS glossary definition and didn't make sense. It is just thrown around so freely on here that I was curious what the actual explanation was.

Well I think there is some precip "drag" of cooler air...but I agree in that it does not properly explain the real and more significant mechanism. I feel like it was created as a layman's term to help people understand the process, but it is not entirely correct. The posts above explain it well.

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Quck question. When 850mb temperatures start to fall below zero is that the upper level front coming through or is it CAA taking place? I'm thinking it might be CAA with the dynamic cooling as the heavier precip starts to fall.

A "front" is a sharp boundary between air masses. If the change in temperature is rapid, it is probably a front. If it is more gradual, then likely not. CAA always occurs behind a cold front, after the front passes.

Dynamic cooling is not the same concept as CAA. CAA (cold air advection) means that cold air is moving in from another location. Dynamic cooling means that the air itself is cooling off without necessarily moving anywhere.

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Precipitation drag is a real physical phenomenon. It's magnitude depends on the size of the water droplets, but in essence, precipitation drag disrupts hydrostatic balance as droplets bring lower theta/e air downward...this can help enhance downdrafts and it also can curb updrafts and decrease condensation for precipitation development/rates. A negative feedback...that the models do poorly at parameterizing most times.

Dynamical cooling is a different animal. Any time there is ascent created by large or small scale systems, such as an upper trof or isentropic lift related to conveyor belt theory, etc...the air parcels involved in the lift cool as they ascend. Many times closed lows over the desert SW will remain relatively precip free even though the air lifted condenses...by the time these closed lows reach the deep south...much higher theta/e air is being lifted or "dynamically cooled" to saturation through a relatively deep layer...thus these lows seem to "make their own" moisture.

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FWIW

Here is what I posted back on:

Posted 20 March 2006 - 02:57 PM

http://www.easternuswx.com

"Operationally, I've always understood that dynamic cooling was when a closed low at 500 mb deepened with the corresponding lowering of critical (for a change over) heights.

The mechanism is always additional cyclogenesis to an existing surface cyclone.

The only time anyone pays attention to this phenomena was/is in cases of whether rain would change to snow and there was no lower level cold air within a few hundred miles to get the job done.

I might add, like others have pointed out, what truly is amazing about "Dynamic Cooling" is the extreme paucity of any significant article/study done on same.

The AMS Glossary also has no definition."

http://www.easternuswx.com/bb/index.php?/topic/24814-forecasts-for-dynamic-cooling/page__st__40

http://www.easternuswx.com/bb/index.php?/topic/24814-forecasts-for-dynamic-cooling/

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Gents - great discussion.  Did a Google search on dynamic cooling and got this thread.  As someone who is very familiar with momentum and heat transfer from a different perspective (PhD in chem eng'g), I have to ask a simple question.  I get that rising parcels of air will move towards lower pressure and undergo adiabatic expansion and cooling, but if one looks at the column as a whole, doesn't this process essentially reverse itself in the column, as parcels containing the precip generated in the snow growth region descend, via gravity and undergo adiabatic compression and warming? 

 

I would think these processes would cancel each other out if doing an energy balance on the column and all the inputs and outputs going on in the column.  On top of those possibly balanced processes, one should also have heat being generated by nucleation/vapor phase condensation, creating snow crystals in the snow growth region, but I imagine that only warms that region up marginally - and more importantly, I would think that the immense numbers of falling snow crystals, which are well below 32F in the snow growth region, would have an impact on the temperature of the column below, essentially "dragging down" some cold air with them and cooling the column at least a bit.  In addition, as the crystals near the warmer surface and some melting occurs that would further cool the column, at least near the surface. 

 

Maybe I'm missing something here, but that's my simplistic way of looking at the column and the processes going on in it.  I'd be interested in comments...

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Gents - great discussion.  Did a Google search on dynamic cooling and got this thread.  As someone who is very familiar with momentum and heat transfer from a different perspective (PhD in chem eng'g), I have to ask a simple question.  I get that rising parcels of air will move towards lower pressure and undergo adiabatic expansion and cooling, but if one looks at the column as a whole, doesn't this process essentially reverse itself in the column, as parcels containing the precip generated in the snow growth region descend, via gravity and undergo adiabatic compression and warming?

 

Except the parcels do not descend, only the hydrometeors descend since they have a greater density than air. As solids and liquids, these precipitation particles do not change volume (significantly, at least) so no work is done adiabatically to warm them.

 

I would think these processes would cancel each other out if doing an energy balance on the column and all the inputs and outputs going on in the column.  On top of those possibly balanced processes, one should also have heat being generated by nucleation/vapor phase condensation, creating snow crystals in the snow growth region, but I imagine that only warms that region up marginally - and more importantly, I would think that the immense numbers of falling snow crystals, which are well below 32F in the snow growth region, would have an impact on the temperature of the column below, essentially "dragging down" some cold air with them and cooling the column at least a bit.  In addition, as the crystals near the warmer surface and some melting occurs that would further cool the column, at least near the surface. 

 

Maybe I'm missing something here, but that's my simplistic way of looking at the column and the processes going on in it.  I'd be interested in comments...

 

The latent heating due to condensation/fusion will prevent the parcels from cooling adiabatically. This situation is called a moist process where the parcels will conserve equivalent potential temperature (though not entirely). There is still a net cooling of air parcels of 6 degrees C per kilometer (a rough, average value), less than the 10 degrees C cooling per kilometer under purely dry adiabatic motion.

 

You are correct that the hydrometeors will that form in potentially cooler regions of the atmosphere at higher altitudes will cool the column somewhat as they fall. This is a valid cooling process as the air parcels come into contact with the cooler raindrops or snowflakes. I'm not sure it is a huge effect though, relative to the adiabatic cooling.

 

Melting of frozen hydrometeors will cool the column as well though again, this effect is not as great as latent cooling from evaporation.

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If I may add to this, there is also another situation which refers to 'dynamic cooling' and it happens mainly in the winter.

 

You may have a situation where you have an elevated moist layer from which snow is falling and it is moving over a shallow surface layer which is above freezing but is very dry. The snow falling through the dry air will evaporate (virga) and this will do two things; it will add moisture to the dry layer which will eventually lower tward to surface and, it will also begin to cool the air due to evaporative cooling. (the opposite of latent heating). If the lower layer temps are close to freezing and the precip is heavy/persistent enough, the dynamic cooling may be enough to drop the temps below freezing so once the snow reaches the surface, it  will accumulate.

 

Jeff

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Gents - great discussion.  Did a Google search on dynamic cooling and got this thread.  As someone who is very familiar with momentum and heat transfer from a different perspective (PhD in chem eng'g), I have to ask a simple question.  I get that rising parcels of air will move towards lower pressure and undergo adiabatic expansion and cooling, but if one looks at the column as a whole, doesn't this process essentially reverse itself in the column, as parcels containing the precip generated in the snow growth region descend, via gravity and undergo adiabatic compression and warming? 

 

There are two potential processes involved here, adiabatic (reversible) and diabatic (irreversible). "Heavy_wx" has already covered the diabatic process, so I won't duplicate his comment here. In the event the rising parcel does not generate precipitation, the cooling is completely reversible.

 

Think of a somewhat dry airmass and a small hill. As the dry air is advected over the terrain feature, adiabatic cooling occurs. If we make the airmass sufficiently dry and the hill sufficiently short, such that the advected air does not produce clouds or precipitation, the parcels will warm on the descent side of the hill to their original thermodynamic values from the upwind side of the hill.

 

--Kevin

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