What makes a model hurricane head east?

Whether or not a hurricane strikes land is determined largely by the timing of its recurvature-the typical switch from a west or northwest path at low latitudes to a northeastward track at midlatitudes. Although track forecasts of tropical cyclones have improved steadily in recent decades, forecasting the time and location of recurvature still poses a challenge in some cases.

In the May 2004 issue of Monthly Weather Review, Adam O'Shay and T. N. Krishnamurti (Florida State University) analyze the factors that control recurvature in a numerical model. The scientists employed FSU's Global Spectral Model to simulate the life cycles of hurricanes Cindy and Dennis, both from 1999. Each was studied using eight versions of the FSU model: one full run, plus seven partitioned runs that intentionally omitted one or more of the physical and dynamical processes driving the atmosphere. These partitioned runs were adjusted at each time step with output from the full run. According to the authors, this technique "achieves a far greater understanding of how the storm recurvature occurs via each particular partitioning component, while simultaneously maintaining the integrity of the fields."

All of the FSU runs with full dynamics managed to recurve Cindy and Dennis. The tracks became progressively more accurate with the addition of three physical components: deep convection (showers and thunderstorms), shallow convection, and surface fluxes. The authors noted little improvement from the model's depiction of large-scale precipitation and radiation.

"Further research on tropical cyclones that 'may' recurve but eventually do not is an important and necessary extension to this work," write the authors.