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Tropical cyclone activity index in IPCC simulations

PI: Mike Fiorino
Institution: Lawrence Livermore National Laboratory / PCMDI
Tropical cyclone (TC) activity is typically defined as the number of significant cyclones (wind speeds ≥ 34 kt or15 m/s) that form over the tropical oceans on an annual basis. While TC activity has been well observed in the satellite era (1970s to present), counting and characterization of cyclones in climate model solutions is more difficult due mainly to coarse spatial resolution of the models. An alternative to direct measurements is to use an index based on atmospheric parameters that have strong observational and physical linkages to TC formation.

One such index is the “seasonal genesis parameter” proposed by Gray (1979) and refined by Royer et al. (1998). This index has two factors, one accounting for the dynamical controls of vertical wind shear and low-level vorticity, and the other the thermodynamic source of TC energy including upper-layer ocean heat content, mid-tropospheric moisture and low-level moist static energy. The innovation of Royer et al. was to use model convective precipitation for the thermodynamic factor and to scale the convective precipitation so as to produce current-climate (observed) TC activity. Thus, the Royer-style index can measure slow changes in TC activity if we assume that the physics of the relationship between the dynamical/thermodynamic controls–TC activity varies at a much slower rate than the climate temporal scales of interest. For interannual, decadal variations and century-scale trends it seems unlikely that underlying physics would change so that if over-ocean tropical convection increased in a warmer climate, then TC activity would increase if the dynamical parameters remained constant.

TC activity in the IPCC AR4 coupled model simulations will be analyzed using the Royer scheme for three scenarios: 1) present climate control (pdcntrl); 2) climate of the 20th century (20c3m); and 3) 1%/year CO2 increase to doubling (1pctto2x). TC activity will first be analyzed using the mean annual cycle of a 30-y period for the control and the current climate to determine the convective precipitation scale factor. We will also use total precipitation and the ratio of convective/total precipitation from reanalysis to determine sensitivity to the thermodynamic part of the index. The second phase will consider climate variations in the index, specifically interannual, decadal and century trends using filters.

The date requirements are relatively modest: 1) monthly means; 2) atmospheric variables of 850 and 200 hPa winds (ua,va; dynamics), convective and/or total precipitation (prc and pr; thermodynamics). The Royer scheme has been implemented and tested for the NCEP R1 & R2 and the ECMWF ERA-40 reanalyses.

Gray W.M., 1979: ‘Hurricanes: their formation, structure and likely role in the tropical circulation’ in Shaw. D.B. (ed.), Meteorology over the Tropical Oceans, Royal Meteorological Society, J. Glaisher House, Grenville Place, Bracknell, Berkshire, UK, pp. 155-218.
Royer, J.-F., F. Chauvin, B. Timbal, P. Araspin and D. Grimal, 1998: A GCM study of the impact of greenhouse gas increase on the frequency of occurrence of tropical cyclones. Climate Change, 38, 307-343.

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