Strategy for Regional Seasonal Forecasts
191
(condensation, convection, radiation). To advance a variable h by a time increment,
t:..t, from a time level 1 to another time level 1+1, a tentative value h. is calculated in
the predictor step:
h. = h t + (LP + HP + DP) . /::,.t
(30)
Let LF* denote these terms at the time level, 1, computed from the tentative values h •. In the corrector step, h't+ 1 , is obtained by first computing
hul = ht+[(1-a)·LP+a·LF·]+[(1-~)·HP+~·HF·]
(31)
and next adding the forcing effect AD't+1 to the above. Values assigned to the
weight parameters a and ~ are 0.506 and 2.5, respectively. Using these weights, the
low frequency waves are preserved in time marching, whereas the amplitudes of
high frequency waves are strongly suppressed 1.
This Hurricane Prediction System (GFDL) was adopted in 1995 by the US NWS
(National Weather Service) as an official tropical cyclone prediction technique. In
both the Atlantic and eastem Pacific basins, storm track predictions in 1995 were
substantially improved compared with predictions from several other models, in
particular for the period beyond 36 hours. For example, improvements in the track
forecasts in the Atlantic at 36,48 and 72 hours are 14, 19 and 25 %, respective1y,
relative to the next best NWS guidance at each forecast time.
10.4.2 Eta-model prediction
The dynamical part of the Eta-model is designed following the approach of
Arakawa for maintaining a variety of chosen integral constraints of the continuous
atmosphere (e.g. Janic 1984, Mesinger et al. 1988). Differencing schemes are of
second or approximately second order accuracy.
The current operational program of NCEP's Environmental Modeling Center
uses three models; Eta 48 km, Eta 29 km, and Eta 10 km models. These models are
identical except for the grid size and a single constant, the "horizontal diffusion"
coefficient.
The lateral boundary conditions are provided by a global spectral GCM of resolution T126L28; triangular truncation at 126 wave numbers and 28 verticallevels
(The Medium Range Forecasting Model, MRF, see Kanamitsu et al. 1991). This
run at an earlier time and is referred to as the "Aviation model". The Eta-model is
initialized with a static analysis using the global analysis as the first guess, and
other nested grid model used at NCEP is the NGM (Nested Grid Model), which has
80 km/L18 inner grid (Philips 1979).
1. HF = h~ + (Coriolis force, metric force, pressure gradient force for h=u,v, energy
at
conversion terms for h=D
191
(condensation, convection, radiation). To advance a variable h by a time increment,
t:..t, from a time level 1 to another time level 1+1, a tentative value h. is calculated in
the predictor step:
h. = h t + (LP + HP + DP) . /::,.t
(30)
Let LF* denote these terms at the time level, 1, computed from the tentative values h •. In the corrector step, h't+ 1 , is obtained by first computing
hul = ht+[(1-a)·LP+a·LF·]+[(1-~)·HP+~·HF·]
(31)
and next adding the forcing effect AD't+1 to the above. Values assigned to the
weight parameters a and ~ are 0.506 and 2.5, respectively. Using these weights, the
low frequency waves are preserved in time marching, whereas the amplitudes of
high frequency waves are strongly suppressed 1.
This Hurricane Prediction System (GFDL) was adopted in 1995 by the US NWS
(National Weather Service) as an official tropical cyclone prediction technique. In
both the Atlantic and eastem Pacific basins, storm track predictions in 1995 were
substantially improved compared with predictions from several other models, in
particular for the period beyond 36 hours. For example, improvements in the track
forecasts in the Atlantic at 36,48 and 72 hours are 14, 19 and 25 %, respective1y,
relative to the next best NWS guidance at each forecast time.
10.4.2 Eta-model prediction
The dynamical part of the Eta-model is designed following the approach of
Arakawa for maintaining a variety of chosen integral constraints of the continuous
atmosphere (e.g. Janic 1984, Mesinger et al. 1988). Differencing schemes are of
second or approximately second order accuracy.
The current operational program of NCEP's Environmental Modeling Center
uses three models; Eta 48 km, Eta 29 km, and Eta 10 km models. These models are
identical except for the grid size and a single constant, the "horizontal diffusion"
coefficient.
The lateral boundary conditions are provided by a global spectral GCM of resolution T126L28; triangular truncation at 126 wave numbers and 28 verticallevels
(The Medium Range Forecasting Model, MRF, see Kanamitsu et al. 1991). This
run at an earlier time and is referred to as the "Aviation model". The Eta-model is
initialized with a static analysis using the global analysis as the first guess, and
other nested grid model used at NCEP is the NGM (Nested Grid Model), which has
80 km/L18 inner grid (Philips 1979).
1. HF = h~ + (Coriolis force, metric force, pressure gradient force for h=u,v, energy
at
conversion terms for h=D