Crashing Dryline?

[StephenHenry]

Daytime dryline propagation has always been a tricky thing for me to get an intuitive feel for. My (potentially flawed) understanding is that as the sun warms the moist air just east of the dryline, its gets mechanically stirred up - drawing together and mixing in the dry air above it. This mixing is what propagates the dryline. So unlike cold/warm fronts wedging under/over, my simplified minds-eye picture of dryline is kind of like acid eating away a substance. It's bubbly and complicated at the boundary where the acid is reacting, and the acid advances by eating away the substance rather than pushing it (i.e. the dry air "eats away" at the boundary of the moist air).

First question: is that at all a fair way to think about dryline propagation?

Second question: does a dryline ever advance similarly to a crashing cold front (i.e. driven so that it literally wedges under the slightly less dense moist air)?

Third question: Are there any cool 3D visualizations of dryline propagation? I'd love to see something like that.

 

[David Williams]

I do not perfectly understand dryline propagation either but I thought it did act sort of like a wedge because moist air is less dense than dry air and is pushed up into the atmosphere by the dry air. As the dryline moves forward it does mix with the moist air to a certain degree, but that is determined by the dew point spread between the two air masses and the speed of the dryline's propagation. So all that may not be totally accurate so let's hope that some of the more knowledgeable guys throw down their answers too

Also, I'm not aware of any 3D animations, but I'd like to see one if anyone has a link.

 

[StephenHenry]

I could stare at this for a long time. A relatively stationary dryline, so it doesn't really help defend or refute my acid-erosion intuition, but man those little eddies are interesting. Source

 

[StephenHenry]

I'm going to selfishly bump this thread back up now that the board is a little more active.

Still very curious if my dryline propagation intuition outlined in the first post is way off base.

And David Duncan brought up another very interesting question: Are there major differences in how drylines propagate depending on the moisture differential across the boundary? Does a 45Td/60Td dryline behave very differently from a 40Td/70Td (all other things equal)?

 

[Jeff Duda]

Yes, drylines move by mixing rather than advection.

In reality the dryline is just where the top of the moist PBL coming off the Gulf meets the sloping terrain of the Plains. Rich moisture only gets so high in the atmosphere. The horizontal location of the dryline is the location where the height at the top of the rich moisture is about the same as the terrain height. If you ever see a vertical cross section of moisture across a dryline you would note a generally progressively narrowing depth of moist air as you approach the dryline. Given the mixing that occurs during the day, moisture is mixed out more quickly just ahead of the dryline than well ahead of it, so the dryline jumps east (rather than smoothly advecting eastward) as the moisture progressively mixes out.

This is not 100% fully descriptive, as there is an advective component to dryline activity. Also, the vertical mixing and pooling of moisture tends to cause upward bulges in the top of the rich moisture in the PBL. I'm not totally sure what the impacts of this are, but it can cause not obvious things to happen.

 

[Bob Schafer]

After chasing for a pretty long time, I still don't understand why moisture sinks. The atmosphere is mostly N2 and O2. N2 has a molecular weight of 28, and O2 has a molecular weight of 32. H2O has a molecular weight of 18 (or 20?), so why does it sink? Sorry for going OT, something at which I am notoriously adept.

Also, it has long been my understanding that there is much greater heating behind (west of) the DL, so that the subsequent rising thermals explains the mixing that occurs, as well as the initiation (excessive simplification!).

 

[StephenHenry]

Not OT at all Bob. I've been thinking about a similar question. Something along the lines of "Why does moisture pool at the surface?" Maybe the answer is "It doesn't. It just comes from the surface." Definitely curious if others have insight.

Jeff, thanks so much for the response. You were definitely one of the people I was hoping to get some wisdom from. I had been forgetting the sloping-terrain aspect of a dryline. I've seen idealized cross-sections like this before, but never really thought of them in a useful operational sense. It makes a lot of intuitive sense why things mix out more quickly near the dryline, and why the dryline can jump forward.

Do you have any sense for how "horizontal" the top of the moist layer really is? Is it really horizontal or is there an overall upward slope in top of the moist layer as you move towards the moisture source? I'll pull some soundings to try and get a sense for this (maybe some MAF-DRT-CRP and AMA-FWD-LCH sounding trends).

And you can't just tease me by mentioning "not obvious" effects of upward bulges. What do you think those effects might be?

 

[Jeff Duda]

Preface: I don't know for sure the answers to any of these questions, but here's a guess after some critical thinking.

Moisture pools in a similar way that water does when a continuous stream is blocked by something in the flow. If you put a big rock in a stream, you'll see the water level bulge upward just a little bit where the incoming water collides with the rock surface. The water has to divert around the rock rather than go through it. This causes convergence of water flow in the region where the streamflow is colliding with the rock. More water is coming into that region in the stream than is going back out, so there's a net gain of water mass in that spot, at least until a new equilibrium is achieved (this occurs rather quickly with liquid water). A dryline acts like the rock in this analogy, except it's a very long rock. So the moisture can pool up in a line rather than just at a point. This may also indirectly or partially explain the upward bulge in the top of the moist layer near the dryline (although I still think vertical mixing plays a stronger role in causing that). Since the dryline is moving and the atmosphere is otherwise evolving, the analogy isn't perfect, but on time scales of a few hours (pre-convective hours on the day of an event), I think it holds just fine.

