Dryline Question

[James Gustina]

Hey guys I have another question to pose to ya. Recently i've been hearing a person call a dryline a pressure trough. Now i have never heard a dryline called a pressure trough and I'm unsure as to whether or not it is. Can anyone shed some light on why this guy is calling it a pressure trough and if it is or not? Thanks.

 

[Andrew Ryan]

A pressure trough is caused by the density discontinuity found at the meeting point between two different air masses. That is why fronts are drawn in pressure troughs. They mark the division of these two air masses and the density discontinuity.

Getting back to your questions, I guess technically it's correct, but I don't know why anyone would call a dryline, a pressure trough? But drylines don't always have to be placed in pressure troughs. So, they are incorrect in that respect. But calling a dryline, a pressure trough, is like calling a thunderstorm, a surface convergence storm.

 

[James Gustina]

Thanks Andrew, that helps a lot. Yeah the guy has been beating it in that a dryline is a pressure trough. Here's an example:
"So a dryline is not a pressure boundary yet chasers converge on dryline bulges because of what? Gradient pressure forced by an approaching front or the convergence of a rearward trough which sandwich the dry line.

If it bends then it is the changes in atmospheric pressure either at the PBL and upward associated the transportation of air right?

It's not a a cold or warm trough - I never said that. I commented on the fact that they are pressure troughs - they have to be it's physics!"

I think what he said is a little bit...off.

 

[Greg Blumberg]

I've seen instances where drylines have been placed in pressure troughs, and it makes sense for it to be by Margules Frontal Slope equation (which proves the relationship between the pressure troughs and the density discontinuity Andrew was talking about.) I would guess that by shrinking the resolution of the surface analysis contours (and spacing of the station plots) you could identify the pressure trough related to the dryline a little bit better.

I would guess that the "pressure trough" terminology used would be to indicate the existence of some sort of boundary initially and then maybe to be later identified as a dryline. Do you have any examples as to how the terminology was used and in what situation?

I'd call on Patrick Marsh to clarify anything else...I'd be willing to bet he has more knowledge on the subject.

 

[Paul Knightley]

Most, if not all 'fronts' reside in pressure troughs. That doesn't mean that fronts *are* pressure troughs.

 

[Patrick Marsh]

Most, if not all 'fronts' reside in pressure troughs. That doesn't mean that fronts *are* pressure troughs.

It can be derived quite easily from Margules' Frontal Slope Equation that the pressure along a front must be a relative minimum. Thus, a front must *always* be placed in a surface pressure trough. This is why synopticians will instruct students to "kink" isobars when they intersect a front. As you state, however, a surface pressure trough does not necessarily mean you have a surface front. The surface pressure trough could easily be a surface reflection of a front aloft.

How does this all tie into the dryline? Well, to understand the dryline let's think about an idealized scenario...Place an incline wedge into a large pan and then pour water into the pan on the side with the smallest edge of the wedge. Notice how as the water depth increases on the small edge of the wedge, it appears to be "advecting" up the incline plane to the higher edge? (I use scare quotes around advecting, because if we were to look at the water from the frame of reference of the surface of the incline plane, it would appear as if water was being advected to the higher side!) As we pour water into the pan, the top of the water will intersect the wedge at a given point. In the atmosphere, we have a special name for this intersection, which I'll say below.

The geography of the central US can be crudely approximated as that of the incline wedge. For example, AMA would be the high side of the wedge, DFW would be in the middle, and HOU would be on the flat side. If moisture in HOU becomes "deeper" (similar to us pouring water into the pan above), it will spread uphill toward DFW and AMA. On a surface map, we'd call this moisture advection. At some location, the "top of the moisture" will intersect the sloped terrain. We give this intersection a special name: "Dryline".

So, because of this, the dryline is *not* a front and therefore should *not* have isobars kinked when they intersect it. However, with that said, it is possible to have a surface pressure trough associated with a dryline...such as when there might be an 850 front co located with the dryline. In this instance, you *would* kink the isobars...not because of the dryline, but because of the surface pressure trough.

Have you ever notice how a wind shift often precedes a dryline? This is actually the result of the surface pressure trough racing ahead of the dryline - most likely because of a front located just above the surface. Margules' Frontal Slope equation also states that in a "front", the winds should change directions cyclonically as the front passes and the wind speeds should increase -- which is precisely what happens along a "wind shift" that races out ahead of a dryline.

 

[Patrick Marsh]

"So a dryline is not a pressure boundary yet chasers converge on dryline bulges because of what? Gradient pressure forced by an approaching front or the convergence of a rearward trough which sandwich the dry line.

If it bends then it is the changes in atmospheric pressure either at the PBL and upward associated the transportation of air right?

It's not a a cold or warm trough - I never said that. I commented on the fact that they are pressure troughs - they have to be it's physics!"

Dryline bulges are *not* directly caused by "gradient pressure forced by an approaching front or a rearward trough which sandwich the dry line".

Dryline bulges are the result of jet streaks aloft moving across a dryline. Contrary to popular belief, air does not flow across constant pressure surfaces, nor along constant height surfaces. If the air is dry, it flows along a constant theta surface, or isentropic surface. The air behind a dryline is warmed as it it is compressed "adiabatically". This results in a vertical temperature profile behind a dryline to become "dry adiabatic". Another name for a dry adiabat on a Skew-T/Log-P is "isentrope". In other words, as a thermal profile becomes dry adiabatic, it means the dry adiabatic layer lies on a plane of constant theta...also known as an isentropic surface.

Recall, I started by saying that dry air flows along a constant adiabatic surface...this means, if you have a dry adiabatic layer, you'll bring the stronger wind speeds atop that layer down to the bottom of that layer.

Putting this altogether: As a jet streak approaches a dryline, the faster mid-level winds are brought down to the ground along the dry adiabat. This increase wind speed on one side (typically west side, but it is possible to have different orientations) of the dryline causes mixing to increase locally at the location of stronger winds. The net result is the dryline surges eastward locally, giving the "bulge" shape to the dryline. (Your friend is half right, the jet streak is present as a result of "gradient forces" and some of the other words he used...)

So, why then do chasers favor the northside of a bulge? This is where your surface winds back the most (just think orientation of the dryline bulge), you have cyclonic shear at the surface, and you are typically in the left exit region of the jet streak that induced the dryline bulge/surge.

 

[James Gustina]

Thank you very much Patrick. Yeah i don't know what the guy was trying to point out there. Would you mind if i used some of your post to help clear it up a bit? I'd use something other than friend to describe him. All he was trying to do was to hijack a thread on TVN i think to make people feel and look stupid.

 

[Patrick Marsh]

Feel free to use any of this. They aren't my ideas...they are meteorological principles!

 

[James Gustina]

Thank ya. Yeah i'm not really one for being able to "fluently" explain principles as in depth as you can. Thanks very much again man.