Abstract
Kinetic energy spectra from the mesoscale numerical weather prediction (NWP) model ALADIN with horizontal resolution 4.4 km are split into divergent and rotational components which are then compared at horizontal scales below 300 km and various vertical levels. It is shown that about 50% of kinetic energy in the free troposphere in ALADIN is divergent energy. The percentage increases towards 70% near the surface and in the upper troposphere towards 100 hPa. The maximal percentage of divergent energy is found at stratospheric levels around 100 hPa and at scales below 100 km which are not represented by the global models. At all levels, the divergent energy spectra are characterised by shallower slopes than the rotational energy spectra, and the difference increases as horizontal scales become larger. A very similar vertical distribution of divergent energy is obtained by using the standard ALADIN approach for the computation of spectra based on the extension zone and by applying detrending approach commonly used in mesoscale NWP community.
Acknowledgements
We would like to thank our ALADIN and HIRLAM colleagues Lisa Bengtsson-Sedlar, Magnus Lindskog, Nils Gustafsson, Neva Pristov and Filip Vàña, as well as Ted Shepherd for discussions of the spectra and models. We appreciate comments from three anonymous reviewers which helped to improve the paper. VB gratefully acknowledges the support of the Slovenian Research Agency through PhD funding. The Centre of Excellence for Space Sciences and Technologies SPACE-SI is an operation part financed by the European Union, European Regional Development Fund and Republic of Slovenia, Ministry of Higher Education, Science, Sport, and Culture.