Mesoscale Convective System

The Parts Turn Over. The Whole Does Not.

The One Relation Doing All the Work

A cold pool spreads because it is heavy. For a pool of uniform buoyancy the speed of its leading edge is fixed by the buoyancy stacked above that edge:

which for a pool carrying a total buoyancy deficit I spread over a length L is just C = √(2I / L). That single line is the whole engine of this panel. A pool that is being fed holds its speed. A pool that only spreads slows down. Everything you watch happen follows from it.

The rest is accounting. Cells are born at the front and injected buoyancy deficit into the pool once their downdrafts start, about forty percent of the way through their lives. The pool loses length and buoyancy together on one timescale, because air far behind the front is warmed by the ground and stops being part of the current. And a new cell appears at the front only when the front is lifting hard enough to get a parcel through its inhibition, which is the condition

at the CIN of 50 J/kg used here. That threshold is the hinge of the whole page. Below it the loop is open and the system is a handful of ordinary storms. Above it the loop closes and the system becomes a thing with a life of its own.

What It Does

One hundred and sixty-three storms, none of which lasted an hour, and a structure that was still going at hour twelve with a steady speed and depth. The bar at the bottom of the raster is the system. Every short bar above it is a storm. That picture is the claim of the card and there is nothing else to it.

Press “Kill Every Cell Now”

Every storm in the system dies at once. And then:

Nothing happened. The system did not notice, because the system was never the storms. The storms were the mechanism by which the cold pool kept itself fed, and a cold pool already built does not need the storms that built it in order to make more. Now press Cut off the supply, which leaves every cell alive and takes away the unstable air instead. The last cell is dead forty-nine minutes later and nothing replaces it.

Those two buttons are the whole reading. The system is indifferent to losing all of its parts and fatal-to-lose its supply. It is a closed loop drawing on an open one.

It Is Easier to Keep Than to Start

The map is measured, not drawn. For each value of the shear, the panel bisects on CAPE twice: once asking whether a parent cluster of ordinary storms can organise into a system at all, and once asking whether a system that is already running can survive being moved into that environment. The two answers are not the same.

Almost a factor of three. A system that exists can survive in air that could never have created it, which is the sharpest thing the model says and the reason the shear window has the same shape: a system can start in shear between 4.7 and 44.6 m/s, and keep going in shear between 4.3 and 59.9. Real systems do exactly this. They organise in the afternoon over heated ground and then march all night into air that has been cooling for hours and could not have started anything.

Where the Basin Actually Lives

Two of the experiments above look similar and are not. Kill every cell now destroys every part of the system and changes nothing. Cut off the supply leaves every part alive and kills it inside an hour. Put those together and they locate the basin precisely: the state that has to be inside it is the cold pool, not the convection. The pool is the whole memory of the system. Everything else is traffic.

You can see the edge of that basin directly with the kick slider. At x₁* = 1.55 a kick of +0.30 to the zonal index recovers and a kick of +0.55 does not, and the panel bisects the boundary at +0.457. That is not a statement about the storms. It is a statement about how far the pool can be pushed and still come back.

The second thing the pair of numbers above says is stranger, and it is why this card’s basin is not the same shape as the magnetic pendulum’s. The set of environments a system survives in is strictly larger than the set it can form in, by a factor of nearly three in CAPE. So the basin is not a region of the environment at all. You cannot look at the air a system is sitting in and say whether a system should be there. You have to know where it came from.

That is the sense in which the structure is doing something. It is not a consequence of the environment it is in. It is a consequence of the environment it was born in, plus its own persistence, and the gap between those two is the 1219 against the 430.

Why the Card Is Blue and Not Red

The taxonomy asks whether the system’s own activity releases the energy that drives it. Here it plainly does. The storms release latent heat by condensing vapour, and that released heat is what drives the updrafts; the rain they produce evaporates and that evaporation is what makes the cold pool; the cold pool is what lifts the air for the next storm. Every step of that is the system acting on itself. It is the flame case, not the convection-roll case, and the card is blue for the same reason the hurricane card is.

And it is not sovereign, for the reason the hurricane is not. The loop is closed but it is not self-supplying: the CAPE comes from somewhere else, made by sunlight on wet ground over the preceding day, and the system spends it and cannot make more. Cut off the supply is a forty-nine-minute death sentence. Whether such a thing is a base or a coordination attractor the card leaves open, exactly as the gate box on the Weather page says it does.

What This Model Is Not

It is not a published model. The pieces are: the density-current speed and the shear-balance idea are Rotunno, Klemp and Weisman; the gust-front triggering is Houze; the thirty-to- sixty-minute cell and the many-hour system are the standard description. The budget that links them was written for this page. It has never been compared to an observed squall line and nothing here should be read as if it had.

Two constants were fitted rather than derived. The buoyancy each cell injects was chosen to put the steady cold pool speed at about 25 m/s, which is where observed squall-line cold pools sit. The shape of the shear-balance efficiency curve is a Gaussian in log(C/ΔU) centred at 1.2, chosen so that it stays reasonably high across the 0.5 to 3 range that Bryan, Rotunno and Weisman describe as favourable. Both are fits to reported numbers, not results.

It never dies on its own. Left alone in a fixed environment it reaches a steady state and runs forever, and real systems do not. This is the same class of failure as the blocking panel’s: what actually ends an MCS is running out of unstable air, moving over ground that has been cooling all night, or losing the large-scale lift that was feeding it, and none of those exist in a model with no map. You can end it here only by taking the fuel away yourself.

There is no line. This is a cross-section through one direction. A real MCS is a line a hundred kilometres or more long, with cells at different stages along it, bowing segments, a rear-inflow jet and a trailing stratiform region with its own mesoscale circulation. The grey shelf in the picture is decoration; nothing in the budget produces it.

RKW theory itself is argued about. The strict “optimal state” at C/ΔU = 1 has been softened considerably since 1988, partly because the original simulations used domains small enough to distort the result. Mid-level shear matters, rear-inflow jets change the balance, and the criterion applies only to cold-pool-driven systems in the first place. The curve used here follows the broadened version, and it is still a curve someone drew.

Rotunno, R., Klemp, J. B. & Weisman, M. L. (1988). A theory for strong, long-lived squall lines. Journal of the Atmospheric Sciences, 45(3), 463–485.
Weisman, M. L. & Rotunno, R. (2004). “A theory for strong, long-lived squall lines” revisited. Journal of the Atmospheric Sciences, 61(4), 361–382.
Bryan, G. H., Rotunno, R. & Weisman, M. L. (2012). What is RKW theory? 26th Conference on Severe Local Storms, American Meteorological Society. Source of the broadened favourable range of C/ΔU and of the list of objections above.
Houze, R. A. Jr. (2004). Mesoscale convective systems. Reviews of Geophysics, 42, RG4003. Source of the definition and of the gust-front triggering description.
NOAA National Severe Storms Laboratory, Severe Weather 101: Types of Thunderstorms. Source of the thirty-to-sixty-minute cell and the system that lasts many hours.