Nested grid

Nested grid capability This is required for multiscale simulations with a very fine model resolution over the areas of interest. 

Yamada, T. and Bunker S., 1988. Development of a nested grid, second moment turbulence-closure model and an application to the 1982 ASCOT Brush Creek data simulation, J. Atmos. Sci., Tl, pp. 562-578. 

Nested grid A grid system consists of fine inner mesh grids imbedded in a coarse mesh grid. 

The three levels nested grids method is adopted here (Fig. 3). The large model includes the Bohai sea mostly. The middle model covers the Bohai Gulf. The small model contains the sea area near the Caofeidian Harbour. The tidal prediction tool in MIKE was used to produce the tidal boundaries of the large model, which provides tide levels for middle model. The small model gets the tide levels in boundary from middle model. The minimum grid step of the small model is 5 m. 

Fig. 19.1 The model areas used for the coarse grid run yielding the forcing data (a) and for the nested simulations for the case low pressure system over Europe (b). The positioning of the nested area is shown by the frame in (a)...

The simulations nested in the coarse grid METRAS results agree in a similar way with the reference simulation in the second half of the simulation time (after about 21 h i.e. 18 CET of 29 August 2003). At this time the performance of simulation 5 is somewhat closer to the other nested simulations than before. This might be a hint that the nesting becomes less relevant and the situation is more locally driven. In the first 21 h of the simulation the two simulations with constant update intervals (3 h, 6 h) are closest to the reference case, while the adaptive update simulations (3, 4) show a high variability in performance. This is a hint that the acceleration is probably not a reliable measure to determine update intervals. The best performance is received in the present case study for a nesting every 3 h. 

The initial field data of the prediction model is the U U NCEP data. Two layer nested area is chosen as the forecast area. The horizontal network points of the first area is 75 x 75, the grid point distance is 81 kilometers the horizontal network points of the second area is 31 31, the grid point distance is 27 kilometers. The center point of the forecast area is located in 32°N and 118°E. The model vertical resolution selection is the inhomogeneous 31 layers (two layers in the surface layer), the iron tower and the radar observation point are both located in the second forecast area, its horizontal coordinate point is 22 its vertical coordinate point is 16. The initial time is at 00 00 GMT, 25 May 2002, the forecast time is 48 hours. For the stabilization of the calculation, the split-time-step method is adopt to cope with the acoustic item (u, V, w) in the forecast equation, which uses several short time step to replace the long time step for predicting the velocity field and the pressure field. The forecast areas are illustrated in Figure 3. 

The numerical model used in this study is a nonhydrostatic, primitive equation with terrain-following sigma(o) vertical coordinate MM5 version 3. The simulation performed for this study was run on two-way interactive nesting model. They consist of a 6-km grid (Doj, mesh size of 181 X 181), a 18-km grid (Doj, mesh size of 121 x 121) and a 54-km grid (Doj,... 

The existence of this expansion is hard to prove theoretically but easy to monitor within the adaptive algorithm. Solving the problem with different internal time stepsizes Atj = ATfjyj = ,..., and two nested space grids (Ari = AR, Av2 = AR/2) allows to eliminate the leading error coefficients by a coupled extrapolation in space and time. This extraploation... 

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