storms are imbedded within steering currents produced
by large-scale motions (spatial scales greater than about
5,000 km and temporal scales greater than five days)
within the atmosphere. These steering currents roughly
coincide with many of the average seasonal circulation
features around the globe, producing east to west
motions at latitudes between 30
latitude north and
south and curving toward west to east motions at latitudes poleward of 35
latitude north and south. Tropical
cyclones move with the general speed of these largescale currents in which they are imbedded. The winds
in the tropical cyclone and large-scale wind system are
superposed vectorially. Relative to the earth’s surface,
this creates higher wind speeds on the side of the storm
moving in the same direction as the large-scale motion
and lower wind speeds on the side moving against the
large-scale motion. At the same time, the vector addition changes the wind directions slightly (typically less
than 10
). The overall effect is to create an asymmetric
wind field with generally stronger(weaker) winds on the
right (left) side of a tropical cyclone in the northern
hemisphere and stronger (weaker) winds on the left side
of a tropical cyclone in the southern hemisphere.
Features within a tropical cyclones
• The eye and eye wall: As shown in Figure 2, there is an
often cloudless region of relatively calm winds, termed
the “eye” of the storm, around which the rest of the
storm rotates. In the eye, the atmospheric pressure typically reaches its minimum values accompanied by the
warmest temperatures. The warming in this region is
caused by slowly sinking air from the high pressure
region above. In well-developed storms, the eye is
surrounded by an eyewall, as illustrated in Figure 2.
The eyewall is where the strongest winds and highest
rainfall rates are usually located. The existence of the
eye and eyewall is related to the dynamics of flow
within tropical cyclones. As air nears the center of the
storm, the centrifugal acceleration would become very
large if wind speeds around the eye remained constant.
The pressure gradient required to maintain a
cyclostrophic balance cannot be maintained inside the
eyewall area and the slowly sinking air inside the
eyewall slowly merges with the air in the eyewall,
where it rises to complete the cycle.
• Rain bands: Although many well-developed tropical
cyclones are relatively symmetric, lower intensity or
storms during the early formative stage of developments can be much less symmetric and contain more
irregular organized motions. This creates regions of
higher and lower winds throughout the storm, leading
to somewhat unique patterns of clouds and winds in
tropical cyclones. One prevalent feature related to these
organized variations is termed a rain band. Rain bands
are heavily convective regions containing high rainfall
rates and typically higher wind speed than adjacent portions of the storm. These bands form long, narrow cloud
systems which are oriented in the direction of the wind.
Along the lower level of these bands, convergence is a
maximum, leading to some low-level subsidence in
adjacent areas. In many cases the spiral bands converge
into the storm center merging into the eyewall.
Extratropical transition
When tropical cyclones encounter pronounced horizontal
temperature gradients in the atmosphere, as typical of
many ocean areas poleward of 35
north or south, they
begin to undergo extratopical transition (Jones et al.,
2003). This introduces a new source of energy into the
energy balance, termed a baroclinic energy source, which
is the same mechanism that supplies energy to
extratropical storms. Most hurricanes along the US East
Coast are undergoing extratropical transition by the time
they get into the New York Bight; consequently, it should
not be a surprise that Hurricane Sandy was deriving a substantial part of its energy from baroclinic sources (Shen
et al., 2013) when in made landfall. This also means that
the MPI is not a viable concept for quantifying the maximum storm intensity at this latitude.
Impacts of tropical cyclones
Tropical cyclones represent a major threat to property,
livelihood and safety around the world. Critical threats
are underestimated in many areas due to their relative rarity; and even in areas frequently exposed to tropical
cyclones, an extreme cyclone can produce inundation,
inland flooding, mudslides and wave run-up which significantly exceed the experience of local inhabitants. For this
reason, such storms often produce catastrophic, unanticipated impacts, even when appropriate warnings are
issued. The principal contributors to damages from tropical cyclones are storm surge, damaging waves, strong
winds and inland flooding.
• Storm surge: The rise of water level generated by the
storm winds and waves is termed the storm surge and
is responsible for the majority of deaths in most areas.
