Understanding and Predicting Tropical Cyclone Risks for the AsiaPacific Region

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Understanding and Predicting Tropical Cyclone
Risks for the Asia-Pacific Region
Johnny Chan
Guy Carpenter Asia-Pacific Climate Impact Centre
City University of Hong Kong
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Outline

• Understanding
• Review of what you learned last year
• Some new results in understanding global warming
  and tropical cyclone activity
• Prediction
• Traditional method
• New Approaches
• Summary

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Understanding (1)What you learned last year
(R) Red 0 (G) Green 87 (B) Blue 166
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Number of TCs in WNP (from JTWC)
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Standardized Number of Cat 4 5 Typhoons
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Variations of Landfall in Each Area at Various
Oscillation Periods
South China, Philippines and Vietnam
standardized anomalies
standardized anomalies
East China
standardized anomalies
standardized anomalies
Japan/Korea
standardized anomalies
standardized anomalies
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1977-88
Patterns of TC occurrence anomalies
TC occurrence anomalies
Pattern 1
1964-76
Pattern 2
Pattern 3
1989-97
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Summary

• Tropical cyclone activities (frequency, intensity
  and track) in the western North Pacific Ocean do
  not follow the trend of global warming.
• Instead, such activities oscillate with periods
  of one or more decades
• Such variations or oscillations are caused by
  similar variations in the atmosphere and/or the
  ocean

9
Adding a Custom Blue Color Value to FontsThe
color is the same for both the white and grey
templates
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a font,please do the following Highlight the
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Understanding (2)Some New Results
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Background

• Tropical cyclone formation and development
  depends on two sets of factors
• thermodynamic (heat energy and the
  conduciveness of the atmosphere to the
  development of strong convection)
• dynamic (wind flow and degree of rotation)

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Background

• Thermodynamic conditions
• ocean temperature
• energy available for convection
• atmospheric stability conduciveness of
  atmosphere to the development of strong convection

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Background

• Dynamic conditions
• extent of cyclonic rotation of the wind flow
• vertical wind shear (wind at 15 km minus that at
  1.5 km) strong shear will tear off the vertical
  integrity of the cyclone

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Background

• Global warming leads to
• an increase in the temperature near the earths
  surface (land and ocean)
• an increase in the amount of water vapour in the
  atmosphere due to an increase in ocean
  temperature
• No study has definitively demonstrated that the
  dynamic factors are modified by global warming
  (although some have suggested an increase in
  vertical wind shear).

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Background

• Due to global warming, the thermodynamic factors
  have become more favourable for tropical cyclone
  formation and development.
• To determine whether global warming has an impact
  on the frequency of occurrence of tropical
  cyclones or of intense cyclones, we need to
  examine whether the thermodynamic factors are
  related to the variations on such frequencies.
• A good proxy of the thermodynamic factors is the
  Maximum Potential Intensity (MPI)

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Background

• MPI f(ocean temperature, outflow temperature,
  net amount of energy available for convection)
• Because MPI gives the maximum possible intensity,
  a higher value of MPI summed over the ocean basin
  and over a season should imply a more
  thermodynamically energetic atmosphere, and more
  TCs could reach higher intensities

? a season with a higher value of MPI should have
more intense TCs if the dominant control is
thermodynamic
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MPI - Atlantic
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MPI Western North Pacific
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Correlations with MPI
21-year running correlations with NCat45
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21-year running correlations with NCat45
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21-year running correlations with NCat45
Atlantic
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21-year running correlations with NCat45 ENP
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21-year running correlations with NCat45 WNP
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Summary

• Thermodynamic control on the frequency of intense
  TCs is important only in the Atlantic
• Estimating the effect of global warming on the
  frequency of intense TCs therefore must also
  assess such an effect on the dynamic processes.

