M Odelling This Article Appeared in the East Asia Special 2001 Edition of Global Reinsurance
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M odelling This article appeared in the East Asia Special 2001 edition of Global Reinsurance. Improving typhoon predictions Mark Saunders and Paul Rockett describe techniques to better estimate future storm activity. ropical cyclones are the most costly cause the vast majority of the damage and industry and, understandably, confidence and deadly type of natural disaster death from tropical cyclones in east Asia. in their quality needs building before Taffecting much of Japan, South The 1991-2000 period has seen an annual insurance and reinsurance executives Korea, Taiwan, the Philippines and coastal average of 9.1 intense typhoons and 17.8 employ them routinely in business areas in other east Asian countries. The typhoons in the northwest Pacific basin, and decisions. The purpose of this article is to annual damage bill and fatality rate impacts an annual average typhoon strike rate on describe the typhoon predictions now in east Asia 1990-1999 averages respectively Japan, Taiwan and the Philippines of 2.4, available, their methodologies and skill, US$3.3bn and 740 deaths. However, 1.7 and 1.9 respectively. Already the 2001 and the prospects for further substantial inter-annual variability exists in season has seen several typhoons make improvements in the coming years. regional tropical cyclone losses. For landfall on east Asian shores (figure 1). 2001 has been a busy year for tropical example, in 1991 and 1988 Japan Skilful typhoon predictions will benefit storm landfalls in east Asia. This GMS experienced losses of $7.5bn and zero insurers and reinsurers, as well as society, satellite image (Figure 1) shows typhoon respectively, measured in 2000 dollars. government and other businesses by Utor striking Hong Kong and southern Typhoons, described as one minute of reducing the risk, uncertainty and financial China on 5 July, 2001. Utor killed at least sustained winds of over 73mph (63 knots), volatility inherent to varying active and 23 in China, 121 in the Philippines and and intense typhoons one minute of inactive tropical storm seasons. Predictions one person in Taiwan. It was one of the sustained winds of over 110mph (95 knots), are a relatively new resource for the strongest storms to strike the region in www.globalreinsurance.com statistical analysis of the past behaviour of probabilistic in nature. They are derived storms (statistical forecasts) or from an from a statistical assessment of the major application of our knowledge of the climatic factors affecting tropical cyclone physics of the atmosphere and ocean activity in each region. These factors may (dynamical forecasts). be either contemporaneous or lagged. Lagged factors are climatic conditions Seasonal forecasts prevailing at the time of the forecast that Seasonal forecasts provide an idea of the are known to be precursors of typhoon level of typhoon activity to anticipate activity in the coming season. during the coming season. In the Contemporaneous factors are climatic northwest Pacific, typhoons can occur in conditions that prevail during the main all months of the year, but historically 90% typhoon season itself. In order for of activity happens after 1 June each year, contemporaneous factors to be useful in making this, for practical purposes, the seasonal typhoon forecasts they must be start of the main typhoon season. sufficiently predictable themselves at the A number of organisations now issue necessary lead times. ENSO (El Niño- seasonal forecasts for the Atlantic, Southern Oscillation) is an example of a northwest Pacific and Australian basin contemporaneous predictor useful in regions. Northwest Pacific seasonal typhoon seasonal forecasts. High (low) typhoon forecasts are issued by the typhoon activity is observed when ENSO University of Hong Kong (see is in its positive (negative) phase during the http://aposf02.cityu.edu.hk/~mcg/index.ht main typhoon season from July to m) and by the TropicalStormRisk.com October. (TSR) venture (see http://www.tropicalstormrisk.com). These Intensity forecasts forecasts began routinely in 2000. The Accurate typhoon forecasts not only save University of Hong Kong predictions are lives, but can be used as a risk issued in early April, and are updated in quantification tool for insurers and June. The TSR forecasts are issued in late reinsurers once a storm has formed. If the May/early June, and for the 2002 season risk of landfall can be quantified in near will be available updated monthly from real-time, enterprising executives can use early April to early August. These forecasts loss estimates to optimise capital, organise provide an indication of the numbers of claims response units and even trade tropical storms of various