Transcript Tropical Cyclones Characteristics and Forecasting Horace H. P. Burton and Selvin DeC.

Tropical Cyclones
Characteristics and Forecasting
Horace H. P. Burton and Selvin DeC. Burton
Caribbean Institute for Meteorology and Hydrology
Caribbean Disaster Mitigation Project
Caribbean Institute for Meteorology and Hydrology
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 depression
A 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)
• Hurricane
A tropical cyclone with highest sustained winds 74
mph (119 km/h or 65 kt) or more
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Saffir-simpson Scale
Sustained Winds
Category
Pressure
Knots
Km/hour
Millibars
1
65 – 82
119 – 15
> 980
2
83 – 97
154 – 177
965 - 979
Moderate
3
98 – 113
179 – 209
945 - 964
Extensive
4
114 – 135
211 – 249
920 - 944
Extreme
5
> 135
> 249
< 920
Damage Level
Low
Catastrophic
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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
Forecast Interval (hr)
0
12
24
36
48
72
Track in km (1986-1995)
26
91
173
252
335
506
Intensity in kt (1990-1995)
3
7
10
13
16
19
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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
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