Sensitivity of HWRF to Outer Domain Size and Multiple Moving Nests

The Hurricane Weather Research and Forecast model (HWRF), a regional numerical forecasting system, has been used operationally at the National Hurricane Center (NHC) since 2007 to provide guidance for prediction of track and intensity of tropical cyclones (TC) in the North Atlantic and East Pacific hurricane basins. The model has undergone numerous improvements through the years and since 2012, HWRF has utilized a doubly-nested movable domain, i.e., a movable multi-level nest (MMLN) system within an outer domain which remains static during each forecast cycle. The latest operational versions have produced some of the most skillful intensity forecasts of the various North Atlantic basin models and track forecasts competitive with some of the global models. In order to increase the flexibility of the HWRF system a parallel research version was developed, called the Basin-Scale HWRF (aka HWRF-B), incorporating two main improvements: 1) a much larger outer domain that includes all of the North Atlantic, East Pacific and Central Pacific hurricane basins and 2) the capability for several MMLN to enable simultaneous forecasts of more than one TC per cycle. This study tests the impacts of these two key improvements using the 2015 versions of HWRF-B and a special version of the operational HRWF (both with ~27 km, ~9 km and ~3 km (27:9:3) horizontal resolutions of the outer domain and MMLN, respectively) for an assortment of TC’s from 2012-2015. Track and intensity verifications for the overall sample are presented. In addition, various individual cases and stratifications are examined, in particular looking at the handling of cycles with multiple TCs to see how the different versions handle direct, indirect, and remote TC-TC interactions.