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Validation Test Report: Coupled Ocean/Atmosphere Mesoscale Prediction System-Tropical Cyclone Ensemble (COAMPS-TC Ensemble) v2021


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In this transition, the 11-member COAMPS-TC ensemble is upgraded from v2018 to v2021. As detailed in this report, this transition constitutes a significant upgrade. Included with this transition are: adjusted synoptic-scale initial time and lateral boundary perturbation magnitudes; updates to the shallow cumulus parameterization on grids 1 and 2; modifications to tcinit to produce smaller, more realistic tropical cyclones (TCs); the implementation of graupel-radiation interaction; initialization off of 0.25 deg GFS (versus 0.50 deg GFS as is presently done); GFS downscaling for weak TCs to produce a more realistic, less symmetric vortex for weaker storms; an updated surface drag coefficient; an improved 1-dimensional sea surface temperature cooling parameterization; changes to the interaction between grids 2 and 3 with the grid 1 blendzone; and increased diffusion in the first 6 h of the forecast for weak TCs. Versions v2021 and v2018 are compared against one another using a sample of 412 retrospective forecast cases for the unperturbed control member and 180 cases run using the full 11-member ensemble. Model upgrades in this transition collectively decrease mean absolute error (MAE) and bias for both track and intensity, improve rapid intensification (RI) statistics, improve the pressure-wind relationship and the intensity forecast distribution, and improve the 34, 50, and 64 kt wind radii. Probabilistic verification metrics show, for example, that uncertainty discrimination for track is slightly degraded but uncertainty discrimination for intensity is significantly improved.



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