TROPICAL REVOLVING STORMS OR TROPICAL CYCLONES (OCCURENCES AND AVOIDING ACTIONS)” PDF

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KARARY UNIVERSITY COLLEGE OF MARITIME STUDIES TROPICAL REVOLVING STORM BY FAISAL MUHAMMAD ALI SPONSORED BY: AZHAR SHAHBAZ 1 ‫ميحرلا نمحرلا هللا‬ ‫بسم‬ TROPICAL REVOLVING STORMS OR TROPICAL CYCLONES (OCCURENCES AND AVOIDING ACTIONS)” INTRODUCTION Gentlemen, Tropical storms are intense areas of low pr...

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KARARY UNIVERSITY COLLEGE OF MARITIME STUDIES

TROPICAL REVOLVING STORM BY FAISAL MUHAMMAD ALI

SPONSORED BY: AZHAR SHAHBAZ

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‫م‬

‫بسم‬

TROPICAL REVOLVING STORMS OR TROPICAL CYCLONES (OCCURENCES AND AVOIDING ACTIONS)” INTRODUCTION Gentlemen, Tropical storms are intense areas of low pressure which developes in certain tropical oceanic areas and cause very high winds and heavy seas. The wind does not revelove around the low pressure center in concentric circle but has spiral movement inwards towards the center. The air circulation is anti clockwise in Northern latitude and clockwise in Southern latitude. With this brief introduction the topic of my research paper today is “TROPICAL REVOLVING STORMS OR TROPICAL CYCLONES (OCCURENCES AND AVOIDING ACTIONS)” In next 30 minutes are so I will be covering the various tropical cyclones,their stages, areas of Occurance and actions to avoid them. My scheme of presentation is as flashed: IMPORTANT DEFINATION Three of the most significant imaginary lines running across the surface of the earth are the equator, the Tropic of Cancer, and the Tropic of Capricorn. While the equator is the longest line of latitude on the earth (the line where the earth is widest in an east-west direction), Tropics The tropics are the two lines where the sun is directly overhead. at noon on the two solstices - near June and December 21. The sun is directly overhead at noon on the Tropic of Cancer on June 21' (the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere) and the sun is directly overhead at noon on the Tropic of Capricorn on December 21 (the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere). Tropical Area The area bounded by the Tropic of Cancer located at 23.5" North of the equator on the north and Tropic of Capricorn at 23.5" South of the equator is known as the "tropics." This area does not experience. seasons because the sun is always high in the sky. Only higher latitudes, north of the Tropic of Cancer and south of the Tropic of Capricorn; experience significant seasonal variation in climate Coriolis Force The apparent force, caused by the earth's rotation, which deflects moving air (and water to a lesser extent) to the left in the Southern Hemisphere and to the right in the Northern hemisphere. 2

ITCZ The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ITCZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe. It is formed by the vertical ascent of warm, moist air from the latitudes above and below the equator. As the air ascents it cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms. The position of the ITCZ varies with the seasons. Trade Winds or Tropical Easterlies (From 0-30 degrees latitude) Steady and persistent winds which blow on the Equatorial side of the subtropical high pressure systems in both Hemispheres. In the northern hemisphere the Trade Winds blow from the north east called North Easterlies while in the southern hemisphere they blow from the south east called South Easterlies. The direction of the winds is influenced by land masses so the Trade Winds tend to be more uniform over the oceans. Subtropical high-pressure belts: Belts of persistent high atmospheric pressure trending east-west and centered about latitude 30° N and S. Westerlies or Prevailing Westerlies (From 30-60 degrees latitude) Surface winds blowing from a generally westerly direction in the midlatitude zone, but varying greatly in direction and intensity. Air Mass An air mass is a large body of air with fairly uniform temperature and moisture characteristics. Air masses acquire their characteristics from their source regions Front A given air mass usually has a sharply defined boundary between itself and a neighboring air mass. This boundary is termed a front. Cold Front In a cold front, a cold air mass lifts a warm air mass aloft. The upward motion sets off a line of thunderstorms. The front is steep and fast moving. Warm Front In a warm front, warm air rides up and over colder air. The front has a shallow gradient and moves slowly.

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Occluded Front In an occluded front, a warm front is overtaken by a cold front. The warm air is pushed aloft, and it no longer contacts the ground. Abrupt lifiting by the denser cold air produces precipitation. This is the process that determines the development of wave cyclones.

