Hurricanes and Tropical Storms

1
Hurricanes and Tropical Storms
2
Naming Convention

• Hurricanes extreme tropical storms over Atlantic
  and Eastern
• Pacific Oceans
• Typhoons extreme tropical storms over western
  Pacific Ocean
• Cyclones extreme tropical storms over Indian
  Ocean and
• Australia

3
Ocean Temperatures and Hurricanes
Hurricanes depend on large pools of warm water
4
Annual Hurricane Frequency

• There are no hurricanes in the southern Atlantic
  Ocean!
• The strongest hurricanes occur in the western
  Pacific Ocean
• (often referred to as super-typhoons.)

5
Hurricane Characteristics

• Definition Hurricanes have sustained winds of
  120 km/hr (74 mph) or greater.
• Size Average diameters are approx. 600 km (350
  mi). This is 1/3 the size of a mid-latitude
  cyclone (synoptic storm system).
• Duration Days to a week or more.
• Strength Central pressure averages 950 mb but
  may be as low as 870 mb (lower the pressure
  stronger the storm).
• Power The power released by a single hurricane
  can exceed the annual electricity consumption by
  the US and Canada combined.

6
Hurricane Seasons

• Hurricanes obtain their energy from latent heat
  release in cloud formation processes.
• Hurricanes occur where a deep layer of warm water
  exists during the time of the highest SSTs (sea
  surface temps).
• Northern Hemisphere August through September are
  most active months.
• Southern Hemisphere January through March are
  the most active months

7
Latent Heat Release Me!

• Hurricanes draw their power from warm, extremely
  humid air found only over warm oceans.
• The key energy source is the latent heat that's
  released when water vapor condenses into cloud
  droplets and rain. Tropical storms and hurricanes
  grow best in a deep layer of humid air that
  supplies plenty of moisture.
• When a substance changes phase, energy must be
  supplied in order to overcome the molecular
  attractions between the constituent particles.
  This energy must be supplied externally, normally
  as heat, and does not bring about a change in
  temperature.

When water is in the vapor state, as a gas, the
water molecules are not bonded to each other.
They float around as single molecules.
When water is in the liquid state, some of
the molecules bond to each other with hydrogen
bonds.  The bonds break and re-form continually.
8
Latent Heat

• We call the energy needed to change phases latent
  heat (the word "latent" means "invisible"). The
  latent heat is the energy released or absorbed
  during a change of state.
• To get the molecule of water vapor to become
  liquid again, we have to take the energy away,
  that is, we have to cool it down so that it
  condenses (condensation is the change from the
  vapor state to the liquid state).  When water
  condenses, it releases latent heat.

9
Latent Heat Values

• The amount of latent heat involved depends to
  some extent on the temperature at which the
  process is occurring. The figures below are those
  normally found in meteorology texts and are for
  temperatures found in the atmosphere, such as 0
  Celsius (32 F).
• Latent heat of condensation (Lc) Refers to the
  heat gained by the air when water vapor changes
  into a liquid. Lc2500 Joules per gram (J/g) of
  water or 600 calories per gram (cal/g) of water.
• Latent heat of fusion (Lf) Refers to the heat
  lost or gained by the air when liquid water
  changes to ice or vice versa. Lf333 Joules per
  gram (J/g) of water or 80 calories per gram
  (cal/g) of water.
• Latent heat of sublimation (Ls) Refers to the
  heat lost or gained by the air when ice changes
  to vapor or vice versa. Ls2833 Joules per gram
  (J/g) of water or 680 calories per gram (cal/g)
  of water.
• Latent heat of vaporization (Lv) Refers to the
  heat lost by the air when liquid water changes
  into vapor. This is also commonly known as the
  latent heat of evaporation. Lv -2500 Joules per
  gram (J/g) of water or -600 calories per gram
  (cal/g) of water.

