The HWRFX Modeling System: Recent Developments in Hurricane Structure and Intensity Forecasting Research in NOAA

Forecasting intensity changes in Tropical Cyclones is a complex and challenging multi-scale problem because the factors that are known to influence these changes may vary in scales ranging between several thousands of kilometers (synoptic scale) to millimeters (microphysical scale). The hurricane forecast improvement project (HFIP) is a unified NOAA approach to guide and accelerate improvements in hurricane track, intensity and structure forecasts, with an emphasis on rapid intensity change. An integral component of the HFIP will be the development of improved coupled atmosphere-ocean-land surface, high-resolution non-hydrostatic regional models. A general introduction of NOAAs high-resolution, simple mixed-layer ocean coupled version of the experimental Hurricane WRF system, called the HWRFX, will be presented. The model has an option to provide both real-data and idealized simulations as well as operate in real time.

Some results of various tests using HWRFX will be presented here. Based on the High Resolution HFIP test results of 69 cycles of numerical forecasts from the 2005 and 2007 hurricane seasons, without ocean coupling, our findings indicate that, when compared to an operational resolution of 27 km domain with a 9 km moving nest (indicated by 27:9), higher and down-scaled resolution of 9:3 km improves track forecast for lead times, 24-48 h and improves intensity forecasts for lead times 0-30 h. HWRFX was also used for real time predictions in the 2009 season at 9:3 km. Track performance is comparable to other regional models, while the intensity bias is higher than other regional model for the 2009 season. Our initial testing, as well as those from all the HFIP tests, indicates that better resolution may be important to improve forecasts of vortex-scale motions, but the hurricane intensity change forecasting problem is beyond a simple model grid resolution problem. Better representation of physical processes in numerical models is pivotal for improving hurricane forecasting of the inner core. Ongoing work includes the development of multiple moving nests capable of providing an inner core resolution down to 1 km and the implementation of alternate physical packages that can operate at 1 km resolution. We will report some of our findings related to structure and intensity changes at 1 km resolution for ideal and real cases.