Climate Theory and Tropical Cyclone Risk Assessment

Abstract

The links between the ability of general circulation models to simulate tropical cyclones and the development of a climate theory of tropical cyclone formation are explored, with an emphasis on the potential of general circulation models (GCMs) and theory for tropical cyclone hazard and risk assessment. While GCMs can now generate a reasonable simulation of the observed tropical cyclone formation rates and intensity distributions, they are very expensive to run. Simpler methods involving statistical relationships between climate variables and tropical cyclone formation have been developed and have been used for hazard assessment, but like other methods used for projections, such as downscaling or GCMs, they do not constitute a theory of tropical cyclone formation. An outline is given of some of the possible characteristics of such a theory and its potential utility for climate science and risk.

Appendix: Formulation of GPIs

Emanuel and Nolan (2004):

$$ GP={\left|{10}^5\eta \right|}^{\frac{3}{2}}{\left(\frac{H}{50}\right)}^3{\left(\frac{V_{pot}}{70}\right)}^3{\left(1+0.1{V}_{shear}\right)}^{-2} $$

• η = absolute vorticity at 850 hPa (s⁻¹)

• H = Relative humidity at 700 hPa (%)

• V _pot = potential intensity (m s⁻¹) calculated using a routine provided by Emanuel (http://eaps4.mit.edu/faculty/Emanuel/)

• V _shear = vertical shear from 850 to 200 hPa (m s⁻¹)

Emanuel (2010):

$$ GP={\left|\eta \right|}^3{\chi}^{-\frac{4}{3}}\max {\left(\left({V}_{pot}-35\right),0\right)}^2{\left(25+{V}_{shear}\right)}^{-4} $$

χ = saturation deficit

Tippett et al. (2011):

$$ TCS=\exp \left(b+{b}_{\eta}\eta +{b}_HH+{b}_TT+{b}_VV+\log \left( cos\phi \right)\right) $$

• η = clipped absolute vorticity at 850 hPa in 10⁵ s⁻¹ (η =  min (η, 3.7))

• $T= SST-{\overline{SST}}^{\left[{20}^{{}^{\circ}}S-{20}^{{}^{\circ}}N\right]}$ in °C

• ϕ = latitude

• H = Relative humidity at 600 hPa (%)

• V = vertical shear from 850 to 200 hPa (m s⁻¹)

The constants used are those from line 6 of Tippett et al. (2011)’s Table 1 and are those used by Menkes et al. (2012):

$$ b=-5.8;{b}_{\eta }=1.03;{b}_H=0.05;{b}_T=0.56;{b}_V=-0.15 $$

Bruyere et al. (2012):

$$ CGI={\left(\frac{V_{pot}}{70}\right)}^3{\left(1+0.1{V}_{shear}\right)}^{-2} $$

Since the CGI does not include a vorticity term to remove TCs that form close to the equator, its values are set to zero between 0 and 5°N.