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The overview of the operational forecast using ALADIN model with NH dynamics

The nonhydrostatic and hydrostatic primitive equation versions of the dynamical kernel are implemented in the ALADIN model as a single package activated through a logical switch. Since both can be used with the same horizontal and vertical discretization, time-stepping and advection schemes (there is a choice for each of these schemes in ALADIN), it is possible to distinguish the differences in the model forecast due to introduction of nonhydrostatic component in the context of operational numerical weather prediction. Operational numerical weather prediction in the Croatian Meteorological and Hydrological Service relies on a high resolution dynamical adaptation of wind field that has proven to be essential for the prediction of severe wind events in mountainous terrain along the Adriatic coast. In the operational suite, a 2 km resolution 24 hour forecast was established since 1 July 2011, that uses non-hydrostatic (NH) dynamics and the full parametrization set, including radiation, microphysics and convection schemes. It runs on the same domain as the hydrostatic dynamical adaptation. The NH run uses semi- implicit semi-lagrangian scheme to solve the dynamical and advection terms and semi- lagrangian horizontal diffusion. ALADIN uses a simple microphysics scheme with prognostic cloud water and ice, rain and snow and a statistical approach for sedimentation of precipitation. The operational radiation scheme that describes the transfer, scattering, absorption and reflection of the shortwave solar radiation and longwave thermal radiation of the Earth's surface and clouds is simple and computationally cheap since it uses only one spectral band for long-wave and one for short- wave radiation computations. The turbulent exchange coefficients are computed using prognostic values of turbulent kinetic energy (TKE). The surface scheme used in the operational forecast is ISBA (Interaction Soil Biosphere Atmosphere). The parametrization of convection is used even in the 2 km resolution forecast since its formulations disables double counting for the convection in the resolved and stratiform part. The deep convection is a prognostic mass-flux scheme where convective processes are treated with the use of prognostic variables for updraft and downdraft vertical velocities and mesh fractions. The NH run has been able to predict short duration events of severe bura as well as calm periods in the long bura events that were not predicted by other runs in the operational suite. However, its forecast also yields errors due to wrong timing of such events (by few hours). The errors in the model forecast have revealed the need for better representation of the surface characteristics especially in the mountainous terrain.

operational forecst ; nonhydrostatic dynamics

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