The Role of Elevated Mixed Layers in Severe Weather

[It's Always Sunny]

Well low-level lapse rates are very important, it's just they have little do do with EML's.  

 

I have class in 10 min so don't have time to explain further but perhaps someone else can

Low level lapse rates are important because it allows air in the lower levels to rise until it hits the cap aloft. Most of the time you need a "trigger" to break the cap so thunderstorms can start developing (like the passage of a cold front). On some occasions, convection may be so strong that the cap will naturally erode and you'll see t-storms flare up that way. Having low level lapse rates below the cap let's "energy build up" as I like to say in simple terms. Once that cap breaks, convection will erupt. Weak low level lapse rates below a weak cap more times than not will not produce as good of t-storms than if you had strong low level lapse rates and a weak cap. 

 

The strength of the cap is important too because it sets the bar as to "how much energy can you build up" before the cap breaks. One thing to remember though is having too strong of a cap may not be good because there may not be a mechanism that can break it. That's why weak to moderate are always good.

 

As for wind shear, personally I don't look for how much room it has to mix things, but rather how strong it is (speed/direction), and how high up in the atmosphere it goes.

[ThompNickSon]

You know, folks, what I would like?  I would like it if one of you wise fellows would start a series of posts to this site, or a blog, called "SkewT of the Day."   I promise I would go there every day, read it rapaciously, and purchase anything advertized there.  SHMG.  Well, ALMOST anything.

 

You could start with today"s Skew T from  topeka, KS, which is NE of a triple point (I think that's the correct term) and shows a classic EmL. so far as my inexpert eyes can perceive.  TOP is at the SW extreme of a hail watch zone. 

 

 

Here is the mesoscale analysis showing where TOP is in relation to weather systems and showing that the predicted hazard is HAIL.

 

What I want to know is what do you wise folks see in this SKEW and why does it lead to a prediction of HAIL rather than tornadoes, etc. 

 

Im serious about the blog.  It would be a real gift to humanity.

 

N

[ThompNickSon]

There is a "feature" tracking across the southern plains and into the deep south, which is supposed to produce Tornadoes in AR tomorrow, and further east, later in the week..   The feature includes an EML, I think, and I would like to get some discussion going on the role of the EML.     It would be nice to post the PREDICTED skew't for AR tomorrow.  I used to know how to find those, but don't any more.  Can anybody help me locate those?  Bufkit used to produce them, but I can't make Bufkit work on my computer any more. 

 

Here, is the current skewT for Dallas, as  a substitute.  Since the whole thing is drifting eastward, I figure Dallas is as good a proxy as any other station. 

 

I would love to know what you wise folks see here.  That is a classic EML, right?  Moist warm air below, "hot" dry air above.  The dry line at the surface runs N/S at about the longitude of the eastern edge of the Texas Panhandle, with a diffuse area of overrunning "hot" dry air extrending eastward to the MI river.  My inexpert eyes see some nice cape, but also a lot of convective inhibition.  The air at the surfact would have to be moved upward almost 150 mb before it could climb freely, right? 

 

Any comments?  

 

N 

[ThompNickSon]

The situation in Arkansas seems to be developing on schedule. There is a developing low over Eastern Co with pressures falling broadly to the east of it.   The dry line is surging eastward through central OK and TX, with three-hour dewpoint falls of 20-30 degrees.  This "hot"  dry air is overrunning east of the dryline as shown in the mid-level lapse rate chart.  (For reasons I don't understand this feature is depicted on the mesoanalysis bulletin as a cold front, even tho it is over running. Could someone please comment on this depiction?). 

 

So, here is my inexpert nomination for the SKEW-T of the day.  It is from Shreveport, which is just south of the area of concern.  The central Arkansas station (Fayetteville?) is not very dramatic at all, so Shreveport it is.  Please comment.

http://www.spc.noaa.gov/exper/soundings/14042700_OBS/SHV.gif

 

 

Thank you in advance for any thoughts you might have, particularly with reference to the contribution of an EML to this situation. N

[OceanStWx]

The situation in Arkansas seems to be developing on schedule. There is a developing low over Eastern Co with pressures falling broadly to the east of it.   The dry line is surging eastward through central OK and TX, with three-hour dewpoint falls of 20-30 degrees.  This "hot"  dry air is overrunning east of the dryline as shown in the mid-level lapse rate chart.  (For reasons I don't understand this feature is depicted on the mesoanalysis bulletin as a cold front, even tho it is over running. Could someone please comment on this depiction?). 

