Impact Study Of The Vertical Coordinate On A High-resolution Meso-scale Numerical Prediction Model

Now, studies of the meso-scale non-hydrostatic high-resolution model become hot issues. Some of new techniques about meso-scale non-hydrostatic numerical weather prediction are addressed, and one of them is the impact of the vertical coordinate system on the NWP. At the same time, the vertical coordinate is focused in the design of an atmospheric (oceanic) dynamical model, because it is directly associated with boundary conditions, the property of phydical conservation and gradient computational errors etc.In the field of NWP, many coordinate systems have been used. The terrain-following coordinate system which is generally used in a high-resolution meso-scale numerical prediction model, is focused and studied, including terrain-following height coordinate (height coordinate) and pressure-based terrain-following coordinate (mass coordinate). The obvious differences between height coordinate and mass coordinate are addressed. The computation of pressure gradient force (PGF) in the mass coordinate is sensitive to the computational error of surface pressure; PGF errors are a little difference between the two coordinates in lower levels, but PGF errors of the height coordinate are obviously smaller than those of the mass coordinate in higher levels. In the ideal experiment numerical computational results of the two coordinates are sensitive to the resolution, the slope of terrain and the distribution of physical property with the height. Two heavy terrain storm processes are simulated and disgnosed, and the simulation result are analysed in the different coordinates.Through theoretical analysis, ideal experiment and pratical case simulation , the simulated results in the high-resolution meso-scale numerical prediction model are sensitive to the choice of vertical coordinates. The error of surface pressure in the mass coordinate causes the difference of vertical velocity computation. Due to the error of surface pressure, the computational error value of vertical velocities reach 1.5 m/s and that of precipitation is 50mm. The impact study of the vertical coordinate on a non-hydrostatic meso-scale high-resolution model indicates that the errors are a little difference between the height coordinate and the mass coordinate in lower levels, but the errors of the height coordinate are obviously smaller than those of the mass coordinate in higher levels. The correlative coefficient and RMSE analysis of the 24hr simulated prediction show that the prediction ability of height coordinate is better than that of the mass coordinate in the high-resolution meso-scale numerical prediction model. The impact study of the vertical coordinate on a multi-scale unified model indicates that the forecast fields of physical variables between the height coordinate and mass coordinate are distinct in the different resolutions, and the forecast fielddifference between the two coordinates become bigger in the higher resolution. For example, the biggest difference value of vertical velocities between the two coordinates is 0.15m/s in the 30km resolution, but the biggest difference value of vertical velocities between the two coordinates reaches 4.5m/s in the 2.5km resolution. The impact study of the different vertical coordinate on terrain precipitation is simply discussed. The simulation results point out that the terrain precipitation in the different vertical coordinate systems is closely related with the model resolution. The higher is the resolution, the more obvious is the prediction precipitation difference in the different vertical coordinates.From above, the research results in this paper are meaningful for the non-hydrostatic high-resolution multi-scale unified model.