Intensity Forecast Experiment of Hurricane Rita (2005) with a Cloud-Resolving, Coupled Hurricane–Ocean Modelling System

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

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

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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, 2000; Hong et al., 2000). In of the WRF model, so that the two grid systems exactly addition, evaporation and precipitation are handled as match. Twenty-two hybrid layers are used in the vertical, source (sink) and sink (source) of salinity (mass) in the and the K-Profile parametrization (KPP: Large et al., 1994) is ocean mixed layer. The wind stress and all the fluxes used for describing the vertical mixing processes. The initial are computed in the WRF model and passed to the and boundary conditions for the ocean model come from HYCOM. This is accomplished by exchanging the ESMF the daily analysis of the eddy-resolving (0.08◦ at Equator) fields contained in the export (import) state of the WRF basin-scale Atlantic HYCOM data assimilation system (HYCOM) model component through an ESMF coupler (Chassignet et al., 2007). To minimize spin-up time, the component (Figure 2). The opposite is applied to SST, bathymetry is also interpolated from the Atlantic HYCOM which is kept constant in the WRF model during each domain. coupling time step. 2.3. Experimental design 2.1. Hurricane model All the numerical experiments (Table I) start from 0000 TheWRFmodelisconfiguredwithonecomputational UTC 20 September 2005 and are run for 72 hours. In the domain of 4 km horizontal resolution. The domain includes control experiment (CPLCTRL), models of the atmosphere the GOM, northwest part of the Caribbean Sea and the and the ocean are fully coupled. In experiment UCPLTSK, West Atlantic basin (Figure 3). Physical processes used the initial SST is from the GFS analysis of skin temperature are the Rapid Radiative Transfer Model (RRTM) scheme and is held constant. To isolate the effect of initial SST for long-wave radiation and Dudhia scheme for short- and ocean coupling, experiment UCPLSST is carried out, wave radiation, the Purdue Lin cloud microphysics scheme, where the initial SST is from the HYCOM analysis and held and the Yonsei University (YSU) planetary boundary-layer constant. In experiment CPLHRES, the WRF model uses parametrization. Calculation of surface stress and fluxes uses an additional 1.33 km resolution, vortex-following domain, the new bulk formulation in WRF V3.1, which is in line with which covers an area of 400 km × 400 km. The moving nest the recent research results under hurricane wind conditions starts after 30 hours simulation of the outer 4 km domain, (Donelan et al., 2004; Black et al., 2007). just prior to the onset of rapid intensification. This nest is The WRF model is initialized at 0000 UTC 20 September two-way nested with the 4 km domain, which is coupled 2005. The boundary and initial conditions for the 4 km do- to the HYCOM. Based on experiment CPLHRES, two extra main come from a previous run of 12 km coarse domain, experiments (SEN DCPL and SEN LRES) are carried out to

