Hurricanes: Their Nature and Impacts on Society - Climate Science ...

HURRICANE FORECASTS 105
uses information from the National Weather Service (NWS) global prediction
model (NBC 90; McAdie 1991), <strong>and</strong> several which solve mathematical
equations for atmospheric flow including VICBAR (Aberson <strong>and</strong> DeMaria
1994), BAM (Marks 1992), the Geophysical Fluid Dynamics Lab (GFDL)
model (Bender et al. 1993), <strong>and</strong> the National Meteorological Center (NMC)
Aviation Medium Range Forecast (MRF) global forecast models (Lord 1993).
During the 1992 <strong>and</strong> 1993 seasons, the GFDL model track forecasts were
superior to the other track models (Aberson <strong>and</strong> DeMaria 1994); the
improved forecasts of hurricane track using this model are also summarized
by Sawyer (1995).
Track forecasts are very sensitive to how the actual hurricane is initially
defined in the model (Leslie <strong>and</strong> Holl<strong>and</strong> 1995). For instance, track predictions
depend on the choice of hurricane radius <strong>and</strong> strength, <strong>and</strong> its initial
location. Improved measurements of temperature, wind, <strong>and</strong> humidity in <strong>and</strong>
around tropical cyclones have contributed to improved operational track
forecasts (Burpee et al. 1996). An example of a 72-hour track forecast for
Hurricane Hugo for different models is given in Figure 4.8. This figure
illustrates that despite significant progress in hurricane forecasting, exact track
prediction remains fraught with difficulties. Hugo actually made l<strong>and</strong>fall near
Charleston, South Carolina. Hurricane track models are also summarized in
Puri <strong>and</strong> Holl<strong>and</strong> (1993).*
Figure 4.9 illustrates the trend <strong>and</strong> accuracy of 24-hour, 48-hour, <strong>and</strong>
72-hour forecasts of storm position between 1970 <strong>and</strong> 1992. As of 1997,
average forecast errors are on the order of 115 miles (185 kIn) for 24-hour
forecasts, 230 miles (368 kIn) for 48-hour forecasts, <strong>and</strong> 345 miles (552 kIn)
for 72-hour forecasts (C. L<strong>and</strong>sea, 1997, personal communication). Note that
an improvement of only about 20 miles (42 kIn) has been achieved in 24-hour
position forecasts over 23 years, despite the great advances both in monitoring
these storms (e.g. radar, satellite, reconnaissance aircraft) <strong>and</strong> in computer
power to process <strong>and</strong> analyze the data.
.Four criteria have been proposed for accurate track forecasts using models such as VICBAR,
BAM, the GFDL model, <strong>and</strong> the Aviation global model (Elsberry 1995). These are: adequate
initial specifications of the environmental wind field, the symmetric <strong>and</strong> asymmetric cyclone vortex
structure, <strong>and</strong> the adequacy of the prediction models to forecast the time evolution of the vertical
<strong>and</strong> horizontal wind field. An accurate representation of the diabatic heating of the atmosphere by
the hurricane, <strong>and</strong> the prediction of winds <strong>and</strong> temperature in the upper troposphere are also
essential to accurately characterize hurricane-environmental interactions (WU <strong>and</strong> Kurihara
1996). When the large-scale weather pattern is changing with time, these reqnirements are difficult
to achieve with sufficient accuracy.
The MRF model is used for general weather forecasting in addition to its 'application for
tropical cyclone track prediction. VICBAR, BAM, NHC90 <strong>and</strong> the GFDL model use forecast
fields from the MRF model for input. One version of NHC90 (referred to as UK90) uses output
from the United Kingdom Meteorological Office global forecast model.
The GFDL model includes the most physical realism in its formulation, including moving
nested grids which translate with the cyclone <strong>and</strong> a sophisticated vortex initialization scheme with
the finest horizontal grid interval of 20 km (DeMaria 1995; Bender et aI. 1993).