Convection and waves on Small Earth and Deep Atmosphere

TitleConvection and waves on Small Earth and Deep Atmosphere
Publication TypeMiscellaneous
Year of Publication2015
AuthorsSemane, N, Bechtold, P
Secondary TitleTechnical Memorandum
Number752
Abstract

A scaled version of the ECMWF spectral hydrostatic forecast model (IFS) has been developed with full physics using an Aqua planet configuration. This includes Kuang et al's Small Earth Diabatic Acceleration and REscaling (DARE/SE) approach bringing the synoptic scale a factor $\gamma$ closer to the convective scale by reducing the Earth radius by $\gamma$, and increasing the rotation rate and all diabatic processes by the same factor. Furthermore, the scaled version also provides an alternative system to DARE/SE, dubbed 'Deep Atmosphere Diabatic Acceleration and REscaling' (DARE/DA), which reduces the gravity by a factor $\gamma$ and thereby increases the horizontal scale of convection by $\gamma$, while only weakly affecting the large scale flow. \\ The two approaches have been evaluated using a T159 spectral truncation and $\gamma=8$ with the deep convection scheme switched off. The evaluation is against the baseline unscaled model at T1279 spectral resolution without deep convection parametrization, as well as the unscaled T159 model using the deep convection parametrization. It is shown that the DARE/SE and DARE/DA systems provide fairly equivalent results, while the DARE/DA system seems to be the preferred choice as it damps divergent modes, providing a better climatology, and is technically easier to implement. However, neither of the systems could reproduce the motion range and modes of the high-resolution spectral model. Higher equivalent horizontal resolution in the 1-10 km range and the full non-hydrostatic system might be necessary to successfully simulate the convective and large-scale explicitly at reduced cost.