Monday, September 10, 2007

Hurricanes, typhoons, and cyclones: Basics

Cyclones, anticyclones, and fronts. Near the Earth's surface, vertical convection often creates localized low-pressure areas where surface air is pulled in towards a center of exceptionally low pressure, rises strongly toward the tropopause, then flows aloft outward from the low-pressure center. Compensating such low(er)-pressure areas are areas of high(er)-pressure where air aloft flows toward an center of exceptionally high pressure, flows down toward the surface, then outward along the surface from the high-pressure center.

In the Earth's rotating frame, any vertical or north-south air motion is deflected by the Coriolis pseudoforce.
  • In the northern hemisphere, low-pressure systems are set spinning counterclockwise. In the southern hemisphere, they are set spinning clockwise.
  • In the northern hemisphere, high-pressure systems are set spinning clockwise. In the southern hemisphere, they are set spinning counterclockwise.
Low-pressure systems (marked by "L" on a weather map) are also called cyclones and often bring with them rain and strong winds. If they form, as they often do, over water or humid land, the strongly heated, rising air at their centers takes with it a lot of moisture, which condenses into large cloud banks that eventually dump their moisture as precipitation (rain or snow). High-pressure systems (marked by "H" on a weather map) are also called anticyclones. Their air is typically much drier than the air in a low-pressure system, being pulled from aloft. They compensate for the presence of cyclones not only with respect to horizontal air flows, but also with respect to angular motion. They ensure that the atmosphere has no net angular momentum.*

Cyclones can be broadly divided between those associated with weather fronts (see here for more about fronts) and independent storms. A weather front is a sharp boundary between two air masses, one typically colder and drier, the other typically warmer and more humid. A cold front is a front where the colder air mass is pushing the warmer air mass; a warm front, a front where the warmer air mass is doing the pushing. Where a cold and a warm front meet, a "hinge" forms that gets spinning (by the Coriolis force) and forms a "low." A weather front can bring its own precipitation and strong winds, driven by the temperature, humidity, and pressure differences across the front boundary.

Front-associated cyclones and anticyclones are dragged along by their fronts in a direction determined by the basic Hadley wind pattern associated with the climate zone in question. Fronts in the temperates are pushed eastward. In the tropics and polars, they are pushed westward.

Tropical cyclones. Independent cyclones form away from any front boundary and live out a typical lifecycle that strongly depends on latitude and season. Some form near the equator (0o lat) and, while still in the tropics, drift westward and (steered by the upper air tropical Hadley cell flow) away from the equator. Once they cross into the temperate zone, the prevailing eastward Hadley cell air flow takes over, and they reverse course to eastward, while continuing to move poleward.** Tropical cyclones are universally called "tropical storms." If they feature winds above a certain threshold, they are called hurricanes in the Atlantic (after the Carib storm god, Hurikán), and typhoons in the Pacific.

Tropical cyclones derive their power from upward convection of warm, moist air. They pick up this power at night, when the warmer nighttime ocean water dumps heat by upward convection into the cooler nighttime atmosphere. (Recall that during the day, the heat flow right over the water is downward, supplying the latent heat needed for evaporation - the daytime air temperature is higher than the daytime water temperature.) The longer they move over warm tropical water, they more powerful they get. They lose that power rapidly when they cross over land. The strong upward convection of warm, moist air at the cyclonic center, combined with the Coriolis force, gets the cyclone to spinning faster and faster, with a well-defined and relatively cloud-free vertical column of upward convection (the "eye" of the storm) whose walls have high circulating winds. If these winds exceed a certain limit, the storm is classified as a hurricane/typhoon. While Atlantic hurricanes can be spectacular and destructive, Pacific typhoons are typically stronger and more frequent, because the Pacific features a much larger region of unobstructed warm tropical water.

Northern hemisphere tropical cyclones form during the period June-November (from right after summer solistice to almost winter solistice), but mainly in August and September. It is during these months that the nighttime contrast between warmer water and cooler air temperatures is largest. The upward convection of warm, moist air is most enhanced in these months, leading to the highest frequency of storms and the strongest storms. In the rest of the year, conditions are not right for tropical cyclone formation. For southern hemisphere storms, shift by six months.

Extratropical cyclones. Independent cyclonic storms also form near the ±60o lat lines. They drift westward in the polar zone, but switch to eastward if they cross into the temperate zone. Depending on geography, they can become trapped and sit for weeks or even months before dissipating. They typically pull warmer moist air from the south, convert it to rain or snow, then dump it at higher latitudes. They play an important role in transferring moisture to subarctic regions and replenishing polar ice caps. At somewhat lower latitudes, they form the nucleus of long-lived springtime storm systems that can afflict the northeast American and Asian coasts.†

The basic driving force of these systems is the contrast between the warmer, moister air of the high temperate zones and the colder, drier air of the polar regions. This contrast reaches its maximum (in the northern hemisphere) in the middle to late spring (April to early June), exactly the period when these storms are most frequent and strongest. For the southern hemisphere, again shift by six months.

Poleward mass, moisture, and heat transfer. The overall effect of these moist and warm low-pressure cyclones is to help in the transfer of excess heat and humidity from the tropical zone towards the poles. There is no net mass transfer, but rather a conveyor belt where mass transfer evens out to zero because of the balancing of low- and high-pressure systems.

While poleward heat transfer in the atmosphere is not as powerful as it is in the oceans, the effect is still significant. The net heat transfer is a result of the fact that the net air flow is, rising at the equator and falling at the poles. The presence of three Hadley cells per hemisphere, rather than one, merely complicates the details. Poleward atmospheric moisture transfer is also crucial in maintaining polar ice and snow covers. Water is evaporated at lower, hotter latitudes and dumped as rain and snow at higher, cooler latitudes.
* That is, as seen by an observer rotating around the Earth's axis with the surface. As seen by an outside inertial observer, the Earth's atmosphere does have a net angular momentum associated with its overall rotating motion.

** If the temperate eastward air flow ("westerlies") is weakened, a hurricane can keep drifting westward, even as it continues to move northward. From the Atlantic, such storms get into the Caribbean Sea and Gulf of Mexico, where they can wreak exceptional havoc in a small confined area. Such storms also get new strength from the warm, shallow waters of the Gulf.

If the temperate "westerlies" are strong, OTOH, Atlantic hurricanes get strongly deflected back eastward once they get into the temperate zone. They can then miss landfall altogether. As they move over the colder north Atlantic waters, they rapidly lose strength.

† These include the famous New England-Maritime-Greenland "nor'easters."

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