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No successful tropical cyclone warning and response system can be finely tuned and objectively defined to the finest detail. The facts of life are that each tropical cyclone embedded in its broader environment exhibits a physical eccentricity and predictability generally unlike any of its predecessors, whilst the human and physical geography of the most threatened areas are constantly changing. Warning strategies thus have to be sufficiently flexible to enable forecasters, emergency services and affected communities to respond effectively under conditions in which there is not a precise model and the hazard potential is constantly changing. Unlike many other natural hazards, however, tropical cyclones do have a development life cycle and seasonal occurrence which assists in logical planning and in allocation of required resources. A solid effort devoted to assessment of demographic and economic vulnerability, and to the community response capacity, will prove to be a very good investment when a tropical cyclone occurs.
The warning and response system objectives are threefold: to alert communities of a potential danger, to identify areas likely to be affected, and to call the community to action by warning messages and recommendations of specific actions. The strategies adopted follow the principles in Section 7.2 by an evolutionary, persuasive process starting with pre-season education and awareness programs, to issue of early alerts of potential, but uncertain threats, and supported by increasingly detailed and urgent information as the cyclone approaches or develops, and finishing with review of the system. The major components of the warning and response system required to support these objectives are:
Tropical Cyclone Forecast: A scientific prediction of the future location, motion and intensity, and main parameters of a tropical cyclone. These predictions must be in a form useful for the derivation of warnings for the public and be capable of objective verification.
Tropical Cyclone Warning: An imperative tropical cyclone forecast that consists of several stages: Early Alert of potential threat more than 48 h away, especially for industrial and emergency service operations; Watch for a tropical cyclone that is likely to affect the community, but not within 24 h; and Warning of imminent onset of tropical cyclone conditions within 24 h. This graded system consists of messages compiled in meaningful laymen's language, which conveys the personal danger imposed by the destructive elements of an approaching cyclone, first raising the community awareness of the threat, then inviting and finally demanding active response commensurate with the urgency and severity of the threat.
Utilisation: Adaptation of warnings by recipients into required preparedness measures and announcements in accord with contingency plans for the threatened areas.
Emergency Response. Implementation of community emergency procedures under the coordination of the relevant counter-disaster organisations.
System Evaluation. Review and evaluation of the total system to identify deficiencies and effect improvements in both warning and response functions.
Figure 5: Warning and response system effectiveness by combination of the effectiveness of each component.
The warning and response system thus consists of several interrelated components, all of which must function effectively and in a complementary manner for effective operation of the whole system. Perfect warnings are of little use if no response organisation exists. An objective approach (Fig. 5) is to take the effectiveness of implementation of the warning inputs and of the community response organisation on a scale of 1-10, then multiply these to arrive at the percentage effectiveness of the overall warning-response system. Thus an excellent warning effectiveness of 9 with a poor community response of 1 would result in a poor 9% effectiveness of the overall system. A very satisfactory result and overall practical objective would be to achieve a 9 for each component giving an 81% measure of effectiveness. This requires a co-ordinated approach, which includes the following essential ingredients:
As has been discussed in Chapter 6, effective warning-response strategies cover all aspects from pre-season review and preparation to post-event evaluation. From the perspective of a forecaster, ten major phases may be identified.
(i) The Pre-Season Check Phase: All aspects of the system are checked and reviewed, including: observational and communication equipment, operational procedures, emergency service organisation contacts and revised procedures. Available techniques are updated and forecasters review their own methodologies. Community awareness campaigns are conducted in conjunction with counter-disaster authorities.
(ii) Routine Monitoring Phase: Continuous surveillance by a national meteorological or tropical cyclone warning centre using twice-daily (or more frequent as necessary) scientifically-based procedural checks for signs of potential cyclone activity.
(iii) Cyclone Information Phase: Media and preparedness authorities are advised that a cyclone has formed near or within the area of warning responsibility but is not forecast to cause dangerous conditions within a specified time, often 48 h. The information is contained in low-key statements issued once or twice daily with the aim of arousing initial interest and creating a climate of expectancy should the system move to the next phase. Very specific information may be passed to highly vulnerable areas such as offshore oil operations and fishing fleets.
(iv) Cyclone Watch or Alert Phase: The alert phase commences when an existing or potential cyclone poses a threat within 36-48 h, but not within 24-36 h. The objective is to build public awareness of the increasing threat without making definitive predictions that are beyond the forecast system capacity at this early stage. The frequency of warning advices is increased, generally to 6 h, but provided only in general terms, for example, potential landfall along extended coastal sectors several times the lateral dimensions of gale or storm force winds and which incorporate the degree of uncertainty in the forecast. This phase may activate costly public counter-disaster plans, such as the setting up of emergency operations centres and initial deployment of resources. Preparedness recommendations include return of fishing craft to home ports and preliminary precautions by residents.
