GFDL - Geophysical Fluid Dynamics Laboratory

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 180×180 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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