Intensity forecast experiment of hurricane Rita (2005) with a cloud-resolving, coupled hurricane ocean modelling system

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1 Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. (2011) 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 Tan a * and John Michalakes b a Key Laboratory of Mesoscale Severe Weather/MOE, and School of Atmospheric Sciences, Nanjing University, China b Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado, USA c College of Marine Science, University of South Florida, St Petersburg, Florida, USA *Correspondence to: Z.-M. Tan, School of Atmospheric Sciences, Nanjing University, Nanjing , China. zmtan@nju.edu.cn 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 Received21 July 2010; Revised3 July 2011; Accepted7 July 2011; Published online in Wiley Online Library Citation: Qiu X, Xiao Q, Tan Z-M, Michalakes J Intensity forecast experiment of hurricane Rita (2005) with a cloud-resolving, coupled hurricane ocean modelling system. Q. J. R. Meteorol. Soc. DOI: /qj Introduction It is well known that latent heat flux through the ocean atmosphere interface is the main source of energy for tropical cyclones (TCs). As a TC intensifies, however, turbulent mixing due to shear induced by surface-wind stress across the ocean mixed layer (OML) leads to entrainment below the OML and a subsequent decrease in the seasurface temperature (SST). This effect weakens the overlying storm and presents a negative feedback mechanism in the TC ocean coupled system. In addition to storm intensity and translation speed, the degree of SST cooling depends strongly on the thermal content of the underlying ocean (e.g. Price, 1981). When encountering oceanic mesoscale features of high heat content, such as the Loop Current (LC) and the Warm-Core Eddies (WCEs) in the Gulf of Mexico (GOM) region, the negative feedback of SST cooling is significantly reduced and rapid intensification of the overlying storm may result (Hong et al., 2000). Spatial variation of ocean heat content along the TC track is now widely accepted as one of the most important factors that modulate TC intensity (Goni and Trinanes, 2003; Scharroo et al., 2005; Mainelli et al., 2008). Among other factors, TC internal dynamics is of unparalleled significance. Increasing research literature on the role of convective asymmetries has led to an emerging view that TC intensity change involves life Copyright c 2011 Royal Meteorological Society

2 X. Qiu et al. Figure 1. Six-hourly Best-Track estimated positions and intensities (onthesaffir Simpson scale) ofhurricanerita from20 to25september 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 (Wang and Wu, 2004; Nguyen et al., 2008). Not surprisingly, failure to include an accurate description of the TC ocean feedbacks and the broad scales of internal processes in operational TC prediction systems would undoubtedly limit the skills of intensity forecasts. Previous real-case simulations have demonstrated that the use of a coupled hurricane ocean model significantly improved the intensity forecasts for hurricanes of moderate strength (Bender and Ginis, 2000). With insufficient model resolution, dynamically important convective features (and the associated internal processes) inside the TC circulation are poorly resolved, which is considered to be the most probable explanation for the underestimation of intensity variability among strong hurricanes (Davis et al., 2008). In recent years, the capability of computational resources has increased to the point where numerical models can be run at resolutions capable of resolving both the innercore dynamics of the hurricane and the oceanic mesoscale processes. This enables us to investigate the effects of hurricane ocean coupling and increasing the horizontal resolution of the atmospheric model on improving hurricane intensity prediction in a coherent context. Hurricane Rita (2005) intensified rapidly from Saffir Simpson Category 1 to Category 5 in less than 36 hours, when translating over the warm waters of the LC in the southeastern GOM and within an environment of weak vertical wind shear. After reaching its maximum strength (897 hpa), Rita abruptly weakened to Category 4. Due to increasing southwesterly wind shear associated with an approaching upper-level trough, it continued to weaken before making landfall as a Category 3 hurricane (Knabb et al., 2006). Of particular interest is the rapid intensification 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 of Rita during this period coincided with its successive encounters with oceanic mesoscale features of contrasting thermodynamic properties (i.e. the LC and the Cold-Core Eddies; see Figure 1), indicating that strong air sea interactions occurred between Rita and the upper ocean. Moreover, Rita underwent complex eyewall structure changes (i.e. genesis of a secondary eyewall and the subsequent concentric eyewall replacement) later on 22 September after it reached its maximum intensity. The primary objectives in this study are: (1) to assess the coupled system s ability to reproduce the observed intensity and structure changes of hurricane Rita (2005) and the SST cooling in the GOM, and (2) to examine the importance of hurricane ocean coupling and horizontal resolution of the atmospheric model in hurricane