Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: 2149–2156, October 2011 B Intensity forecast experiment of hurricane Rita (2005) with a cloud-resolving, coupled hurricane–ocean modelling system Xin Qiu,a,b Qingnong Xiao,b,c Zhe-Min Tana* and John Michalakesb aKey Laboratory of Mesoscale Severe Weather/MOE, and School of Atmospheric Sciences, Nanjing University, China bMesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado, USA cCollege of Marine Science, University of South Florida, St Petersburg, Florida, USA *Correspondence to: Z.-M. Tan, School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China. E-mail: [email protected] A cloud-resolving, coupled hurricane-ocean modelling system is developed under the Earth System Modeling Framework using state-of-the-art numerical forecasting models of the atmosphere and the ocean. With this system, the importance of coupling to an eddy-resolving ocean model and the high resolution of the atmospheric model for the prediction of intensity change of hurricane Rita (2005) is demonstrated through a set of numerical experiments. The erroneous intensification in the uncoupled experiments could be eliminated when taking account of the negative feedback of sea-surface temperature cooling. Moreover, the deepening rate of Rita becomes larger with higher resolution of the atmospheric model. Nevertheless, the horizontal resolution of the atmospheric model with grid spacing at least less than ∼4 km is required in order to predict the rapid intensity changes of hurricane Rita in the fully coupled experiment. This strong dependence is found to arise from the potential interaction between the ocean coupling and internal processes of Rita, thus indicating that both high resolution and the ocean coupling are indispensable for the future improvement of hurricane intensity prediction. Copyright c 2011 Royal Meteorological Society Key Words: tropical cyclone; intensity change; air-sea interaction Received 21 July 2010; Revised 3 July 2011; Accepted 7 July 2011; Published online in Wiley Online Library 11 August 2011 Citation: Qiu X, Xiao Q, Tan Z-M, Michalakes J. 2011. Intensity forecast experiment of hurricane Rita (2005) with a cloud-resolving, coupled hurricane–ocean modelling system. Q. J. R. Meteorol. Soc. 137: 2149–2156. DOI:10.1002/qj.899 1. Introduction (e.g. Price, 1981). When encountering oceanic mesoscale features of high heat content, such as the Loop Current It is well known that latent heat flux through the (LC) and the Warm-Core Eddies (WCEs) in the Gulf ocean–atmosphere interface is the main source of energy of Mexico (GOM) region, the negative feedback of SST for tropical cyclones (TCs). As a TC intensifies, however, cooling is significantly reduced and rapid intensification of turbulent mixing due to shear induced by surface-wind stress the overlying storm may result (Hong et al., 2000). Spatial across the ocean mixed layer (OML) leads to entrainment variation of ocean heat content along the TC track is now below the OML and a subsequent decrease in the sea- widely accepted as one of the most important factors that surface temperature (SST). This effect weakens the overlying modulate TC intensity (Goni and Trinanes, 2003; Scharroo storm and presents a negative feedback mechanism in the et al., 2005; Mainelli et al., 2008). Among other factors, TC TC–ocean coupled system. In addition to storm intensity internal dynamics is of unparalleled significance. Increasing and translation speed, the degree of SST cooling depends research literature on the role of convective asymmetries has strongly on the thermal content of the underlying ocean led to an emerging view that TC intensity change involves life Copyright c 2011 Royal Meteorological Society 2150 X. Qiu et al. Figure 1. Six-hourly Best-Track estimated positions and intensities (on the Saffir–Simpson scale) of hurricane Rita from 20 to 25 September 2005. Spatial coverage of the Loop Current (‘LC’), the Warm-Core Eddy (‘WCE’) and the two Cold-Core Eddies (‘CCE 1’ and ‘CCE2’) is based on the sea-surface height anomaly (SSHA) analysis at 0000 UTC 20 Sep from Colorado Center for Astrodynamics Research (CCAR). Inset: Best-Track estimates of the minimum sea-level pressure (blue line) and the maximum surface wind speed (red line) of Rita from 20 to 25 September. cycles of various sub-storm-scale wave and vortex structures of Rita during this period coincided with its successive (Wang and Wu, 2004; Nguyen et al., 2008). Not surprisingly, encounters with oceanic mesoscale features of contrasting failure to include an accurate description of the TC–ocean thermodynamic properties (i.e. the LC and the Cold-Core feedbacks and the broad scales of internal processes in Eddies; see Figure 