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 4 + 1.33 5SENDCPL Same as CPLHRES except that the atmospheric model is decoupled from the ocean model from 2100 UTC 21 Sep 6SENLRES Same as CPLHRES except that the moving nest is removed from 2100 UTC 21 Sep examine the sensitivity of intensity and structure changes of of 996 hPa (4 hPa higher than observation). Time series of Rita to the ocean coupling and the horizontal resolution of both the MSLP and MAXW compare well with the Best- the atmospheric model during the rapid weakening phase. Track estimate until hour 36, when the vortex reaches an MSLP of 940 hPa. After this point, the intensification rate 3. Results slows down and the storm obtains an MSLP of 920 hPa andMAXWof67ms−1 at hour 57, while the observations 3.1. Track indicate Rita underwent even more rapid intensification. The simulated storm weakens during the next 15 hours, with The simulated tracks in all experiments are similar, with the the MSLP increased from 921 hPa to 923 hPa. While the largest spread less than 30 km. Therefore, only the six-hourly intensity change during the last 36 hours is not as dramatic storm centres from the control experiment (CPLCTRL) are as observed, the 4 km resolution, coupled experiment has at shown in Figure 3, along with the Best-Track estimates least marginally reproduced a weakening tendency. obtained from the National Hurricane Center (NHC). The two uncoupled experiments start from the same Because of the BDA initialization, the storm centre at vortex as that in CPLCTRL. The intensity of the UCPLSST initial time is nearly at the same location as observed vortex is nearly the same as that in CPLCTRL during the (within the accuracy of model grid spacing). The simulated first 20 h of simulation. After that, the simulated storm in storm translation speed generally compares well with the UCPLSST grows stronger than that in CPLCTRL, indicating observation. Both the simulation and observation show fast that the effect of SST cooling becomes obvious only when the movement over the first 48 h and then slowing down during storm is strong enough. Since the SST is generally higher than ◦ the last 24 h when Rita rounded the southwestern tip of 29 C along the simulated track (figure not shown) and fixed the Western Atlantic Ridge. However, the simulated storm in time, the storm in UCPLSST intensifies monotonically to moved faster than observed between hours 36 and 60. A an MSLP of 899 hPa, 24 hPa deeper than that in CPLCTRL at northward drift of the simulated track became evident after the end of simulation. The intensity change of the simulated the storm passed Florida Strait and entered the southeastern storm in UCPLTSK is similar to that in UCPLSST, but the GOM. This bias reached a maximum at the end of the deepening rate is larger. This difference is due to the NCEP ◦ second day, while the observations indicated Rita nearly skin temperature being about 0.5 C higher than the SST stayed along the same latitude between 0000 UTC and 1800 from HYCOM analysis (figure not shown). The persistent UTC 21 September. During the last 24 h, the simulated intensification as shown in both uncoupled experiments storm headed northwestward as observed, but with the (UCPLTSK and UCPLSST) suggests that the effect of SST final position 166 km further northwest than that of Best- cooling accounts for the weakening of Rita during the later Track position. Much of this track error is caused by the stage of the control experiment. combination of faster movement (between hours 36 and 60) In order to examine the sensitivity of the simulated and northward drift (during the second model day) of the storm intensity in CPLCTRL to the horizontal resolution simulated storm. of the atmospheric model, experiment CPLHRES is carried Although the track errors in the experiments generally out. With a moving nest of 1.33 km resolution added increase linearly with time (54, 127, 166 km at hours 24, 48, after 30 hours into the 4 km domain simulation, the 72 respectively in the control experiment), they are much intensification rate of the simulated storm is much larger less than the averaged bias for the same lead time in the than that in CPLCTRL and agrees better with observation. present-day operational forecasts. Since the spatial scale of Maximum intensity is obtained at hour 52, with an MSLP both the WCE and the LC is generally O(200 ∼ 400 km), of 909 hPa and MAXW of 75 m s−1. Afterwards, the MSLP the track errors are considered minor, thus providing the fills rapidly to 920 hPa and MAXW decreases to 66 m s−1. basis for accurate intensity predictions in the coupled Compared with CPLCTRL, the higher resolution of the experiments. atmospheric model captures better the rapid intensification, maximum intensity and the subsequent weakening processes 3.2. Intensity and structure of Rita. Moreover, the simulated Rita in CPLHRES also generated a secondary eyewall during 22 September as The 3-day intensity forecasts in terms of minimum sea-level reported by Knabb et al. (2006). After the initiation of pressure (MSLP) and 10 m maximum wind speed (MAXW) the moving nest, a convective outer rain band quickly are shown in Figure 4(a) and (b), respectively. The control develops and spirals cyclonically outward from the right- experiment (CPLCTRL) begins with an enhanced initial quadrant (relative to the storm motion) to the rear-quadrant vortex defined by the BDA procedure, resulting in an MSLP (Figure 5(a)) as Rita traverses the GOM. The convective

Copyright c 2011 Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: 2149–2156 (2011) Intensity Forecast Experiment of Hurricane Rita (2005) 2153

(a) (b)

Figure 4. 72 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.