intensities that catastrophe bonds. can be expected in the season ahead. When a tropical cyclone is active, While it is impossible to give forecasts of forecasts of the storm position and precise positions of landfall at such long maximum winds at 12, 24, 36, 48, 72, 96 range, predictions can be made of the risk and even 120 hours ahead are issued every to broad regions, such as Japan or the south six hours by the meteorological agency China coast. These forecasts are often with warning responsibility in the region of Figure 1: Typhoon Utor striking Hong Kong recent years with flood and wind damage in the southern Chinese province of Guangdong alone estimated at US$290m. Utor also caused most of Hong Kong to shut down for business for two days. (Image courtesy of Tim Olander, University of Wisconsin-Madison Cooperative Institute for Meteorological Satellite Studies, US.) Methodologies Typhoon predictions come in two types: seasonal forecasts of overall activity for the coming season, and track and intensity forecasts for individual events with up to five days lead time. Forecast models are of Source: authors’ own two main forms, based either on a www.globalreinsurance.com “ The only near certainty in forecasting is that the forecast will be wrong.” the storm. For the northwest Pacific, the two main sources are the Japan Meteorological Agency (http://ddb.kishou.go.jp/typhoon/cyclone/c yclone.html) and the US Navy and Army Joint Typhoon Warning Center (JTWC) on Guam the atmosphere by a set of mathematical a result of the well-known ‘Butterfly (http://www.npmoc.navy.mil/jtwc.html). equations, the parameters of which are Effect’. Attempts have been made to use Forecasters at these centres consider calculated. dynamical models to investigate future independent predictions made by a Dynamical models can be applied to the changes in typhoon activity due to global number of dynamical and statistical whole atmosphere (global models) or to warming, but to date the results have prediction models produced both in-house smaller local areas (local models). Despite proved inconclusive. and by other organisations. The JTWC, recent advances in model resolution for example, takes guidance from the UK (current gridded global dynamical models Statistical models Meteorological Office and the NOAA typically have a resolution of about one Statistical methods are used in all types of Geophysical Fluid Dynamics laboratory degree in latitude and longitude with typhoon predictions: seasonal, track and amongst others. around 20 levels through the atmosphere), intensity forecasts. In all cases, analyses of global models are still too coarse to past data are made to quantify the Dynamical models represent the important physical processes probability of an event occurring, given Dynamical models use state-of-the-art within a typhoon. In some global models certain conditions. Linear regression computing techniques to apply our this is overcome by using a ‘bogussing’ analysis is applied to past data to identify knowledge of the physics of the scheme. Under such a scheme, once a links between the desired forecast quantity atmosphere and ocean to forecast what suspect tropical cyclone is identified and potential predictors. Predictors which may occur ahead, knowing its initial state. artificial data is introduced into the model have a strong statistical link to the forecast To achieve this, the atmospheric system to better represent the storm. quantity are incorporated into a linear must be simplified sufficiently to allow the Dynamical models have good uses in prediction model. In the case of seasonal model runs to be completed in time to many operational track forecasting forecasting, the predictors are climatic make a useful forecast. This is done either situations. They are not employed factors which strongly influence tropical by calculating the atmospheric variables generally for seasonal predictions, since cyclone frequency in the regions of (temperature, humidity, windspeed, etc.) at even tiny errors in the initial state of the interest. certain grid points only, or by representing model can become massively amplified The advantage of statistical methods is the distribution of these variables through when forecasting at long lead times. This is that they are easier and quicker to run than dynamical models. In the case of seasonal predictions, they are more useful than Figure 2: typhoon seasonal prediction skill dynamical models because they are free of the large amount of noise that results from the Butterfly Effect inherent in these models. On the downside, since they are based on (often limited) past data, they are likely to have little skill in situations not previously encountered (under changed climate conditions, for example). Dynamical models are not susceptible to this problem. Dynamical track and intensity