One thing I've seen shown in research on drylines is that there can be a "mixing zone" covering a width of a few 10s of km just ahead of the main part of the dryline where dry air from above the moist layer is beginning to mix all the way down to the surface, but the moisture has yet to fully mix out. Storms can preferentially develop along the leading edge of this mixing zone, which would appear as a storm developing a few 10s of km ahead of what many would subjectively analyze as "the dryline". This is optimal for chasing as it puts a storm basically in the rich moisture without worry of the dryline "undercutting" it or just wiping it out. I think the width of the mixing zone increases either with northward or southward extent along the dryline, but I don't remember which particular direction it goes.

 

[JamesCaruso]

We have all seen situations where surface winds are veered from the southwest in the warm sector - i.e., ahead of the dryline as defined by dewpoint - but I have also heard it said that the true dryline is delineated by the leading edge of the southwest winds, even if the lower dews are located further west. So I guess two questions, do you guys agree that the dryline is delineated more by the leading edge of the southwest winds than by the lower dewpoints (in situations where they are not colocated)? And in these situations, what mechanism is at play? Is it just the result of greater mixing and/or a shallower moist layer?

 

[Bob Schafer]

Here's a quick and dirty answer to your question, James: No, the DL is not determined by where the sfc winds veer to SW... or SSW. Anybody who would argue that is corrupting the very definition of dryline. Just as no two storms are alike no two DL's have the exact same characteristics, so some days you have a really poorly defined DL, but please don't define it by sfc wind direction.

 

[Jeff Duda]

We have all seen situations where surface winds are veered from the southwest in the warm sector - i.e., ahead of the dryline as defined by dewpoint - but I have also heard it said that the true dryline is delineated by the leading edge of the southwest winds, even if the lower dews are located further west. So I guess two questions, do you guys agree that the dryline is delineated more by the leading edge of the southwest winds than by the lower dewpoints (in situations where they are not colocated)? And in these situations, what mechanism is at play? Is it just the result of greater mixing and/or a shallower moist layer?

I'm pretty sure any difference between the dewpoint gradient and wind shift is the result of that mixing zone I discussed in my previous post.

 

[JamesCaruso]

Jeff, agreed, except that it seems like the distance between the dryline and the wind shift can often be more than the 10's of kilometers you noted in your post, what do you think? Also I agree that IF a storm goes up on the wind shift out ahead of the dryline it is in a good moist airmass without fear of being undercut by the dryline... But it seems to me that veering winds in the warm sector out ahead of the dryline most often mean that the setup is falling apart and likely to disappoint, as compared to a nice sharp colocated dryline and wind shift. Admittedly anecdotal evidence on my part...

Jim

 

[Jeff Duda]

I guess I can't really think of an example setup where I saw that occur.

 

[Lanny Dean]

Just looking back at older threads and wanted to chime in on this one as I myself had a few questions:
1) Jeff, by definition, if DLs move totally by mixing rather than advection can you explain a westward moving DL? The scenario might unfold as a perturbation "bump" during the late afternoon located near the caprock - with or without severe weather being produced. As conditional cooling takes place, the DL makes a westward retreat. Would this be due to mixing? Or advection?
2) Knowing that differential pressures and temperatures drive the air and mass movement of such towards the lowest energy state (equilibrium) this would be considered advection correct?

 

[Jeff Duda]

Just looking back at older threads and wanted to chime in on this one as I myself had a few questions:
1) Jeff, by definition, if DLs move totally by mixing rather than advection can you explain a westward moving DL? The scenario might unfold as a perturbation "bump" during the late afternoon located near the caprock - with or without severe weather being produced. As conditional cooling takes place, the DL makes a westward retreat. Would this be due to mixing? Or advection?
2) Knowing that differential pressures and temperatures drive the air and mass movement of such towards the lowest energy state (equilibrium) this would be considered advection correct?

I should've been more clear about that. During the daytime (when diabatic and subsequent sensible heating are strong), the dryline moves by mixing drier air down from a loft rather than by advection of dry air. However, at night when sensible heat flux weakens significantly or becomes negative, you don't really have that turbulence to drive continued mixing. Also the cT air mass behind the dryline tends to have lower pressure than the mT airmass ahead of the dryline, so there is a pressure gradient directed towards the cT air mass and little to no vertical mixing to counter that. Thus, the westward propagation during the evening is generally due to advection.

I have never personally witnessed the dryline move westward during the daytime, however. If you see it and it's near the caprock, I would suspect topography may be playing a mesoscale role in causing that, but I'm not totally sure what that could be.