Enhanced wind variability in the orographic gravity wave parameterisation and its influence on dynamics

Comprehensive global climate simulations are still conducted in fairly low resolution. Current general circulation models therefore rely on gravity wave parameterisations to simulate atmospheric dynamics correctly. Among other parameters, the surface wind determines gravity wave launching in orographic gravity wave parameterisations. However, the mountainous terrain in regions where orographic gravity waves occur suggests larger surface wind variability on unresolved topography than the model grid box wind can provide. To account for this variability, we here present a stochastic modification of the low-level wind direction when it is used in the orographic gravity wave scheme of the EMAC (ECHAM MESSy Atmospheric Chemistry) model. For our first application, we implemented a random normal function to evoke a modest deviation of the wind direction at each time step when it is used in the subgrid scale scheme.

An EMAC simulation shows that this gravity wave modification locally leads to significant changes of orographic gravity wave drag, but this does not result in significant annual or seasonal differences in temperatures or winds. However, the Arctic polar vortex is stretched and its center shifts in February. Moreover, we find a shift in the Antarctic polar vortex breakdown date, resulting in a significant zonal mean temperature change in October and possibly in an alleviation of the EMAC low bias in Antarctic polar vortex strength. In this presentation, we discuss our results, the method and possible further developments like allowing gusts in the modified scheme.