Since 1995, the GFDL huricane model has been an official operational product of the NWS, providing forecast guidance on track and intensity for the National Hurricane Center (NHC). The model was originally developed as a research tool, by scientists at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, to help understand the behavior and structure of tropical cyclones, and such topics as hurricane formation, decay and intensification. To adequately represent the structure of the hurricane and its inner core, the GFDL hurricane model required high resolution (distance between the individual grid points where the atmosphere equations of motion are solved), compared to other models of the atmosphere that define processes over the entire globe (typically called general circulation or global models). Also, advanced physics were required to correctly reproduce the processes that occur in the hurricane core, as well as the interaction with the ocean below.
In the early 2000s scientists at the NWS National Center for Environmental Prediction (NCEP) began to develop a new state of the art hurricane model using the most advanced numerical techniques available, to more accurately solve the mathematical equations that represent the processes of the atmosphere. This model (named HWRF, or Hurricane WRF) became operational in 2007, as an official product of the National Weather Service. Since then, improvements have been made to the HWRF modelling system every year, resulting in a steady reduction in track and intensity forecast errors. The recently upgraded HWRF model implemented for the 2014 hurricane season has shown much reduced track forecast errors compared to the 2013 version of HWRF for a large sample of North Atlantic basin tropical cyclones, with its performance comparable to the NHC Official Forecasts (Figure 1).
A major accomplishment is the significant reduction of intensity errors from the HWRF model in the past three years since the model was upgraded to run using cloud-permitting, 3 km resolution nests (see Figure 2), making it a primary model for intensity forecast guidance for NHC. Much of the increased skill seen in the HWRF model over the past 3 years was due to the successful collaboration between agencies within NOAA (GFDL, NCEP, AOML, ESRL) and partners within the academic community (such as the University of Rhode Island), that was made possible through the coordinated efforts and support from NOAA's Hurricane Forecast Improvement Project (HFIP). Apart from providing operational forecast guidance to the NHC for the Atlantic and Eastern Pacific basins, the HWRF model is also run in real-time for all global oceanic basins, providing forecast guidance to the US Navy's Joint Typhoon Warning Center (JTWC). All real-time forecast products from operational HWRF can be accessed from the HWRF website.
At the same time, scientists at GFDL have also upgraded the GFDL hurricane modeling system, with major improvements made operational in 2014, particularly to improve the prediction of hurricane intensity as shown in Figure 3. Note that the improvements made to the GFDL hurricane model reduced the error in the prediction of the storm maximum wind about 15% in the 3 to 5 day forecast time period, for a set of forecasts rerun from the 2008, 2010, 2011, and 2012 Atlantic hurricane seasons, using both the 2013 version of the GFDL model and the newly upgraded model.
The National Hurricane Center continues to support both of these operational regional hurricane models (HWRF and GFDL) since the forecast error of both models is often not correlated (individual model errors often go in different directions). Numerous scientific studies suggest that the average forecast from models that are well behaved produce errors that are less than those from the individual models. This has led to an increase in the use of model ensembles (models with slightly different initial conditions or different physics). For example, as shown in Figure 4, a consensus made up of the average of the intensity forecasts from the 2014 versions of the GFDL and HWRF (solid black line) results in an intensity forecast error that is significantly less than either the HWRF or GFDL model at every forecast lead time. Note that the average intensity forecast error of the 2-model consensus is even less than the HFIP 5 year goal at days 4 and 5, established in 2010.
Both the upgraded GFDL and HWRF modeling system did well for track and intensity forecasts for Hurricane Arthur, the first hurricane of the 2014 Atlantic hurricane season (Figure 5). The new GFDL and HWRF had very low track errors although the sample size was small, with the average intensity errors comparable to the other two top NWS intensity prediction models (Decay SHIPS and the LGEM). HWRF forecasts for Hurricane Arthur indicated perfect landfall location, timing and intensity at landfall, and the advanced products from HWRF model have been helpful to the NHC forecasters and NWS WFOs. An example of HWRF simulated composite radar reflectivity animation along with storm specific products are shown in Figure 6.
With continuous advancements to the NCEP hurricane models supported by HFIP, and enhanced computational resources available for operational models, we anticipate further improvements in track and intensity forecasts through improved hurricane physics, advanced inner core data assimilation, and increased horizontal and vertical resolutions. Apart from coupling the atmospheric model to the ocean model, future efforts also include coupling to wave, land surface, hydrology, and surge and inundation models for improved prediction of land falling storms.