Abstract : 2G.1
Impact of the non-hydrostatism and the new Semi-Lagrangian horizontal diffusion scheme on high-resolution forecast of bura using Aladin model

Martina Tudor
Croatian meteorological and hydrological service

The importance of non-hydrostatic effects in the case of stratified flow over isolated mountain has been studied in several (mostly academic) studies using various analytical and numerical models. These effects will be studied using Aladin model in 2 km horizontal resolution with hydrostatic and non-hydrostatic dynamics for the real flow over real mountains on several real cases of bura (Adriatic bora) in the area of Dynaric Alps (Velebit mountain).

The horizontal diffusion schemes in operational numerical models remove the energy accumulated due to finite truncation of a model spectrum acting as a numerical filter. The usual 4th order numerical horizontal diffusion has been replaced with the Semi-Lagrangian horizontal diffusion (SLHD) based on physical properties of the flow. SLHD is a stable and efficient non-linear horizontal diffusion, based on the control of the degree of interpolation needed for the semi-Lagrangian advection scheme that depends on the local flow. SLHD provides a flow-dependant horizontal diffusion a sort of first step towards a 3D turbulence.

The simulated wind speed 10 m above ground in 2 km horizontal resolution is lower with non-hydrostatic dynamics than with the hydrostatic one for a case of severe bura on the steepest mountain slopes. Introduction of SLHD has a strong impact on the simulated hydraulic jump and reduces the 10 m wind speed below it, downstream of the mountain obstacle. Both non-hydrostatic dynamics and SLHD have a strong impact on the PV (potential vorticity) field in the low troposphere reducing the high PV values in the vicinity of the orographic obstacles.