 

So, here is my inexpert nomination for the SKEW-T of the day.  It is from Shreveport, which is just south of the area of concern.  The central Arkansas station (Fayetteville?) is not very dramatic at all, so Shreveport it is.  Please comment.

http://www.spc.noaa.gov/exper/soundings/14042700_OBS/SHV.gif

 

Thank you in advance for any thoughts you might have, particularly with reference to the contribution of an EML to this situation. N

 

FWIW, I still think the best 12z sounding was FWD.

 

700-500 mb lapse rates are 9.5 C/km, very nearly perfectly dry adiabatic. You want to look towards where the air is coming from. For AR today the lapse rates aloft (and the EML) is coming from the SW towards FWD. The low level moisture is coming from the SHV area, but not the temp profile aloft. So in effect you have deep low level moisture from SHV being overtopped by steep lapse rates from FWD. A very good recipe for some very strong storms.

[ThompNickSon]

Thank you. I hope others will comment.

Have edited this message to remove a stupid error in the earlier vsn. It is the Shreveport SKEW-T, that has no convective inhibition, not the DFW. So, I guess I am wondering, if the DFW parameters are so good, why we don't expect severe weather in DFW? Is the idea that by the time we get to maximum heating, those conditions will have passed east?

Another thing: I swear I remember that there used to be a site where one could get PREDICTED skew-t's for any station. Did I dream that, or does it exist.

Thanks for your help. I am sorry for wasting your time with the earlier screwed up message. I was looking at SHV and talking about FWD.

N

[ThompNickSon]

I have not been able to stay on this all day, but I have one question of you-all about the way knowledgeable people talk about the dry line. Sometimes it appears as a cold front and sometimes it appears as a warm front. Which is it? Now, everything I read would suggest that a dryline is a place were an advancing hot, dry air mass is displacing and OVERRUNNING a mass of merely-warm, moist air. That would make it a WARM front, right? There is, I gather, a cold front operating in the present situation, but it is further north and west and is not the same thing as the dry line, at all. Am i correct about any of this? N

[ThompNickSon]

After posting the above message, I ran across this meso analysis that distinctly shows a cold front following along behind the dry line in East Texas.

http://www.spc.noaa.gov/products/md/mcd0441.gif

I would post the gif myself, but for some reason the editor isn't working.

So, I would assume that the cold front consists of cold dry air undercutting hot dry air, and that the dry line consists of hot dry air overrunning warm moist air. So, the space between the cold front and the dry line, is an odd sort of warm sector where the air is extremely hot and dry. East of the dryline, is the EML floating (for the moment) on a layer of warm moist air. Further east, is another warm front, where the warm moist air is overrunning colder, drier air. I have a feeling somebody is going to object to that characterization, but I cannot think why.

Note that the dryline is here drawn with rounded symbols, usually reserved for warm fronts.

Does anybody have an idea why the editor isn't working tonight? Or is it a me-thing.

N

[calm_days]

 

Thank you for continuing to work with this topic!  In a faraway view, what can each of us see , either directly or tangentially related to EML phenomena?

I see a lot but i want to wait to hear what others intuit!

 

also this article is loosely referencing a similar phenomenon/phenomena you are exploring:

http://www.redorbit.com/news/science/1113123531/noctilucent-clouds-evidence-teleconnections-mesosphere-041714/

 

[ThompNickSon]

Wow! that is some anomaly!

While I have you, can I ask a question? Now that you guys have educated me re midlevel lapse rates, etc., it has made me even more aware than I was before of how "laminated" our atmospheres are. So THAT is making me wonder, what I am looking at when I am looking at that watervapor loop. I assume that an eml does not show up on a water vapor loop because there is at least one moist layer between it and the ground. But that is just an assumption. Is it correct? I assume that a wv loop bounces off the highest layer of cloud that it encounters, which is what makes it so useful for seeing the jet stream.

