ESCI 241 –Meteorology Lesson 19 – Tropical Cyclones Dr

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ESCI 241 –Meteorology Lesson 19 – Tropical Cyclones Dr. DeCaria

READING: Chapter 16

GENERAL A tropical cyclone is a large, low-pressure system that forms over the tropical oceans. Tropical cyclones are classified by wind speed as ο Tropical disturbance – wind less than 25 knots ο Tropical Depression (TD ) – wind 25 to 34 knots ο Tropical Cyclone (TC ) – wind greater than 35 knots In the North Atlantic and North Pacific, tropical cyclones are further classified as ο Tropical storm (TS ) – wind 35 to 64 knots ο Hurricane – wind 65 knots or greater (North Atlantic and Eastern North Pacific) ο Typhoon – wind 65 knots or greater (Western North Pacific) Other than the two areas mentioned above, the rest of the world doesn’t use terms like “hurricane” or “typhoon”. Instead, any tropical system with sustained winds of greater than 34 knots is called a tropical cyclone .

DYNAMICS OF TROPICAL CYCLONES The energy that drives hurricanes is primarily the latent heat of condensation. The source of the latent heat is evaporation of moisture from the ocean. ο Tropical cyclones can only form over very warm water, where there is plenty of moisture to feed the thunderstorms. ο Tropical cyclones do not form if the sea-surface temperature is less than 26 °°°C (79 °°°F) ο Hurricanes weaken rapidly when moving over colder water, or over land. The air-flow through a tropical cyclone is as follows: ο Air spirals inward at low levels, picking up moisture (latent heat) and sensible heat from the ocean surface. ο The air rises in the thunderstorms either in the spiral rain bands, or in the eye wall, where it releases a tremendous amount of latent heat. The release of the latent heat can serve to strengthen the surface low and the upper-level anticyclone, because it pushes the low-level pressure surfaces downward, and pushes the upper-level pressure surfaces upward. ο The air is ejected from the top of the cyclone (almost like an exhaust vent). Tropical cyclones are warm-core lows. ο The cyclone becomes less intense at higher levels in the atmosphere. ο In the upper troposphere there is an anticyclone, or high-pressure system above the surface cyclone. The anticyclone is associated with the divergence at the top of the tropical cyclone. Tropical cyclones are very sensitive to vertical wind shear. The upper-level anticyclone needs to remain over the surface cyclone, or the system will begin to weaken. ο They can only form in regions of low vertical wind shear.

TROPICAL CYCLONE FORMATION AND GROWTH Tropical cyclone formation requires: ο High sea-surface temperature (> 26 °°°C) ο Deep ocean mixed layer (> 60 m) ο Pre-existing cyclonic disturbance ο Low vertical wind shear ο High mid-level humidity The conditions required for tropical cyclone formation limit the regions and times of year in which they form. ο For instance, the North Atlantic tropical cyclone season is from June 1 to November 30. ο Tropical cyclones don’t form during the winter and spring in the North Atlantic because of the strong westerly winds aloft.

2 One region of great tropical cyclone activity is the Eastern North Pacific off of the Pacific Coast of Mexico. ο This region is very active, but since ships rarely travel here the cyclones don’t get as much attention. If conditions are right, a disturbance will grow to a tropical depression . ο In the depression stage, a surface circulation will form, or become better defined, and the convection becomes more organized. The depression may continue to grow to the tropical storm stage. ο At this stage the tropical cyclone may begin to have well-developed feeder bands (also called rain bands ). As the tropical storm approaches hurricane intensity, an eye begins to form in the middle of the storm. ο There is strong downward motion (subsidence) in the eye. This is why the eye is relatively cloud free. ο The eye is surrounded by the eye wall , an intense band of thunderstorms.

TROPICAL CYCLONE MOTION Tropical cyclones move as a result of three effects, listed in order of importance. Environmental steering ο To a first approximation the tropical cyclone can be thought of as being pushed in the direction of the tropospheric mean flow, much like a cork-in-a- stream . ο Environmental steering accounts for the large majority of the movement of a tropical cyclone. The Beta-effect ο Because the Coriolis parameter is not constant with latitude, there is a tendency for large circulations to drift westward, or northwestward, against the mean flow. ο This is called the beta-effect because the derivative of the Coriolis parameter with latitude ( ∂∂∂f/ ∂∂∂y) is commonly denoted with the Greek letter, βββ. ο This effect is larger for larger (diameter) systems. Asymmetric pressure falls 3 ο The convection in a tropical cyclone is not always symetric. The side that has more convection will experience greater surface pressure drops. The cyclone will appear to move toward these pressure drops. ο This effect is responsible for much of the wobble seen in tropical cyclone tracks. Since most tropical cyclones form below the subtropical ridge where the mean flow is easterly, they usually travel toward the west. If there is a break, or trough, in the subtropical high the hurricane may move northward, or recurve around the western edge of the high. ο The most difficult part of forecasting hurricane tracks if forecasting if and when they will recurve. After recurvature the cyclone is usually out of a favorable region because ο There are strong westerlies aloft ο The sea-surface temperatures are cooler After recurvature the cyclone may either dissipate, or it may undergo extratropical transition . ο Extratropical transition refers to a tropical cyclone gaining the characteristics of an extratropical cyclone (i.e., becoming tilted with height, forming fronts, etc.) Extratropical transition doesn’t mean the cyclone is any less dangerous!

HURRICANE DAMAGE Most damage and destruction from hurricanes is not caused by the wind directly, but by the flooding caused by the torrential rains and storm surge . The storm surge from a typhoon in Bangladesh in 1970 killed at least 200,000 and may have killed up to 500,000 people! The right hand side of the storm (relative to its direction of motion) is where the highest winds are found. ο This is because the storms speed of advance is added to the wind circulating around the storm. The intensity of a tropical cyclone is expressed using the Safir-Simpson scale, which related to wind speed 4 ο The Safir-Simpson scale ranges from 1 to 5, with 1 being the least amount of damage and 5 being the worst.

HURRICANE AWARENESS AND SAFETY The National Weather Service issues watches and warning for hurricanes and tropical storms. These are as follows ο Hurricane watch – Hurricane force winds (>64 knots) are possible within 36 hours ο Hurricane warning – Hurricane force winds are expected within 24 hours. ο Tropical storm watch – Tropical storm force winds (>33 knots) are possible within 36 hours ο Tropical storm warning – Tropical storm force winds are possible within 24 hours. The key to hurricane preparedness is not to be caught near the coast or in low- lying areas. HEED ALL EVACUATION ORDERS BY THE AUTHORITIES! ο Many people are reluctant to leave their homes during a hurricane. They often will use the excuse that “we’ve ridden them out before”. What they fail to consider is that they may have been through a weak hurricane, or they may have only experienced the tropical storm force winds farther from the eye. They are not prepared to be hit by a category 4 or 5 hurricane dead on! ο Many people also refuse to evacuate because they are tired of false alarms. Unfortunately, hurricane forecasting is difficult, and is not exact. The government forecasters are providing the best forecasts scientifically possible, but they often will err on the conservative side (as well they should).