Tropical cyclones are one of the major threats for lifes and
livelihoods particularly in the tropical coastal regions. Each
year, they cause tremendous damage and are estimated to be
responsible for the death of approximately 10,000 people. Besides
producing extremely powerful winds and torrential rain, tropical
cyclones are also able to cause high waves and damaging storm
surges as well as spawning tornadoes.
Climate change is expected to increase the problem: The NOAA
(U.S. National Oceanic and Atmospheric Administration Geophysical
Fluid Dynamics Laboratory) states that "the strongest hurricanes in
the present climate may be upstaged by even more intense hurricanes
over the next century as the earth's climate is warmed by
increasing levels of greenhouse gases in the atmosphere".
The societal impacts of tropical cyclones are numerous and easy
to identify. However, it is often difficult to evaluate them
quantitatively and/or qualitatively.
The "Typhoon Trigger", developed by DHI for the Philippines,
provides decision support in connection with damage assessment
following tropical cyclones. After all, the Philippines and its
surrounding areas record 20 tropical cyclones on average per
The overall aim of the Typhoon Trigger is to provide a near
real-time classification of exposure to actual typhoons, during or
immediately after the event, in order to classify it.
Classification as an "extreme" event will then e.g. result in a
pay-out by an insurance company.
The Typhoon Trigger covers the Philippines at municipal level
based on the probability of both rain and wind exposures during a
typhoon event. The main challenge during development of the project
was to integrate different kinds of information to form a coherent
picture of the typhoon hazard in the Philippines. Required data
include a qualitative and quantitative description of historical
events with emphasis on the exposure (wind and rainfall) as well as
data describing the vulnerability of the municipalities towards
typhoons, taking the local economy and the level of adaptation to
extreme events into account.
Typhoons and the Philippines
A typhoon is a severe tropical cyclone formed in the western
North Pacific. At maturity it is one of the most intense and feared
storms of the world with ranges from 100 to 1000km. The wind field
pattern is that of a circularly symmetric spiral added to a
straight current in the direction of propagation of the cyclone.
The cloud and rain patterns vary from storm to storm, but in
general there are spiral bands in the outer vortex, while the most
intense rain and winds occur in the eyewall, the circle of strong
thunderstorms that surrounds the eye.
Figure 1: Typhoon Hagupit, September 2008 (Source: NOAA)
Within the northwestern Pacific there are no official typhoon
seasons as tropical cyclones form throughout the year. Like any
tropical cyclone, there are six main requirements for typhoon
formation and development: sufficiently warm sea surface
temperatures, atmospheric instability, high humidity in the lower
to middle levels of the troposphere, enough Coriolis force to
develop a low pressure center, a pre-existing low level focus or
disturbance, and low vertical wind shear. Typhoons development over
large bodies of warm water and lose strength if they move over
land. Their average lifetime is around four to five days from
formation to dissipation.
Nearly one-third of the world's tropical cyclones form within
the western Pacific. Therefore, the area just northeast of the
Philippines is the most active place on Earth for the development
of tropical cyclones.
Figure 2: Tracks of all tropical cyclones in the Northwest Pacific
Ocean from 1980 to 2005. The points show the locations of the
storms at six-hourly intervals. (Source: Nilfanion on
The Philippines and its surrounding seas, the Philippines Area
of Responsibility (PAR), record 20 tropical cyclones per year on
average. While many typhoons follow a west-northwestward track
around and across the Philippines, due to the mountain ranges,
typhoons crossing the Philippines can have trajectories and
development dynamics particularly complex when compared with
offshore typhoons and typhoons passing plain areas.
Establishment of the trigger
The Typhoon Trigger recognizes a typhoon event from the moment a
tropical depression hits the PAR with maximum wind speeds superior
to 30 knots. Damages associated with the event are assessed from
the time the typhoon enters the PAR until the typhoon leaves the
PAR or until the maximum sustained wind speed drops below 30 knots
Wind and rainfall were identified as the two major causes of
loss and threshold values - so called triggers - set up for these
two parameters. That enables DHI to categorize the severity of a
weather event into a 10-, 15- or 20-year event for each
municipality. In this project, an extreme event (the trigger value)
is selected as a typhoon event having a return period larger than
In order to accurately assess the rainfalls and the winds
associated with the storm events, different datasets were used:
PAGASA Rain Gauge
Data: The PAGASA operates several networks of meteorological
stations around the Philippines. Each measuring station includes a
simple but robust rain gauge, collecting and measuring rainfall
with an accuracy in the order of millimetres. Data made available
to this study were limited to daily rainfall (mm/day). Spatial as
well as temporal interpolation techniques facilitate the
representation of station (point) data to complete area coverage.
Station data is important to validate other applied data since
these are considered very accurate for localized measurements.
TRMM Remote Sensor
Data: The TRMM is a joint mission between NASA and the Japan
Aerospace Exploration Agency (JAXA), intended to monitor rainfall
in the tropics.
The TRMM Multi-satellite Precipitation Analysis (TMPA) uses four
different satellites, each one using a different kind of sensor for
precipitation measurement. Data are available around 6 hours after
their acquisition and on a 3 hours basis, which enables an analysis
of the high rain events within a few hours of the actual event.
TMPA data are available in grids of 0.25°, which are approximately
squares of 27x27 km. Hence, the Philippines are covered by 852 data
squares, each containing the level of rain for this area.
Figure 3: TRMM satellite coverage, dated 25 May 2004 (Source:
Journal of Hydrometeorology, Huffman et al., 2007)
The use of TRMM data enables dense coverage of the Philippines,
both in time and space. After quality control operations, TRMM data
are regarded as a reliable qualitative and totally independent data
source useful to the hazard analysis and the development of the
In order to evaluate the rain trigger, extreme value statistics
are estimated from the 24hr TRMM rainfall data for each
municipality. Combining the maximum 24 hour rainfall to the
calculated 20-year extreme rain, the event return period is
calculated for each of the municipalities. Finally, checking each
of the municipalities against the trigger criteria, the triggered
municipalities are sought out.
Tropical cyclone information including analysis, data provision
and forecasting of tropical cyclones is provided by the RSMC in
Tokyo. RSMC is operated by JMA (Japan Meteorological Agency) within
the framework of the World Weather Watch program of WMO. The RSMC
provides two main categories of data, Tropical Cyclone (TC)
analyses and TC forecasts.
From the moment a tropical cyclone of tropical storm intensity
is present in the RSMC's area of coverage, data information on the
TC main characteristics from satellite imagery (ex. MTSAT,
QUIKSCAT) and surface observations by in situ sensors are provided
After a tropical cyclone dissipates, warning centres conduct
re-analysis with all available data producing "best track data"
which are the official records of tropical cyclones and the most
accurate and reliable data regarding typhoon characteristics in the
TC best track data have been then used as the basis to derive
wind speed values, classify municipalities' exposure and develop
the wind trigger. However, TC best track data are not available in
short-term period. Therefore, during operation TC analyses could be
used to feed the real-time trigger system.
Similar to the rainfall trigger, an extreme value analysis is
performed on wind speed, providing the 20-year return period wind
for all municipalities using the TC best track data as input. Then,
for each event, the maximum sustained wind speed is calculated
overlapping the calculated wind fields with the geographical extent
of each municipality and compared against the trigger threshold
At the end, the trigger for a "20 year Typhoon event", based on
the available data, is calculated by adding the probabilities from
a 20-year extreme rain and a 20-year wind extreme event.
Figure 4: Trigger evaluation for rain exposure
Figure 5: Trigger based upon central estimate of 20-year return