Chapter 4 summery Flashcards
Tropical storm:
An organized center of low pressure that originates over warm tropical oceans
The maximum sustained surface winds of tropical storms
Range from 63 to 118 kmph
Tropical storms generally develop
During the warmer months (late summer and early autumn)
The main parts of a tropical cyclone:
- Rain bands
- The eye
- Air spiral in toward the center in a counter-clock wise pattern in NH and out the top in the opposite direction in the center of the storm air sinks forming an eye
- Mostly cloud free
- The eyewall
- The strong rotation of air around the cyclone balances inflow to the center causing air to ascend about 16-32 km from the center forming the eyewall
- Consist of tall thunderstorms that produce
- Heavy rains
- The strongest winds
The destructive high winds accompanying tropical cyclones produce:
Phenomenal seas (wave height larger than 14 m) which are dangerous both for vessels out at sea and those moored in harbors.
How are swells created?
The strong winds associated with offshore storms blow on the surface of the sea
Swells:
Waves that have been created sometime before by the winds blowing in an area some considerable distance away
The size of the swell depends on:
- The intensity of the wind within a storm
- Fetch
- The amount of distance the wind blows across the water
- The amount of time the wind blows at a sustained speed
The long swell ahead of a tropical storm is sometimes experienced at a distance
Of more than 1000 miles from the storm’s center
The long swell ahead of a tropical storm travels
- Much faster than the storm itself
- Cause storm tide
- Result in disastrous flooding on the coast
- Cause storm tide
A storm generates
Winds that impact a region over open water
Fetch:
The “length” (distance) wind blows over open water
Sea:
Area where wind waves are generated, mixed period and wavelengths
Ocean swell:
Refers to series of ocean surface waves that were not generated by the local wind
Tropical cyclone is a major hazardous weather system for:
Ships travelling in the oceans
Important prediction for tropical cyclones approaching:
- Cyclone intensity
- Movement
Essential information to understand for mariners for safe voyage
- Wind
- Wave distribution in the vicinity of the tropical cyclone
Tropical cyclone route and air flow in the NH:
- Travel from east to west
- Air flow towards the center of a tropical cyclone in a counter-clockwise direction
Dangerous semicircle:
- High winds and waves are normally most prominent on the right hand side of the moving cyclone
- The speed of forward movement of the cyclone itself is added on top of the wind circulating around the cyclone
During the tropical storm season the navigator must
Exercise constant vigilance to ensure that he is not caught unaware in the path of the storm
The mariner’s aim must be to
Avoid getting near the center of the storm
Certain common features of these storms have enabled:
The evolvement of guidance ‘rules’ (warning signs) to help in keeping the ship out a serious trouble
Warning signs include:
- Barometric pressure
- Appearance of the sky
- Swell
- Visibility
- Winds
Note: except for the variation of the barometric pressure, any one of the above warning signs, if taken alone is only an uncertain indication of the approach of a TS. All these warning signs must be considered together.
Barometric Pressure:
If there is a marked departure in the barometric pressure (3 hPa or more below the mean pressure) from the diurnal variation there is a risk of a tropical storm forming or developing.
- The barometric pressure is as low as 5 hPa below normal, it should be taken to mean that a TS is almost certainly somewhere in the area and probably within 200 miles of the observer.
Appearance of sky:
Vivid (bright) coloring at sunrise and sunset are often a warning feature, accompanied or followed by high cirrus clouds of the eye.
- Cirrus may first appear when the storm is from 300 to 600 miles away and is often the first warning of a TS.
Swell:
There will be a long swell coming from the direction of the storm center provided there is no land intervening between the storm and the ship.
- The swell travels faster than the storm and usually extends more than 400 miles and sometimes 1000 miles from the center.
Visibility:
Exceptionally good visibility frequently precedes a TS
Wind:
During the storm season an appreciable increase in wind force and/or direction should be regarded as a possible indication of the approach of a TS.
All ocean areas that are affected by tropical storms are now covered by
Radio weather bulletins and storm warnings issued by meteorological services.
Radars and meteorological satellites provide
Valuable information about existing storms and their development and in some areas reconnaissance aircraft keep in contact with each storm.
Radars can locate the
Center of a tropical cyclone if it comes within the effective range of the weather radar of a ship or land station.
Radars can display
The eye of a TS as a dark circular area in the center and the surrounding rain belts.