Also, two other questions: (1) Is there a source for PREDICTED skew-T's? I used to get them from buffkit, but buffkit got beyond me, and now I can't. (2) Do you know why the html toolbar has gone grey on me. I can only write text. I have a lovely skew-t i want to share, and I can't do it.

Looking forward to your thoughts.

Nick

[calm_days]

 

Here is the same image today.  some of the structures have changed quite a lot, the locations have not changed too much. 

 

I would say that your word "laminated" is very accurate. 

 

The atmospheric changes that seemed to reach a dynamic turning point in 2010, and continue to change onward have created, mixed layers of clouds, where what appear like almost computer-generated smallish cirrus clouds are often what is overhead glued to some nondescript stratus type clouds only to often have a few airplane vapor trails that have fallen from the higher altitude attached in between.  Thunderstorm cumulonimbi often have surplus higher altitude versions of scud clouds that just float alongside them almost stationary.

 

  The appearance of gravity waves in clouds has become extensive, when it was once visible only in larger cloud structures in satellite images; the perpetuation of gravity waves has been explained to involve the mesosphere, thermosphere, and ionosphere.

It does seem entirely possible that part of what keeps the EML structurally intact is some type of electromagnetic stasis achieved in higher levels of the atmosphere in conjunction with the stasis achieved in the troposphere and stratosphere. 

 

 

 

[ThompNickSon]

Wow, again! 

 

There are two particularly remarkable features of that amazing loop. The first is that enormous pocket of arctic dry air pouring down the high plains toward TX. The second is the sharp line of cloud along the Gulf Coast. Can you explain how that works? How does air that is read as "dry" over the Gulf become so obviously and dramatically "wet" as it crosses the shoreline. I could imagine how that worked, perhaps, if the satellite was seeing water droplets, but it's seeing water VAPOR, right? I looked at the low and midlevel lapse rate diagrams on the exper pages of spc and, indeed, there is an enormous patch of midlevel dry air over the gulf. but why that air doesnt get in trained in the circulation around the midwest low and drawn into LA, AL, and GA is a mystery to me..  See the two pix below: 

 

The first is the mid-level lapse rate:  The second is the low level lapse rate. 

How do you co=ordinate in your head the two ways of looking at the atmosphere of the south, the wv loop and the lapse rate diagrams?

 

Have to run,

 

Nick
 

[heavy_wx]

Wow, again! 

 

There are two particularly remarkable features of that amazing loop. The first is that enormous pocket of arctic dry air pouring down the high plains toward TX. The second is the sharp line of cloud along the Gulf Coast. Can you explain how that works? How does air that is read as "dry" over the Gulf become so obviously and dramatically "wet" as it crosses the shoreline. I could imagine how that worked, perhaps, if the satellite was seeing water droplets, but it's seeing water VAPOR, right? I looked at the low and midlevel lapse rate diagrams on the exper pages of spc and, indeed, there is an enormous patch of midlevel dry air over the gulf. but why that air doesnt get in trained in the circulation around the midwest low and drawn into LA, AL, and GA is a mystery to me..  See the two pix below: 

 

The first is the mid-level lapse rate:  The second is the low level lapse rate. 

How do you co=ordinate in your head the two ways of looking at the atmosphere of the south, the wv loop and the lapse rate diagrams?

 

Have to run,

 

Nick

 

 

Water vapor satellite measures radiation emitted at a wavelength very effectively absorbed (and emitted) by water vapor molecules. The radiation at that wavelength received by the satellite then correlates to a temperature. Generally, the brighter parts of the image indicate colder effective radiative temperatures of water vapor in the atmosphere.

 

Since water vapor is such an effective emitter (and absorber) of radiation at this wavelength, what is 'seen' by the satellite is a weighted mean of the radiation emitted by the water vapor through the entire atmospheric column. If there is more water vapor in the upper troposphere than normal, most of the radiation sensed by the satellite will be from those levels meaning temperatures will be colder. Generally, the effective radiative level is in the middle of the troposphere unless the atmosphere is abnormally dry through the entire column and more of the radiation emitted at this wavelength will be from the lower troposphere.