1), indicating that strong air–sea interac- operational TC prediction systems would undoubtedly limit tions occurred between Rita and the upper ocean. Moreover, the skills of intensity forecasts. Rita underwent complex eyewall structure changes (i.e. gen- Previous real-case simulations have demonstrated that esis of a secondary eyewall and the subsequent concentric the use of a coupled hurricane–ocean model significantly eyewall replacement) later on 22 September after it reached improved the intensity forecasts for hurricanes of moderate its maximum intensity. strength (Bender and Ginis, 2000). With insufficient model The primary objectives in this study are: (1) to assess the resolution, dynamically important convective features (and coupled system’s ability to reproduce the observed intensity the associated internal processes) inside the TC circulation and structure changes of hurricane Rita (2005) and the SST are poorly resolved, which is considered to be the most cooling in the GOM, and (2) to examine the importance probable explanation for the underestimation of intensity of hurricane–ocean coupling and horizontal resolution of variability among strong hurricanes (Davis et al., 2008). the atmospheric model in hurricane intensity prediction In recent years, the capability of computational resources in a coherent context. To fulfil these objectives, a cloud- has increased to the point where numerical models can resolving, coupled hurricane–ocean modelling system is be run at resolutions capable of resolving both the inner- developed under the Earth System Modeling Framework† core dynamics of the hurricane and the oceanic mesoscale (ESMF), which is a flexible and highly efficient software processes. This enables us to investigate the effects of framework designed to ease the developing of multi- hurricane–ocean coupling and increasing the horizontal component Earth-science modelling applications (Hill et al., resolution of the atmospheric model on improving 2004). A brief description of the coupled system will be hurricane intensity prediction in a coherent context. presented in section 2, along with the experimental design. Hurricane Rita (2005) intensified rapidly from Saf- The numerical results will be shown in section 3, followed by fir–Simpson Category 1 to Category 5 in less than 36 hours, the summary of results and concluding remarks in section 4. when translating over the warm waters of the LC in the southeastern GOM and within an environment of weak 2. System configuration, initialization and experimental vertical wind shear. After reaching its maximum strength design (897 hPa), Rita abruptly weakened to Category 4. Due to increasing southwesterly wind shear associated with an The Weather Research and Forecasting model (WRF Version approaching upper-level trough, it continued to weaken 3.1: Skamarock et al., 2008) and the Hybrid Coordinate before making landfall as a Category 3 hurricane (Knabb Ocean Model (HYCOM Version 2.2: Bleck, 2002; Chassignet et al., 2006). Of particular interest is the rapid intensification et al., 2003; Halliwell, 2004) form the hurricane and the and subsequent weakening event of Rita between 0000 UTC 20 September and 0000 UTC 23 September, which becomes the focus of the present study. The rapid intensity change †http://www.earthsystemmodeling.org/ Copyright c 2011 Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: 2149–2156 (2011) Intensity Forecast Experiment of Hurricane Rita (2005) 2151 30οN 27οN 24οN 21οN 18οN 96οW90οW84οW78οW72οW Figure 3. Six-hourly storm centres of hurricane Rita forecasted by the control experiment (grey line) and from the Best-Track estimates (black line) between 0000 UTC 20th and 0000 UTC 23rd September 2005. which is initialized at 1200 UTC 19 September 2005 with 1◦ × 1◦ Global Forecast System (GFS) analysis from National Centers for Environmental Prediction (NCEP). In order to better represent the vortex structure and intensity of Rita at the initial time of the 4 km domain, the Bogus Data Assimilation (BDA) procedure (Xiao et al., 2009) is conducted. 2.2. Ocean model HYCOM is a three-dimensional primitive-equation ocean Figure 2. Schematic diagram of HYCOM–WRF coupling through ESMF. circulation model, whose vertical coordinates are isopycnal in the open, stratified ocean, but smoothly revert to terrain- following in shallow coastal regions, and z-level in the ocean model components of the coupled system used in mixed-layer and/or unstratified seas (Bleck, 2002). The this study. The coupling is based on the conservation of horizontal domain coverage and grid resolution (0.04◦ at momentum, sensible and latent heat through the air–sea Equator) of the HYCOM are configured the same as those interface (Bender and Ginis,