Confluence, Diffluence, Convergence, Divergence

[Steve Dedman]

Since I am a rank amateur, and fairly new to really getting into the science of meteorology, I need to make sure I have a grasp of these concepts.

As I understand it, they can be described thusly:

Confluence - a car full of air (a parcel) is getting ready to jump on to the wind flow expressway. The point where that on-ramp meets the main flow is the point of confluence. Much like when the Arkansas River meets the Mississippi River.

Diffluence - exactly the opposite. The point where the off-ramp leaves the main flow is the point of diffluence.

What exactly is the significance of these points? Is it related to vorticity?

Convergence - like a wind blowing between buildings. It accelerates (venturi effect?), but also rises because there is less resistance to the wind flow above than on the sides where it is being constricted. Is this caused by pressure gradients? When you talk about convergence in forecasting severe wx, what scale are you talking about? Storm scale? Synoptic scale? Why is it inhibitive in the upper levels?

Divergence - like wind exiting the valley between buildings...it spreads out and decelerates. But why is it conducive to severe genesis at the upper levels, but not the lower levels?

Thanks all.

Also, while I am envious of the education a lot of you have, I haven't been there. So please, type slowly and use small words.

 

[Adam Boggs]

I'm in the same boat as you are, but think I can add some non-technical insight.

The importance of these concepts seems to be around forcing. Severe storms are not created by simple convection like regular storms. Instead, they require the power of jet streams to lift them. The rule I (think I) remember is "convergence below, diffluence aloft" for severe storms.

Convergence and divergence lead to a change in speed, because you're compressing or expanding the air. This can be a linear action, as in stronger air moving into weaker air, and is not necessarily two separate streams joining. As air converges, as you mention, it is pushing more air into the same area which speeds it up and creates high pressure. The high pressure has nowhere to go but UP.

Similarly, diffluence aloft creates a low pressure, effectively sucking the surface air up to the higher levels. (My understanding is that divergence allows the air to rise, but doesn't suck it up quite like diffluence does.) Diffluence and divergence at the surface causes sinking, which is not as useful for severe weather genesis. (I think convergence aloft and divergence at the surface can lead to Fohn wind scenarios as the strong jet gets forced to the surface where it is heated adiabatically.)

When you have winds converging at the surface, and diffluent winds aloft, a huge amount of upward forcing occurs.

This is purely one novice to another, so if there are corrections or clarifications to anything I said here, I would be happy to hear them too!

 

[Chris Allington]

Similarly, diffluence aloft creates a low pressure, effectively sucking the surface air up to the higher levels. (My understanding is that divergence allows the air to rise, but doesn't suck it up quite like diffluence does.) Diffluence and divergence at the surface causes sinking, which is not as useful for severe weather genesis. (I think convergence aloft and divergence at the surface can lead to Fohn wind scenarios as the strong jet gets forced to the surface where it is heated adiabatically.)

When you have winds converging at the surface, and diffluent winds aloft, a huge amount of upward forcing occurs.

This is purely one novice to another, so if there are corrections or clarifications to anything I said here, I would be happy to hear them too!

Exactly... as to the way these impact severe weather, like Adam said divergence aloft and convergence at the surface create lift for an air parcel.

On a day where there is significant capping, divergence aloft or convergence at the surface along a boundary can be just enough to intiate convection.

 

[Scott Weberpal]

Glad to see someone asking about the difference between diffluence and divergence; so many times I see people using "divergence" when they should be using "diffluence" and vice versa.

upper diffluence necessarily cause rising air while divergence does.

This site has a good explanation and diagram showing the difference between diffluence and divergence...

http://www.theweatherprediction.com/advanced/habyextra6/

 

[Adam Boggs]

upper diffluence necessarily cause rising air while divergence does.

Scott, could you elaborate/clarify this? I think you mean "upper diffluence DOES NOT necessarily cause rising air", which means I got this wrong in my previous post. I think I'm still confused on why divergence causes lift but diffluence does not though.

I like the link you posted as well.. I think most of what I learned about this stuff was from that site! I think I need to review it a bit more.

 

[Scott Weberpal]

Scott, could you elaborate/clarify this? I think you mean "upper diffluence DOES NOT necessarily cause rising air", which means I got this wrong in my previous post. I think I'm still confused on why divergence causes lift but diffluence does not though.

I like the link you posted as well.. I think most of what I learned about this stuff was from that site! I think I need to review it a bit more.

You can have a diffluent pattern (height contours spreading out) on a map (which leads to divergence allowing the "wind" to spread out), but if there is stronger winds running into weaker winds in that diffluent pattern (convergence), that will not be conducive to rising air. Haby describes this toward the bottom of the page...

The effect of convergence and divergence occurring at the same time is no vertical motion. The air is merely being deformed into a new shape. The air is spreading out, but it is not rising or sinking. It is upper level divergence that causes rising air. The two best examples of upper level divergence are PVA and divergence associated with the right rear and left front quadrants of a jet streak. Upper level diffluence by itself does not cause rising air.

 

[Tim Vasquez]

Severe storms are not created by simple convection like regular storms. Instead, they require the power of jet streams to lift them.

Severe storms are definitely possible without a jet stream. Delta Flight 191 at DFW in 1985 was downed by a pulse thunderstorm (pardon the term), and I am fairly certain there was no jet stream or strong winds aloft. Severe storms (mostly hail, wind, and rarely, tornadoes) happen all the time with weak wind profiles and weak dynamics.

Rising motion from short wave troughs, jet streams, and so forth is only on the order of centimeters per second, not enough to influence a storm. But over the span of hours, this rising motion acts to prepare the environment for thunderstorms. Lifting a column of air adiabatically weakens inversions (the warm part rises and cools adiabatically, and the air underneath that replaces it is cooler, thus no more inversion) and if this kind of lifting results in latent heat release in the lower troposphere but not higher up (due to dry air up there) then the instability is increased.

PVA (also known as CVA) is a diagnostic measure. For it to work there has to be increasing PVA with height and warm air advection. Those things often cancel out. There are other tools like q-vectors and omega, but the use of them has been in a state of decay for years and I do wish more was done with them. Other diagnostics are looking at satellite images for patterns associated with upward motion, as well as surface and radar (extensive areas of virga and mid level cloud over NM is one sign of vertical motion about to come out into the Plains, for example). As far as forecast vertical motion diagnosis, the new meteorological cancer du jour seems to be looking solely at precip areas "breaking out" and ending the diagnosis there, and not looking at some of the other quantities the models have to offer.

Also regarding diffluence, I am pretty sure that this as a severe weather indicator comes largely from Miller's 1972 forecast rules, and it's kind of dated. In reality diffluence/confluence are often cancelled out by convergence/divergence, which is why we have objective analysis tools to measure those fields. I think Doswell has written some about this, I believe in his Pet Peeves area, which I recommend checking out.

Even with increasing jet stream winds, the strengthened vertical motion can sometimes take an opposite sign: downward motion. You can sometimes be ahead of upper-level processes that produce subsidence, like the classic short wave ridge. This strengthens the cap. I am convinced that a good share of busts are caused by undiagnosed or undiagnosable areas of weak subsidence acting on the target area.

In summary, strong upper-level dynamics act to:
1. Get rid of caps and to a certain extent destabilize the air mass, if associated with lift.
2. Strengthen caps, if associated with subsidence.
3. Help impart various amounts of shear, helicity, and horizontal vorticity into the environment, which produces an assortment of long-lived structures.
4. Make things more complicated, of course.

Tim