 

What you are usually seeing then is mid-level dry air, which may or may not be correlated with dry low-level air. In fact, severe weather is often associated with an EML (dry mid-level air) and high low-level moisture, the later of which would not be apparent from water vapor satellite imagery.

[ThompNickSon]

Hlo,

 

One of you put me onto the storms prediction centers experimental page, for which I am eternally grateful.  It is an amazing resource.  See http://www.spc.noaa....r.php?sector=19#, for example.  It would be very interesting if a bunch of us could bring to bear these resources on particular situations as they develop. 

 

Notice that while the late April outbreak in the south seemed to have a lot to do with dry lines and eml's, the recent mother's day outbreak did not, at least so far as I could see.    I would love to see that paradox discussed.  Apparently, complex layering in the atmosphere is not essential to severe storms development?

 

N

[calm_days]

Here are some recent water vapor images; although not pertaining to severe thunderstorms, there does seem to be some views of large scale behavior of mixing layers.  I missed one day and am glad for that because it emphasizes what changed.

 

 

 

[Hoosier]

Hlo,

 

One of you put me onto the storms prediction centers experimental page, for which I am eternally grateful.  It is an amazing resource.  See http://www.spc.noaa....r.php?sector=19#, for example.  It would be very interesting if a bunch of us could bring to bear these resources on particular situations as they develop. 

 

Notice that while the late April outbreak in the south seemed to have a lot to do with dry lines and eml's, the recent mother's day outbreak did not, at least so far as I could see.    I would love to see that paradox discussed.  Apparently, complex layering in the atmosphere is not essential to severe storms development?

 

N

 

 

Not sure there wasn't an EML present for 5/11.  The soundings I looked at seemed to have it.  Not the best one you'll ever see but still.

 

A robust EML is not absolutely necessary for severe weather (a classic example of this would be a landfalling tropical system when mid level lapse rates are usually terrible, but it also pertains to non-tropical systems) but it certainly helps, and I'd say the ceiling on a severe weather outbreak tends to be higher when it is present.

[ThompNickSon]

Thanks for this response. 

 

Not sure there wasn't an EML present for 5/11.  The soundings I looked at seemed to have it.  Not the best one you'll ever see but still.

 

A robust EML is not absolutely necessary for severe weather (a classic example of this would be a landfalling tropical system when mid level lapse rates are usually terrible, but it also pertains to non-tropical systems) but it certainly helps, and I'd say the ceiling on a severe weather outbreak tends to be higher when it is present.

 

Perhaps the next time around, we could watch this together and post some soundings.  I am sure you would teach me lots if we could manage the patience to do some of this. 

 

  In my way of thinking, a large thunderstorm is a device for  exploiting gradients in moisture and [potential] temperature to produce mechanical energy.  The simple model I carry around in my head is a chimney model.  You need a fire at the bottom (supply of warm, moist air), you need a chimney (which is more or less provided by mid-level dry air -- metaphor is a bit hinkey, here) and you need a brisk wind accross the top of the chimney ( sheer).    This model is obviously pretty primitive.  I am curious what models others bring to bear when they think about these storms. 

 

Does anybody within the sound of my voice know what a "Benard Cell"  is?   See http://en.wikipedia.org/wiki/Rayleigh%E2%80%93B%C3%A9nard_convection .  Is it something that young meteorologist learn about  or not?

[calm_days]

Yes, something similar to the appearance of Benard cells is in the latest water vapor images ; moreso in the second one which is from today. 

 

 

http://www.esrl.noaa.gov/psd/outreach/education/science/convection/RBCells.html

[heavy_wx]

Thanks for this response. 

 

 

Perhaps the next time around, we could watch this together and post some soundings.  I am sure you would teach me lots if we could manage the patience to do some of this. 

 

  In my way of thinking, a large thunderstorm is a device for  exploiting gradients in moisture and [potential] temperature to produce mechanical energy.  The simple model I carry around in my head is a chimney model.  You need a fire at the bottom (supply of warm, moist air), you need a chimney (which is more or less provided by mid-level dry air -- metaphor is a bit hinkey, here) and you need a brisk wind accross the top of the chimney ( sheer).    This model is obviously pretty primitive.  I am curious what models others bring to bear when they think about these storms. 