intensity prediction in a coherent context. To fulfil these objectives, a cloudresolving, coupled hurricane ocean modelling system is developed under the Earth System Modeling Framework (ESMF), which is a flexible and highly efficient software framework designed to ease the developing of multicomponent Earth-science modelling applications (Hill et al., 2004). A brief description of the coupled system will be presented in section 2, along with the experimental design. The numerical results will be shown in section 3, followed by the summary of results and concluding remarks in section System configuration, initialization and experimental design The Weather Research and Forecasting model (WRF Version 3.1: Skamarock et al., 2008) and the Hybrid Coordinate Ocean Model (HYCOM Version 2.2: Bleck, 2002; Chassignet et al., 2003; Halliwell, 2004) form the hurricane and the

3 Intensity Forecast Experiment of Hurricane Rita (2005) 30 ο N 27 ο N 24 ο N 21 ο N 18 ο N 96 ο W 90 ο W 84 ο W 78 ο W 72 ο W 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 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 Ocean model Figure 2. Schematic diagram of HYCOM WRF coupling through ESMF ocean model components of the coupled system used in this study. The coupling is based on the conservation of momentum, sensible and latent heat through the air sea interface (Bender and Ginis, 2000; Hong et al., 2000). In addition, evaporation and precipitation are handled as source (sink) and sink (source) of salinity (mass) in the ocean mixed layer. The wind stress and all the fluxes are computed in the WRF model and passed to the HYCOM. This is accomplished by exchanging the ESMF fields contained in the export (import) state of the WRF (HYCOM) model component through an ESMF coupler component (Figure 2). The opposite is applied to SST, which is kept constant in the WRF model during each coupling time step Hurricane model The WRF model is configured with one computational domain of 4 km horizontal resolution. The domain includes the GOM, northwest part of the Caribbean Sea and the West Atlantic basin (Figure 3). Physical processes used are the Rapid Radiative Transfer Model (RRTM) scheme for long-wave radiation and Dudhia scheme for shortwave radiation, the Purdue Lin cloud microphysics scheme, and the Yonsei University (YSU) planetary boundary-layer parametrization. Calculation of surface stress and fluxes uses the new bulk formulation in WRF V3.1, which is in line with the recent research results under hurricane wind conditions (Donelan et al., 2004; Black et al., 2007). The WRF model is initialized at 0000 UTC 20 September The boundary and initial conditions for the 4 km domain come from a previous run of 12 km coarse domain, HYCOM is a three-dimensional primitive-equation ocean circulation model, whose vertical coordinates are isopycnal in the open, stratified ocean, but smoothly revert to terrainfollowing in shallow coastal regions, and z-level in the mixed-layer and/or unstratified seas (Bleck, 2002). The horizontal domain coverage and grid resolution (0.04 at Equator) of the HYCOM are configured the same as those of the WRF model, so that the two grid systems exactly match. Twenty-two hybrid layers are used in the vertical, and the K-Profile parametrization (KPP: Large et al., 1994) is used for describing the vertical mixing processes. The initial and boundary conditions for the ocean model come from the daily analysis of the eddy-resolving (0.08 at Equator) basin-scale Atlantic HYCOM data assimilation system (Chassignet et al., 2007). To minimize spin-up time, the bathymetry is also interpolated from the Atlantic HYCOM domain Experimental design All the numerical experiments (Table I) start from 0000 UTC 20 September 2005 and are run for 72 hours. In the control experiment (CPLCTRL), models of the atmosphere and the ocean are fully coupled. In experiment UCPLTSK, the initial SST is from the GFS analysis of skin temperature and is held constant. To isolate the effect of initial SST and ocean coupling, experiment UCPLSST is carried out, where the initial SST is from the HYCOM analysis and held constant. In experiment CPLHRES, the WRF model uses an additional 1.33 km resolution, vortex-following domain, which covers an area of 400 km 400 km. The moving nest starts after 30 hours simulation of the outer 4 km domain, just prior to the onset of rapid intensification. This nest is two-way nested with the 4 km domain, which is coupled to the HYCOM. Based on experiment CPLHRES, two extra experiments (SEN DCPL and SEN LRES) are carried out to

