A new gravity-wave-drag scheme incorporating anisotropic orography and low-level wave breaking: Impact upon the climate of the UK Meteorological Office Unified Model

A new anisotropic gravity-wave-drag parametrization-scheme which represents high-drag states modelled on hydraulic jump, flow blocking and internal-wave-reflection theory, including trapped lee-waves, is presented. The scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of PYREX case studies. Extended simulations of the Meteorological Office Unified Model at climate resolution are presented, showing the impact of the new scheme and a combination of the scheme with a new orographic-roughness parametrization. Results show a changed distribution of surface-gravity-wave surface-stress, mostly due to mountain anisotropy, and a greater tendency for lower-troposphere wave-drag. Inclusion of the orographic-roughness parametrization halves the gravity-wave stress, the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow.