Basic Storm Structures

So I know I promised in my first post that I wouldn't ramble on and on about super-science nerdy stuff that nobody cares about. But - this is an exception to my rule. When talking about our chases in V2 it will probably be somewhat helpful if the reader has an idea what I'm talking about when I reference specific parts of the storm. So with that in mind I wanted to use this last post before deployment to give a brief overview of the structure of the storms we'll be targeting.

Nearly all tornadoes are generated in what is called a supercell (cheesy moniker, serious storm). Supercells are composed of several "parts" (for lack of a better word) - which when assembled form a picture in real life that is often fairly close to textbook. A simple Google search will turn out a diagram of a supercell much like the one below:


The storm has several key components - the most important of which is the updraft (denoted in this image by a "U"). The updraft acts as the engine of the storm. It's constantly ingesting new air (denoted by the bold arrow) which is unstable enough to rise upward through natural buoyancy in a process called convection. If you take away the updraft the storm ceases to exists. As air rises in the updraft if will cool to the point where condensation occurs and eventually rain starts to form. The heaviest of this rain will fall out directly adjacent to the updraft (indicated by the shaded closed circular region). In supercells the rising air will eventually be above the freezing level and hail can form. Often times the region of intense precipitation will be called the "hail core" because of the presence of golf ball sized and larger hail. It is our strong desire to avoid the hail core when chasing storms.

Two other regions of the storm are indicated by FFD and RFD - which stand for Forward Flank Downdraft and Rear Flank Downdraft respectively. These regions develop when precipitation formed in the updraft gets thrown away from the center of the storm. You can think of it kind of like pouring pancake batter in a pan, except for upside down. Eventually the rising air parcels are either no longer buoyant or they hit the top of troposphere and the air starts to spread outward. When the air moves outward away from the center of the updraft it carries the cloud droplets and smaller rain droplets with it. This gives the storm it's characteristic flat top which is called an anvil (for obvious reasons). Eastern Illinois University has a nice schematic of this on their web page:


Rain falling from these clouds will often fall into areas of air that are subsaturated - that is, they can hold more water vapor than what is currently in them. This results in some of the smaller rain droplets evaporating until the air is no longer subsaturated. If you reach way back in your memory of high school chemistry you'll recall that evaporation is actually a cooling process, and the evaporation of these small rain drops can substantially lower the temperature of the air. Cooler air is more dense and will thus sink towards the ground (called a downdraft conveniently) and spread outward (again, imagine pouring pancake batter onto the ground). This cooler, denser air will spread out from the storm in a process called "outflow" or sometimes a "gust front". This is indicated on the supercell image by the teethed lines that are often used to represent cold fronts on your 6pm tv weather forecast. You'll hear me talking often about getting passed over by the gust front - which is often accompanied by a strong gust of wind, a shift in the wind direction (from inflow to outflow) and a steep drop in the temperatures on the ground. Sometimes you can see the gust front moving outward on radar images because the strong winds can kick up dust and bugs dense enough to be observed by a weather radar.

Perhaps the key signature to identifying a supercell on radar is its distinctive "hook" pattern that surrounds the updraft. What separates a supercell from an ordinary cellular thunderstorm is the turning of the updraft column as the air rises upward. This turning will advect some of the precipitation around the updraft until it forms what looks like the end of a fishing hook. If a tornado forms in a supercell, it's most likely location is directly in this hook pattern of precipitation. Lots of time on tv during severe weather the forecasters will be keen to point out this hook pattern as an indicator of a possible tornado developing. Better understanding the process by which rotation in the updraft is converted into a tornado is one of the main goals of V2.

I know this post is getting technical and lengthy but I just wanted to take one more paragraph to highlight how this supercell diagram actually plays out in real life during V2. Paul Markowski from Penn State University cooked up this handy little diagram below which outlines our observation strategy for measuring a supercell storm.



If you look close you can find the four balloon icons, which indicate the ideal launch positions for NCSU's team of researchers. Compared to most of the armada we're relatively far away from "the business end" of the storm. I'm quite happy that I haven't been volunteered to attempt to get a radar within a few kilometers of the hail core. Luckily for those taking observations close up the U.S. government does have hail insurance on all of our vehicles.

Well that's all for now. Hopefully now when I mention a particular storm had a weak hook or the gust front shook the truck you'll have a better idea what I'm talking about. My next post will likely come from Norman Oklahoma later this week as we prepare the trucks and get ready for day one of operations on Saturday.