ATMOS 301 (Final) – Deep Convection and Thunderstorms

Saturday, December 7, 2013
8:38 p.m.

Well, I’m back! I’ve been through heck and back over the past few weeks with schoolwork, but I now have some time to write on this blog, and write I shall.

OK, well that wasn’t really true. I actually have an atmospheric science 301 FINAL on Monday, and I need to study for it. I studied for my midterm by doing this same sort of blogging method, and I actually did quite well on the midterm. And it wasn’t just because of my previous exposure to weather… I’m learning many things in this class that I’ve never even come across before.

Alright, so we got several topics to cover. This blog will cover thunderstorms, but then we’ll move onto the dynamics of horizontal atmospheric flow, numerical weather prediction, and radiation/the greenhouse effect/why we are glad we aren’t in a perpetual nuclear winter (yet). Here’s thunderstorms.

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I’m sure we’re all familiar with cumulus clouds. Every thunderstorm starts as a cumulus cloud. There are two types of cumulus clouds that we tend to talk about: cumulus humilis and cumulus congestus. Humilis are known as “fair-weather cumulus,” because they are pretty short and fluffy due to a stable atmosphere overhead preventing them from growing them much further. 
Cumulus congestus, on the other hand, are the cumulus that have the potential to grow into cumulonimbus clouds that have the potential to produce lightning, tornadoes, and even sharknadoes. I haven’t seen that movie yet, but I need to see it immediately. The atmosphere above their LCL (lifting condensation level) is much more unstable, as as a result, the air parcel visibly denoted by the cumulus congestus cloud is a lot more buoyant. 
Take a look at the two skew-T plots below. The one on the left is for humilis, and the one on the right is for congestus. Notice how much more unstable the congestus skew-T is.
Cumulus humilis
Cumulus congestus

 Oh yeah, at some point I should probably show you what they look like. I don’t know where these were taken, but they probably weren’t taken in the 21st century. It’s good to have some old-school photographs now and then in this digital age of ones and zeros. Can you guess which cloud is which?

The top one is humilis, and the bottom is congestus.

Now that we’ve got our cumulus background, lets looks at the environmental controls for the formation of deep convection and thunderstorms.

Temperature and Moisture Stratification:

There are three conditions necessary for deep convection:
   1.)   A deep, conditionally unstable environmental lapse rate
   2.)   Deep boundary-layer moisture
   3.)   Low-level convergence (or lifting) sufficient to “release” the instability

Convection feeds on the potential energy inherent in the temperature in the moisture stratification, and this potential energy is known affectionately by the meteorological community as CAPE (Convective Available Potential Energy) and is in J/kg.

It is defined as the integral from the LFC (level of free convection) to the EL (equilibrium level) of F/(rho)’dz. F is the upward buoyancy force per unit volume on the rising air parcel due to the temperature difference between the parcel and its environment, rho’ is the density of the parcel, and dz indicates that we are taking the integral with respect to height. We do some weird derivation stuff, and we find out that the integral is essentially the area on the skew-T plot from the LFC to EL and pounded by the temperature sounding on the left and the moist adiabat on the right. CAPE values of 0-1000 are considered marginal for convection, but when values get above 4000, look out.

*When it comes to wind, storms generally move at the average direction and speed of all the horizontal wind vectors in their component of the atmosphere. But it’s not just horizontal wind profiles that drive storms… vertical ones have a huge one too, and I’ll discuss those shortly.

Structure and Evolution of Convective Storms:

There are three types of storms: single-cell, multi-cell, and supercell.

1.) Single-cell storms

These types of storms are the most common type of storm, and they are also the weakest. If multi-cell storms are like “Daniel’s Broiler” and supercells are “Canlis,” these guys are just “Papa John’s.” These storms are short-lived because they “self-destruct” very quickly in that their downdraft quickly cuts off their updraft and the supply of warm air necessary to keep the storm alive.

2.) Multi-cell storms

The gust front is the boundary between the warm, moist air from within the boundary layer and the denser, evaporatively-cooled downdraft from the storm itself and can be thought of as the “leading edge” of the storm. This advancing gust front is an example of what would be the leading edge of a “gravity current,” which is where a mass of high-density fluid flowing along a horizontal bottom displaces a fluid of lower density. This displacement causes lift, allowing new cells to continuously form.
This allows for the whole multi-cellular system to exist for quite some time. The picture below shows this “conveyor belt” effect pretty well.
Multi-cell storms can be pretty damaging. “Derechos” are a type of multi-cell system that can devastate areas for hundreds of miles. Just because they don’t produce torndaoes doesn’t mean they can’t cause massive destruction. The June 2012 North American Derecho killed 28 people and caused 2.9 billion dollars in damage as it tracked from the Midwest to the Atlantic Coast.
3.) Supercell storms:
Supercell storms are defined by having a rotating updraft. This rotating updraft forms when wind shear in the lower atmosphere gets a vortex of air parallel to the ground rotating in the atmosphere. This vortex is then lifted into an upright column by an updraft in the unstable atmosphere, and this rotating column of air becomes the rotating updraft.
When the updraft lifts this vortex up, we now have two vortices in the storm. The high pressure prefers to be on the upshear side of the low pressure, so the storm actually ends up splitting into a “left-mover” and a “right-mover” so that this preference is conserved. 
The right mover is generally the “favored” mover in the US. I give myself a headache trying to visualize and explain it, so I’ll just give you the diagrams and let you do the dirty work yourselves. The vertical pressure gradient helps to reorganize the storm so that the updraft is on the right and the downdraft is on the left once the vortex of spinning air has been lifted upright by the initial updraft. I’ll just leave it at that.
Supercells are pretty complex in their anatomy and have a lot of little (and not-so-little) quirks that set them apart from other thunderstorms, so take a look at the drawing below, copy it, etc. and you should be fine. You’ll know you’ll be in the green when you can draw it using an Etch-A-Sketch
That wasn’t too bad… mostly qualitative stuff. Now it’s time to move on to something a little more intimidating…
Charlie
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