TROPICAL REVOLVING STORMS or TROPICAL CYCLONES INTRODUCTION Cyclone A weather system with a low-pressure area in the center around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere There are three major types of large cyclones: 1) Extra-tropical cyclones 2) Tropical cyclones 3) Sub-Tropical Cyclones Tropical Cyclones Cyclone originating in the tropics or subtropics is called the “Tropical Cyclone”. Extra –Tropical Cyclones Cyclone originating in the higher latitudes is called the “Extra-Tropical Cyclone”. Subtropical cyclone Is a low pressure system that develops over subtropical oceans and initially has a non-tropical circulation but exhibits some elements of the cloud patterns associated with tropical cyclones It is often considered a hybrid system with some characteristics of tropical and extra-tropical cyclones STAGES OF DEVELOPMENT Tropical Disturbance It is a discrete system of apparently organized convection, generally 100 to 300 miles in diameter, having a non-frontal migratory character, and having maintained its identity for 24 hours or more. It may or may not be associated with a detectable disturbance of the wind field. It has no strong winds and no closed isobars i.e., isobars that completely enclose the low. 4

Tropical Depression At its next stage of development it becomes a tropical depression which has one or more closed isobars and some rotary circulation at the surface. The highest sustained (1-minute mean) surface wind speed is 33 knots. Tropical Storm The next stage is a tropical storm which has closed isobars and a distinct rotary circulation. The highest sustained (1-minute mean) surface wind speed is 34 to 63 knots. Hurricane Fully developed form of a cyclone is hurricane which has closed isobars, a strong and very pronounced rotary circulation, and a sustained (1-minute mean) surface wind speed of 64 knots or higher. AREAS OF OCCURRENCE Tropical cyclones occur almost entirely in six distinct areas, four in the Northern Hemisphere and two in the Southern Hemisphere. The name by which the tropical cyclone is commonly known varies somewhat with the locality. North Atlantic.

A tropical cyclone with winds of 64 knots or greater is called a hurricane.

Eastern North Pacific

The name hurricane is used as in the North Atlantic.

Western North Pacific

A fully developed storm with winds of 64 knots or greater is called a typhoon or, locally in the Philippines, a baguio.

North Indian Ocean

A tropical cyclone with winds of 34 knots or greater is called a cyclonic storm.

South Indian Ocean

A tropical cyclone with winds of 34 knots or greater is called a cyclone.

Southwest Pacific

A severe tropical cyclone originating in the Timor Sea and moving southwest and then southeast across the interior of northwestern Australia is called a willy-willy.

NOTE Tropical cyclones have not been observed in the South Atlantic or in the South Pacific east of 140°W.

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ORIGIN, SEASON AND FREQUENCY OF OCCURENCES IN INDIAN OCEAN Origin, season, and frequency of occurrence of the tropical cyclones in Indian Ocean is as follows: North Indian Ocean Tropical cyclones develop in the Bay of Bengal and Arabian Sea during the spring and fall. Tropical cyclones in this area form between latitudes 8°N and 15°N, except from June through September, when the little activity that does occur is confined north of about 15°N. These storms are usually short-lived and weak; however, winds of 130 knots have been encountered. They often develop as disturbances along the Intertropical Convergence Zone (ITCZ); this inhibits summertime development, since the ITCZ is usually over land during this monsoon season. However, it is sometimes displaced southward, and when this occurs, storms will form over the monsoonflooded plains of Bengal. On the average, six cyclonic storms form each year. These include two storms that generate winds of 48 knots or greater. Another 10 tropical cyclones never develop beyond tropical depressions. The Bay of Bengal is the area of highest incidence. However, it is not unusual for a storm to move across southern India and reintensify in the Arabian Sea. This is particularly true during October, the month of highest incidence during the tropical cyclone season. It is also during this period that torrential rains from these storms, dumped over already rainsoaked areas, cause disastrous floods. South Indian Ocean Over the waters west of 100°E, to the east African coast, an average of 11 tropical cyclones (tropical storms and hurricanes) form each season, and about 4 reach hurricane intensity. The season is from December through March, although it is possible for a storm to form in any month. Tropical cyclones in this region usually form south of 10°S. The latitude of re-curvature usually migrates from about 20°S in January to around 15°S in April. After crossing 30°S, these storms sometimes become intense extra-tropical lows.

