Yesterday you must have read my post regarding Cyclone Phallin...All you need to know: @
Now in continuation to that I have prepared notes for theory on Cyclones:All about it:
This will be helpful for Geography Optional also.
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All you need to know about Cyclones:
-Definition
The term cyclone refers to a wide
variety of broad, low pressure system with cyclonic rotation, that is counterclockwise if the system is in the northern hemisphere and
clockwise in the southern hemisphere.
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- The first type to explore is called a tropical cyclone.
- These are the most intense cyclones in terms
of both pressure (in all cyclones, a lower pressure means a stronger
storm) and wind speed.
- Tropical cyclones originate from disorganized
areas of low pressure that move over warm ocean water, where they feed on
moisture and become stronger and better organized.
- These storms can only develop over warm ocean
water and weaken rapidly if they hit land.
- These tropical systems tend to be called cyclones in the southern hemisphere, and they
rotate in a clockwise direction.
- In the northern hemisphere, where cyclones
occur in the Atlantic Ocean, Caribbean and northeastern Pacific Ocean they
are called hurricanes and those in
the South China Sea and regions of Asia are called typhoons.
- Northern hemisphere cyclones rotate
anti-clockwise.
- Weaker tropical cyclones in
these regions are referred to as tropical storms if sustained winds are between 63
and 118 km/h (39 and 73 mph) and
as tropical depressions if winds are less than 63 km/h.
- In addition to producing
damaging winds these systems often produce heavy flooding. Tropical
cyclones usually form in the summer of early autumn when the oceans are
warmest.
Cyclone intensity is measured by wind
speed, and cyclones are accorded a category based on this:
Hurricanes, which are cyclones in the
northern hemisphere are classified according to the Saffir-Simpson Scale, which is also
based on wind speed - not wind gusts, but rather sustained wind
speeds.
The highest classification on this
scale is category 5, in which sustained winds exceed 251 km/h (156 mph).
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Next up are mid-latitude or extratropical cyclones.
- Unlike tropical cyclones these
systems are powered by temperature
gradients, or changes in temperature across a distance.
- Because they do not need as much moisture as tropical cyclones
these systems can develop over land or over cool water.
- These systems form along fronts,
or boundaries between air
masses of different temperatures and dew points (a measure of absolute
humidity).
- As a mid-latitude cyclone develops, a warm
front will usually develop on the western side of the low while a cold
front develops on the equator ward side (this is the southern side in the
northern hemisphere and northern side in the southern hemisphere).
These systems are usually not as intense as tropical cyclones but are typically larger in geographic extent, especially
if you include the fronts they connect with. They do sometimes produce
gale force and even hurricane force winds, especially at sea. Unlike
tropical cyclones these cyclones usually occur in fall, winter, or spring, when
temperature contrasts are greatest.
Depending on where and when they
occur these systems can cause blizzards, flooding,
or outbreaks of severe weather and tornadoes.
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There are also subtropical cyclones, which have characteristics
of both tropical and extratropical cyclones. These typically follow the same
naming conventions as tropical cyclones. A cyclone can transition between these
three types.
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Mesocyclone
- A mesocyclone is a dense, swirling
pack of cloud and winds between half a mile and six miles wide.
- To the eye, it looks like a thin,
vertical band of black clouds that spins from beneath thunderclouds.
- A mesocyclone turns into a tornado
if it hits the ground and continues to churn up wet, warm air.
- The United States experiences
approximately 1,700 mesocyclones a year, with 50 percent of these turning
into tornadoes.
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Polar or Arctic Cyclones
- Arctic or polar cyclones occur in
Antarctic regions and can reach up to 1,200 miles wide.
- Polar cyclones differ with others because
they are not seasonal.
- They can occur at any time of the year,
unlike in the Gulf of Mexico when during late summer, the risk of a
hurricane increases.
- Polar cyclones can also form quickly
(sometimes less than 24 hours), and their direction or movement cannot be
predicted.
- Plus, they can last from a day up to
several weeks.
- Most frequently, polar cyclones develop
above northern Russia and Siberia.
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Miscellaneous
Some terms related to Tropical Cylcones...to understand it furthur
!!!
What is a "CDO"?
- "CDO" is
an acronym that stands for "central dense overcast".
- This
is the cirrus cloud shield that results from the thunderstorms in the
eyewall of a tropical cyclone and its rainbands.
- Before
the tropical cyclone reaches very severe cyclonic storm (64 knots,),
typically the CDO is uniformly showing the cold cloud tops of the cirrus
with no eye apparent.
- Once
the storm reaches the hurricane strength threshold, usually an eye can be
seen in either the infrared or visible channels of the satellites.
- Tropical
cyclones that have nearly circular CDO's are indicative of favourable, low
vertical shear environments.
What is the "eye"? How is it formed and maintained? What
is the "eyewall"? What are "spiral bands"?
EYE
- The "eye" is
a roughly circular area of comparatively light winds and fair weather
found at the centre of a severe tropical cyclone.
- Although
the winds are calm at the axis of rotation, strong winds may extend well
into the eye.
- There
is little or no precipitation and sometimes blue sky or stars can be
seen.
- The
eye is the region of lowest surface pressure and warmest temperatures
aloft - the eye temperature may be 10°C warmer or more at an altitude of
12 km than the surrounding environment, but only 0-2°C warmer at the
surface in the tropical cyclone.
- Eyes
range in size from 8 km to over 200 km across, but most are approximately
30-60 km in diameter.
EYEWALL
- The
eye is surrounded by the "eyewall", the roughly
circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone.
- The
eye is composed of air that is slowly sinking and the eyewall has a net
upward flow as a result of many moderate - occasionally strong - updrafts
and downdrafts.
- The eye's warm temperatures are due to compressional
warming of the subsiding air.
- Most
soundings taken within the eye show a low-level layer, which is relatively
moist, with an inversion above - suggesting that the sinking in the eye
typically does not reach the ocean surface, but instead only gets to
around 1-3 km of the surface.
1.
The exact mechanism by which the eye
forms remains somewhat controversial. One idea suggests that the eye forms as a
result of the downward directed pressure gradient associated with the weakening
and radial spreading of the tangential wind field with height (Smith,
1980).
2.
Another hypothesis suggests that the
eye is formed when latent heat release in the eyewall occurs, forcing
subsidence in the storm's centre (Shapiro and Willoughby, 1982).
It is possible that these hypotheses
are not inconsistent with one another. In either case, as the air subsides, it
is compressed and warms relative to air at the same level outside the eye and
thereby becomes locally buoyant. This upward buoyancy approximately balances
the downward directed pressure gradient so that the actual subsidence is
produced by a small residual force.
SPIRAL BANDS
- Another
feature of tropical cyclones that probably plays a role in forming and
maintaining the eye is the eyewall convection.
- Convection
in tropical cyclones is organized into long, narrow rainbands which are
oriented in the same direction as the horizontal wind.
- Because
these bands seem to spiral into the centre of a tropical cyclone, they are
called"spiral bands".
- Along
these bands, low-level convergence is a maximum, and therefore,
upper-level divergence is most pronounced above.
- A
direct circulation develops in which warm, moist air converges at the
surface, ascends through these bands, diverges aloft, and descends on both
sides of the bands.
- Subsidence
is distributed over a wide area on the outside of the rainband but is
concentrated in the small inside area.
- As
the air subsides, adiabatic warming takes place, and the air dries.
- Because
subsidence is concentrated on the inside of the band, the adiabatic
warming is stronger inward from the band causing a sharp contrast in
pressure falls across the band since warm air is lighter than cold
air.
- Because
of the pressure falls on the inside, the tangential winds around the
tropical cyclone increase due to increased pressure gradient. Eventually,
the band moves toward the centre and encircles it and the eye and eyewall
form.
Thus, the cloud-free eye may be due to a combination of
dynamically forced centrifuging of mass out of the eye into the eyewall and to
a forced descent caused by the moist convection of the eyewall.
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Why there are very few Tropical Cyclones during southwest monsoon
season?
- The
southwest monsoon is characterized by the presence of strong westerly winds in the lower troposphere
(below 5 km) and very strong easterly winds in the upper troposphere
(above 9 km) .
- This
results in large vertical wind shear. Strong vertical wind shear inhibits
cyclone development.
- Also
the potential zone for the development of cyclones shifts to North Bay of
Bengal during southwest monsoon season.
- During
this season, the low pressure system upto the intensity of depressions
form along the monsoon trough, which extends from northwest India to the
north Bay of Bengal.
- The
Depression forming over this area crosses Orissa – West Bengal coast in a
day or two.
- These
systems have shorter oceanic stay which is also one of the reasons for
their non-intensification into intense cyclones.
What is the life period of cyclones? Which tropical cyclone lasted
the longest?
- Life
period of a Tropical Cyclone over the north
Indian Ocean is 5-6 days.
- It
will have hurricane intensity for 2-4 days as against 6 days of global
average.
- Life
period of the longest lived Tropical cyclone in Indian seas is 14 days (2nd -15th Nov,
1886 & 16th - 29th Nov, 1964).
