8.2 Strange linear features in WRF clouds and precipitation: Diagnosis and correction

Mass, Clifford, and David Owens, University of Washington

During the past year it became obvious that the model output over the Pacific Ocean often showed strange, and often persistent, linear features, particularly in a post-frontal environment in which stability was reduced. In a comprehensive series of experiments, in which various boundary-layer schemes, vertical resolutions, and cumulus parameterizations were, it was found that these lines were highly sensitive to the convective parameterization, with only the SAS scheme providing satisfactory results. Other sets of strange linear features, mainly over land were also noted, and it was found they originated in reflections off the upper boundary (100 hPa) in the model. Lifting the model top to 50 hPa and using the WRF Rayleigh damping scheme with a sufficient number of levels near the model lid, radically reduced the phantom lines and improved the structure and tilt of mountain waves.