Hurricane Juan (2003). Part II: Forecasting and Numerical Simulation

Abstract

The landfall of Hurricane Juan (September 2003) in the Canadian Maritimes represents an ideal case in which to study the performance of operational forecasting of an intense, predominantly tropical feature entering the midlatitudes. A hybrid cyclone during its genesis phase, Juan underwent a tropical transition as it drifted slowly northward 1500 km from the east coast of the United States. Shortly after reaching its peak intensity as a category-2 hurricane, the storm accelerated rapidly northward and made landfall near Halifax, Nova Scotia, Canada, with maximum sustained winds of 44 m s Ϫ1 . Although the forecasts and warnings produced by the U.S. National Hurricane Center and the Canadian Hurricane Centre were of high quality throughout Hurricane Juan's life cycle, guidance from numerical weather prediction models became unreliable as the storm accelerated toward the coast. The short-range, near-surface forecasts from eight operational models during the crucial prelandfall portion of Juan's track are investigated in this study. Despite continued improvements to operational numerical forecasting systems, it is shown that those systems not employing advanced tropical vortex initialization techniques were unable to provide forecasters with credible near-surface guidance in this case. A pair of regional forecasts, one successful and one from the failed model set, are compared in detail. Spurious asymmetries in the initial vortex of the deficient model are shown to hamper structural predictions and to cause nonnegligible track perturbations from the trajectory implied by the well-described deep-layer mean flow. The Canadian Mesoscale Compressible Community model is rerun with an improved representation of the hurricane's vortex in the initial state. The hindcast produced following the tropical cyclone initialization contains reduced track, structure, and intensity errors compared with those generated by the model in real time. The enhanced initial intensity produces a direct improvement in the forecast storm strength throughout the period, and the symmetrization of the vortex eliminates the interactions that plague the operational system. The southeastward relocation of the implanted vortex to Juan's observed location eliminates a significant northwestward track bias under the influence of a broad area of southerly steering flow. The study concludes that the initialization of Hurricane Juan's structure and position adds value to numerical guidance even as the storm accelerates poleward at a latitude where the implantation of a quasi-symmetric vortex may not be generally valid.

Modelling Extreme Storm-Induced Currents over the Grand Banks

“…To demonstrate how errors in a wind field can impact the model results, we consider storm-generated winds. The accurate analysis or simulation of the wind field over the ocean during an intense storm requires careful treatment of the initial condition in the weather forecast model (McTaggart-Cowan et al, 2006). Assuming the error in the wind field is 5% of the peak wind speed, we obtain an error of the order of 1.0 m s −1 for Storm 22 which has a maximum wind speed of 23 m s −1 .…”

Sensitivity of Tropical Cyclone Feedback on the Intensity of the Western Pacific Subtropical High to Microphysics Schemes

“…While the TC position forecast ''busts'' can be related to errors in the structure and intensity of the TC vortex (e.g., McTaggart-Cowan et al 2006), errors in the environmental wind appear to be dominant on errors in the TC track forecast (Galarneau and Davis 2013). For example, in the forecasts of TC Ike (2008) based on three operational global models, the environmental wind field in all three models steered TC Ike into southern Texas instead of recurving it over the Gulf of Mexico (Brennan and Majumdar 2011).…”

Diagnosing Forecast Errors in Tropical Cyclone Motion

“…While TC position forecast ''busts'' can be related to errors in the structure and intensity of the TC vortex (e.g., McTaggart-Cowan et al 2006), errors in the environment wind appear to have the greatest effect on TC position errors. For example, position forecasts for TC Ike (2008) from three operational global models 1 initialized at 0000 UTC 9 September 2008 all steered TC Ike into south Texas, rather than recurved Ike over the Gulf of Mexico (Brennan and Majumdar 2011).…”