Improved Classification of Hurricane Impact by Inclusion of Rainfall Hazards

Introduction

This is the fourth decade in which the Saffir-Simpson hurricane scale (SSHS) has been used as a convenient approach for meteorological scientists and decision makers to categorize hurricanes based on surface wind (Jordan Ii and Clayson, 2008). However, the 2004 Atlantic hurricane season shows that the SSHS cannot measure the true destructive potential of hurricanes (Kantha, 2006, Jordan Ii and Clayson, 2008, Hebert et al., 2010), when Category 2 hurricane Frances (2004) caused US$ 12B damage and 49 fatalities compared with category 3 hurricane Jeanne (2004), with only US$ 7B damage and 5 fatalities, while both made landfall at the same location, just 2 miles apart. Further, other existing hurricane scales also do not classify the impact of these hurricanes properly. This suggests an opportunity to develop a new tropical cyclone or Hurricane scale methodology to address this issue. Our hypothesis is that inclusion of rainfall in the hazard scale will lead to significant improvement in categorizing potential damage and risk to life.

 

Although, the aim is that the methodology can be used at any location in the world, the Atlantic Hurricane Basin (ATL) has been selected as the geographical area. There are 19 Hurricanes which have made landfall in U.S.A in the period 2003-2012. Precipitation Estimation from Remote Sensing Information Using Artificial Neural Network (PERSIANN) from the Centre for Hydrometeorology and remote sensing (CHRS), University of California, Irvine was selected as the rainfall database. H*Wind from the Hurricane Research Division (HRD) was selected as the wind database due to its accuracy, high resolution and availability.

 

Methodology

Equations (1), (2) and (3), present the function used to estimate the hazard from hurricanes due to their wind field, potential storm surge and their rainfall characteristics, respectively.

 

 V is the surface wind speed and V= 34, 50, 64 and 87 kts are considered as wind speed thresholds. A is the area of each wind speed threshold (and rainfall intensity threshold) and C is the tropical cyclone forward velocity. Items with subscript “0” correspond to reference values. The combined Tropical Cyclone Hazard Scale (TCHS) was derived by testing different model formulations, including linear and non-linear relationship between TCWS, TCRS, and TCSS with the final scale as proposed in equation (4).

Result and discussion:

Figure (1) shows the regression between the three hazard scales versus damage and death toll for the 17 hurricanes (excluding Katrina and Sandy which have site specific reasons for very high damage and fatalities). The result shows that both damage and death toll have a better regression with TCRS with 0.43 and 0.74, respectively, compare to SSHS. This is consistent with (Rappaport, 1999) who showed that death caused by freshwater flooding is dominant in comparison with other hurricane hazards such as strong wind or coastal flooding.

Figure 1: TCRS and SSHS against Damage (USD billion, left hand side) and Death toll (right hand side) and the final TCHS model scenarios.

 

Figure 2: Ranked hurricanes according to their actual damage rate, left, and death toll, right, the SSHS and TCHS, respectively.

 

 

The severity or hazard of each hurricane is ranked according to actual damage and fatality and plotted in figure 2, together with the rank predicted by SSHS and TCHS. In addition, the new model also can estimate the amount of damage and the number of fatalities likely caused by a given hurricane, which is not possible using SSHS. This will enable improved hazard planning, emergency response and may also be useful for insurance and risk management. The table shows that the new model is able to rank existing hurricanes by damage and death toll with about a 2 position error compared with roughly a 6 position errors for SSHS. Specifically, the model classifies hurricanes Frances and Jeanne (green and red bars in figure 2, respectively), much better than the SSHS which categorizes the hazard from those systems in reverse the actual hazard. Likewise, SSHS  categories  hurricane Ike (blue bar in figure 2) at rank position 7 for both damage and death toll, while in fact Ike was the most costly and the second most deadly hurricane in selected set. The TCHS provides far better estimates of both severity (rank) and also good estimates of the actual damage and death toll.

Table 1: Model accuracy compare with SSHS.