CDMP/CIMH Storm Hazard Atlas Introductory Text

Note: The following text was included as the introduction for each of the eastern Caribbean storm hazard atlases produced by the Caribbean Institute for Meteorology and Hydrology (CIMH) in the fall of 1999. Further information about this activity is available in the CDMP progress bulletin entitled "National-level storm hazard mapping".

Introduction

In the Caribbean tropical storms and hurricanes are annual occurrences. Although little, if anything, can be done to change the character or frequency of these events, the damage and loss of life that they cause can be significantly reduced through changes in the location and the quality of design of construction, and through appropriate contingency planning for emergency events. Damaging winds, coastal storm surges, heavy rains and waves can each inflict severe damage during a tropical storm or hurricane, affecting different locations or combining to significantly increase damages in a highly vulnerable site. Prior hazard awareness and risk assessment practices must be employed well before any immediate threat in order to reduce the impacts of these hazards.

With appropriate information on storm surge and wind hazard risks in the Caribbean region, emergency managers, development planners and meteorologists can better prepare for and respond to destructive storms. Appropriate and cost-effective vulnerability reduction measures - such as building setbacks, higher building standards and construction of protective structures - can be better identified and implemented using accurate information on hurricane hazard risks.

In September 1993, the Unit of Sustainable Development and Environment of the OAS started implementing the Caribbean Disaster Mitigation Project (CDMP), a six year technical cooperation project funded by the US Office of Foreign Disaster Assistance of the US Agency for International Development. One of the objectives of the CDMP is to improve the understanding and management of the risk posed by tropical storms and hurricanes to population, housing, infrastructure, and economic activity in the Caribbean. To this end, CDMP supported the development of a storm hazard model, developed by Charles Watson, called The Arbiter Of Storms (TAOS), for assessing the impact of storm surge and wave action on coastal areas throughout the region. The first version of the model was installed at the Caribbean Institute for Meteorology and Hydrology (CIMH) in December 1994 and based on CIMH’s experiences, its capabilities and ease of use have been enhanced.

Within the CDMP, TAOS has been used to map storm-related hazards in numerous hazard assessments. A 1995 study of Parham Harbour, Antigua, included modelling of storm surge and inland flood hazards. In 1996, TAOS was used to assess coastal surge and inland wind hazards in Belize, and, as part of a CDB-funded coastal rehabilitation project, to assess storm surge and wave effects on the west coast of Dominica. At CIMH, TAOS has been used to assess wind, wave and surges for at least two real-time cases – Hurricane Luis in September 1995 and Hurricane Georges in September 1998. In 1998 a pilot project to develop maps of Maximum Envelope of Water (MEOW) for the islands of Antigua and Barbuda was undertaken by CIMH. In December 1998 CDMP sponsored a national workshop in Antigua to discuss these maps and to develop guidelines for the use of the maps in development and emergency planning. As a result of the success of the Antigua pilot project, the programme was extended to cover the English speaking countries in the Eastern Caribbean. This Atlas presents the results of the work of this project.

Tropical Cyclones - Definitions, Characteristics and Forecasting

A tropical cyclone is defined as an area of low pressure which develops over tropical or subtropical waters. These systems form over all tropical oceans with the exception of the South Atlantic and the eastern South Pacific east of about 140oW. In their most intense state these storms are called hurricanes in the Atlantic, typhoons in the western North Pacific and cyclones in the Bay of Bengal. These low pressure systems draw their energy from the very warm sea-surface waters. As the warm, moist air spirals counterclockwise in toward the centre, the wind speeds increase, reaching their maximum values in the region surrounding the almost calm centre of the cyclone.

Over the Atlantic a tropical cyclone is classified according to its intensity as a tropical depression, tropical storm, or hurricane, defined as follows:

The Saffir-Simpson Hurricane Scale (Table 1) is used to give an estimate of the potential property damage along the coast from a hurricane landfall.

In the North Atlantic Ocean, including the Caribbean Sea and the Gulf of Mexico, the official hurricane season is June through November. On average, nine tropical cyclones of at least tropical storm strength form in this region each year of which six reach hurricane intensity. Although tropical cyclones occasionally form as early as May, the most active months are August and September.

Tropical cyclones which develop in the deep tropics initially move towards the west or west northwest before recurving and moving towards the east. Recurvture generally occurs in the western and northern parts of the Atlantic Basin.

