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Richard M. Yablonsky, Isaac Ginis, Biju Thomas, Vijay Tallapragada, Dmitry Sheinin, and Ligia Bernardet

Abstract

The Princeton Ocean Model for Tropical Cyclones (POM-TC), a version of the three-dimensional primitive equation numerical ocean model known as the Princeton Ocean Model, was the ocean component of NOAA’s operational Hurricane Weather Research and Forecast Model (HWRF) from 2007 to 2013. The coupled HWRF–POM-TC system facilitates accurate tropical cyclone intensity forecasts through proper simulation of the evolving SST field under simulated tropical cyclones. In this study, the 2013 operational version of HWRF is used to analyze the POM-TC ocean temperature response in retrospective HWRF–POM-TC forecasts of Atlantic Hurricanes Earl (2010), Igor (2010), Irene (2011), Isaac (2012), and Leslie (2012) against remotely sensed and in situ SST and subsurface ocean temperature observations. The model generally underestimates the hurricane-induced upper-ocean cooling, particularly far from the storm track, as well as the upwelling and downwelling oscillation in the cold wake, compared with observations. Nonetheless, the timing of the model SST cooling is generally accurate (after accounting for along-track timing errors), and the ocean model’s vertical temperature structure is generally in good agreement with observed temperature profiles from airborne expendable bathythermographs.

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Hyun-Sook Kim, Carlos Lozano, Vijay Tallapragada, Dan Iredell, Dmitry Sheinin, Hendrik L. Tolman, Vera M. Gerald, and Jamese Sims

Abstract

This paper introduces a next-generation operational Hurricane Weather Research and Forecasting (HWRF) system that was developed at the U.S. National Centers for Environmental Prediction. The new system, HWRF–Hybrid Coordinate Ocean Model (HYCOM), retains the same atmospheric component of operational HWRF, but it replaces the feature-model-based Princeton Ocean Model (POM) with the eddy-resolving HYCOM. The primary motivation is to improve enthalpy fluxes in the air–sea interface, by providing the best possible estimates of the balanced oceanic states using data assimilated Real-Time Ocean Forecast System products as oceanic initial conditions (IC) and boundary conditions.

A proof-of-concept exercise of HWRF–HYCOM is conducted by validating ocean simulations, followed by the verification of hurricane forecasts. The ocean validation employs airborne expendable bathythermograph sampled during Hurricane Gustav (2008). Storm-driven sea surface temperature changes agree within 0.1° and 0.5°C of the mean and root-mean-square difference, respectively. In-storm deepening mixed layer and shoaling 26°C isotherm depth are similar to observations, but they are overpredicted at similar magnitudes of their ICs. The forecast verification for 10 Atlantic hurricanes in 2008 and 2009 shows that HWRF–HYCOM improves intensity by 13.8% and reduces positive bias by 43.9% over HWRF–POM. The HWRF–HYCOM track forecast is indifferent, except for days 4 and 5, when it shows better skill (8%) than HWRF–POM. While this study proves the concept and results in a better skillful hurricane forecast, one well-defined conclusion is to improve the estimates of IC, particularly the oceanic upper layer.

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