4 X. Qiu et al. Table I. A summary of numerical experiments. No. Name Initial SST Coupled Resolution (km) 1 CPLCTRL HYCOM SST Yes 4 2 UCPLTSK NCEP Skin Temperature No 4 3 UCPLSST HYCOM SST No 4 4 CPLHRES HYCOM SST Yes SEN DCPL Same as CPLHRES except that the atmospheric model is decoupled from the ocean model from 2100 UTC 21 Sep 6 SEN LRES Same as CPLHRES except that the moving nest is removed from 2100 UTC 21 Sep examine the sensitivity of intensity and structure changes of Rita to the ocean coupling and the horizontal resolution of the atmospheric model during the rapid weakening phase. 3. Results 3.1. Track The simulated tracks in all experiments are similar, with the largest spread less than 30 km. Therefore, only the six-hourly storm centres from the control experiment (CPLCTRL) are shown in Figure 3, along with the Best-Track estimates obtained from the National Hurricane Center (NHC). Because of the BDA initialization, the storm centre at initial time is nearly at the same location as observed (within the accuracy of model grid spacing). The simulated storm translation speed generally compares well with the observation. Both the simulation and observation show fast movement over the first 48 h and then slowing down during the last 24 h when Rita rounded the southwestern tip of the Western Atlantic Ridge. However, the simulated storm moved faster than observed between hours 36 and 60. A northward drift of the simulated track became evident after the storm passed Florida Strait and entered the southeastern GOM. This bias reached a maximum at the end of the second day, while the observations indicated Rita nearly stayed along the same latitude between 0000 UTC and 1800 UTC 21 September. During the last 24 h, the simulated storm headed northwestward as observed, but with the final position 166 km further northwest than that of Best- Track position. Much of this track error is caused by the combination of faster movement (between hours 36 and 60) and northward drift (during the second model day) of the simulated storm. Although the track errors in the experiments generally increase linearly with time (54, 127, 166 km at hours 24, 48, 72 respectively in the control experiment), they are much lessthantheaveragedbiasforthesameleadtimeinthe present-day operational forecasts. Since the spatial scale of both the WCE and the LC is generally O( km), the track errors are considered minor, thus providing the basis for accurate intensity predictions in the coupled experiments Intensity and structure The 3-day intensity forecasts in terms of minimum sea-level pressure (MSLP) and 10 m maximum wind speed (MAXW) are shown in Figure 4(a) and (b), respectively. The control experiment (CPLCTRL) begins with an enhanced initial vortex defined by the BDA procedure, resulting in an MSLP of 996 hpa (4 hpa higher than observation). Time series of boththemslpandmaxwcomparewellwiththebest- Track estimate until hour 36, when the vortex reaches an MSLP of 940 hpa. After this point, the intensification rate slows down and the storm obtains an MSLP of 920 hpa and MAXW of 67 m s 1 at hour 57, while the observations indicate Rita underwent even more rapid intensification. The simulated storm weakens during the next 15 hours, with the MSLP increased from 921 hpa to 923 hpa. While the intensity change during the last 36 hours is not as dramatic as observed, the 4 km resolution, coupled experiment has at least marginally reproduced a weakening tendency. The two uncoupled experiments start from the same vortex as that in CPLCTRL. The intensity of the UCPLSST vortex is nearly the same as that in CPLCTRL during the first 20 h of simulation. After that, the simulated storm in UCPLSST grows stronger than that in CPLCTRL, indicating that the effect of SST cooling becomes obvious only when the storm is strong enough. Since the SST is generally higher than 29 C along the simulated track (figure not shown) and fixed in time, the storm in UCPLSST intensifies monotonically to an MSLP of 899 hpa, 24 hpa deeper than that in CPLCTRL at the end of simulation. The intensity change of the simulated storm in UCPLTSK is similar to that in UCPLSST, but the deepening rate is larger. This difference is due to the NCEP skin temperature being about 0.5 C higher than the SST from HYCOM analysis (figure not shown). The persistent intensification as shown in both uncoupled experiments (UCPLTSK and UCPLSST) suggests that the effect of SST cooling accounts for the weakening of Rita during the later stage of the control experiment. In order to examine the sensitivity of the simulated storm intensity in CPLCTRL to the horizontal resolution of the atmospheric model, experiment CPLHRES is carried out. With a moving nest of 1.33 km resolution added after 30 hours into the 4 km domain simulation, the intensification rate of the simulated storm is much larger than that in CPLCTRL and agrees better with observation. Maximum intensity is obtained at hour 52, with an MSLP of 909 hpa and MAXW of 75 m s 1.Afterwards,theMSLP fills rapidly to 920 hpa and MAXW decreases to 66 m s 1. Compared with CPLCTRL, the higher resolution of the atmospheric model captures better the rapid intensification, maximum intensity and the subsequent weakening processes of Rita. Moreover, the simulated Rita in CPLHRES also generated a secondary eyewall during 22 September as reported by Knabb et al. (2006). After the initiation of the moving nest, a convective outer rain band quickly develops and spirals cyclonically outward from the rightquadrant (relative to the storm motion) to the rear-quadrant (Figure 5(a)) as Rita traverses the GOM. The convective

5 Intensity Forecast Experiment of Hurricane Rita (2005) (a) (b) Figure h model forecasts and Best-Track data of (a) minimum sea-level pressure and (b) maximum wind speed at 10 m between 0000 UTC 20th and 0000 UTC 23rd September 2005 Figure 5. Snapshots of simulated reflectivity (units dbz) at 3 km altitude in experiment CPLHRES ((a) (c)) and experiment SEN DCPL ((d) (f)), showing the different behaviors with respect to eyewall evolution in the two experiments.