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Adding a Custom Blue Color Value to FontsThe
color is the same for both the white and grey
templates
Should you need to apply the custom blue color to
a font,please do the following Highlight the
text you want to colorize and then go to the
Format pull down menu to access the preferences
for Font. Once the Font window appears, select
the Color option and then choose More Colors
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Prediction (1)Traditional Method
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Statistical method

• Identify a list of variables relating to the
  atmospheric and oceanographic conditions prior to
  the season that significantly correlate with
  seasonal tropical cyclone activity
• Perform regressions to derive prediction equations

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Examples of Predictors used in the CityU Forecasts

• Index of the westward extent of the subtropical
  high over the western North Pacific
• Index of the strength of the India-Burma trough
  (15-20oN, 80-120oE)
• Primary mode of low-frequency variability over
  the North Pacific
• Sea surface temperature (SST) anomalies in the
  NINO3.4 region (5oS-5oN,170-120oW)
• Sea surface temperature (SST) anomalies in the
  NINO4 region (5oS-5oN, 160oE-150oW)
• Equatorial Southern Oscillation Index (Equatorial
  SOI)
• Equatorial Eastern Pacific SLP - Indonesia SLP
  (standardized anomalies)

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All tropical cyclones
Typhoons
Tropical storms and typhoons
Forecasts of Annual Tropical Cyclone Activity
over the western North Pacific (Deviations from
Observations)
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Adding a Custom Blue Color Value to FontsThe
color is the same for both the white and grey
templates
Should you need to apply the custom blue color to
a font,please do the following Highlight the
text you want to colorize and then go to the
Format pull down menu to access the preferences
for Font. Once the Font window appears, select
the Color option and then choose More Colors
Under the Custom tab you will see value areas
where you can type in the RGB values shown to the
right. Once finished, click the OK button. This
will apply the color to your highlighted type.
Prediction (2)Statistical Dynamical Method
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Statistical vs. Statistical-dynamical Methods

• Problem with the statistical method
• Relate the past events and future conditions by
  statistics
• Inherent problem
• assumes the future would behave the same as the
  past, which may not be correct
• Statistical-dynamical method partly solves the
  inherent problem by
• relating dynamical model predictions with future
  conditions

Dynamical atmospheric model
Predicted future conditions
Integrate over time
statistical prediction
TCs
Observations
statistical prediction
Time
several months
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Tracks of EC landfalling TCs 1980 2001, Aug
Sept
Subtropical High
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GC
Tracks of FL/GC landfalling TCs 1980 2001,Aug
Sept
Subtropical High
Subtropical High
FL
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Observed vs. PredictedEast Coast
Single model CERFACS
Multimodel
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Observed vs. PredictedGulf Coast
Single model LODYC
Multimodel
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Observed vs. PredictedFlorida
Single model LODYC
Multimodel
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Adding a Custom Blue Color Value to FontsThe
color is the same for both the white and grey
templates
Should you need to apply the custom blue color to
a font,please do the following Highlight the
text you want to colorize and then go to the
Format pull down menu to access the preferences
for Font. Once the Font window appears, select
the Color option and then choose More Colors
Under the Custom tab you will see value areas
where you can type in the RGB values shown to the
right. Once finished, click the OK button. This
will apply the color to your highlighted type.
Prediction (3)Regional Climate Model
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Dynamical method

• Run a global circulation model (GCM) with a
  relatively coarse resolution
• Solutions from the GCM are used as boundary
  conditions for a regional model with a higher
  resolution that can resolve a tropical cyclone
• Integrate the regional model to predict seasonal
  activity.

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Regional Model Simulations of 1997 and 1998 TCs
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Summary

• Statistical methods can provide some clues on
  tropical cyclone activity but suffers from an
  inherent problem of predicting future events
  based only on past conditions
• Statistical-dynamical methods can provide
  predictive information and therefore should give
  better results, but still suffers from the
  statistical nature of the method.
• Dynamical model forecasts should be the way
  forward to predict tropical cyclone risks
  although more research is still necessary on
  fine-tuning the regional model.

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http//www.cityu.edu.hk/gcacic/