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CONDITIONS FOR DEVELOPMENT & INTENSIFICATION OF CYCLONE The process by which a tropical cyclone develops and subsequently intensifies depends on at least six conditions explained below: 1.

A pre-existing surface disturbance with thunderstorms.

2. Relatively moist atmospheric layers in the middle troposphere, approximately 10,000-20,000 ft above the earth’s surface. Dry air at this level of the atmosphere is not favorable for continued development of the required thunderstorm activity in a disturbance. 3. Warm (at least 79ºF or 26ºC) ocean temperatures with a mixed layer depth of about 200 feet. This mixed ocean layer allows warm water to remain available to a developing system even after the wind has begun to increase in speed and the sea surface begins to get churned up by the developing cyclone. 4. Light winds aloft that do not change much in direction and speed throughout the depth of the atmosphere (low vertical wind shear). Tropical cyclones rely on a vertically stacked structure inorder to grow or maintain in intensity. In other words, the ideal tropical cyclone will have its cyclonic circulation in the middle & upper levels of the atmosphere located directly above the cyclonic circulation of the surface & low levels of the atmosphere. Increases in wind speed with height will tilt the vertical structure of a tropical cyclone not allowing the system to remain stacked throughout the troposphere. If this vertical tilting of the system persists, growth will become inhibited and the tropical system will decay. 5. Must be poleward of about 5 degrees north/south latitude in order to meet minimum threshold values for the Coriolis Force. 6. Upper-level outflow over a system serves to remove mass from the top of the vertical column in a tropical cyclone. As a system develops, low-level cyclonic flow pulls more mass towards the center of the system; the flow then turns upward in intense vertical motions associated with thunderstorms in the area. Without a method to dispose of this mass from above the tropical cyclone, low-level converging flow toward the center of the system will be halted and the system will “suffocate”. In a complex relationship, these six factors are interdependent. The absence or change in one of the ingredients often results in a change or loss in one or more of the other factors. If nature allows these conditions to remain favorable over a period of time, it can produce a spectacular atmospheric event of catastrophic proportions. During an idealized case of tropical cyclogenesis, the following events would occur on the order of days with different factors occurring simultaneously or nearsimultaneously throughout the developing phase of a tropical cyclone.

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HOW DOES CYCLONE FORMS a) Initially, heat and therefore energy for the storm are gathered by the disturbance through contact with warm ocean waters.Low Level Convergence collects latent energy and causes air to rise Water Vapor Condenses and latent energy is converted to internal energy and the air warms Warming generates upper level divergence Upper level divergence removes mass and surface pressure decreases b) Thunderstorm activity begins to develop and define the vertical structure above the tropical disturbance. c) Soon the Coriolis Force begins to act on the system, aiding in the development of a cyclonic circulation with winds near the ocean surface now spiraling into the disturbance’s developing low pressure area. d) The warm ocean waters and their sufficient mixed layer depth will continue to add moisture and heat to the air that rises in the updrafts of convection near the disturbance. As the moisture condenses into drops, more heat is released into the atmosphere,adding energy to power the storm. f) Thunderstorms begin to take on a curved banding structure as they organize around the low-level center of the system. As these thunderstorms grow higher into the troposphere, relatively light winds at those high levels will allow the vertically stacked warm core of the storm to remain intact and continue to strengthen. CYCLONE STRUCTURE The main parts of a cyclone are the Rainbands, the Eye, and the Eyewall. The Eye At the very center of the storm, air sinks, forming the warm core and relatively cloudfree eye. The cyclone’s center is a relatively calm, clear area usually 10-40 nautical miles wide containing the lowest surface pressure in the tropical cyclone The Eyewall The innermost convective ring of thunderstorms that surrounds the eye of a cyclone is known as the eyewall. This region is home to the most intense winds and fiercest rains within a tropical cyclone and has a typical width of approximately 10-15 NM. Additionally, it is the most significant contributor in the vertical transport of warm moist air from the lower levels of the storm into the middle and upper levels of the troposphere over a tropical cyclone. Eyes and eyewalls are generally observed only in systems with winds of strong tropical storm force or greater

