Can Limited Area Mesoscale Models Forecast Tropical Cyclones?

The forecast position and intensity of a tropical cyclone (TC) drive the tropical storm/hurricane watches and warnings issued by NOAA. The small-scale inner core, commonly known as the “eye” of the hurricane, and the large-scale tropical circulation interact to trigger TC motion. TCs exhibit features on multiple scales ranging from the inner core, which is on the order of 10 km, to the steering flow, which can range from 1,000 to 10,000 km. Since numerical weather prediction models form the basis of the National Hurricane Center’s decisions, it is imperative that models be able to simulate multi-scale phenomena as accurately as possible. The current standard solution, employed by the Hurricane Weather Research and Forecasting (HWRF) model, is to embed a higher resolution limited-area model within a global model and utilizing one-way influence at the lateral boundaries. Lateral boundary conditions (LBC) play a crucial role in nested grids such as the HWRF parent grid as they provide the communication between the large-scale flow outside the limited-area model domain to its interior.

This study evaluates the effects of the HWRF LBC by comparing the flow inside the “blending zone” to the NCEP Global Forecast System (GFS) – the global model that forces the HWRF forecast. The departure of the GFS from the HWRF represents an error in the large-scale steering flow that moves the HWRF TC. By comparing the steering motion vector in both the HWRF and GFS at the center of the TC, it can be found that the differences between the two increase with time, explaining the greater error in the HWRF TC position forecast.