As with any hurricane season, you’ve probably heard some terminology this year that you may not have understood. Below are a few terms that I’ve heard a lot over the past few weeks, with Harvey and now with Irma. I hope this helps you understand those terms and, in turn, the hurricanes a little bit better! As always, let me know if you have any questions, or if there’s a term you’ve heard that you didn’t quite understand. I’d be glad to add it to this list!
Eyewall Replacement Cycle
With all the talk about eyewall replacement cycles (ERC), you may be wondering what exactly an ERC is. First of all, we don’t fully understand the whole ERC and much research is needed to fully understand these cycles. We do know they naturally occur in intense tropical cyclones. At some point in the life cycle of the hurricane, the eye begins to contract and grow smaller. At the same time, some of the outer rainbands begin to move inward to create a new eye wall. While this process takes place, the cyclone weakens. The new eyewall eventually organizes and the storm can then regain its original intensity or may even become stronger.
So, basically the eyewall begins to collapse while the outer rainbands move in to take its place. That’s the gist of an ERC.
Divergence Aloft
Some of you may have noticed the fanning out of clouds over the top of Irma (or any hurricane). I’ll try to explain why.
First, air has to rise in order for clouds to form. The air rises, cools, and condenses to form clouds. We all know air temperature decreases with height. It gets colder the higher up you go (think about climbing a mountain with snow on top). As the warm air rises, it cools and reaches its dew point. The dewpoint is the temperature air must be cooled to in order to condense. For example, right now my air temperature is 64 but my dewpoint is 48. That means I would have to cool the air outside to 48 in order to saturate the air (100% humidity).
In order to get this air to rise, you have to have a way for it to be carried away aloft (divergence). As with any storm, air has to come together at the surface (convergence) and move away (divergence) aloft. For severe weather on the mainland, the jet stream serves this purpose by carrying the air aloft away. That provides the divergence. In the tropics, it gets a little trickier. I’ll try to keep it simple.
As thunderstorms erupt around the center of the hurricane, they release a lot of heat. That is because the process of condensation (vapor to water) releases heat. This causes the air to be very warm at the center of the hurricane, and that causes pressure to slowly drop with increasing altitude. The pressure outside the storm drops more rapidly with height (as it normally does). This causes the pressure at the middle of the storm to be higher relative to the pressure around the edges of the storm. Since wind always blows from higher pressure to lower pressure, the air moves away from the center of the storm and to the areas of lower pressure around the edges. This creates a vacuum for more rising air to fill. And, again, that rising air forms clouds, storms, etc. creating a tropical cyclone. The faster that air rises, the more robust the t-storms can become. That’s why we measure the pressure in the hurricanes. That tells us how fast the air is rising. The lower the pressure, the faster the air is rising.
This outward flow is diverging away from the storm and is anticyclonic (flows counter-clockwise). Diverging air flows anticyclonically, while converging air flows cyclonically. This is why if you look closely, you’ll notice the cirrus clouds around the edge of a hurricane spin in the opposite direction of the storm itself. Check it out the next time you look at a hurricane spin on satellite (esp on visible satellite).
Stadium Effect
This is another complicated aspect of a hurricane but I will try to keep it simple. This effect is often seen by hurricane hunters, especially in strong hurricanes. It is when the eyewall widens with height as it spirals upward, creating a look similar to that of a stadium. One reason for this is due to the air parcels moving outward as they rise. They do this, in part, because of less friction with increasing altitude. As a result of less friction, the wind speeds increase and “fling” (centrifugal force) the parcels outward more with height. Studies have shown that these winds increase by 1-4%, which is enough to fling the parcels out enough to create this cool effect.
Brown Ocean
We usually see tropical systems weaken when they make landfall. Occasionally, this isn’t the case. If a hurricane moves over an area with high moisture content from evaporation, such as marshes or swamps, it might not realize that it is no longer over the ocean. Saturated soils may create conditions similar to what the cyclone experienced over the ocean. This can cause the tropical system to either hold on to its intensity or even strengthen if conditions are right.