Toward High Horizontal and Vertical Resolution Convection Resolving Modeling in Operational Hurricane Forecasting

As one of NCEP’s primary operational hurricane guidance models, the Hurricane Weather Research and Forecast (HWRF) modeling system is a sophisticated, high-resolution, air-sea coupled model designed to resolve inner-core features of hurricanes and to improve our understanding of multi-scale, spatial-temporal interactions between the storm and large-scale environment. Over the past few years, tremendous efforts had been made to increase horizontal and vertical resolutions for operational HWRF configuration, moving towards convection resolving modeling. In FY2012 HWRF implementation, a major accomplishment was the development of a 3-km resolution third nest (covering the inner core and most of surrounding circulation of the storm) and its transition to operations. This increased the model horizontal resolution from the 27/9-km nesting to the 27/9/3-km triple-nested configuration. In 2014, the vertical level configuration of operational HWRF upgraded from 43 levels with a 50-hPa model top to 61 levels with a 2-hPa model top. During the 2015 implementation, HWRF was upgraded from 27/9/3-km resolution to a higher resolution operating at 18/6/2-km resolution. In FY2017 HWRF implementation, the vertical resolution was further increased from 61 vertical levels (with a 2hPa model top) to 75 levels (with a 10-hPa model top). The increasing horizontal and vertical resolutions allows the model to better resolve the fine-scale hurricane inner-core structure and provide improved track, intensity, size and structure forecasts. Diagnosing and analyzing the real-time operational HWRF performance for the past several seasons demonstrate that the upgrades with increased horizontal and vertical resolutions together with the improvements in other aspects (e.g., model physics, vortex initialization and data assimilation, air-sea coupling) played important roles in advancing operational HWRF forecasting skills for both track and intensity forecasts. Experiments for idealized tropical cyclone simulations are conducted to investigate model sensitivity to different horizontal and vertical resolution configurations. In addition, retrospective tests for 2017 North Atlantic storms are conducted to assess the impact of further increasing horizontal resolution from 2-km grid spacing to 1.5-km grid spacing on hurricane track, intensity, size and structure forecasting.