Short-Range Prediction in Isentropic Coordinates with Filtered and Unfiltered Numerical Models

Abstract

Two limited-area prediction models in isentropic coordinates, one based on potential vorticity conservation and one using the primitive equations of motion, are tested on 50 synoptic cases chosen from winter and spring of 1972/73. Both models disregard diabatic processes, but incorporate variable terrain height. Numerical instabilities caused by overturning or entwining coordinate surfaces appear to be no problem in this approach, and the gravitational noise generated at the lower boundary in the primitive equation model is shown to remain well within acceptable limits. Skill scores based on displacement errors of surface cyclones and on correlations between predicted and observed sea-level pressure gradients indicate that neither model at this stage can compete with the six-level hemispheric model used by the National Weather Service. Of the two isentropic models, the one using primitive equations shows better skill than the potential vorticity model.