Hurricanes vary in size from 300 to 1500 km (180 miles to 900 miles) in diameter. The mature system is characterised by a central portion called the ‘eye’ which is usually enclosed by a circular mass of cloud referred to as the eye wall or wall cloud. The outer portion of the hurricane is comprised of a number of bands of cloud which spiral into the centre and are referred to as spiral bands or rainbands.

Hurricane force winds generally extend out about 100 km (60 miles) from the centre and storm force winds may extend out as much as 500 km (300 miles). The maximum wind speeds are usually found between 20 and 50 km (12 and 30 miles) from the centre, depending upon the intensity of the system. The eye of the hurricane is characterised by clear skies, calm winds, and the lowest pressure. On average the eye is about 20 km (12 miles) in diameter and the eyewall between 30 and 50 km (20 to 30 miles) in width. The spiral bands also contain regions of intense thunderstorms and strong gusty winds away from the core of the system.

Forecasting the future location and intensity of tropical cyclones is considered to be one of the most important functions of tropical cyclone warning centres. One reason for the importance is the potential damage and loss of life which can occur with the passage of an intense tropical cyclone. Early and precise warnings do not necessarily remove the risk of damage or loss or life, but the effects may be significantly reduced.

Over the years forecast methods have moved from simple subjective deductions based on observations of specific parameters such as cloud types and motions, sea swells, and pressure, to more sophisticated objective techniques using complex computer models of the atmosphere.

A tropical cyclone forecast involves the prediction of several interrelated features, including the track, winds, rainfall, storm surge and, of course, the areas threatened. In practice, the National Hurricane Center (NHC) in Miami, which is responsible for tropical cyclone forecasting in this area, issues official forecasts which are based on output form the various models, present and historical performance of the models and personal experience.

Unfortunately, like other forecasts, tropical cyclone forecasts are not perfect. Errors arise from a lack of a full understanding of the formation and growth of tropical cyclones and from the limitations of the forecasting techniques themselves. It is important that all users of the forecasts, whether they are meteorologists or disaster planners, be cognisant of the limitations of the information provided in the various forecasts.

Tropical cyclone forecast errors increase remarkably with increasing time, but are smaller for lower latitude cyclones moving westward than for the higher latitude systems in the westerlies and for those which are recurving.

Errors in forecast, although showing a slow and steady decrease, are still substantially large. The National Hurricane Center's official track average forecast errors during the 10-year period 1986 to 1995 ranged from approximately 90 km for the 12-hour forecasts to 500 km for 72-hour forecasts. The official intensity errors during the 5-year period 1990 to 1994 for similar forecast hours range from approximately 8 to 20 knots.

Storm Surge

Storm surge is the increase in water levels caused by a tropical storm. Wind blowing across a stretch of open water creates waves on the surface of the water. As long as the wind-produced waves remain in deep water or some distance from the coast line, there is little mass transport of water along the wave front. As the tropical cyclone approaches land, several factors combine to cause the sudden rise in the level of the sea - the storm surge.

The height of the storm surge depends on the complex interaction of several factors including the wind field, the pressure anomaly, the size and speed of motion of the system, the bottom topography near the storm's landfall point, and the astronomical tides. The most important factor, however, in determining the maximum storm surge heights is the maximum wind speed which in turn is closely related to the minimum sea-level pressure. Depending upon a site’s location relative to the storm track, maximum storm surge can precede, coincide with or follow the arrival of the maximum winds generated by the storm. Storm surges may be as little as 1 m or less if only a few of the factors are making their maximum contribution, and 5 m or more if all the factors are making the maximum possible contribution to the total deviation of the sea surface.

Storm Surge is usually understood to be made up of a pressure setup, a wind setup, a wave setup and the astronomical tide.

• Astronomical tide is the increase due to lunar and solar tides. Astronomical tide effects are typically small in the Caribbean.
• Pressure setup is the increase in water level due to the lower atmospheric pressure in the interior of a storm.
• Wind setup is the increase in water level due to the force of the wind on the water. The wind causes currents in the surface layers of the water. This is a factor in shallow areas, where water levels will rise when the water pushed by the winds cannot flow out of the area as quickly as it arrives.
• Wave setup is the increase in still water levels resulting from mass transport by breaking waves.

The area where the inertia of breaking waves carries water up a beach is referred to as the wave runup.

The TAOS Model

TAOS is a computer based numerical model that produces estimates of maximum sustained winds at the surface, and still water surge heights at the coastline and wave heights in deep water. Model runs can be made for any historical storm, for probable maximum events associated with different return periods, or using real-time tropical storm forecasts from the US National Hurricane Center (NHC). The model consists of three basic components: the input data set, the processing modules, and the output data sets.

