Strategy for Regional Seasonal Forecasts
189
10.3.4 Lateral mesh nesting
A fine mesh (FM) is embedded in a larger coarse mesh (CM). There are two versions of FM-CM connection, Le., one-way nesting and two-way nesting. Often
there is an overlapping (transitional) zone. In general, the lateral boundary treatment should serve two purposes. It should be capable oftransmitting the large scale
flow resolved by the external boundary data smoothly into and out of the limited
domain, and it should also adequately represent the outgoing gravity waves and
fine-mesh-scale flow (see the review by Wamer et al. 1997).
For the FM-CM connection, three additional considerations are:
1) An overlapping zone is normally required in both one-way and two-way nesting schemes. 2) In two-way nesting, the conservation of fluxes is essential for first
order quantities, such as momentum and vertical velocity, and desirable for the second order quantities, such as kinetic energy and/or enstrophy, though this may not
be required in one-way connection. 3) For open boundary conditions, some
schemes treat the flow differently dependent on its direction, inflow to or outflow
from FM. See Kurihara, Tripoli and Bender (1979).
In order to satisfy these requirements, different methods have been developed:
a. radiation condition: representative phase velocity is estimated numerically, see
Orlansky (1976).
b. wind direction dependent scheme: gravity waves should be sufficiently suppressed, and fields smooth near the transition zone. See Kurihara, Kerr and Bender
(1989), and the next section.
c. boundalY zone: Substantial damping is applied to reference fields in broad
overlapping zone: Davies (1976), or tendency weighting (predicted and specified),
Perkey and Kreitzberg, (1976).
10.4 Nested predictions
Two examples of nested prediction are given in this section. One is a two-way
ne sting for hurricanes forecast (Kurihara et al. 1997), and the other is the one-way
ne sting for short-range weather forecast at NCEP (National Centre for Environmental Prediction, USA (Mesinger et al., 1988». For seasonal prediction, the latter
approach will be used (Ji and Verneckar, 1997). However, the frrst example will be
shown here because of its remarkable achievement as a nested calculation.
10.4.1 Hurricane prediction
Forecast from the NCEP global model are used to specify the time-dependent lateral boundary conditions for the MMM model. Mesh C is a one-way from the global model, while meshes M and F are two-way nested in meshes C and M,
respectively.
Mesh movement: The interior meshes F and M are relocated so that their centers
are near the forecast storm center at all times. Mesh F moves in the zonal and
189
10.3.4 Lateral mesh nesting
A fine mesh (FM) is embedded in a larger coarse mesh (CM). There are two versions of FM-CM connection, Le., one-way nesting and two-way nesting. Often
there is an overlapping (transitional) zone. In general, the lateral boundary treatment should serve two purposes. It should be capable oftransmitting the large scale
flow resolved by the external boundary data smoothly into and out of the limited
domain, and it should also adequately represent the outgoing gravity waves and
fine-mesh-scale flow (see the review by Wamer et al. 1997).
For the FM-CM connection, three additional considerations are:
1) An overlapping zone is normally required in both one-way and two-way nesting schemes. 2) In two-way nesting, the conservation of fluxes is essential for first
order quantities, such as momentum and vertical velocity, and desirable for the second order quantities, such as kinetic energy and/or enstrophy, though this may not
be required in one-way connection. 3) For open boundary conditions, some
schemes treat the flow differently dependent on its direction, inflow to or outflow
from FM. See Kurihara, Tripoli and Bender (1979).
In order to satisfy these requirements, different methods have been developed:
a. radiation condition: representative phase velocity is estimated numerically, see
Orlansky (1976).
b. wind direction dependent scheme: gravity waves should be sufficiently suppressed, and fields smooth near the transition zone. See Kurihara, Kerr and Bender
(1989), and the next section.
c. boundalY zone: Substantial damping is applied to reference fields in broad
overlapping zone: Davies (1976), or tendency weighting (predicted and specified),
Perkey and Kreitzberg, (1976).
10.4 Nested predictions
Two examples of nested prediction are given in this section. One is a two-way
ne sting for hurricanes forecast (Kurihara et al. 1997), and the other is the one-way
ne sting for short-range weather forecast at NCEP (National Centre for Environmental Prediction, USA (Mesinger et al., 1988». For seasonal prediction, the latter
approach will be used (Ji and Verneckar, 1997). However, the frrst example will be
shown here because of its remarkable achievement as a nested calculation.
10.4.1 Hurricane prediction
Forecast from the NCEP global model are used to specify the time-dependent lateral boundary conditions for the MMM model. Mesh C is a one-way from the global model, while meshes M and F are two-way nested in meshes C and M,
respectively.
Mesh movement: The interior meshes F and M are relocated so that their centers
are near the forecast storm center at all times. Mesh F moves in the zonal and