The superposition of storm surge on local tides is sometimes termed the storm tide. Inundation due to the storm
tide, or total water level, is driven by three main processes: (1) lower atmospheric pressures at the water surface, termed the “inverted barometer effect” (roughly
1 m of sea-level rise for a 100 mb lowering of the atmospheric pressure at the surface), (2) direct momentum
transfer from wind to water, and (3) fluxes into the
water column from breaking waves approaching the
coast. Additional, factors affecting surge include: storm
size, forward velocity, angle of coastal incidence, wave
stress (Irish et al., 2009), coastal bathymetry (Resio and
Westerink, 2008) and interactions with rivers (Kerr
et al., 2013; Resio et al., 2013). In areas with broad continental shelves, direct wind forcing is usually the dominant surge generation mechanism, while on steep
coasts, typical of many island areas around the world,
waves and wave run-up are often the dominant flooding
332
HURRICANES AND TYPHOONS
by large-scale motions (spatial scales greater than about
5,000 km and temporal scales greater than five days)
within the atmosphere. These steering currents roughly
coincide with many of the average seasonal circulation
features around the globe, producing east to west
motions at latitudes between 30
latitude north and
south and curving toward west to east motions at latitudes poleward of 35
latitude north and south. Tropical
cyclones move with the general speed of these largescale currents in which they are imbedded. The winds
in the tropical cyclone and large-scale wind system are
superposed vectorially. Relative to the earth’s surface,
this creates higher wind speeds on the side of the storm
moving in the same direction as the large-scale motion
and lower wind speeds on the side moving against the
large-scale motion. At the same time, the vector addition changes the wind directions slightly (typically less
than 10
). The overall effect is to create an asymmetric
wind field with generally stronger(weaker) winds on the
right (left) side of a tropical cyclone in the northern
hemisphere and stronger (weaker) winds on the left side
of a tropical cyclone in the southern hemisphere.
Features within a tropical cyclones
• The eye and eye wall: As shown in Figure 2, there is an
often cloudless region of relatively calm winds, termed
the “eye” of the storm, around which the rest of the
storm rotates. In the eye, the atmospheric pressure typically reaches its minimum values accompanied by the
warmest temperatures. The warming in this region is
caused by slowly sinking air from the high pressure
region above. In well-developed storms, the eye is
surrounded by an eyewall, as illustrated in Figure 2.
The eyewall is where the strongest winds and highest
rainfall rates are usually located. The existence of the
eye and eyewall is related to the dynamics of flow
within tropical cyclones. As air nears the center of the
storm, the centrifugal acceleration would become very
large if wind speeds around the eye remained constant.
The pressure gradient required to maintain a
cyclostrophic balance cannot be maintained inside the
eyewall area and the slowly sinking air inside the
eyewall slowly merges with the air in the eyewall,
where it rises to complete the cycle.
• Rain bands: Although many well-developed tropical
cyclones are relatively symmetric, lower intensity or
storms during the early formative stage of developments can be much less symmetric and contain more
irregular organized motions. This creates regions of
higher and lower winds throughout the storm, leading
to somewhat unique patterns of clouds and winds in
tropical cyclones. One prevalent feature related to these
organized variations is termed a rain band. Rain bands
are heavily convective regions containing high rainfall
rates and typically higher wind speed than adjacent portions of the storm. These bands form long, narrow cloud
systems which are oriented in the direction of the wind.
Along the lower level of these bands, convergence is a
maximum, leading to some low-level subsidence in
adjacent areas. In many cases the spiral bands converge
into the storm center merging into the eyewall.
Extratropical transition
When tropical cyclones encounter pronounced horizontal
temperature gradients in the atmosphere, as typical of
many ocean areas poleward of 35
north or south, they
begin to undergo extratopical transition (Jones et al.,
2003). This introduces a new source of energy into the
energy balance, termed a baroclinic energy source, which
is the same mechanism that supplies energy to
extratropical storms. Most hurricanes along the US East
Coast are undergoing extratropical transition by the time
they get into the New York Bight; consequently, it should
not be a surprise that Hurricane Sandy was deriving a substantial part of its energy from baroclinic sources (Shen
et al., 2013) when in made landfall. This also means that
the MPI is not a viable concept for quantifying the maximum storm intensity at this latitude.
Impacts of tropical cyclones
Tropical cyclones represent a major threat to property,
livelihood and safety around the world. Critical threats
are underestimated in many areas due to their relative rarity; and even in areas frequently exposed to tropical
cyclones, an extreme cyclone can produce inundation,
inland flooding, mudslides and wave run-up which significantly exceed the experience of local inhabitants. For this
reason, such storms often produce catastrophic, unanticipated impacts, even when appropriate warnings are
issued. The principal contributors to damages from tropical cyclones are storm surge, damaging waves, strong
winds and inland flooding.
• Storm surge: The rise of water level generated by the
storm winds and waves is termed the storm surge and
is responsible for the majority of deaths in most areas.
The superposition of storm surge on local tides is sometimes termed the storm tide. Inundation due to the storm
tide, or total water level, is driven by three main processes: (1) lower atmospheric pressures at the water surface, termed the “inverted barometer effect” (roughly
1 m of sea-level rise for a 100 mb lowering of the atmospheric pressure at the surface), (2) direct momentum
transfer from wind to water, and (3) fluxes into the
water column from breaking waves approaching the
coast. Additional, factors affecting surge include: storm
size, forward velocity, angle of coastal incidence, wave
stress (Irish et al., 2009), coastal bathymetry (Resio and
Westerink, 2008) and interactions with rivers (Kerr
et al., 2013; Resio et al., 2013). In areas with broad continental shelves, direct wind forcing is usually the dominant surge generation mechanism, while on steep
coasts, typical of many island areas around the world,
waves and wave run-up are often the dominant flooding
332
HURRICANES AND TYPHOONS