 

Does anybody within the sound of my voice know what a "Benard Cell"  is?   See http://en.wikipedia.org/wiki/Rayleigh%E2%80%93B%C3%A9nard_convection .  Is it something that young meteorologist learn about  or not?

 

Thunderstorms are stabilizing mechanisms in the atmosphere. They convert potential energy (really convective available potential energy - CAPE) to kinetic energy. The more warm and moist the low-levels are and the more cold and dry the mid-levels are, the more potential instability there will be and thus greater CAPE that can be realized in theory. In reality, air parcels will mix with environmental air so really dry mid-levels will entrain drier air and lower the actual CAPE.

 

As others have mentioned, an EML is useful because it provides a negatively buoyant layer (the cap) that prevents parcels from reaching the level of free convection (LFC) earlier in the day when the environment is more stable (cooler surface air). When daytime heating at the surface reaches a certain threshold and/or when the cap weakens due to advection/large-scale lifting in that layer/differential diabatic heating/etc. surface parcels will be able to reach the LFC and there will now be much more instability to work with.

 

When these parcels are lifted by some type of convergent boundary in temperature or moisture, they will ascend and condense at their LCL and accelerate upwards past their LFC. As this happens, condensation will continue to occur and rain will be produced, warming mid-levels and cooling the surface thereby decreasing the CAPE. The buoyant forces that accelerate parcels and increase kinetic energy as a result will stabilize the atmosphere, decreasing the potential energy.

 

If there is no ambient wind shear, thunderstorms will convert most of the CAPE to parcel motion and then will be left in a stabilized environment, causing the thunderstorm to dissipate shortly after precipitation production. With shear, the storm will be organized in such a way that the source of CAPE will not be disrupted as the storm matures.

 

Rayleigh-Bernard convection is an idealized model for dry convection. The model is used to determine when dry convection will initiate between two differentially-heated surfaces, with molecular diffusion (or turbulent diffusion in the atmosphere) opposing the organization of convection. With a preferred wind direction, rolls will develop given a certain critical temperature difference while cells will develop with no preferred wind. It can be useful for describing boundary layer processes in the atmosphere but not really for looking at deep-moist convection. It wasn't taught to me as an undergrad, probably for this reason and because of the math involved (wave solutions of the linearized Boussinesq system).

[ThompNickSon]

Thanks, Heavy WX for this wonderfully lucid and complete post.   There are just a couple of details I would like to clear up, if you have a few more moments:

 

You wrote: In reality, air parcels will mix with environmental air so really dry mid-levels will entrain drier air and lower the actual CAPE.

 

Is there a slip of the  pen, in there somewhere.  If I understand what is going on here, the movement of the "bubbles" of moist air up through the dry layer (once the cap has been broken) is not entirely frictionless, so some of the moist/dry contrast is lost on the way up.  Parcels aren't quite balloons. They aren't surrounded by membranes.   Shouldn't one of the "driers" in the previous sentence be a "moister" ?

 

You also wrote:  an EML is useful because it provides a negatively buoyant layer (the cap) that prevents parcels from reaching the level of free convection (LFC) earlier in the day when the environment is more stable (cooler surface air).

 

I think you have arrived at the crux of the [my?] confusion about what a cap actually is.  Do you agree that a less buoyant layer can NEVER overlie a more buoyant layer, because if it did, the two would immediately exchange places?  So the metaphor of a "lid", which many weather people deploy on TV broadcasts, just doesn't quite work, right?  A "lid" works because it is heavier than the stuff pushing up under it.    What actually makes the cap is that the overlying dry layer is much MORE buoyant than the moist layer below it.   So any parcel of warm air rising within the lower layer that tries to cross the boundary is LESS buoyant than its "environment" and falls back.  But as soon as the rising air in the lower level becomes moist and warm enough so that it is slightly MORE buoyant than the air at the bottom of the EML, it will rise unfettered all the way to the top of the EML layer, realizing all its potential heat along the way.   After that, all your comments about sheer take effect.

