December 10, 2010
My meteorology 111 final is coming up on Tuesday, so the next few articles will help me in studying for the test. Today we tackle thunderstorms.
I think it's safe to say that we've all experienced a thunderstorm in our lifetime, and most of us have probably heard of a supercell thunderstorm. The big difference is at the core of the storm. A regular old, garden variety thunderstorm is formed as warm air rises into the atmosphere, gets above the freezing level, condenses to form a nice tall cloud and then the air rushes back down toward the ground as a downdraft. Regular mid-summer afternoon storms have a life cycle of about 45 minutes to an hour before the rain that is falling, cools the air below the storm enough that it no longer rises, robbing the storm of the lift it needs to continue on, and it rains itself out. Regular thunderstorms don't usually need a strong trigger(i.e., frontal lifting or convergence lifting) to get going, they can pop anywhere that the atmosphere will support the lifting air.
A supercell storm is a little different kind of storm. The ingredients are all the same for a supercell, they are just much stronger. A storm will be classified as a supercell if the core of the storm is rotating, also known as a "rotating updraft". Supercells usually require some kind of "trigger" in the atmosphere to get the strong lifting they need. Triggers like a strong cold front, or moist tropical winds converging tend to do the trick. The air that is drawn into the storm and forced upward will begin to spin, thanks mainly to wind shear in the atmosphere. Once it gets spinning, if the core of the storm is tall enough, it may be able to tap into the jet stream winds aloft and pull them into the storm, they usually get forced down to the surface by the existing downdraft, making it that much stronger. Supercells have a much longer life cycle as the lifting mechanism that caused the storm also helps to continually provide the warm air needed to feed the updraft. The downdraft from a supercell can spread out for miles after it hits the ground, sometimes strong enough to case damage. Imagine pouring a bucket of water out on the floor. The descending air from the downdraft does the same thing when it hits the ground. The rotating updraft of a supercell thunderstorm is also the main cause for tornadoes, which we will get to later on. Don't get me wrong, both types of thunderstorms can cause tornadoes, for example, the November 17th storms that hit northeast Baltimore with an EF1 tornado were not supercells. The odds are much greater for a supercell to put down a tornado.
The thing to remember here is rotation. "Regular updraft"= regular thunderstorm. "Rotating Updraft" = Supercell.