Bangladesh extends between 20°34' to 26°38' North latitude and 88°01' to 92°41' East longitude. The Bay of Bengal is to the south side of the country. The total area is 147,570 sq km and size of population is around 130.62 million (2001). The coastal land of Bangladesh (710 Km long) is of recent origin formed out of the process of sedimentation. Most parts of the area are, therefore, low lying which can be subjected to inundation even under ordinary circumstances of tides. Bangladesh, due to its unique geographic location, suffers from devastating tropical cyclones frequently. The funnel-shaped northern portion of the Bay of Bengal causes tidal bores when cyclones make landfall, and thousands of people living in the coastal areas are affected. Some of the most devastating natural disasters in recorded history with high casualties were tropical cyclones that hit the region now forming Bangladesh. Among them, the 1970 Bhola cyclone alone claimed more than 500,000 lives.
Bangladesh lacks any weather satellites of its own. The three satellite ground stations, located in Betbunia, Talibabad, and Mohakhali, are used to receive feeds from other satellites. Bangladesh Space Research and Remote Sensing Organisation (SPARRSO), a Government agency under the Ministry of Defence, provides storm predictions and early warnings using feeds from NASA and NOAA's satellites. The warnings are usually given in a scale of 10, with the number 10th being given for the deadliest storms.
A detailed programme for storm prevention has been taken by the Government following the cyclone of 1991. A Comprehensive Cyclone Preparedness Programme (CPP) is jointly planned, operated, and managed by the Ministry of Disaster Management and Relief and the Bangladesh Red Crescent Society, and a volunteer force of more than 32,000 are trained to help in warning and evacuation in the coastal areas. Around 2,500 cyclone shelters have been constructed in the coastal regions. The shelters are built on elevated platforms, and serve the dual role of schools or community Centers during normal weather.
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Tropical Cyclones
Wednesday, July 16, 2008
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1.2 Tropical Cyclone
A tropical cyclone is a relatively small, intensely developed low pressure cell that usually occurs over warm oceans. Its diameter can range between 200 and 2000 km. It is characterized by a warm centre, very steep pressure gradients and strong cyclonic winds near the Earth's surface. Tropical cyclones with a maximum wind speed of less than 60 km/h are called tropical depressions; when the maximum wind speed ranges between 60 and 110 km/h, they are tropical storms, and when the maximum wind speed exceeds 110 km/h, they are called tropical cyclones. (In the North Atlantic and eastern North Pacific regions it is called “Hurricanes”, in the western North Pacific “Typhoons”.
In meteorology, a tropical cyclone (or tropical storm, typhoon or hurricane, depending on strength and location) is a type of low-pressure system which generally forms in the tropics. While some, particularly those that make landfall in populated areas, are regarded as highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which move heat from the equatorial region toward the higher latitudes.
Structurally, a tropical cyclone is a large, rotating area of clouds, wind, and thunderstorm activity. The primary energy source of a tropical cyclone is the release of heat of condensation from water vapour condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.
This use of condensation as a driving force is the primary difference setting tropical cyclones apart from other meteorological phenomena, such as mid-latitude cyclones, which draw energy mostly from pre-existing temperature gradients in the atmosphere. To drive its heat engine, a tropical cyclone must stay over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle.
In meteorology, a tropical cyclone (or tropical storm, typhoon or hurricane, depending on strength and location) is a type of low-pressure system which generally forms in the tropics. While some, particularly those that make landfall in populated areas, are regarded as highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which move heat from the equatorial region toward the higher latitudes.
Structurally, a tropical cyclone is a large, rotating area of clouds, wind, and thunderstorm activity. The primary energy source of a tropical cyclone is the release of heat of condensation from water vapour condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
The ingredients for a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon.
This use of condensation as a driving force is the primary difference setting tropical cyclones apart from other meteorological phenomena, such as mid-latitude cyclones, which draw energy mostly from pre-existing temperature gradients in the atmosphere. To drive its heat engine, a tropical cyclone must stay over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle.
1.3 Origin of storm terms
The term cyclone is derived from the Greek word 'Kyklos' meaning coil of snakes. The British-Indian scientist and meteorologist Henry Piddington coined the word 'Cyclone' to represent whirling storms expressing sufficiently the tendency to circular motion in his book “The Sailor's Horn-book for the Law of Storms”, published in 1848. Other meteorologists of the world immediately accepted the term and it is still current today. Satellite pictures of cyclones show that the nomenclature is very appropriate. Technically a cyclone is an area of low pressure where strong winds blow around a centre in an anticlockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere. Cyclones occurring in the tropical regions are called tropical cyclones and those occurring elsewhere are called extratropical cyclones. Tropical cyclones are usually destructive and affect Bangladesh and its adjoining areas.
1.3 Naming
Storms reaching tropical storm strength were initially given names to eliminate confusion when there are multiple systems in any individual basin at the same time which assists in warning people of the coming storm. In most cases, a tropical cyclone retains its name throughout its life; however, under special circumstances, tropical cyclones may be renamed while active. These names are taken from lists which vary from region to region and are drafted a few years ahead of time. The lists are decided upon, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues), or by national weather offices involved in the forecasting of the storms. Each year, the names of particularly destructive storms (if there are any) are "retired" and new names are chosen to take their place.
