Limited-area forecasting model based on semi-Lagrangian semi-implicit scheme leading to solving the separable elliptic equations

A significant number of atmospheric models use a semi-implicit algorithm in order to carry out the time integration. In the semi-implicit scheme, the size of the time step is no longer restricted by the phase speed of external gravity waves; it is limited mainly by the magnitude of the wind associated with advection. Combining a semi-Lagrangian scheme with the semi-implicit technique introduces the possibility of integrating atmospheric models using relatively large time steps. In this paper, the semi-implicit algorithm in association with the semi-Lagrangian technique is presented. To solve the equations of semi-implicit schema we have used method of dimension reduction which reduces the three-dimensional problem to two-dimensional problems. The original contribution of this way is that semi-implicit approximation leads to the solution of two-dimensional separable partial differential equations. A detailed description is given and the model is tested by using the Lambert projection and the initial data provided by the ARPEGE model. The results are very satisfactory.