Gravity wave stress parameterization in a mesoscale sea breeze model

Linear wave theory is used to evaluate the stress on the atmospheric flow produced by breaking gravity waves launched by a sub-grid scale two-dimensional ridge in a mesoscale sea breeze simulation. The ridge is parallel to the coast line. The parameterization scheme is tested in the LADM diffusion model developed by the Division of Atmospheric Research of the CSIRO in Aspendale, Australia. Wave breaking is assumed to occur where either the gradient of potential temperature is negative or flow blocking occurs. It is shown that these two criteria are identical. The dissipation of the waves and the accompanying increase in turbulence occurs below the sea-breeze critical level where the mean horizontal wind speed is zero. The resulting turbulence leads to increased downward fluxes of momentum, heat, and pollutant concentration.

The ridge is loacted 20 km inland from the coast. Ridge sizes tested range in widths from 600 to 1800 m and heights from 100 to 300 m. The plumes originate 10 km inland from the coast, and plume heights are at 50, 100, 200, and 400 m. Model results show that breaking gravity waves can lead to significant increases in ground-level concentrations above what is expected for flat terrain, even for plumes as high as 400 m above the ground surface. It is concluded that failure to account for terrain generated gravity wave effects can lead to significant errors in dispersion calculations over regions of complex terrain or coastal environments.