10
Latent Heat

• As water vapor evaporates from the warm ocean
  surface, it is forced upward in the convective
  clouds that surround the eyewall and rainband
  regions of a storm. As the water vapor cools and
  condenses from a gas back to a liquid state, it
  releases latent heat. The release of latent heat
  warms the surrounding air, making it lighter and
  thus promoting more vigorous cloud development.
• The release of latent heat warms the surrounding
  air, making it lighter and thus promoting more
  vigorous cloud development.
• Animation http//svs.gsfc.nasa.gov/vis/a000000/a0
  01600/a001605/cloud.mov

11
Energy of Latent Heat
Air parcel dew point temperature (oC) Approximate amount of water vapor (g) in an air parcel (kg) at saturation (by the way, in text books referred to as the saturation mixing ratio) Approximate amount of potential heating due to latent heat release (calories) if all the water vapor condenses
0 4 2360
10 8 4720
20 16 9440
30 32 18880
40 64 37760

• Condensation releases latent heat. This causes
  the temperature of a cloud to be warmer than it
  otherwise would have been if it did not release
  latent heat. Anytime a cloud is warmer than the
  surrounding environmental air, it will continue
  to rise and develop.
• The more moisture a cloud contains, the more
  potential it has to release latent heat.
• Temperatures in the middle of the rising air in
  cumulonimbi there may be as much as 15-20C
  (28-38F) warmer than air outside the storm. For
  Hurricane Rita, observations showed that, at 700
  mb (approximately 10000 feet), the interior
  temperature of Rita was 31C and about 20C warmer
  than the outside of the storm at the same
  elevation.

12
Hurricane Structure

• A central eye is surrounded by large cumulonimbus
  thunderstorms occupying
• the adjacent eyewall.
• Weak uplift and low precipitation areas are
  separated by individual cloud
• bands.

13
Temperature Structure

• Hurricanes characterized by a strong thermally
  direct circulation with rising of warm air near
  center of storm and sinking of cooler air
  outside.
• The warm core of the hurricane serves as a
  reservoir of potential energy which is
  continually being converted to kinetic energy by
  thermally direct circulation

14
Pressure Structureof Hurricanes

• The horizontal pressure gradient with altitude
  decreases slowly

• From surface to 400 mb cyclonic circulation.

• At about 400 mb, the pressure inside the storm is
  approx. that of outside of the storm.

• From 400 mb to tropopause anticylonic
  circulation.

• The upper portions of the storm are blanketed by
  a cirrus cloud cap due to overall low
  temperatures.

15
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16
Hurricanes and Tropical Storms
17
Naming Convention

• Hurricanes extreme tropical storms over Atlantic
  and Eastern
• Pacific Oceans
• Typhoons extreme tropical storms over western
  Pacific Ocean
• Cyclones extreme tropical storms over Indian
  Ocean and
• Australia

18
Ocean Temperatures and Hurricanes
Hurricanes depend on large pools of warm water
19
Annual Hurricane Frequency

• There are no hurricanes in the southern Atlantic
  Ocean!
• The strongest hurricanes occur in the western
  Pacific Ocean
• (often referred to as super-typhoons.

20
Hurricane Characteristics

• Definition Hurricanes have sustained winds of
  120 km/hr (74 mph) or greater.
• Size Average diameters are approx. 600 km (350
  mi). This is 1/3 the size of a mid-latitude
  cyclone (synoptic storm system).
• Duration Days to a week or more.
• Strength Central pressure averages 950 mb but
  may be as low as 870 mb (lower the pressure
  stronger the storm).
• Power The power released by a single hurricane
  can exceed the annual electricity consumption by
  the US and Canada combined.

21
Hurricane Seasons

• Hurricanes obtain their energy from latent heat
  release in cloud formation processes.
• Hurricanes occur where a deep layer of warm water
  exists during the time of the highest SSTs (sea
  surface temps).
• Northern Hemisphere August through September are
  most active months.
• Southern Hemisphere January through March are
  the most active months

22
Latent Heat Release Me!

• Hurricanes draw their power from warm, extremely
  humid air found only over warm oceans.
• The key energy source is the latent heat that's
  released when water vapor condenses into cloud
  droplets and rain. Tropical storms and hurricanes
  grow best in a deep layer of humid air that
  supplies plenty of moisture.
• When a solid substance changes phase, energy must
  be supplied in order to overcome the molecular
  attractions between the constituent particles.
  This energy must be supplied externally, normally
  as heat, and does not bring about a change in
  temperature.

When water is in the vapor state, as a gas, the
water molecules are not bonded to each other.
They float around as single molecules.
When water is in the liquid state, some of
the molecules bond to each other with hydrogen
bonds.  The bonds break and re-form continually.
23
Latent Heat

• We call the energy needed to change phases latent
  heat (the word "latent" means "invisible"). The
  latent heat is the energy released or absorbed
  during a change of state.
• To get the molecule of water vapor to become
  liquid again, we have to take the energy away,
  that is, we have to cool it down so that it
  condenses (condensation is the change from the
  vapor state to the liquid state).  When water
  condenses, it releases latent heat.