- Hurricane/Typhoon
John lasted 31 days as it traveled both the Northeast and Northwest
Pacific basins during August and September, 1994. (It formed in the
Northeast Pacific, reached hurricane force there, moved across the
dateline and was renamed Typhoon John, and then finally recurved back
across the dateline and renamed Hurricane John again.)
- Hurricane
Ginger was a tropical cyclone for 28 days in the North Atlantic Ocean back
in 1971. It should be noted that prior to the weather satellite era (1961)
many tropical cyclones' life cycles could be underestimated.
How are Tropical Cyclones monitored by IMD?
- IMD
has a well-established and time-tested organization for monitoring and
forecasting tropical cyclones.
- A
good network of meteorological observatories (both surface and upper air)
is operated by IMD, covering the entire coastline and islands.
- The
conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite
systems.
- INSAT imagery obtained at hourly
intervals during cyclone situations has proved to be immensely useful in
monitoring the development and movement of cyclones.
Why do 'tropical cyclones' winds rotate counter-clockwise
(clockwise) in the Northern (Southern) Hemisphere?
- As the
earth's rotation sets up an apparent force
(called the Coriolis force) that pulls the winds to the right in the
Northern Hemisphere (and to the left in the Southern Hemisphere).
- So,
when a low pressure starts to form over north of the equator, the surface
winds will flow inward trying to fill in the low and will be deflected to
the right and a counter-clockwise rotation will be initiated. The opposite
(a deflection to the left and a clockwise rotation) will occur south of
the equator.
- This
Coriolis force is too tiny to effect rotation in, for example, water that
is going down the drains of sinks and toilets.
- The
rotation in those will be determined by the geometry of the container and
the original motion of the water.
- Thus,
one can find both clockwise and counter-clockwise flowing drains no matter
what hemisphere you are located. If you don't believe this, test it out
for yourself.
What causes each cyclone to have a different maximum wind speed
for a given minimum sea-level pressure?
- The
basic horizontal balance in a tropical cyclone above the boundary layer is between the sum of the Coriolis 'acceleration' and the
centripetal 'acceleration', balanced by the horizontal pressure gradient
force.
- This
balance is referred to as gradient balance, where the Coriolis
'acceleration' is defined as the horizontal velocity of an air
parcel, v, times the Coriolis parameter, f.
Centripetal 'force' is defined as the acceleration on a parcel of air
moving in a curved path, directed toward the centre of curvature of the
path, with magnitudev2/r, where v is
the horizontal velocity of the parcel and r the radius of
curvature of the path.
- The
centripetal force alters the original two-force geostrophic balance and
creates a non-geostrophic gradient wind.
- The
reason that different peak winds can result in different central pressures
is caused by the fact that the radius, r, of the peak wind
varies. A storm with 40 m/s peak winds with a 100 km RMW will have a much
lower pressure drop than one with a 25 km RMW.
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What is a Storm Surge?
- Storm
Surge is an abnormal rise of sea level as
the cyclone crosses the coast.
- Sea
water inundates the coastal strip causing loss of life, large scale
destruction to property & crop.
- Increased
salinity in the soil over affected area makes the land unfit for
agricultural use for two or three seasons.
- Storm
surge depends on intensity of the cyclone (Maximum winds and lowest
pressure associated with it and Coastal bathymetry (shallower coastline
generates surges of greater heights).
- The
storm surge is predicted by IMD using nomograms and dynemic model
developed by IIT, Delhi. Both
these models taken into consideration different characteristics, the
cyclones and the coastal bathymetry to predict the storm surge.
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System
|
Pressure deficient
hPa
|
Associated wind speed
Knots
(Kmph)
|
Low
pressure area
|
1.0
|
<17 span="">
|
Depression
|
1.0-
3.0
|
17-27
(32–50)
|
Deep
Depression
|
3.0
- 4.5
|
28-33
(51–59)
|
Cyclonic
Storm
|
4.5-
8.5
|
34-47
(60-90)
|
Severe
Cyclonic Storm (SCS)
|
8.5-15.5
|
48-63
(90-119)
|
Very
Severe Cyclonic Storm
|
15.5-65.6
|
64-119
(119-220)
|
Super
Cyclonic Storm
|
>65.6
|
>119(>220)
|
What are the super cyclone, super-typhoon, a major hurricane and
an intense hurricane?
- When the maximum sustained 3 minutes surface winds are more than 119
knots, the low pressure system is called as "Super Cyclone" over
north Indian Ocean.
- Similarly, “Super-typhoon" is a term utilized by
the U.S. Joint Typhoon Warning Centre for typhoons that reach maximum sustained 1 minute surface winds of at least 130 knots (65 m/s).
- This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical
cyclone in the Australian basin.
