Cyclone Analysis

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.

Typhoon Trigger

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 year.

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.

typhoon hagupit

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.

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 Wikipedia)

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 (event dissolution).

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 20 years.

In order to accurately assess the rainfalls and the winds associated with the storm events, different datasets were used:

Precipitation data

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.

trmm satellite

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 trigger.

Rainfall Trigger

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.

Wind data

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 every 3hours.

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 Philippines.

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.

Wind Trigger

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 value.

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.

trmm satellite

Figure 4: Trigger evaluation for rain exposure

trmm central estimate

Figure 5: Trigger based upon central estimate of 20-year return period.