24
Latent Heat Values

• The amount of latent heat involved depends to
  some extent on the temperature at which the
  process is occurring. The figures below are those
  normally found in meteorology texts and are for
  temperatures found in the atmosphere, such as 0
  Celsius (32 F).
• Latent heat of condensation (Lc) Refers to the
  heat gained by the air when water vapor changes
  into a liquid. Lc2500 Joules per gram (J/g) of
  water or 600 calories per gram (cal/g) of water.
• Latent heat of fusion (Lf) Refers to the heat
  lost or gained by the air when liquid water
  changes to ice or vice versa. Lf333 Joules per
  gram (J/g) of water or 80 calories per gram
  (cal/g) of water.
• Latent heat of sublimation (Ls) Refers to the
  heat lost or gained by the air when ice changes
  to vapor or vice versa. Ls2833 Joules per gram
  (J/g) of water or 680 calories per gram (cal/g)
  of water.
• Latent heat of vaporization (Lv) Refers to the
  heat lost by the air when liquid water changes
  into vapor. This is also commonly known as the
  latent heat of evaporation. Lv -2500 Joules per
  gram (J/g) of water or -600 calories per gram
  (cal/g) of water.

25
Latent Heat

• As water vapor evaporates from the warm ocean
  surface, it is forced upward in the convective
  clouds that surround the eyewall and rainband
  regions of a storm. As the water vapor cools and
  condenses from a gas back to a liquid state, it
  releases latent heat. The release of latent heat
  warms the surrounding air, making it lighter and
  thus promoting more vigorous cloud development.
• The release of latent heat warms the surrounding
  air, making it lighter and thus promoting more
  vigorous cloud development.
• Animation http//svs.gsfc.nasa.gov/vis/a000000/a0
  01600/a001605/cloud.mov

26
Energy of Latent Heat
Air parcel dew point temperature (oC) Approximate amount of water vapor (g) in an air parcel (kg) at saturation (by the way, in text books referred to as the saturation mixing ratio) Approximate amount of potential heating due to latent heat release (calories) if all the water vapor condenses
0 4 2360
10 8 4720
20 16 9440
30 32 18880
40 64 37760

• Condensation releases latent heat. This causes
  the temperature of a cloud to be warmer than it
  otherwise would have been if it did not release
  latent heat. Anytime a cloud is warmer than the
  surrounding environmental air, it will continue
  to rise and develop.
• The more moisture a cloud contains, the more
  potential it has to release latent heat.
• Temperatures in the middle of the rising air in
  cumulonimbi there may be as much as 15-20C
  (28-38F) warmer than air outside the storm. For
  Hurricane Rita, observations showed that, at 700
  mb (approximately 10000 feet), the interior
  temperature of Rita was 31C and about 20C warmer
  than the outside of the storm at the same
  elevation.

27
Hurricane Structure

• A central eye is surrounded by large cumulonimbus
  thunderstorms occupying
• the adjacent eyewall.
• Weak uplift and low precipitation areas are
  separated by individual cloud
• bands.

28
Temperature Structure

• Hurricanes characterized by a strong thermally
  direct circulation with rising of warm air near
  center of storm and sinking of cooler air
  outside.
• The warm core of the hurricane serves as a
  reservoir of potential energy which is
  continually being converted to kinetic energy by
  thermally direct circulation

29
Pressure Structureof Hurricanes

• The horizontal pressure gradient with altitude
  decreases slowly

• From surface to 400 mb cyclonic circulation.

• At about 400 mb, the pressure inside the storm is
  approx. that of outside of the storm.

• From 400 mb to tropopause anticylonic
  circulation.

• The upper portions of the storm are blanketed by
  a cirrus cloud cap due to overall low
  temperatures.

30
Hurricane Eye and Eyewall

• A shrinking eye indicates storm intensification

• The eyewall is moves at a speed of 20 km/hour and
  the calm weather associated with the eye will
  last less than 1 hour

• The hurricane eye is an area of descending air,
  relatively clear sky and light winds 25 km (15
  mi) diameter on average.

