Workshop: Stratospheric predictability and impact on the troposphere

Part of gravity wave research is motivated by the need to improve the
representation of their impacts on the large-scale circulation in climate
models. As a major portion of the gravity wave spectrum is subgrid-scale,
parameterizations are responsible for this. Gravity wave parameterizations
generally share a common framework, which includes common assumptions on
their propagation (columnar only) and their sources (tropospheric only). These
assumptions are very justified to leading order and parameterizations have been
successful in allowing models to reproduce a number of middle atmospheric
features. Once this framework is setup, the choice of the characteristics of
the sources is a necessary step but it remains fairly arbitrary, particularly for
non-orographic sources, and hence constitutes a prime suspect for errors and
uncertainty. As sources are poorly constrained, they are conveniently tuned
to improve the modelled atmospheric circulation. Consequently, significant
efforts have been carried out to better quantify the sources of gravity
waves, combining modeling and observations. This has stimulated formidable
progress in our description and understanding of atmospheric gravity waves.
Transfer to parameterizations however is not straightforward: knowledge of the
characteristics of lower stratospheric gravity waves does not directly translate
into input parameters for parameterizations. The example of intermittency
is used to illustrate the potential impact of a shift in the parameterization
framework, leading to a redistribution of the resulting forcing in the middle
atmosphere. The better knowledge of atmospheric gravity waves, obtained
in recent years, highlights a number of phenomena which fall outside of
the classical framework of parameterizations, notably lateral propagation
and secondary generation. This growing evidence calls for investigations
to determine which of these phenomena may have systematic and robust
implications for the larger-scale flow.