Model simulated changes in maximum TC intensity due to global warming

The impact of global climate change on the maximum intensity of tropical cyclones (TCs) has become a topic of frequent investigation. The relationship between SST and TC intensity is well documented, with data indicating a general upward trend in TC intensity with increasing SST. Therefore, it is logical to hypothesize that if SSTs increase due to global warming, the possibility exists that TCs could attain greater intensity if other regulating factors remain unchanged. Global circulation models (GCMs) are often used to investigate climate change, and although they are able to crudely represent TCs, the GCM-simulated TC intensity is too weak due to coarse model resolution. Therefore, using the GCMs themselves to analyze future TC intensity is not a viable approach.

In this study, we explore possible changes in future TC intensity by performing idealized, high-resolution simulations with the Weather Research and Forecasting (WRF) model. The model is initialized with an idealized TC vortex, and atmospheric conditions consistent with either (i) average current conditions or (ii) estimates of future climate as predicted by a suite of GCMs. Changes in maximum intensity and TC structure are analyzed. This approach combines observations, high-resolution model simulations and GCM projections of climate change in order to examine possible changes in maximum TC intensity that may occur due to global warming.

The run based on current observations is initialized using an areal average for a subsection of the Atlantic main development region (MDR), utilizing 2.5º NCAR/NCEP reanalysis data for the atmospheric temperature and moisture values, and 0.5º sea-surface temperature (SST) data for September 2005. The year 2005 was chosen because conditions were highly favorable for the formation of strong TCs. The model was run in a no-shear, maritime domain for 15 days in order to allow the storm to reach maximum intensity in the model.

For comparison, a similar run was performed utilizing 21st century forecasts from the suite of 20 models used in the IPCC Fourth Assessment Report (AR4). The A1B emission scenario was selected, and the maximum potential intensity (MPI) for each model run was calculated using a subroutine written by Prof. Emanuel. The year with the highest MPI, averaged over all the GCMs in the aforementioned region, was found to be 2089. Anomalies of temperature, moisture and SST were computed and these were added to the current conditions in order to calculate an ambient TC environment consistent with the changes projected by the GCMs. To summarize the major changes, the suite of GCMs forecast an increase in SST of ~2.0 C, along with a general warming of the troposphere, with the most warming found in the upper-troposphere (300-200 hPa). This profile of warming is consistent with other studies, and was also consistent between the GCMs. The impact of shear is not considered here.

The MPI subroutine calculates minimum central pressure (maximum 10-m wind speed) values of ~900 hPa (~79 m s-1) for the current climate, and ~906 hPa (83 m s-1) for the future climate. These changes in maximum intensity are less than would be expected based solely on the SST increase; using the temperature and moisture profile from 2005 along with the future SST value, the MPI increases substantially, to 860 hPa, or 91 m s-1. However, the upper-tropospheric warming and increased stabilization evidently offsets some of the intensification that would be found due to the increase in SST. Although the MPI subroutine has been show to provide realistic MPI values, full-physics, high-resolution model simulations provide a means to analyze possible changes in TC structure, and an independent test of the change in MPI. 15-day WRF simulations were performed, utilizing 2-km grid spacing. Initial model experiments, with the future climate values determined only by 3 GCMs produced results similar to the subroutine, with a modest increase in TC intensity found in the future simulation. More tests will be performed utilizing the ensemble of GCMs, and the sensitivity to initial vortex choice, model grid spacing and model physics will be analyzed to test the robustness of the results. Differences in TC structure between the simulations will also be investigated, in order to better understand how changes in the ambient environment, due to global warming, may impact the structure of future, intense TCs.