Tropical%20Cyclones%20Characteristics%20and%20Forecasting

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Tropical CyclonesCharacteristics and Forecasting
Horace H. P. Burton and Selvin DeC.
Burton Caribbean Institute for Meteorology and
Hydrology
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Outline

• Introduction
• Definitions
• Characteristics
• Forecast Methods
• Forecast errors
• Probability forecasts

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Definitions

• Tropical cyclone An area of low pressure which
  develops over tropical or subtropical waters
• Tropical depressionA weak tropical cyclone in
  which the maximum surface wind is 38 mph (62 km/h
  or 33 kt) or less

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Definitions

• Tropical storm A tropical cyclone in which the
  maximum surface wind ranges from 39 to 73 mph (63
  to 118 km/h or 34 to 64 kt)
• HurricaneA tropical cyclone with highest
  sustained winds 74 mph (119 km/h or 65 kt) or more

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Saffir-simpson Scale
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Tropical Cyclone Characteristics

• Season - June to November
• Average - 9 tropical storms - 6
  hurricanes
• Most active months - August and September
• Movement - west or west northwest
  - recurve to east generally in western
  and northwestern Atlantic

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Tropical Cyclone Characteristics

• Size - 300 to 1500 km in diameter
• Horizontal structure
• Eye - 20 km
• Eyewall - 30 to 50 km
• Spiral bands

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Tropical Cyclone Characteristics

• Hurricane force winds generally extend out about
  100 km (60 miles) from the centre
• Storm force winds may extend out as much as 500
  km (300 miles)
• Maximum wind speeds between 12 and 50 km from
  centre

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Tropical Cyclone Forecasting

• Early and precise warnings (forecasts) do not
  necessarily remove the risk of damage or loss or
  life, but the effects may be significantly
  reduced
• Current inaccuracies in forecast dictate over
  warning
• False warnings can produce an attitude of
  scepticism

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Forecasting Methods

• Forecasting methods have evolve to become more
  complex
• Pre 1960 - simple subjective methods
• cloud types and motion, swells, pressure
• Post 1960 - more sophisticated objective methods
• complex statistical techniques and computer models

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Forecasting Methods

• A tropical cyclone forecast involves the
  prediction of several interrelated features,
  including
• The track, winds, rainfall, storm surge, areas
  threatened
• NHC normally issues a forecast every 6 hours for
  period extending out to 72 hours
• Official forecast is based on the guidance
  obtained from a variety of subjective and
  objective models

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Forecast Errors

• Forecast errors arise from
• A lack of a full understanding of the formation
  and growth of tropical cyclones
• The limitations of the forecasting techniques
• Users of forecasts must be aware of the
  limitations of information in advisories

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Forecast Errors - characteristics

• Still substantially large although showing a slow
  and steady decrease
• Increase remarkably with increasing time
• Are approximately 30 of the cyclone movement
  over the same time
• Large year-to-year variations

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Official Forecast Errors for Atlantic
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Initial forecast errors

• Average official initial forecast errors
• 26 km for position
• 3 kt for intensity
• Range of initial forecast errors
• 10 km for good eye to more 180 km for poorly
  defined centre
• as much as 30 kt for intensity

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Strike probability forecasts

• One method use to objectively define uncertainty
  inherent in forecasts
• Derived from knowledge of past cyclones and
  forecast errors in the region of interest
• Provide valuable early guidance in estimating the
  risk of tropical cyclones affecting important or
  vulnerable facilities

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Use of strike probability forecasts

• To extend the usable length of forecasts
• To provide a quantitative assessment of the
  threat posed by a tropical cyclone
• To compare the relative threat
• To cause a consistent response
• As a tool in risk analysis

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Probability ellipses - Georges 1998
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Strike probability forecasts

• Uncertainty increases as the forecast interval
  increases
• 72 hours probability less than that 12 hours
• Maximum probability at 72 hours is about 10
  compared to about 70 at 12 hours
• Longer lead-time actions must be based on smaller
  probabilities