• The eyewall is comprised of the strongest winds,
  the largest clouds and the heaviest precipitation
  with rainfall rates as high as 2500 mm/day (100
  inches/day).

31
Hurricane Formation

• Tropical Disturbance Clusters of small
  thunderstorms.
• Tropical Depression When at least 1 closed
  isobar is present (organized center of low
  pressure).
• Tropical Storm Further intensification to wind
  speeds of 60 km/hr (37 mph).
• Hurricane Hurricane status is gained when the
  winds reach a sustained 120 km/hr (74 mph).

32
Tropical Disturbances and Easterly Waves

• Some tropical disturbances form from mid-latitude
  troughs migrating
• towards lower latitudes, some form from ITCZ
  convection, but most
• develop from easterly waves.
• Easterly Waves, or undulations in the trade wind
  patterns, spawn
• hurricanes in the Atlantic.
• Only 10 of tropical disturbance become more
  organized, rotating
• storms.

33
Conditions necessary for Hurricane Formation

• Hurricanes only form over deep (several 10s of
  meters) water layers with temperatures in excess
  of 27 degrees C.
• Poleward to about 20 degrees, water temperatures
  are usually below this threshold.
• Coriolis effect is an important contributor,
  hurricanes do not form from equator to 5 degrees.
• Need unstable atmosphere available in western
  parts of oceans but not in eastern parts of
  ocean.
• Strong vertical shear must be absent. (both
  magnitude and direction).
• Vertical shear changing of wind speed and/or
  direction with height of the atmosphere.

34
Hurricane Movement

• Tropical depressions and disturbances are largely
  regulated by trade wind flow move westward.
• Tropical storms hurricanes upper-level winds
  and ocean temperatures gain importance.
• Fully developed hurricanes will move poleward
  (stronger upper winds will steer the hurricanes
  northward).

35
Hurricane Dissipation

• After making landfall, a hurricane may die
  completely within a couple of days (no longer has
  moisture to feed into storm).
• Even as storm weakens, it can still have large
  effects on land (especially flooding).
• Hurricanes will also dissipate over cooler waters
  or if they encounter strong vertical shear.
• Animation http//svs.gsfc.nasa.gov/vis/a000000/a0
  01600/a001605/coldwater.mov

36
Hurricane Damage

• Heavy rainfall
• Strong winds
• Tornadoes (generally F0-F2)
• Storm surge rise in water level induced by the
  hurricane.

37
Tornado formation in hurricanes

• Most hurricanes contain clusters of tornadoes.
• Most tornadoes occur in right front quadrant of
  the storm.
• It appears that slowing of wind by friction at
  landfall contributes to the formation of these
  tornadoes.

38
Storm Surges

• Process 1 Hurricane winds drag surface waters
  forward and pile water near coasts.
• Process 2 Lower atmospheric pressure raises sea
  level (for every 1mb pressure decrease, sea level
  raises 1 cm)
• Storm surges raise sea level by a 1-2 m for most
  hurricanes, as much as 7 m (a real problem when
  coastal locations are at or below sea-level)

39
Hurricane Wind Structure

• Winds and surge are typically strongest at right
  front quadrant of storm where wind speeds combine
  with speed of storms movement to create the
  highest area of potential impact.

40
Trends for Atlantic

• Mid 1990s-now A significant increase in the
  numbers of hurricanes and intense hurricanes
  making landfall in US.
• 1970s-mid 1990s Lower than normal incidence of
  Atlantic Hurricanes.
• Debate Is the recent increase in hurricanes part
  of a natural cycle, global climate change or a
  combination?

41
Forecasting

• National Hurricane Center responsible for
  Atlantic and eastern/central Pacific.
• Data gathered through satellite, surface
  observations and aircraft using dropsondes
  (dropping of instruments through hurricane)
• Computer models assist in predictions.

42
Hurricane Watch and Warning

• Hurricane Watch if an approaching hurricane is
  expected to make landfall within 24 hours.
• Hurricane Warning if the time frame is less,
  then its a warning.

43
Naming of Hurricanes

• When a tropical disturbance reaches a tropical
  depression, the storm will be given a name.
• The name comes from an A-W list given by World
  Meteorological Organization (WMO).
• The names of hurricanes with devastating effects
  are retired.

44
Hurricane Intensity Scale

• Saffir-Simpson Scale
• Five Categories the larger numbers indicate
  lower central
• pressures, greater winds and stronger storm
  surges