 

The mystery is, of course, how can the dry air of the EML be more buoyant than the moist air below it, given that water vapor is lighter than nitrogen.  The answer, is that it is so very much WARMER.  (By the way, the terms "warmer" and "colder" here are used in their "altitude-relative" sense; we are talking about "potential temperature", here,  the temperature the parcel WOULD have if it is brought to the surface. ) 

 

This relates to a confusion I think I detect in the way people talk about dry-lines, as if they were cold fronts, pushing under the moist gulf air to the east.  In fact, they are WARM fronts.  You can see that by comparing maps of the surface lapse rates with those of the middle level lapse rates.   In severe situations, the steep-lapse-rate air runs out ahead of the dry line at midlevels and provides the "capping" function that makes for severe weather just ahead of the dry line. 

 

Cold fronts often do play a role in severe outbreaks.  Here I am a bit hazy, but what I want to say is that the "shock waves" a head of a on rushing continental polar air mass can contribute to the process of destabilization described above. 

 

Am I right about any of this?   I am supposed to be packing to go back east, so I better quit, now.  Please be quick to correct any misconceptions I have. 

 

N

[ThompNickSon]

Apparently, tornadoes occurred west of Albany, NY, today in an environment of relatively low instabiity.  I don't have time, now to check for an EML, but I am guess there wasn't one.    Look at this report from a Hartford Meteorologist, who clearly knows an eml when he sees one.  Another Tornado West of Albany, NY

 

hope we can discuss this later when I get time to look at SPC's experimental page. 

 

N

[ThompNickSon]

Here, for future reference is the sounding from albany earlier today near where the tornadoes occcured.

 

 

http://www.spc.noaa.gov/exper/soundings/14052212_OBS/

 

n

[JC-CT]

Apparently, tornadoes occurred west of Albany, NY, today in an environment of relatively low instabiity. I don't have time, now to check for an EML, but I am guess there wasn't one. Look at this report from a Hartford Meteorologist, who clearly knows an eml when he sees one. Another Tornado West of Albany, NY

hope we can discuss this later when I get time to look at SPC's experimental page.

N

I don't mean to be that guy, but since Ryan hasn't actually posted in this particular thread...he is a mod and an active poster here at AmWx, as you probably know already, screenname CT Rain.

[ThompNickSon]

Dr. Ranrahan's blog is fabulous.   I hope I didn't say anything to suggest otherwise.  I highly recommend it.  It was from him, for instance, that I first began to understand about EML's  My point was only that that if he didn't see an EML in that sounding, there probably wasn't one.  And no, I didn't know his screenname.  I will be on the look out for it.  Thanks,  N

[heavy_wx]

Thanks, Heavy WX for this wonderfully lucid and complete post.   There are just a couple of details I would like to clear up, if you have a few more moments:

 

You wrote: In reality, air parcels will mix with environmental air so really dry mid-levels will entrain drier air and lower the actual CAPE.

 

Is there a slip of the  pen, in there somewhere.  If I understand what is going on here, the movement of the "bubbles" of moist air up through the dry layer (once the cap has been broken) is not entirely frictionless, so some of the moist/dry contrast is lost on the way up.  Parcels aren't quite balloons. They aren't surrounded by membranes.   Shouldn't one of the "driers" in the previous sentence be a "moister" ?

 

I think my wording was a bit unclear. Warm, moist surface-based parcels will entrain the drier mid-level air as they ascend, lowering the actual CAPE. Hopefully that is a bit less confusing.

 

You also wrote:

  an EML is useful because it provides a negatively buoyant layer (the cap) that prevents parcels from reaching the level of free convection (LFC) earlier in the day when the environment is more stable (cooler surface air).

 

I think you have arrived at the crux of the [my?] confusion about what a cap actually is.  Do you agree that a less buoyant layer can NEVER overlie a more buoyant layer, because if it did, the two would immediately exchange places?  So the metaphor of a "lid", which many weather people deploy on TV broadcasts, just doesn't quite work, right?  A "lid" works because it is heavier than the stuff pushing up under it.    What actually makes the cap is that the overlying dry layer is much MORE buoyant than the moist layer below it.   So any parcel of warm air rising within the lower layer that tries to cross the boundary is LESS buoyant than its "environment" and falls back.  But as soon as the rising air in the lower level becomes moist and warm enough so that it is slightly MORE buoyant than the air at the bottom of the EML, it will rise unfettered all the way to the top of the EML layer, realizing all its potential heat along the way.   After that, all your comments about sheer take effect.