The TAOS model is designed to read from and write to standard Geographic Information System data formats as input data sets. The required input data consist of:

• Bathymetric data for the ocean and topographic data for the land ;
• Storm track or wind field data including the storm location, wind speed, radius of maximum wind or the central pressure; and
• Characteristics the underlying surface for computing the effects of friction associated with wind and water.

The model consists of processing modules which compute wind, surge and waves. Although each module has its own user selected components, the results from one module may be used in another module. The wind module computes the wind at 5 metres above the surface. The wave module uses a simple parametric wave model to compute the height of waves generated by the wind in deep water. A water flow module is used to compute the surge heights and related effects. The tidal effect is included in the surge. The model can output results of individual storm tracks or for maximum envelope of water/wind (MEOWs) produced from runs of multiple storm tracks.

Verification of TAOS predictions against actual storm measurements shows that over 95 percent of the peak surges predicted by the model have an error of less than 15 per cent when compared to measured or observed values. Over half of the predictions were within 3.5 percent of the actual peak values. These validation results compare favourably with those of other currently accepted surge models. The model generates results within 0.3 meters (less than 1 foot) 80% of the time, and less than 0.6 meters (about 2 feet) 90% of the time.

MEOWs

Individual storm model runs are useful to understand the effects of historical storms or active storms, using the current forecast track. Individual storm runs can also be used to generate realistic scenarios for emergency management exercises. A storm’s track is a significant determinant of a hurricane’s storm surge and wind effects on a specific area. Since storm tracks are difficult to predict more than a day before landfall, individual storm runs are insufficient for long-term planning purposes, whether for development control or for emergency management. Maximum envelopes of water/wind (MEOWs) are more useful for long term planning.

A MEOW depicts the maximum water level (or wind speed) produced by a storm of a chosen intensity, forward speed, and track. MEOWs are produced by running the model for multiple storm tracks, spaced a fixed distance apart, for a selected intensity, speed, and direction. The results of the individual runs are then combined, into a single map showing, for each point on the map, the maximum value generated across all of the individual model runs. MEOWs thus provide an easily accessible summary of the "worst case" scenario given the uncertainty in a tropical cyclone forecast.

Atlas - Outline and Interpretation

This Atlas contains MEOWs of various combinations of storm direction, forward speed, and intensity for the islands of the English-speaking Eastern Caribbean. The tropical cyclone climatology of the region was examined to determine the most suitable tracks and speeds, while the Saffir-Simpson scale was used to classify the intensities.

Tracks varying between 270º and 330º, at intervals of 20º, were chosen for this Atlas. Climatology indicates that tropical cyclone tracks over the region tend to vary between 270º and 300º, but to take into account the occasional more northerly motion that is likely to occur over the northern part of the region, tracks were extended to include 310º and 330º. Although forward speeds over the region generally vary between 10 and 15 knots, the value of 12 knots was used for the study because preliminary tests indicated that the surges did not vary significantly with change of speed. A lateral extent of the track of 180 km on either side of the specific location was used. This distance was chosen as representative of the typical horizontal extent of hurricane force winds in a tropical cyclone. Wind speeds used for each category were chosen approximately mid-way in the range of speeds for the specific category. The radius of maximum winds varied with category, with smaller values for the higher categories. (See Table 2)

The coast outline for each country is obtained from the base topography/bathymetric data of the TAOS model. The format of this data accounts for the step-like nature of the outlines.

Individual MEOWs are presented as colour-shaded maps of storm surge heights above mean sea-level at intervals of 0.25 m up to 2 m and at intervals of 0.5 m for higher values. Each value represents the maximum surge likely to be experienced at the particular location under the indicated tropical cyclone conditions. The surge height represents the value over some finite distance determined by the model, which is approximately 1 km. Therefore, the maps should not be used to determine the impact of the surges for any smaller distance along the coast. In addition, values of the surge at any specific location should be considered in relation to the surrounding values and not to the value at that location alone. In other words, the important factor is the typical value in a given area.

It is impossible to account for all possible eventualities. This atlas should be regarded as a good starting point to isolate the potential surge whenever a tropical cyclone is forecast to affect a given coastal region. After a study of potential surges with idealised tracks from the atlas, runs can be made to fine tune a surge forecast, that includes relevant details from the actual cyclone.