Title: Analyzing the representation of hurricanes in SuperBUGS
Kate Musgrave
Anna Harper
Proposal for: Science Research and its Relevance to CMMAP
Solicitation: CMMAP 07-025
General circulation models (GCM’s) can simulate the global climate but currently have
difficulties modeling hurricanes. Hurricanes and other tropical storms are important for
transportation of global moisture and energy. Many GCM’s have grid spacing of order 100km,
which is too large to accurately resolve hurricanes. As grid spacing becomes smaller in GCM’s,
their ability to reproduce hurricanes (or hurricane-like features) improves, but at the cost of
increased computing resources. Hurricanes are traditionally modeled using limited-area
mesoscale models, or in regional models like the NCAR Regional Climate Model (NRCM),
which has been developed from a mesoscale model. Attempting to reduce the grid spacing in
GCM’s down to scales small enough to resolve hurricanes requires extensive computing
resources, generally involving supercomputers like the Earth Simulator in Japan.
The Earth Simulator has been used in connection with multiple GCM’s to perform global
“cloud-resolving” simulations, which should be able to capture mesoscale features like
hurricanes. With at least three different GCM’s run at 20 km grid spacing or smaller (an order of
magnitude lower than most GCM’s are currently run) multiple simulations have been produced
that included hurricanes (see Figure 1; Oouchi et al. 2006, Miura et al. 2007, Satoh et al. 2008).
These simulations have covered a time period ranging from one week to three months, except for
Oouchi et al. (2006) who developed 10 year climatologies of hurricane statistics in their GCM
with a grid spacing of 20 km. These simulations take extensive computing power and data
storage, giving a limited sample of hurricanes. Part of CMMAP’s mission is to bridge the gaps
between traditional GCM’s, which have difficulties producing mesoscale convective features in
the tropics, and cloud-resolving GCM’s, which are too computationally intensive to produce
many simulations.
To provide this bridge, CMMAP is implementing a multi-scale modeling framework
(MMF), in which a cloud resolving model is imbedded in a GCM. One model in particular is
called SuperBUGS. This is a version of the CSU general circulation model (BUGS5). Within
each of SuperBUGS’ gridcells, a line of miniature cells of about 4 km resolution contains cloud
resolving models. Therefore, SuperBUGS is less complex than a global cloud resolving model,
but resolves more complex processes than a global climate model. SuperBUGS also contains a
land surface model, SiB3 (the Simple Biosphere model), which predicts fluxes of heat, moisture,
momentum, and carbon dioxide from the surface to the atmosphere (Sellers et al. 1986; Sellers et
al. 1996; Denning et al. 1996).
During a 10-day simulation with BUGS5, multiple hurricane-like features were produced
in the West Pacific (D. Dazlich, personal communication). The resolution was about 240 km,
preventing the resolution of cyclonic circulation. However, in one feature the model produced a
central pressure of 977 hPa and central winds of 40-50 m/s (90-110 mph) in one gridcell. Since
these are daily means, it is implied that a strong hurricane was occurring. We would expect to
find similar features in SuperBUGS, which has higher resolution convective parameterizations in
its embedded 4 km CRM’s.
We propose to study the representation of hurricanes in SuperBUGS. SuperBUGS
should be able to reproduce the hurricane-like features seen in BUGS5, and with the addition of
the MMF SuperBUGS should provide an improved representation of the convective organization
of the hurricane. To test this, we plan to run SuperBUGS off the same initialization used for the
BUGS5 case. Should this test case work, SuperBUGS would then be run for 20 years under
current climate conditions. This will require the use of NCAR computing for running the
simulation and for storage of the output. Statistics of the hurricanes produced in SuperBUGS
will be generated, including examining the number of hurricanes generated on an annual basis,
the formation locations, duration, size, and intensity of the generated hurricanes. These results
can be compared with both the NRCM and the results of Oouchi et al. (2006), as well as with
observed hurricane statistics.
Often models produce inaccurate hurricane evolution as they move over land. Part of our
goal is to determine if the high spatial resolution and inclusion of SiB3 in SuperBUGS produces
more realistic reaction to landfall. Therefore, we will chose several case studies of landfalling
hurricanes and investigate wind decay profiles, fluxes of sensible and latent heat, and
precipitation. To investigate model realism, we will compare these variables to observations. The
wind decay profiles will be compared to statistical models for intensity change. Precipitation can
be compared to datasets from Trenberth and Fasullo (2007). As precipitation from hurricanes
make up a large percentage of total precipitation in parts of the world, a more accurate
representation can improve long-term drought forecasts and climate change impacts. Fluxes of
sensible and latent heat flux can be compared to theoretical values based on windspeed and
profiles of heat and moisture leading up to hurricane passage.
This work will give us an understanding of hurricane evolution in the multi-scale
modeling framework. Previously, very little research has been done on this important aspect of
tropical convection, which is a very important part of the global energy and moisture budget.
Therefore, it is critical that we improve its representation in climate models. This presents
CMMAP to once again be on the cutting edge of research in resolving previously unresolvable
features in climate models. Previous studies have shown that the MMF can provide an improved
representation of large-scale features like the Madden Julian Oscillation (Khairoutdinov et al.
2005). The comparison of SuperBUGS with BUGS5 can show how the MMF can affect the
representation of even mesoscale aspects of tropical convection, which would not be expected to
be well represented.
This work can also be a stepping stone for many future projects. For example, the MMF
has been implemented in the Community Atmospheric Model (Super-CAM) but hurricanes were
not produced. A model comparison between SuperBUGS and SuperCAM would increase our
understanding of tropical circulation in the models, and how to best represent realistic processes
in the CRM’s.
References
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Drought Stress in the Amazon: Reconciling Models and Observations
Denning, A.S., G.J. Collatz, C. Zhang, D.A. Randall, J.A. Berry, P.J. Sellers, G.D. Colello, D.A.
Dazlich, 1996: Simulations of Terrestrial Carbon Metabolism and Atmospheric CO2 in a
General Circulation Model. Part I: Surface Carbon Fluxes. Tellus, 48B, 521-542.
Khairoutdinov M., D. Randall, C. DeMott, 2005, Simulations of the atmospheric general
circulation using a cloud-resolving model as a superparameterization of physical
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Figure 1. Global view of outgoing longwave radiation produced by a 3.5 km grid spacing global
circulation model on the Earth Simulator (Satoh et al. 2008, Figure 12). Note tropical cyclones
in Pacific Ocean.