 

The mystery is, of course, how can the dry air of the EML be more buoyant than the moist air below it, given that water vapor is lighter than nitrogen.  The answer, is that it is so very much WARMER.  (By the way, the terms "warmer" and "colder" here are used in their "altitude-relative" sense; we are talking about "potential temperature", here,  the temperature the parcel WOULD have if it is brought to the surface. ) 

 

This relates to a confusion I think I detect in the way people talk about dry-lines, as if they were cold fronts, pushing under the moist gulf air to the east.  In fact, they are WARM fronts.  You can see that by comparing maps of the surface lapse rates with those of the middle level lapse rates.   In severe situations, the steep-lapse-rate air runs out ahead of the dry line at midlevels and provides the "capping" function that makes for severe weather just ahead of the dry line. 

 

Cold fronts often do play a role in severe outbreaks.  Here I am a bit hazy, but what I want to say is that the "shock waves" a head of a on rushing continental polar air mass can contribute to the process of destabilization described above. 

 

Am I right about any of this?   I am supposed to be packing to go back east, so I better quit, now.  Please be quick to correct any misconceptions I have. 

 

N

 

Actually, the vertical distribution of mass in the atmosphere can be thought of as a Boltzmann distribution, where air molecules are most likely to be closer to the surface where the gravitational potential energy barrier is lower. Therefore, mass per unit volume, or density, decreases with height in the atmosphere. Even if there is a strong temperature inversion where a cap might be present, the vertical density profile of the environment will still be very close to the Boltzmann mass distribution as decreases in pressure will far out-way the temperature effect on density.

 

The key to evaluating the buoyancy of a parcel is to separate the atmosphere into a base state (environment) and a perturbation field (parcels). Soundings are taken to be a proxy for the environment such that they are horizontally homogeneous enough for the environment to be considered hydrostatic. This hydrostatic approximation will generally hold when the horizontal scale is much greater than the vertical scale of motion. Since the environment is assumed to be hydrostatic (a good approximation) the vertical pressure gradient force will nearly balance gravity acting on the environmental base state.

 

Buoyancy is only defined when considering the parcel density vs. its environment. Parcels lifted from the surface in thunderstorm environments will have relatively lower potential temperatures compared to the cap, where the bottom of the EML is relatively warm (higher potential temperature) due to its origins in the boundary layer of an elevated region. The environmental potential temperature at this cap layer will therefore be greater than the parcel potential temperature meaning the parcel will be negatively buoyant and accelerate downward.

 

During the daytime, the drylines will generally move eastward under quiescent weather conditions. This is due to mixing in the boundary layer from solar heating. The shallow moist air just to the east of the boundary mixes with the much drier air aloft, drying the surface layer. This process will continue until the boundary layer reaches its maximum height and no more mixing occurs. When this happens, the dryline will begin to act more like a density current and propagate back westward. The cooler air, though more moist, will be more dense than the very hot air to the west of the boundary. This will "push" the dryline back towards the west, where new convection may initiate along this boundary due to the low-level forcing.

[JC-CT]

Dr. Ranrahan's blog is fabulous. I hope I didn't say anything to suggest otherwise. I highly recommend it. It was from him, for instance, that I first began to understand about EML's My point was only that that if he didn't see an EML in that sounding, there probably wasn't one. And no, I didn't know his screenname. I will be on the look out for it. Thanks, N

No no, I wasn't implying anything other than I might be introducing you to someone you already know.

Check out the New England regional subforum sometime, he posts basically daily and knows his stuff. He and Wiz have some good conversations, and he's very friendly.

That's what makes AmWx so awesome, you follow some respected meteorologist on their website (or on the nightly news) and then you find out they are an active poster here, shooting the sht with weather weenies like myself.

[ThompNickSon]

Hi, everybody,

 

Many pages back in this forum I got my ears boxed for suggesting there were two kinds of above-surface layers that could contribute to conditional instability; dry layers and cold layers.  I referred to the latter as  'cold pools" that followed lows and resulted in vigorous snow flurry development in the days following new england north easters. .  The person who boxed my ears insisted that the term cold pool referred only to the puddles of rain- and hail-cooled air that puddles under collapsing thunderstorms.  But look what I stumbled on today in the Noaa glossary! 