6 X. Qiu et al. features in the downwind portion of the outer spiral rain band seem to be gradually axisymmetrized by the storm core during the weakening phase (Figure 5(b)), and a secondary convective ring outside the original eyewall began to form several hours later (Figure 5(c)). Different from the primary eyewall, the convection in the secondary eyewall is organized in the form of small-scale cells that are distributed intermittently and tend to be elongated, which is very similar to the airborne Doppler radar observations of the same hurricane (see Fig. 2b in Houze et al., 2007; and Fig. 3a in Didlake and Houze, 2011). At first glance, it is not straightforward to understand why the rapid decay of Rita is better simulated with the aid of higher horizontal resolution of the atmospheric model. The clue lies in the well-established fact that the formation of secondary eyewall commonly weakens the storm intensity (e.g. Willoughby et al., 1982), which could contribute to the rapid weakening in addition to the negative feedback of SST cooling. Therefore, two extra sensitivity experiments (SEN LRES and SEN DCPL) are carried out, and supportive evidence is shown below. After the 1.33 km moving nest is removed, the simulated storm in SEN LRES underwent a rapid adjustment, with the MSLP increased by 4 hpa and MAXW decreased by 5 m s 1. The intensity of the simulated storm then gradually converges to that in CPLCTRL. No secondary eyewall formation is found in experiment SEN LRES. In view of the recent studies on the formation of secondary eyewall which highlight the roles of small-scale convective features (e.g. Terwey and Montgomery, 2008; Qiu et al., 2010; Abarca and Corbosiero, 2011), it is not surprising that all the experiments with 4 km resolution fail to reproduce such an eyewall structure change. However, the simulated storm in SEN DCPL did not generate a secondary eyewall either (Figure 5(d) (f)). Instead, intensification continued as the simulated storm in SEN DCPL was approaching the unmodified waters with much warmer SST. Based on the results above, it is clear that the effect of SST cooling directly leads to the weakening. However, this source alone could not account for the large decaying rate both observed and well simulated in CPLHRES. Therefore, it is reasonable to attribute the extra source of the rapid weakening to the formation of the secondary eyewall, which could only be realistically reproduced with the aid of sufficient resolution of the atmospheric model. Of particular interest are the different behaviours with respect to the eyewall evolution in experiments CPLHRES and SEN DCPL. This might suggest the potential role of hurricane ocean coupling in the formation of secondary eyewall. A heuristic explanation is given here before our attempt toward more detailed understanding. During the rapid intensification period, the inner core of Rita quickly develops an elevated vorticity ring with little vorticity gradient at and beyond the radius of 50 km (see grey line in Figure 6). When Rita began to leave the LC area, the effect of SST cooling became obvious and latent heat flux at the surface was continuously reduced. Weaker secondary circulation and convergence at low level is less conducive to tempering the shear instability associated with the vorticity ring (Nolan, 2001; Montgomery et al., 2002). As a result, redistribution of vorticity along the radial direction could be expected (Schubert et al., 1999; Kossin and Schubert, 2001) and the radial profile of vorticity outside the eyewall was broadened (see black solid line in Figure 6). According to the beta-skirt axisymmetrization hypothesis Figure 6. Calculated mean radial profile of relative vorticity (units 10 3 s 1 ) at 1.5 km altitude using temporally averaged mean quantities. Grey solid line is valid at 2100 UTC 21 September, and black solid (dashed) line is valid at 0700 UTC 22 Sep from experiment CPLHRES (SEN DCPL) (Terwey and Montgomery, 2008), a non-trivial vorticity skirt with negative gradient outside the eyewall is essential to axisymmetrize small-scale vorticity anomalies into a cyclonic low-level jet (a predecessor of the secondary eyewall). In contrast, when the negative feedback of SST cooling is excluded in experiment SEN DCPL, the simulated storm continued to intensify over the ocean of much warmer SST. Enhanced secondary circulation and convergence at low level helps to maintain the elevated vorticity ring structure later into the simulation, and little vorticity gradient is found at and beyond 50 km radius (see black dashed line in Figure 6) Ocean responses To examine whether the simulated rapid weakening phase of Rita in experiment CPLHRES results from a correct ocean response, the SST change in the central GOM between the beginning and the end of the simulation time is shown in Figure 7(a). Immediately following its maximum