(v) Cyclone Warning Phase: This is initiated when cyclonic conditions, in the form of gale-force winds, are expected within at least 24 h at a vulnerable location (some regions may require longer lead times). It is usual at the beginning of this phase to have to place at least 600-800 km length of coastline under expectancy of destructive winds, with perhaps another 200 km remaining under a watch stage at the peripheries. For a cyclone approaching at an acute angle, the warning/watch region may be substantially larger. As a general strategy discrete coastal sectors between named well-known locations should be identified in warning statements. This is the highest level of operation of the warning system and is accompanied by significant cost impact for all concerned parties. Disaster operations centres are staffed on a 24-h basis to implement contingency action plans, which will often include evacuation starting from the more exposed areas. The community is expected to make immediate arrangements for its safety and security and businesses and industry commence shutdown procedures.
The frequency of warnings is progressively increased to 3 h. When practical, such as with radar tracking, abbreviated hourly warnings may be issued to media. The warning system is fully extended to enable rapid response to changes in the cyclone motion and structure. In accord with warning strategy principles in Section 7.2, additional persuasive information is martialled and broadcast to heighten the sense of urgency in the community and to hasten public response in the decreasing safe lead time available. As the landfall lead time shortens more specific information on the destructive power of the cyclone and general warning of abnormal tides (storm surge) are issued, and attention is drawn to unusual features about the storm, such as unusual intensity or speed of approach.
(vi) Imminent-Landfall Phase: At this advanced stage of the warning phase gale or stronger force winds are imminent or have commenced along with heavy rain, rough seas and increasing tides, and the community should be already sheltering in expectation of landfall of the cyclone within 6-8 h. Well-equipped emergency services personnel checking on the safety of people in the most vulnerable areas and the continuing operation of community lifeline facilities should be among the few persons not sheltering inside safe refuges.
Broadcast warnings contain highly pertinently information concerning the impact of the cyclone in the most vulnerable areas. The expected landfall region for the destructive cyclone core should be detailed to within an accuracy of 50-100 km, but it is essential to stress the asymmetric extent of destructive winds and rainfall to ensure communities do not concentrate unnecessarily on the cyclone centre. Preliminary flood warnings may be issued for coastal catchments and river basins. During this phase when increasing gales are being experienced, monitoring and prediction focus on nowcasting. Predictions are usually given on the basis of persistence and current surface synoptic conditions.
Because the track and structure of a cyclone may change in the h approaching landfall, Simpson (1971) advocated last-minute warnings on a "course of least regret". This assumes that the most serious impact will occur close to the region or towns of highest vulnerability. If these locations suffer lesser damage then so much the better. If the situation is marginal then the higher rated cyclone category or public warning signal number should be quoted in warnings. For the same reasons immediate pre-landfall announcements that the cyclone is weakening should generally be avoided, especially as serious misinterpretations may be made with disastrous consequences.
It is recommended that the highest priority, Flash Cyclone Warning prefix be used for the first urgent advice of destructive cyclonic effects within 24-36 h in areas not previously alerted. Flash warnings also should be used to indicate sudden changes in the cyclone track or intensity, which could seriously effect vulnerable communities.
(vii) Post-Landfall Phase. The warning advices continue at 3-h intervals for about 12 h after landfall, advising communities of the location of landfall, the subsequent track of the cyclone and rate of weakening, and potential developments, such as movement back offshore. The emphasis now centres on the issue of warnings for rapid riverine and flash floods, and for tornados, where necessary.
A Final Cyclone Warning should be issued, in consultation with counter-disaster authorities, to indicate the passage of the cyclone threat and recommend a gradual resumption of normal community activities, and deployment of medical and engineering crews to badly hit regions.
(viii) Impact Assessment Phase. This phase is entered as soon as a cyclone has dissipated, or passed into another area of warning responsibility. Meteorological officers, accompanied by disaster personnel, visit the affected areas to gather relevant observational and hazard-related data and discuss warning-response performance with people and officials.
(ix) Documentation Phase. This covers the period from genesis of a cyclone through the sequential warning phases until dissipation or passage of the cyclone from an area of warning responsibility. All relevant information about the cyclone including copies of warnings, press items and damage photographs, criticisms, and a chronology of events, is collected in a substantial case history file for reporting and archival. Selected data are extracted for storage in computer compatible form to facilitate enquiries and research. Supervising meteorologists should keep diaries for strategic planning purposes.
A summary of each season's tropical cyclone activity is prepared annually. Special reports on major disaster-impact cyclones are given wide distribution to interested parties.
(x) System Review Phase. At the end of the cyclone season, a review is conducted of the total performance of the warning and response system. This usually culminates in annual review conferences covering internal procedures as well as external liaison with counter-disaster authorities. As noted earlier, each new cyclone brings unique experience in respect of warning and response aspects which may need to be incorporated into warning procedures and or preparedness contingency plans.
Major innovations involving the community at large may need to be tested for their feasibility before final adoption into national procedures. In some cases, such as when a cyclone has not impacted a country or coastal sector for many years and there has been a substantial breakdown in the warning and response process, the opportunity should be taken for a thorough appraisal of the system.
1. Defined here as "Essential community facilities and services that are vital for the effective response and rapid recovery of a community threatened or impacted by a severe natural hazard".
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