Radars detect tropical cyclones by
Different effective ranges
Different radars have different effective ranges for detecting tropical cyclones depending on
- The wavelength
- Power
- Height of the location of the scanner
Compare 3cm radars and radars with larger ranges
- Average merchant ships (3cm radar)
- Display the eye or the rain belt at a maximum range of 50 km
- Its value in warning of TS is very limited
- Because within the distance the ship will have already become heavily involved in the storm
- 10 cm radars
- Greater range (230-460 km) of detection
Meteorological services ashore also provide very valuable warnings by
- Radio
- Whenever it is available from
- Ships
- Other sources
- Whenever it is available from
Radio warnings include information about:
- The extent
- Wind force
- Existing track
- Speed of movement of a storm
Radio warning information are broadcasted at:
Suitable intervals
Examples of traditional marine radio
- VHF
- MF
- HF
Large number of ships no longer carry qualified radio officers to use traditional marine radio thanks to:
The advancement of the global maritime distress and safety system
The global maritime distress and safety system (GMDS) developed though
The international maritime organization (IMO)
The global maritime distress and safety system (GMDS) uses
- Improved terrestrial and
- Satellite technology
- Ship board radio systems
The following play an important role in storm forecasting
(TS born in the mid ocean)
- Existence of storm
- Behavior of these storms
Masters are required to
- Report the existence of a tropical storm if suspected
- As per the International Convention of safety of life at sea
- Send a safety message as soon as possible to the nearest authority ashore and
- broadcast to shipping in the vicinity
reports should be sent at intervals of
- Not more than three hours until the ship is clear of the storm
- After that the reports are sent t o meteorological authorities at frequent intervals
Practical rules of avoidance:
- 34 kt rule
- 1-2-3 rule
- 1-2-3 rule of thumb
Practical rules of avoidance help
Account for the inherent errors in hurricane forecasting
Practical rules of avoidance are used in order to
Limit the potential of a close encounter between ship and storm
34 kt rule:
- For vessels at sea avoiding 34 kt wind field is nessessary
- 34 kt is chosen as the critical value because as wind speed increases to this speed
- Sea state development approaches critical levels
- Rapidly decreasing limits to ship movement
- Sea state development approaches critical levels
- 34 kt is chosen as the critical value because as wind speed increases to this speed
The state of the sea outside the 34 kt wind radius
Can be significant enough to:
- Limit course
- Limit Speed options
- Must be considered when avoiding hurricanes
1-2-3 rule:
This is the single most important aid in accounting for hurricane Forecast Track Errors (FTE). Understanding & use of this technique should be mandatory for any vessel operating near a hurricane.
The 1-2-3 rule is derived from:
The latest 10-year average FTE associated with hurricanes in the North Atlantic
Application of the 1-2-3 rule requires information from
- Information from the TCM (tropical cyclone forecast/ advisory message)
1-2-3 rule is important to
Remain clear of a hurricane at sea
1-2-3 rule establishes:
A minimum recommended distance to maintain from a hurricane in the Atlantic
Larger buffer zones should be established in situations with:
- Higher forecast uncertainty
- Limited crew experience
- Decreased vessel handling
- Other factors set by the vessel master
The 1-2-3 rule does not account for
- Sudden and rapid intensification of hurricanes
- Could result in an outward expansion of the 34 Kt wind field
- Typical expansion of the wind field as a system transition from hurricane to extratropical gale/storm
The radius of the 34 knot winds extends
Outwards from the storm’s center “synoptic date/time & position”
The radius of the 34 knot wind is measured in:
Nm
The wind radius is given in:
Quadrants
- NE
- SE
- SW
- NW
For easier plotting of each day position
Make the largest quadrant the same for all other quadrants
Steps for plotting the position:
- 34 kt wind radius is drawn from the center (for easier construction and viewing)
- The largest quadrant is equal to all the other quadrant
- Repeat the same process for the 24,48 and 72 hour forecast
- Manually add the buffer zone to complete the danger area to avoid
1-2-3 rule of thumb
- 100 mile error radius for 24hr forecast
- 200 mile error radius for 48hr forecast
- 300 mile error radius for 72hr forecast
Steps to Determine the Hurricane Danger Area
- Plot the initial and forecast hurricane positions on a navigation chart.
- Find the maximum radius of 34 KT winds at the initial, 24, 48, and 72 hour forecast times of the TCM.
- Apply the 1-2-3 rule to each of the radii at the 24, 48, and 72 hour forecast positions.
- Draw a circle around the hurricane initial position with radius equal to the maximum radius of 34 KT winds given in the TCM.
- Draw circles around the 24, 48, and 72 hour forecast positions of the hurricane using the respective radii found in step 3.
- Connect tangent lines to each circle constructed in steps 3 and 4 along both sides of the hurricane track.
- The area enclosed by these tangent lines is known as the danger area of the hurricane and must be avoided as a vessel attempts to navigate in the vicinity of the hurricane.
