Session on Disaster Risk Management. An Introduction to False Tornado Warnings in Canada

Transcription

1 Session on Disaster Risk Management An Introduction to False Tornado Warnings in Canada Dr. S.C. Wirasinghe Ms. Samanthi W. Durage Dr. Janaka Ruwanpura Department of Civil Engineering, Schulich School of Engineering University of Calgary Canada

2 Background T O R N A D O E S

3 What is a Tornado A violently rotating column of air that extends from a thunderstorm cloud to the earth s surface Usually occurs where cold fronts clash with warm fronts Can move at speeds of km/hour Localized and short-lived phenomenon Usually associated with black skies, strong winds, lightning, thunder and heavy rain or hail Has a high potential to create enormous damage to property and large number of deaths and injuries

4 Worldwide Occurrence of Tornadoes Source:(Goliger and Milford,1998)

5 Tornado Distribution in North America Source: (Grosvenor et al, 1998)

6 Tornado Contour Map in Canada Two main clusters of tornado-prone regions Approximately 36 tornadoes are reported on annually on average in the prairie region Disastrous impacts on highly populated, industrialized or agricultural areas Source: Natural Resources Canada (2007)

7 Tornado Warnings Warnings are the culmination of a sequence of actions... that act to alert the public to a heightened probability of high-impact weather, minutes, hours or even days in advance (Stensrud et al., 2009). Tornado warning is issued when a tornado has been reported; or when there is evidence based on radar, or from a reliable spotter that a tornado is imminent (EC, 2012).

8 Activity Network Development Evaluates the process from tornado detection to warning communication and develops an activity network for the Canada-Alberta-Calgary-Household tornado detection, warning decision-making and communication process Recognize the sequence of activities and develop an activity network Models the activity network using the Decision Support Simulation System (DSSS) template in the Simphony Legacy software

9 Collaborating Partners Environment Canada Prairie and Arctic Storm Prediction Centre Spotter Network Alberta Emergency Management Agency Calgary Emergency Management Agency Police, emergency services Local Radio and Television, Internet, Social networks Public

10 Activity Network

11 Network Modeling Approach Model s the network using various tools that represent interrelationships of activities Model s as a stochastic activity network using appropriate PDF s for the activities

12 Network Modeling Approach (contd.) Use simulation-based schedule networking tool - DSSS template of the Simphony Legacy software (Moussa et al, 2007) Use Finish to Start (FS) activity relationships with no time lag from any activity to its successor Use OR and AND logical relationships Monte Carlo Simulation runs Outputs can be visualized through Cumulative Density Functions (CDF)

13 Simulation Output Results There is a 50% chance that tornado detection, warning and communication can be completed within 25 minutes or less. The maximum time predicted through simulation is about 37 minutes.

14 Warning Related Issues Hard to predict Short warning lead times Uncertainty in predicting actual paths, size of forecast regions Warning for a large area although impacts are localized False warnings

15 False Warnings False warnings can be recognized as communications regarding tornado events that have been forecasted but not actually occurred. Tornado No Tornado Warning Given True Warning False Warning No Warning Given Missed Event Status Quo

16 False Warning Impacts False warnings can be costly. False tornado warnings question the level of risk that has to be accepted by the public. False warnings desensitize the public to valid warnings. They can reduce the public s concern for future tornado warnings: cry-wolf effect. Therefore, false warning is an important aspect to understand when planning for future tornado events.

17 Conceptual Model of Warning Accuracy Source: Barnes et al. (2007) (c)american Meteorological Society. Instead of having a yes-no categorization of warnings, this spectrum is used to demonstrate the range of accuracy of warnings.

18 Probability of False Warnings Year Number of tornado warnings Joint occurrence of a tornado and a warning issuance Number of false warnings Total Individual tornado occurrence reports were compared with the tornado warning records to determine the joint occurrence of a tornado and a warning issuance. Absence of such an intersection for a warning is counted as a false warning record. False warning probability 251/266 = 94.3%

19 Warning Decision Tree for a Tornado Decision Event Disutility True Warning T p U 1 =Property damage + Some impact on people W T 1-p False Warning U 2 =Inconvenience + Injuries during evacuation U 2 «U 1 W W =Warning Issued W = No Warning Issued T=Tornado Occurs T =No Tornado Occurs p= P(T/W) T T p 1-p Missed Event Status Quo U 3 =Property damage + Major impact on people U 3»» U 1 U 4 0

20 Factors Behind False Warnings Issuing a warning is much more challenging due to -chaotic nature of tornado development -technological capabilities and limitations -information uncertainty Despite these challenges, A tornado warnings has to be issued in many cases to avoid possible threats to lives and property. However, the storm may not spawn an actual tornado. Then it becomes a false warning.

21 Factors Behind False Warnings in Canada Large area of responsibility for one forecaster - A single forecaster for the area coverage of about ten radars -Lack of capacity to provide a deep analysis on the behaviour of a given storm Inability to verify a tornado occurrence -Tornadoes in sparsely populated areas -Events such as downbursts and wind gusts that have damage potential but are not counted as tornadoes

22 Wording in Tornado Warnings AT 5:30 PM DOPPLER RADAR INDICATED AN INTENSE THUNDERSTORM 40 KM EAST SOUTHEAST OF PORTAGE LA PRAIRIE, NEAR ELIE. RADAR INDICATES ROTATION WITH THIS STORM AND MAY BE CAPABLE OF PRODUCING A TORNADO. (TORNADO WARNING ISSUED AT 5:38 PM CDT TUESDAY 13 JULY 2010.) AT 5:25 PM CST RADAR INDICATES A POSSIBLE TORNADO AS STRONG ROTATION IS DETECTED JUST TO THE NORTHWEST OF MAYFAIR. THIS STORM IS MOVING EASTWARD AT 30 KM/H. THIS IS A WARNING THAT SEVERE THUNDERSTORMS WITH TORNADOES ARE IMMINENT OR OCCURRING IN THESE REGIONS. MONITOR WEATHER CONDITIONS. TAKE IMMEDIATE SAFETY PRECAUTIONS. (TORNADO WARNING ISSUED AT 5:26 PM CST MONDAY 18 JULY 2011.) With these reasonable explanations, if made widely and easily available, the possibility that people will question a false warning is reduced.

23 Conclusion The false warning issue is only one aspect when considering the efficiency of a warning system. With the uncertainties associated with tornadoes, complications faced by human forecasters, & the massive impact of a tornado on the population, the public needs to be patient regarding false warnings. The detection-warning network should be designed so that the probability of a missed event is much lower than that of a false warning.

24 Acknowledgements Calgary Emergency Management Agency (CEMA) Association of Professional Engineers and Geoscientists of Alberta (APEGA)