TRACK PREDICTION MODELS
Forecasters
at the National Hurricane Center use several different computer
models to help forecast hurricane motion. Over the past 20
years errors in the official forecasts have decreased a little
less than 1% per year on average, largely because of improved
forecast models. The models fall into three classes: statistical
models that try to relate the hurricanes' motion to experience
with past storms, dynamical models that solve the fundamental
physical equations that describe atmospheric motions under
various simplifying assumptions, and statistical-dynamical
models that use results from dynamical-model calculations as
input to statistical descriptions of storm motion. The models
being run in 1995 are:
- CLIPER is the only purely statistical model
still in use. The acronym stands for CLImatology and
PERsistence. The only inputs to CLIPER are the storm's current
and previous positions, its motion and intensity, and the time
of the year. Because CLIPER knows nothing about the
meteorological situation, forecasters use it as a "no skill"
prediction against which other models are judged.
- The Aviation Model is a version of NCEP's
(formerly NMC's) operational forecast model. It is a multilevel,
global spectral model with T126 truncation, equivalent to 106 km
horizontal resolution. Hurricane track forecasting capability
is attained by inserting a synthetic vortex into the initial
analysis at the observed hurricane position and tracking the
vortex motion as the model calculation evolves.
- BAM (Beta and Advection Model) assumes
that the hurricane moves with the aviation model's winds
(vertically averaged and filtered to remove the hurricane) plus
a drift toward the pole and westward due to the northward
increase of the Coriolis parameter (the "beta effect"). BAM
comes in three versions, shallow, medium, and deep, depending
upon the depth over which the vertical average is computed.
- The GFDL Model is a full physics model,
developed as a research tool at the Geophysical Fluid Dynamics
Laboratory, that has become fully operational. With its fine
resolution (about 19km) and other special adaptations to the
hurricane problem, it has an excellent forecasting record.
- VICBAR is a specialized forecast model
developed at HRD/AOML. It replaces the complex dynamics of the
atmosphere with a layer of fluid with constant density,
representing the average motions between 850 and 200 hPa.
Despite its simplified dynamics and 50 km resolution, it is also
a powerful forecast model.
- NHC93 was developed at NHC. It is the only
statistical-dynamical model now in use. It uses the output from
the aviation model in a series of statistical regression
relations to predict hurricane motion.
References
Aberson, S. D., and M. DeMaria, 1994: Verification of a nested
barotropic hurricane track prediction model. Mon. Wea.
Rev., 122, 2804-2515.
Bender, M. A., R. J. Ross, R. E. Tuleya, and Y. Kurihara,
1193: Improvements in tropical cyclone track and intensity
forecasts using the GFDL initialization scheme. Mon. Wea.
Rev., 121, 2046-2061.
Lord, S. J., 1993: Recent developments in tropical cyclone
track forecasting with the NMC global analysis and forecast
system. Preprints, 20 Conference on Hurricanes and Tropical
Meteorology, American Meteorological Society, 290-291.
McAdie, C. J., and M. B. Lawrence, 1993: Long-term trends
in National Hurricane Center track forecast errors in the
Atlantic basin. Preprints, 20 Conference on Hurricanes and
Tropical Meteorology, American Meteorological Society,
281-284.