**Analysis of Hurricane Irene's Wind Field Using the Advanced Research Weather Research and Forecast (WRF-ARW) Model**

### **Alfred M. Klausmann**

**Abstract:** Hurricane Irene caused widespread and significant impacts along the U.S. east coast during 27–29 August 2011. During this period, the storm moved across eastern North Carolina and then tracked northward crossing into Long Island and western New England. Impacts included severe flooding from the mid-Atlantic states into eastern New York and western New England, widespread wind damage and power outages across a large portion of southern and central New England, and a major storm surge along portions of the Long Island coast. The objective of this study was to conduct retrospective simulations using the Advanced Research Weather Research and Forecast (WRF-ARW) model in an effort to reconstruct the storm's surface wind field during the period of 27–29 August 2011. The goal was to evaluate how to use the WRF modeling system as a tool for reconstructing the surface wind field from historical storm events to support storm surge studies. The results suggest that, with even modest data assimilation applied to these simulations, the model was able to resolve the detailed structure of the storm, the storm track, and the spatial surface wind field pattern very well. The WRF model shows real potential for being used as a tool to analyze historical storm events to support storm surge studies.

Reprinted from *J. Mar. Sci. Eng.* Cite as: Klausmann, A.M. Analysis of Hurricane Irene's Wind Field Using the Advanced Research Weather Research and Forecast (WRF-ARW) Model. *J. Mar. Sci. Eng.* **2014**, *2*, 33-45.

#### **1. Introduction**

During the period when Hurricane Irene was moving northward along the U.S. east coast, the storm was encountering increasing wind shear and cooler sea surface temperatures and was slowly weakening as it tracked from the Carolinas to New England. Despite this, Irene brought widespread and significant impacts along the east coast, causing severe flooding from the mid-Atlantic states into eastern New York and western New England, widespread wind damage and power outages across a large portion of southern and central New England, and a major storm surge along portions of the Long Island coast. The objective of this study was to perform retrospective numerical simulations with the Advanced Research Weather Research and Forecast model (WRF-ARW) and evaluate how to use the WRF modeling system [1] as a tool for reconstructing the surface wind field for historical storm events to support storm surge studies. The WRF wind and pressure fields can be used to drive a storm surge model such as the Advanced Circulation model (ADCIRC) as part of a storm surge analysis. WRF output may represent a potential source of data for storm surge analysis especially for regions with limited or no observational data.

Storm surge studies typically use a variety of approaches to re-analyze historical storms. These include parametric wind models [2,3] to develop a radial profile of the storm winds based on available data typically issued in National Hurricane Center advisories, including storm central pressure, maximum wind speed, and radius of maximum wind. These parametric-based approaches have been modified recently to account for storm asymmetries by allowing for storm quadrant specific profiles, again using data from available advisories that contain information about wind radii in each quadrant of the storm. Other approaches include hurricane boundary layer models [4] and objective analysis systems such as the Interactive Objective Kinematic Analysis (IOKA) system from Ocean Weather [5] and the HWIND analysis (also denoted as H\*WIND) from the National Oceanic and Atmospheric Administration's (NOAA's) Hurricane Research Division [6]. Data from the HWIND product is dependent on the density and quality of observational data, and to some extent relies on wind speeds extrapolated from flight level observations or dropsonde measurements, so the quality of the analysis may vary throughout a storm's history. Typically, a hurricane's inner core region is well sampled through reconnaissance flights and dropsonde data, while observational data may be more sparse at large distances from the center. Finally, use of nonsteady state dynamic models such as WRF offers another approach to storm reanalysis. There has been some work already done to study the use of the WRF model to drive the ADCIRC model for purposes of providing an improved storm surge prediction system [7]. However, to this author's knowledge, there have been no further applications of WRF as a tool to reanalyze historical storms events specifically for storm surge modeling studies.

WRF has a number of advantages over steady state and objective analysis approaches. The model simulates the evolution of atmospheric systems including tropical cyclones using full physics. It employs a range of physics options to account for cloud microphysics, atmospheric radiation processes, planetary boundary layer and surface layer processes, and parameterization of sub-grid scale moist convection. These capabilities allow WRF to simulate far field winds, spiral rainband structures, and supergradient flow in the inner core region [7], structures generally not resolved by other approaches. The WRF model also has full data assimilation capabilities including four-dimensional data assimilation (FDDA) and three-dimensional variational (3DVAR) approaches for blending the model fields with a diverse set of observational data [8,9]. Other more advanced data assimilation techniques such as four-dimensional variational (4DVAR) and Ensemble Kalman Filter (EnKF) techniques can also be applied to improve the analysis. Unlike HWIND or IOKA, the WRF model is not reliant on observations alone to simulate a storm event, but can use observations when available to help refine the model solutions by adjusting the model fields towards the observed data. The data assimilation process helps to constrain the model fields while preserving important structural features of the storm. Additionally, the model can output wind and pressure fields at both high spatial and temporal resolutions, thus eliminating the need for interpolating between analysis periods. Finally, the application of the WRF model is not limited to tropical cyclones but can also be used to simulate extra-tropical cyclones and hybrid type events such as Superstorm Sandy, where use of parametric modeling approaches are extremely limited. This study presents some results of an ongoing effort to optimize the WRF model for storm surge modeling applications. The focus of the work in this study was on WRF simulations of Hurricane Irene during the period 27–29 August 2011.
