Some design experiments for a nested grid forecast model of western Pacific tropical cyclones.
Ley, Gary William
Elsberry, Russell L.
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A three-dimensional, triply nested tropical cyclone forecast model was initialized from hand-analyzed synoptic-scale wind data. The diagnostic phase forced the mass fields from the wind fields by use of a suitable balance equation. Latent heat parameterization and frictional dissipation were omitted from the model to study movement primarily due to advective processes. For comparison, a three-level uniform coarse mesh grid model was initialized with the same real data. Time averaging of the pressure gradient terms of the momentum equations was incorporated into each model in an attempt to increase the maximum time increment. Movement forecasts of Typhoon Irma to 48-hours with both dynamic models were compared with operational forecasts and the post-analysis "best" track. Very satisfactory movement prognoses were obtained for the three periods forecasted with the dynamic models. The models correctly forecast the passage of a 500-mb trough to the north of the typhoon, apparently preventing recurvature. For movement forecasts, it appears that the nested model can be initialized with linear interpolation of data from the coarse grid to the finer mesh grids. The time increment was increased through use of the pressure averaging in the uniform CMG model; however, no time step increase was realized with the nested model due to lattice separation of the solution.
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