Tropical Cyclones effects

Tropical Cyclones (TCs) are among the most dangerous natural disasters, causing every year extensive damage and deaths in several countries around the world. They have three destructive effects (strong wind, heavy rain and storm surge):

Winds: very strong winds during the passage of a TC could damage and destroy homes, buildings, trees, power lines and vehicles. Moreover several people could be seriously injured or killed by falling trees and flying debris.

Rainfall: extreme rainfall during the passage of a TC could produce floods, flash floods and landslides.

Storm surge: it is an abnormal rise of the sea level above the normal tides during the passage of the TC, caused by strong winds and a drop in the atmospheric pressure. This effect could generate large inundation along the coast.

Current GDACS methodology

TC Information:

An automatic routine that includes the TC bulletins produced by the National Oceanic and Atmospheric Administration (NOAA) and the Joint Typhoon Warning Center (JTWC) into a single database has been developed by the JRC. This system includes the data of all TCs occurring worldwide. The TC bulletins are used in GDACS for the wind impact, while the heavy rain impact is obtained using the NOAA Ensemble Tropical Rainfall Potential (eTRaP) data and the National Aeronautics and Space Administration (NASA) - Global Precipitation Measurement (GPM) data. For the third TC hazard, JRC has developed a storm surge system, introducing the atmospheric forcing in the JRC HyFlux2 code and using as input the TC bulletins (link). JRC has recently developed and implemented several new tools for the analysis of the TC impacts and evaluate their potential risks, including new atmospheric sources (NOAA Hurricane Weather Research and Forecast - HWRF and Global Forecasting System - GFS, global high resolution model of the European Centre for Medium Weather Forecast -ECMWF) (link).

TC Classification:

The TC classification used in GDACS impact tables is based on the equivalent Category of the Saffir-Simpson Hurricane Wind Scale (SSHS) using 1-min sustained winds.

Category	Knots	Km/h
5	≥ 137	≥ 252
4	113 - 136	209 - 251
3	96 - 112	178 - 208
2	83 - 95	154 - 177
1	64 - 82	119 - 153
Tropical Storm	34 – 63	63 - 118
Tropical Depression	≤ 33	≤ 62

TC Impact estimation

Wind

Based on track information provided by the National Oceanic and Atmospheric Administration (NOAA) and the Joint Typhoon Warning Center (JTWC), JRC calculates the areas around the track affected by high winds, as well as the exposed population and critical infrastructure. JRC developed the Vulnerability Indicator for Tropical Cyclone combining indicators describing the Human Development Index (HDI) and the rural populations (Percentage of population in rural areas, Population living below 10m low elevation in coastal zone) and introduced a new Index (Rural Population Index, RPI) for rural population averaging the values of the two chosen indicators for rural population. The countries were ranked by HDI and RPI using the quartile method and then the scores for the two dimension indices are then aggregated into a composite index using arithmetic mean. The countries were finally assigned to 3 classes: (1) High Vulnerability, (2) Medium Vulnerability, and (3) Low Vulnerability.

The GDACS alert levels for the TCs are based on the risk matrix that includes: Hazard (TC 1-min sustained wind speed), Exposed population and Vulnerability of the exposed country

Alert Level	Category	Population	Vulnerability
Green	TS	< 10M	Low – Medium - High
Orange	TS	> 10M	High
Orange	Cat 1-2	> 100K or > 10%	Medium – High
Orange	Cat 3	> 1M	Low
Red	Cat 1-2	> 1M	High
Red	Cat 3	> 100K or > 10%	Medium – High
Red	Cat 4	> 1M	Low

Storm Surge

The first storm surge system for the TCs has been developed by the JRC in 2011, including the atmospheric forcing in the JRC HyFlux2 code used for tsunami modelling. This system includes a Monte Carlo method based on the parametric model of Holland, using as input the wind radii data provided by the JTWC and NOAA NHC (link).

The GDACS alert levels for the storm surge are based on the maximum storm surge height.

GDACS Storm Surge Alert Level	Max storm surge
Red	≥ 3 m
Orange	1 - 3 m
Green	≤ 1 m

The storm surge calculations in GDACS doesn’t include wave, tide and river effects. It is important to note that in the area of a delta river, the storm surge may be higher. The torrential rains that may affect the mountains areas during the passage of a Tropical Cyclone may increase the river flow and its outflow could be blocked by the incoming storm surge. This could create floods in the surrounding areas of the cities close to a delta river. Moreover the estimation of the sea level is strongly dependent on the atmospheric forcing (wind velocity and direction).

TC GDACS Alert Score:

The GDACS Alert score is currently based only on the wind effect. A GDACS storm surge alert level is also estimated, but this is not yet taken into account in the GDACS alert score. The inclusion of all TC effects in the alert algorithm is under development

GDACS Alert Level	GDACS Score (Wind Impact)
Red	2.5
Orange	1.5
Green	0.5

Results and limitations

Overall, GDACS has a good record of assessing the impact of Tropical Cyclones (TCs). All TCs that required the humanitarian intervention or a major national relief effort were alerted by GDACS (TC Red Alert).

GDACS typically provides impact reports 5 days before landfall using the tropical cyclone advisory bulletins. The uncertainty in the meteorological forecasts is propagated to the GDACS results, causing alert levels to change upwards or downwards. Therefore, in order to avoid too much false alerts and changes in the alert level, the GDACS RED alerts forecasted more than 3 days in advance are reduced to Orange Alerts. Currently the maximum GDACS alert for a Tropical Cyclone is the result of the GDACS wind impact estimation and not the result of the three combined effects (wind, rainfall storm surge). A new alert system that will include all these effects is under development.

For each advisory, the alert messages use an alert level related to what is to come, ignoring the previous impact. However, the main impact report on the web site shows the alert status of the whole storm, and is modified with each advisory.

Further work

GDACS is a collaboration platform open to organisations that have data, models or systems that can significantly contribute towards better impact assessment and new information for emergency responders. JRC, as the scientific lead in GDACS, is interested in exploring integration of such products in the existing impact models.

In order to improve the current GDACS system, JRC is developing several new tools for the analysis of the TC impacts and evaluate their potential risks, using new datasets (TC effects, population, vulnerability) and a more complete combined risk and alert system for the GDACS score that will include the three hazards (wind, storm surge and rainfall).

The uncertainty in the meteorological forecasts (TC track and intensity) is propagated to the GDACS alerts, therefore in the future, the alert level uncertainty will be included in the GDACS score.