Mesoscale Convective Complexes- How do they form

[Cap'n Jack]

I read thid from AOPA Flight Training:

MCCs grow and continue after sunset because low-level jet streams bring in warm, humid air from the Gulf of Mexico. They begin blowing when the ground south of the thunderstorms cools after sunset. As this happens, an inversion- air aloft that's warmer than the air at ground level-forms because the air aloft doesn't cool as fast as air next to the ground.
The inversion shuts down the up-and-down air motions that make flights on sunny days bumpy at low altitudes. Meteorologists say that an inversion decouples the upper-altitude winds from the surface. Without slow-moving air rising from below and some of the fast-moving air aloft sinking, the air aloft is no longer "connected" to slow-moving air near the ground. In the morning as the sun heats the ground the inversion is erased and the low-level jet fades away. When this happens, the MCC's thunderstorms weaken and die. But, the MCC itself isn't completely dead. During the night an area of low atmospheric pressure with the winds blowing counterclockwise around it forms 15,000 to 20,000 feet above the ground in the MCC. After the rest of the MCC fades away in the morning, upper air winds carry the swirl of low-pressure air toward the east where it can organize any thunderstorms that form under it into another MCC.

I have a picture in my mind of what's happening...but not how the storms form.

What's the trigger that causes the uplift? Why does the low level jet cause the storm to form?

Thanks much!

 

[Richard]

Basically, pressure and temperature gradients are the engine, LLJS is a fuel source.

Outflow boundaries (high pressure, cool air) and cyclonic rotation (low pressure, warm air) of individual storms organize into large scale disturbances (MCC).

Orogenic topography (mountains), daytime heating, and nighttime cooling are criteria (precursors) to MCC formation. Think of these as the ignition.

 

[Richard]

Time now...2325 PDT local. Thread above was edited at 0129. Wierd.

And I know it hasn't been 56 minutes since.

 

[Cap'n Jack]

Basically, pressure and temperature gradients are the engine, LLJS is a fuel source.

So by "decoupling" from the ground (with the low level jet stream), the lapse rate becomes high enough at night to have thunderstorms? The low level jet stream is the source of warm moist air?

Outflow boundaries (high pressure, cool air) and cyclonic rotation (low pressure, warm air) of individual storms organize into large scale disturbances (MCC).

This is after it has developed; it stays somewhat organized until the next evening. I guess onece a large mass of air starts rotating it has enought momentum to carry it through the day.

Orogenic topography (mountains), daytime heating, and nighttime cooling are criteria (precursors) to MCC formation. Think of these as the ignition.

The nearest mountains I'm aware of out here are the Rockies- those start it off then?

 

[Richard]

So by "decoupling" from the ground (with the low level jet stream), the lapse rate becomes high enough at night to have thunderstorms?

I don't like this term, decoupling. I prefer inversion. The former implies a disconnect as one system is split into two and wholly separate from each other. The latter recognizes a sort of hierarchy within one system. The inversion presents a condition which intensifies the other forces which act in unison to create a MCC. The inversion acts to "stratify", or organize the building blocks which go into forming a MCC.


EDIT: One researcher commented, "It is important to distinguish between organized convection and severe convection, as there are many examples of one without the other." (Zipser, 1982) The point here is there are specific criteria for classification as a MCC. Most meso scale systems do not fulfill those criteria although any meso scale systems is a hazard to aviation.



With that said, it is the inflow brought by the jet which feeds a convection. It's interesting how you put this in terms of "lapse rate" so I should ponder that. I haven't thought of it in that term.

The low level jet stream is the source of warm moist air?

More precisely, the LLJS is the mechanism to bring warm moist air. "...low-level jet streams bring in warm, humid air from the Gulf of Mexico.", as copied from article you quoted in your original post. In this case, the GoM is the source and the jet is the transporter.

This is after it has developed; it stays somewhat organized until the next evening. I guess onece a large mass of air starts rotating it has enought momentum to carry it through the day.

No. I described individual storms before a MCC develops. This is significant because it leads to the understanding that these effects can and do occur independant of, outside of, before, the formation of MCC. However, these individual storms (or line of storms) form in a region of convergence and are fed by warm advection. Think of these as forces which are atypical to "garden variety storms". These forces lend to an organization on a larger scale.

The nearest mountains I'm aware of out here are the Rockies- those start it off then?

It's debatable how influential the Rockies are on MCC formation in the SE USA. But we are talking of large scale weather complexes of a duration of more than 24 hours.



Weather phenomena is interesting. Sever weather even more. It is complex and often misunderstood. Certainly, it's a complex subject.

 

[Richard]

Here is a paper which describes what indicators are used to forecast meso scale severe weather patterns. The value here is to better understand the various forces at work and their relationships in how they propagate meso scale patterns.

A substantially more questionable part of the forecast technique is whether or not the speed and direction of the low-level jet are indicative of the propagation component of MBE (meso beta scale elements) movement. If true, this would be a rather sweeping relationship since propagation can be influenced by many factors such as convective available potential energy, convective inhibition, orographic influences, gravity waves, outflow boundaries, etc.

http://www.spc.noaa.gov/publications/corfidi/mccthes.htm