1.4 Tropical cyclogenesis
Tropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occur are distinctly different from those through which mid-latitude cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment. There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure centre, a pre-existing low level focus or disturbance, and low vertical wind shear. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale).
Palate : Tropical Cyclone Formation Process
1.4.1 General Requirements for Tropical Cyclogenesis
The main requirements for tropical cyclogenesis are as follow:
1) Normally, an ocean temperature of 26.5°C (79.7°F) spanning through at least a 50-metre depth is considered the minimum to maintain the special mesocyclone that is the tropical cyclone. These warm waters are needed to maintain the warm core that fuels tropical systems. This value is well above the global average surface temperature of the oceans, which is 16.1 °C (60.9 °F). However, this requirement can be considered only a general baseline because it assumes that the ambient atmospheric environment surrounding an area of disturbed weather presents average conditions.
Tropical cyclones are known to form even when normal conditions are not met. For example, cooler air temperatures at a higher altitude (e.g., at the 500 hPa level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certain lapse rate is required to force the atmosphere to be unstable enough for convection. In a moist atmosphere, this lapse rate is 6.5 °C/km, while in an atmosphere with less than 100% relative humidity, the required lapse rate is 9.8 °C/km.
At the 500 hPa level, the air temperature averages -7 °C (18 °F) within the tropics, but air in the tropics is normally dry at this level, giving the air room to wet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection. A wetbulb temperature at 500 hPa in a tropical atmosphere of -13.2 °C is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by 1 °C in the sea surface temperature for each 1 °C change at 500 hpa. Under a cold cyclone, 500 hPa temperatures can fall as low as -30 °C, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the troposphere, roughly at the 500 hPa level, is normally a requirement for development. However, when dry air is found at the same height, the wet bulb temperature normally witnessed at 500 hPa does not promote large areas of thunderstorms due to a lack of instability. At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was -77 °C (-132 °F). Recent examples of tropical cyclones that maintained themselves over cooler waters include Delta, Epsilon, and Zeta of the 2005 Atlantic hurricane season.
2) A minimum distance of 500 km (300 miles) from the equator is normally needed for tropical cyclogenesis. The role of the Coriolis force is to provide for gradient wind balance by correcting the interaction of the pressure gradient force (the pressure difference that causes winds to blow from high to low pressure) and geostrophic winds (the force that causes winds to blow parallel to straight isobars) for centripetal acceleration .
3) Whether it be the monsoon trough, a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low.
4) Vertical wind shear of less than 10 m/s (22 mph) between the surface and the tropopause is required for tropical cyclone development. Strong wind shear can "blow" the tropical cyclone apart, as it displaces the mid-level warm core from the surface circulation and dries out the mid-levels of the troposphere, halting development. In smaller systems, the development of a significant mesoscale convective complex in a sheared environment can send out a large enough outflow boundary to destroy the surface cyclone. Moderate wind shear can lead to the initial development of the convective complex and surface low similar to the mid-latitudes, but it must relax to allow tropical cyclogenesis to continue.
5) Limited vertical wind shear can be positive for tropical cyclone formation. When an upper-level trough or upper-level low is roughly the same scale as the tropical disturbance, the system can be steered by the upper level system into an area with better diffluence aloft, which can cause further development. Weaker upper cyclones are better candidates for a favorable interaction. There is evidence that weakly sheared tropical cyclones initially develop more rapidly than non-sheared tropical cyclones, although this comes at the cost of a peak in intensity with much weaker wind speeds and higher minimum pressure. This process is also known as baroclinic initiation of a tropical cyclone. Trailing upper cyclones and upper troughs can cause additional outflow channels and aid in the intensification process. It should be noted that developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet eminating from the developing tropical cyclone.
There are cases where large, mid-latitude troughs can help with tropical cyclogenesis when an upper level jet stream passes to the northwest of the developing system, which will aid divergence aloft and inflow at the surface, spinning up the cyclone. This type of interaction is more often associated with disturbances already in the process of recurvature.
Plate : Schematic representation of flow around a low-pressure area. The pressure gradient force is represented by blue arrows, the Coriolis acceleration by red arrows
1.5 Classification and terminology
Tropical cyclones are classified into three main groups: tropical depressions, tropical storms, and a third group whose name depends on the region.
a. A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 meters per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms.
b. A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34 to 63 knots, 39 to 73 mph, or 62 to 117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present.
c. The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h) varies depending on region of origin, as follows:
Hurricane in the North Atlantic Ocean, the North Pacific Ocean East of the dateline, and the South Pacific Ocean East of 160°E, Typhoon in the Northwest Pacific Ocean West of the dateline. Severe tropical cyclone in the Southwest Pacific Ocean West of 160°E or Southeast Indian Ocean East of 90°E. Severe cyclonic storm in the
1.6 Life Cycle of Tropical Cyclone
The formation of a tropical cyclone is dependent upon a number of favorable environmental conditions which are frequently present in the Inter Tropical Convergence Zone. These include a warm ocean surface (above 260C) and several physical parameters contributing to a deeply humid and unstable atmosphere. The formation process begins in an area of low pressure coinciding with vigorous convective cloud in the tropics between about 50 and 220 latitude. Usually the cloud cluster drifts slowly towards the west as the convection increases and winds begin spiraling in towards the system centre.