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Rainbands The storm’s outer rainbands of thunderstorms,which spiral slowly counterclockwise, range in width from a few miles to tens of miles and can be up to 300 NM long. These rainbands serve as major source of upward vertical motion and therefore play a significant part in the transport process that removes warm moist ocean air and deposits it in the middle and upper troposphere HOW CYCLONES ARE NAMED? Forecasters began naming hurricanes and tropical storms in order to provide ease of communication between them and the general public in areas regarding forecasts, watches and warnings. By naming hurricanes, there is a reduction in confusion about what storm is being described. Prior to 1950, hurricanes were assigned names by the year in which they occurred plus a letter from the alphabet (i.e. 1942A, 1942 B, etc.). It then became the trend to give hurricanes human names. At first, it was only girls’ names, but after 1978, they used boys’ and girls’ names alternately. Experience shows that the use of girls' and boys' names in written and spoken communication is shorter, quicker, and causes fewer mistakes than any other hurricane identification used to date. Now, each year, a potential list of names of each area is prepared for the upcoming hurricane season . The list contains a name for each letter of the alphabet. (The letters Q, U, X, Y, Z are not included because there are not many names beginning with those letters.) These lists are recycled every six years and names are replaced when a hurricane name is retired WARNING/INDICATIONS OF CYCLONE AT SEA There can be four types of observations that may alert the mariner to an approaching tropical cyclone Wind In the absence of any other information, surface winds are normally the best guide to quickly determining the direction to the center of a tropical cyclone. The wind in tropical cyclone flows cyclonically or counter-clockwise around the actual low center. If an observer faces into the direction of the true wind at the surface, the center of lowest pressure, and therefore the center of the cyclone will be to the right hand side, bearing approximately 090 to 120 degrees. This method is a good initial indication of the direction to the cyclone. However, be wary of using this method in the vicinity of thunderstorms and squalls, as these features can temporarily change the wind flow around a tropical system. Wave The direction of the swell encountered over open oceans is indicative of the direction to a tropical cyclone’s center when that swell was originally generated. For example, 9

Northeast swell observed by a vessel in Indian Ocean indicates the strong winds that generated the swell and therefore the tropical system was located NE of the ship when the swell initially developed. However, in shoaling water, this is a less reliable indication of tropical cyclone position as the direction of swell in these areas is often altered by refraction. Clouds With a system 500-1000 NM away from a vessel, skies may appear relatively clear and any low cumulus clouds will have a very shallow vertical extent. As the system and the vessel close to about 300-600 NM in distance, high level cirrus cloudiness will appear as a thin, wispy veil spreading away from the direction of the tropical system. If the separation between the tropical cyclone and the vessel continues to decrease, the cirrus will thicken and lower somewhat taking on the layered appearance of a cirrostratus deck of clouds. Even closer to the storm, layered altostratus clouds will begin to appear at the middle levels of the atmosphere. Finally, rain-showers and thick, heavy walls of cumulonimbus clouds begin to indicate the proximity of outer rain-bands in the tropical cyclone. At this point the center of the system may still be as much as 200-400 NM from the location of the ship. Surface Pressure Small rises and falls in the surface pressure can sometimes be noticed in shipboard barometers as a “pumping action” in the pressure reading. This restlessness of the barometer is related to the intense upward motions and extremely strong wind gusts associated with a tropical cyclone along with the measurably lower surface pressures near the spiral rain-bands surrounding them. These small, yet measurable pressure rises and falls will often be superimposed on the overall pressure fall as the tropical system approaches and can serve as a valuable indication that a tropical system may be nearby. THUMB RULE: If pressure is 5 hpa less than normal it means storm centre is 200 NM from you

HOW TO ESTIMATE POSITION W.R.T CYCLONE Changes in wind direction and speed along with changes to shipboard barometric pressure are the fundamental guides to locating a vessel within the tropical cyclone’s circulation. Winds Veering ( Wind changes direction Anticlockwise) Winds veering over time indicate that the ship is in the right semi-circle (with respect to tropical cyclone motion) of the system.

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Backing Winds (Wind changes direction Clockwise) Backing winds over time indicate that a vessel is in the left semi-circle of a system. Ahead of a Tropical Cyclone If wind direction remains steady but continues increasing in speed, a vessel is likely located ahead of the tropical cyclone. Another indication of this is continue fall of barometric pressure. in some cases quite rapidly as the system center moves closer Rear of the Tropical Cyclone Alternatively, winds that remain steady in direction but decrea...