GFDL - Geophysical Fluid Dynamics Laboratory

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Global models of hurricane frequency

GFDL has a variety of ongoing projects aimed at understanding the influence of global warming on tropical storms. Several of these efforts are summarized here.

One of our central goals is to develop global atmospheric models that are capable of simulating the statistics of tropical storms with sufficient fidelity that they can be used with confidence to study the causes of year-to-year variability in storm activity, recent trends in activity, as well as the predictability of the Atlantic hurricane season. As the credibility of these models improves, based on these comparisons with observations, we will apply these models to predict the effects of global warming on tropical storms.

We have recently made important progress in our global modeling effort, using a model with roughly 50km horizontal grid size. This progress is described in a recent paper submitted for publication in the Journal of Climate. In these simulations, we prescribe the ocean surface temperatures from observations so as to determine how these ocean temperatures control tropical storm frequencies.

The model is designed to address questions of storm genesis and frequency. Its simulation of storm intensity is not yet sufficiently realistic to justify its direct use in studies of how global warming will affect storm intensity.

The simulation of interannual variability of Atlantic hurricane numbers in this model is impressive, as indicated below, and supports the view that the overall activity of the Atlantic hurricane season has substantial predictability, if we can predict ocean temperatures.

Initial global warming simulations, also briefly illustrated below, are for a decrease in the number of Atlantic storms, and an increase in East Pacific storms. But as described in the paper, these regional trends are sensitive to small differences in the pattern of projected ocean temperature change.

An animation of the simulated Outgoing Longwave Radiation at the top of the Atmosphere
during 2005 hurricane season

Simulations of global hurricane frequency climatology

We have completed 4 simulations of the 1981-2005 period using observed sea surface temperature (HadISST) as the lower boundary condition. Below show the tracks of all hurricanes in one of our 4 simulations and the observed hurricane tracks from the International Best Track Archive for Climate Stewardship (IBTrACS) database.

Simulations of interannual variability of basin-wide hurricane frequency

Below are the model simulated interannual variability of Northern Hemisphere basin-wide hurricane frequency (Blue: model ensemble mean) and compared with the observations (red) from the IBTrACS

Simulations of hurricane frequency response to global warming

This model is used to simulate the response to the sea surface temperature anomalies generated by coupled models for the Intergovernmental Panel on Climate Change 4th Assessment Report (IPCC-AR4, CMIP3) A1B scenario late in the 21st century. Below show results for sea surface temperature anomalies computed by averaging over 18 models in the CMIP3 archive.

Model description

This model configuration differs from AM2 in the following:
  • The finite-volume dynamical core on lat-lon grid has been replaced by a finite-volume core using a cubed-sphere grid topology.
  • The number of vertical levels has been increased from 24 to 32.
  • The prognostic cloud fraction scheme has been replaced by a simpler diagnostic scheme assuming a sub-grid scale distribution of total water.
  • The relaxed Arakawa-Schubert convective closure has been replaced by a scheme based on parameterization of shallow convection by Bretherton et. al.

The model retains the surface flux, boundary layer, land surface, gravity wave drag, large-scale cloud microphysics, and radiative transfer modules from AM2. We refer this specific version as C180HIRAM2.1. The notation C180 indicates 180x180 grid points in each face of the cube; the size of the model grid varies from 43.5 km to 61.6 km.

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