1.6.1 Formative stage
Tropical storms form only in or near pre-existing weather systems. Deepening can be a slow process requiring days for the organization of a large area with diffused winds. It can also be explosive, producing a well-formed eye within 12 hours. Winds usually remains below hurricane force in the formative stage. Strongest winds are apt to be concentrated in one quadrant, pole ward and east of the center in deepening waves in the easterlies, move variable in the equatorial trough. Surface pressure drops to about 1000 mb.
Plate: Formative stage of a Tropical Cyclone
1.6.2 Immature Stage
Not all incipient cyclones become hurricanes. Many have been known to die within 24 hours even though winds had attained hurricane force. Others travel long distances as shallow depression. If intensification takes place, the lowest pressure rapidly drops below 1000 mb. Winds of hurricane force form a tight band around the center. The cloud and rain pattern changes from disorganized squalls to narrow organized bands spiraling inward. Only a small area is as yet involved, though there may be a large outer envelope.
1.6.3 Mature Stage
The surface pressure at the centre is no longer falling and the maximum wind speed no longer increasing. Instead, the circulation expands during the mature stage, which may last a full week; whereas winds of hurricane force may below within a 30-50 km in mature storms. The symmetry is lost at the area of gales and bad weather extends much further to the right of the direction of motion than to the left (looking down stream)
It is surprising how much the size of mature storms can vary. Even with central pressure less than 950 mb, the radius of some storms is only 100-200 km. If the surface pressure averages 1000 mb over the storm area, the mass will be 3×1011-1012 tons. In contrast, storm with similar surface pressure can attain a radius of 1000 km of half the size of the United States and a mass of 3×1013 tons, fully two orders of magnitude greater. Only the strongest Icelandic and Aleutian lows have such a size. In the westerlies, the normal cyclone comprises about 5×1012-1×1013 tons.
Plate: Mature stage of a Tropical Cyclone
1.6.4 Terminal Stages
The core of hurricane regularly decreases when they make landfall. Some times the whole storms die within one or two days, some times it continues as a type of “Subtropical cyclone” giving much rain for days. Over Sea, hurricanes have been observed to die, for instance when moving over cold ocean currents as in the northeastern Pacific. Both over land and sea many cyclones “recurve” into the westerlies and take tracks toward northeast and east in the northern hemisphere. It even may be quite unspectacular or it may lead to an enormous blow-up of the general circulation.
1.7 Structure of Tropical Cyclones
1.7.1 Eye
1.7.2 Eye Wall
1.7.3 Spiral Bands
1.7.4 Cirrus Canopy
1.7.2 Eye Wall
1.7.3 Spiral Bands
1.7.4 Cirrus Canopy
1.7.1 Eye
The centre or eye of a tropical cyclone is at the area of lowest pressure and is characterised by little or no wind and often a cloudless sky. In severe cyclones the eye usually shows up as a circular hole in the central cloud mass. The eye is usually about 40 km in diameter, but can vary between less than 10 km and more than 100 km.
Plate: Eye of a Tropical Cyclone
1.7.2 Eye Wall
Surrounding the eye is a wall of dense convective cloud rising about 15 km into the atmosphere. This is the eye wall and is where the most violent winds and heaviest rainfall occur.
1.7.3 Spiral Bands
Radar and satellite observations of tropical cyclones reveal a distinctive pattern of convective cloud bands spiralling into the eye wall. These bands often extend up to 1000 km from the cyclone centre, and contain heavy rain and wind squalls.
1.7.4 Cirrus Canopy
The extremely vigorous uplift of moist air within the convective clouds of the eye wall and inner spiral bands produces a massive outflow of cirrus cloud in the upper atmosphere. This forms a huge canopy over the cyclone, making satellite location of the system centre difficult during the early development stages, before the eye shows through the canopy.
Plate: A mature tropical cyclone clearly displaying the main structural features
1.8 Storm Surge
When a tropical cyclone passed over a sea, for a long period sea surface fluctuation often appears. This phenomenon is called the storm surge. And the storm surge is caused mainly by the suction of seawater due to the pressure drop as well as by the wind drift of seawater. The suction of seawater due to tropical cyclone is independent of water depth, but the rise of seawater due to wind drift is inversely proportional to the water depth. Because of this latter fact, storm surge in general has a tendency to be large in a shallow water region. When the storm surge approaches the edge of the continental shelf, the storm surge increases in height due to the sudden decrease of water depth, and at the same time suffers the effect of wave reflection due to the sea bottom configuration. In addition to the above, the storm surge induces edge waves which propagate along the continental shelf, and other oscillations on the shelf. When the storm surge progresses into the shallower region and enters a bay, wave reflection will occur at the head of the bay, thus a part of the wave energy will escape, but most of the energy will be confined to the bay for a long time. Thus natural oscillation of the bay is developed by the above processes.
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