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Atlantic Ocean (19-39°N, 75-95°W). WRF horizontal grid
spacing was 9 km and the model was integrated at 31
vertical levels. All simulations covered the period August
27-31. WRF simulations were forced by data derived from
the A2 climate change scenario defined by the IPCC (2007),
as simulated by the GISS AOGCM (Russell et al. 1995;
Rangwala et al. 2006). The AOGCM uses a 4° x 5°
horizontal grid at nine vertical atmospheric levels and 13
vertical ocean levels.
there are not larger and more severe hurricanes in the
second half of the 20 th century.
4. Results
Simulations of Hurricane Katrina using the WRF model on a
9 km grid over the Gulf of Mexico and the Southeast US
were analyzed during the period August 27-30, 2005. The
control captured many of the observed characteristics of
Katrina (not shown), although the predicted center was
about 150 km to the actual land falling position. WRF storm
tracks were determined from the locations of minimum sealevel
pressure every three hours. (not shown). The tracks of
the storms in the first half of the century were mostly east of
the simulated (control) 2005 storm. Tracks in the second
half the century were mostly west of the control.
Vertical profiles of the Gulf of Mexico averaged u-wind
component from the GCM for simulations representing the
earlier decades showed a large positive vertical shear
relative to the CTL, reaching maxima between
approximately 12-16 km altitude. Profiles from the latter
decades showed a negative anomaly shear up to about 8-10
km altitude. Each consistent with the differences in tracks
discussed above. The vertical shear of the zonal wind is
proportional to north-south temperature gradients. The
negative shear in the simulations representing the second
half of this century indicates a weakening or a reversal of the
usual south to north temperature gradient. This can occur
when the continent north of the Gulf of Mexico becomes
excessively warm compared to the water temperature. In
contrast, the GCM sea surface temperatures were predicted
to fall in the first two decades, explaining in part the positive
anomalies in the u-wind component during this time.
However, predicted Gulf of Mexico sea surface
temperatures exceeded those of the present decade from the
2030s onward, increasing by about 3 o C by the 2090s.
Fig. 1 compares the horizontal distribution of surface wind
speed of the 2070s to the control. After 54 hours (06 UT on
August 29). the 2070s storm center crossed the Louisiana
coastline at 90°W, displaced further west from the control.
The 2070s storm is more compact than the control. For
example, the diameter within which its wind speeds exceed
30 m/s is about 20% smaller than for the control (2.9° versus
3.6°). Hence, the average wind speeds within, for example,
180 km of the core were less than in the control.
Nevertheless, the future storm maintained its core of very
high winds better than the control as it approached the coast.
The 2070s storm shows narrow rings of wind speeds in
excess of 50 m/s. but unlike the control its bands of wind
speed exceeding 50 m/s completely surround the storm
center. Similar results were obtained for the simulated
storm in the 2080s.
5. Sensitivity Tests
Figure 2 shows WRF simulated hurricane minimum surface
pressures versus Gulf of Mexico sea-surface temperatures.
There is a good relationship between the two, except in the
2090s (when an anomalous track of the storm sent it to close
to the Florida Pan Handle). Hence, one might wonder why
Figure 2. WRF simulated minimum hurricane pressures versus
Gulf of Mexico averaged sea-surface temperatures in each
decade.
Sensitivity tests showed that predicted, decadal increases
in atmospheric temperature anomalies (with peak values
of 12 o C found at about 10 km) prevented further storm
intensification.
6. Conclusions
Results suggest that during the latter part of the 21st
century, the radius of strong winds in a Katrina- like storm
will decrease, but the pressure minimum will be more
extreme and maximum wind speeds will be higher and
more prolonged after landfall. Thus, the extremely low
minimum pressures for 2070s and 2080s storms were
apparently caused by the very high SST the storms
encountered over the Gulf of Mexico, while their
relatively small diameters may reflect the high vertical
thermal stability of the late decades. Our results, using real
boundary conditions modified by a climate change signal,
suggest that the danger to coastal locations from the high
winds of severe hurricanes could increase in the latter half
of this century.
References
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