Thursday, September 15, 2011
I’m doing what would be Friday’s post today since I will hiking around Mt. Rainier tomorrow.
We are now right at or just past the peak of hurricane season, and in a month or so, we should start seeing a more consistent flow of extratropical storms across the Pacific, and they will really ramp up in November. Hurricanes and extratropical cyclones may seem similar at face value; they both cause rain and wind. However, that’s about where the similarities end. The more I have learned about these two types of storms over the years, the more I have marveled about how two completely different sets of circumstances can produce extremely powerful storms.
Hurricanes derive their energy from heat and moisture. There are three main ingredients needed for tropical storm formation: warm SSTs over 80 degrees Fahrenheit, ample moisture, and an area of convergence. Most of the tropical cyclones in the Atlantic start out as thunderstorms in the Intertropical Convergence Zone (ITCZ). The ITCZ, known by sailors as the Doldrums, is the area where the trade winds from the northern and southern hemispheres meet. If a sailor wandered too close to the equator, they often got stuck in the Doldrums. The Doldrums are hot, humid, and calm. Many sailors who got stuck in the Doldrums ended up starving because they could not get catch any wind. Crossing the equator was a risky move for sailors and explorers, and not everybody made it.
The warm SSTs, the hot/humid air, and the low wind shear in all levels of the atmosphere make the ITCZ a prime spot for tropical cyclone formation. When enough thunderstorms develop, a local area of low pressure is formed from the convection that the storms produce. Winds rush to the area of lower pressure and rise, creating more thunderstorms, more convection, and even lower pressure, as more air rises upward from the center rather than filling it in. Additionally, when water turns from invisible water vapor gas to visible cloud droplets, latent heat is released because of condensation, which gives the storm even more energy. If conditions are favorable, this cycle strengthens and a tropical storm (sustained winds 39-73 mph) is born. At some point, the Coriolis Effect, which causes things to deflect to the right, becomes more noticeable, and a center of circulation develops. If the storm continues to strengthen, it will become a hurricane and the center of circulation will be clearly visible. This is called the “eye” of the storm. Even though the eye is where the lowest pressures are found, the winds are calm and there are often sunny skies. Remember, winds are created by differences in air pressure, not simply low pressure itself.
As long as a hurricane has warm water, humid air, and low wind shear, it will generally keep its strength or strengthen. Some hurricanes can last for weeks. A hurricane will die out when wind shear rips it apart, when it runs into dry air, or when the energy source (warm water) is cut off. That is why hurricanes die out so quickly when they reach land. They no longer have access to the warm water that gives them energy.
As you can see from the picture of Hurricane Katrina above, strong hurricanes are very symmetrical, especially when there is low wind shear in the upper atmosphere. The eye wall surrounds the eye, and this is where the heaviest rain and strongest winds are found. Out from that, higher rain clouds give moderate amounts of precipitation, with heavy amounts of precipitation found in individual rain bands. Tropical systems can generate enormous amounts of rain. In fact, Alvin, Texas, received 43 inches of rain in 24 hours when Tropical Storm Claudette stalled over the region, setting a U.S. record that has yet to be broken.
The below diagrams show the formation of the thunderstorms required for tropical systems and the basic anatomy of a hurricane. I got both of these from an MIT page.
Now, I’ll talk about extratropical cyclones.
The first stage is what is called an open wave. A storm starts off with a stationary front dividing warm air from the south and cold air from the north. Because of the Coriolis Effect, cold air will push south and warm air will push north, creating a center of rotation with fronts that spin counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere.
The second stage is a fully developed cyclone. The counter-clockwise rotation that began with the open wave strengthens and the pressure begins to drop. Also, since cold fronts tend to move faster than warm fronts, the storm takes on less of a horizontal appearance and begins to make the familiar common shape that we often see with these types of storms. As warm air from the south is sucked into the center of the storm, it rises, expands, condenses, and cools, while releasing latent heat from the condensation of water droplets in the process. At this stage of the cyclone, there is more air being sucked up than air filling the low pressure center at the surface. Because of this process, additional air molecules are being expelled, creating lower atmospheric pressure throughout the mid and lower levels of the storm.
The third stage is the occluded cyclone. Remember how I said that cold fronts move faster than warm fronts? Eventually, the cold front catches up and overtakes the warm front. This creates a new type of front – an occluded front. Occluded fronts share the characteristics of both warm and cold fronts. This is most intense stage of the cyclone, but sometime into this stage, the cyclone begins to weaken. This is because the air masses on either side of the occluded front are similar in temperature and humidity, and since extratropical storms get their energy from different air masses along a front, they are cut off from their energy source and slowly weaken.
A storm can get especially strong when it is fed by a strong jet stream. Strong jet streams often form amongst stark differences in tropical and polar air masses, meaning that there is a lot of potential energy stored. As previously stated, cyclones deepen when warm air rises into colder air, with more air rising from the center than rushing to it. Additionally, a strong jet stream will speed up this process. Warm air will converge at the center at a faster rate, and the cooled air aloft will diverge faster, making way for new air to fill the void.
The result can be a huge storm with winds that can sometimes approach category 3 hurricane speeds. In addition, these storms are much, much larger and usually move much faster. The Columbus Day Storm of 1962 created hurricane-force winds from California to Vancouver Island, and some storms have stretched from the Pacific Northwest all the way to the International Date Line!
As you can see, these two types of storms are very different! There are a lot of minor details, particularly with extratropical storm formation, which is considerably more complex than tropical storm formation (at least it’s harder for me to explain). Upper level winds fuel a mid-latitude cyclone but can tear a tropical one apart. Extratropical storms usually disintegrate after a couple days due to an occlusion, but tropical storms can exist for a much longer time. And the biggest difference of course, is that tropical cyclones have no fronts. They get their energy from warm air. Extratropical cyclones start because two air masses come together and form a front.
That was a really long post, if you have any questions, comment below and I will be sure to answer them.
Thank you for reading!