intensity, the simulated Rita caused extensive SST cooling over the area bounded by 88 W 92 Wand25 N 28 N. The magnitude of extreme SST cooling is about 4 C, and the location is centred at the point (90 W, 27 N). It is generally not practicable to obtain an observation of SST beneath the eyewall. For comparison, the SST change from TMI-AMSR SST analysis between 0000 UTC 20 September and 0000 UTC 24 September 2005 (the earliest date that shows SST cooling around the area covered by the Rita eyewall at 0000 UTC 23 September) is shown in Figure 7(b). The spatial distribution and magnitude of SST cooling predicted by the coupled system generally compare well with those observed, especially in the extreme cooling area. 4. Summary and conclusions Rita (2005) was a Category 5 hurricane over the central Gulf of Mexico, and its rapid intensity change and the complex evolution of eyewall structure between 20 and 23 September became the focus of the present study. With a cloud-resolving, coupled hurricane ocean modelling system, the realistic intensity change of Rita as well as the response of the upper ocean in the GOM is reproduced. The spatial distribution and the magnitude of SST cooling are similar to those observed. Moreover, the formation of a

7 Intensity Forecast Experiment of Hurricane Rita (2005) (a) Simulated (b) Observed Figure 7. (a) Simulated SST difference between 0000 UTC 20 September and 0000 UTC 23 Sep 2005 from experiment CPLHRES. (b) TMI-AMSR satellite observed SST difference between 0000 UTC 20 Sep and 0000 UTC 24 Sep Shadings are from 1.5to 4 Cwithintervalof0.5 C. secondary eyewall during the rapid weakening phase of Rita is also well simulated. The importance of coupling to an eddy-resolving ocean model and the high resolution of the atmospheric model for the intensity forecast of Hurricane Rita is examined through a set of numerical experiments in a coherent context. The coupling with the ocean model is essential for eliminating the erroneous intensification that occurred in the uncoupled experiments, since Rita caused extensive SST cooling over the area between the Loop Current and the Warm-Core Eddy. However, with lower horizontal resolution of the atmospheric model ( 4 km), the fully coupled experiment (CPLCTRL) has little skill in capturing the observed rapid intensity change. Even when started from a vortex with intensity closer to that observed, the consequent evolution of the storm intensity in experiment SEN LRES gradually converges to that in experiment CPLCTRL. These results are consistent with those from Bender and Ginis (2000), who found that significant improvement of the intensity forecast could only be realized in a category of hurricanes of moderate strength. With higher horizontal resolution of the atmospheric model, the deepening rate is better simulated and compares well against observation. However, the rapid weakening phase of Rita can only be simulated with the horizontal grid spacing of the atmospheric model at least less than 4 km while coupled with the ocean model. Further results from the extended sensitivity experiments lead us to an updated view on the role of air sea interaction in hurricane intensity change. On the one hand, the storm intensity changes as a direct consequence of changes in the heat flux through the ocean atmosphere interface. On the other hand, substantial structure change of the storm could occur under the influence of coupled processes, which in turn would cause storm intensity change. As to the latter point, sufficient resolution is required to represent the internal processes associated with the structure change. Based on the above findings, it is suggested that both high resolution of the atmospheric model and the ocean coupling are indispensable ingredients of the next-generation operational hurricane intensity forecasting systems. Acknowledgements The firstauthorissupported byncar/asp graduate student visiting program during his stay at NCAR. Special thanks go to Alan Wallcraft and Henry Winterbottom for discussing the HYCOM model, and to Jimy Dudhia and Wei Wang for discussing the WRF model. The authors are also very grateful to the NCAR/MMM hurricane group for their scientific insight and comments on the experimental design and results. This research is supported by the National Key Project for Basic Research of China under the grant 2009CB421500, National Natural Science Foundation of China with the grants and , and the National Special Funding Project for Meteorology of China (GYHY ). References Abarca SF, Corbosiero KL Secondary eyewall formation in WRF simulations of hurricanes Rita and Katrina (2005). Geophys. Res. Lett. 38: L07802, DOI: /2011GL Bender MA, Ginis I Real-case simulations of hurricane ocean interaction using a high-resolution coupled model: Effects on hurricane intensity. Mon. 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