 

Cold Pool  A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

 

I don't say this [only] to gloat  (};-)], but to renew the question that I was trying to raise at the time:  How is it that a cold pool remains aloft given that cold air is relatively dense? Just as an EML is heavy to the extent that it is dry, so a "cold pool" ought to be heavy to the extent that it is cold.  So, just as I was led to wonder how and EML stays up there, so I am led to wonder how a cold pool stays up there.   The answer for the EML, ably supplied by the participants in this forum, is that an EML's great potential temperature more than compensates for its dryness.  But because of the boxing, i never insisted on an answer to my question about cold pools.  One possibility is that they are very moist and that their moistness  more than compensates for their coldness. However, this explanation seems very implausible because, as one of you pointed out,  cold air just cannot hold that much moisture and because it takes a large moisture difference to compensate for an opposing small temperature difference. 

 

Does anybody know where these "cold pools " come from and how they are maintained at altitude until needed to explain vigorous snowflurries? 

 

Thanks,  

 

N

[ThompNickSon]

i flew from Albuquerque to Hartford at 39kft today  and encountered a double line of thunderstorms , one over the Adirondacks  and the other over the Berkshires.  Both lines were very large, but the structures were very "frizzy" and indistinct, except for a fine example of overtopping on the storm over NE NY state.   I haven't heard of any severe weather associated with it, but I am interested in the relationship between the observed structure and closest skew T,  the 7 am albany sounding.  I have pasted in the sounding below.  Can any of you wise folks see anything in this sounding to suggest a heavy thunderstorm later in the day?   Perhaps more interesting, does anybody know of a book that relates cloud pictures  with skew-T diagrams ... a sort of a "What the clouds can tell you about layering in the atmosphere."  If not, will one of you get busy and write one before it's too late for me to read it?  I imagine a stunning coffee table book lavish cloud pictures, each overlaid by the properly scaled, simultaneous skewT.  I should perhaps warn you that I am not young. 

 

N

 

 

[calm_days]

 

The next image from may 22 and today's current water vapor image seem to be the most relevant.  Some aspects aren't immediately comprehensible.  In the motion of the water vapor it can be seen that there are strands being pulled by the ULL and that a lot of the pull is coming from the systems in the west atlantic area.  That seems to supply a lot of momentum that does not seem to consistently exist continentally; the two weeks in between these images included the continental ULL over the gulf states sitting there and eventually just sinking south. 

[ThompNickSon]

Dear Calm_Days,

 

Once again you provide me with stunning WV loops; and once again I am left longing for more of your thoughts on their significance.

 

The may 22 image is for two days before the flight from MDW to BDL that I describe in my earlier post.  It's a bit early to depict the conditions on the day of my flight, but it has an absolutely amazing depiction of diffluence over Montana. You would think that  with the atmosphere being ripped assunder over the state on that day, there would be  some convection there, but the SPC prediction for that day was marginal. 

 

The June 4 image would SEEM (again I haven't checked) to be relevant to our ongoing discussion of the role of packets of dry air in severe weather. It appears to depict a large MCC being herded eastward across the US at the nose of a large mass of dry air originating in the Eastern Pacific and flowing across the intermountain west.  We have often noted the fact that WV loops don't give you much of a picture of dry LAYERS in the atmosphere, but I am guessing that overrunning warm dry air may be contributing to that MCC.  Oh, hell;  why guess.  Why not LOOK! (see below).  I guess the answer is yes, although there is not a particularly emphatic dry line. 

 

Firefox is starting to act funny, so I better send this before I lose it. 

 

N

 

 

The next image from may 22 and today's current water vapor image seem to be the most relevant.  Some aspects aren't immediately comprehensible.  In the motion of the water vapor it can be seen that there are strands being pulled by the ULL and that a lot of the pull is coming from the systems in the west atlantic area.  That seems to supply a lot of momentum that does not seem to consistently exist continentally; the two weeks in between these images included the continental ULL over the gulf states sitting there and eventually just sinking south.