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 Figure 6. Calculated mean radial profile of relative vorticity (units − − (e.g. Willoughby et al., 1982), which could contribute to 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) the rapid weakening in addition to the negative feedback line is valid at 0700 UTC 22 Sep from experiment CPLHRES (SEN DCPL). 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 (Terwey and Montgomery, 2008), a non-trivial vorticity removed, the simulated storm in SEN LRES underwent a skirt with negative gradient outside the eyewall is essential to rapid adjustment, with the MSLP increased by 4 hPa and axisymmetrize small-scale vorticity anomalies into a cyclonic MAXW decreased by 5 m s−1. The intensity of the simulated low-level jet (a predecessor of the secondary eyewall). In storm then gradually converges to that in CPLCTRL. contrast, when the negative feedback of SST cooling is No secondary eyewall formation is found in experiment excluded in experiment SEN DCPL, the simulated storm SEN LRES. In view of the recent studies on the formation continued to intensify over the ocean of much warmer SST. of secondary eyewall which highlight the roles of small-scale Enhanced secondary circulation and convergence at low level convective features (e.g. Terwey and Montgomery, 2008; helps to maintain the elevated vorticity ring structure later Qiu et al., 2010; Abarca and Corbosiero, 2011), it is not into the simulation, and little vorticity gradient is found at surprising that all the experiments with 4 km resolution fail and beyond 50 km radius (see black dashed line in Figure 6). to reproduce such an eyewall structure change. However, the simulated storm in SEN DCPL did not generate a secondary 3.3. Ocean responses eyewall either (Figure 5(d)–(f)). Instead, intensification continued as the simulated storm in SEN DCPL was To examine whether the simulated rapid weakening phase approaching the unmodified waters with much warmer of Rita in experiment CPLHRES results from a correct ocean SST. Based on the results above, it is clear that the effect of response, the SST change in the central GOM between the SST cooling directly leads to the weakening. However, this beginning and the end of the simulation time is shown in source alone could not account for the large decaying rate Figure 7(a). Immediately following its maximum intensity, both observed and well simulated in CPLHRES. Therefore, the simulated Rita caused extensive SST cooling over the area it is reasonable to attribute the extra source of the rapid bounded by 88◦W∼92◦Wand25◦N∼28◦N. The magnitude weakening to the formation of the secondary eyewall, of extreme SST cooling is about 4◦C, and the location which could only be realistically reproduced with the aid of is centred at the point (90◦W, 27◦N).Itisgenerallynot sufficient resolution of the atmospheric model. practicable to obtain an observation of SST beneath the Of particular interest are the different behaviours with eyewall. For comparison, the SST change from TMI-AMSR respect to the eyewall evolution in experiments CPLHRES SST analysis between 0000 UTC 20 September and 0000 and SEN DCPL. This might suggest the potential role of UTC 24 September 2005 (the earliest date that shows SST hurricane–ocean coupling in the formation of secondary cooling around the area covered by the Rita eyewall at 0000 eyewall. A heuristic explanation is given here before our UTC 23 September) is shown in Figure 7(b). The spatial attempt toward more detailed understanding. During the distribution and magnitude of SST cooling predicted by the rapid intensification period, the inner core of Rita quickly coupled system generally compare well with those observed, develops an elevated vorticity ring with little vorticity especially in the extreme cooling area. gradient at and beyond the radius of 50 km (see grey line in Figure 6). When Rita began to leave the LC area, 4. Summary and conclusions the effect of SST cooling became obvious and latent heat flux at the surface was continuously reduced. Weaker Rita (2005) was a Category 5 hurricane over the central secondary circulation and convergence at low level is less Gulf of Mexico, and its rapid intensity change and the conducive to tempering the shear instability associated complex evolution of eyewall structure between 20 and 23 with the vorticity ring (Nolan, 2001; Montgomery et al., September became the focus of the present study. With 2002). As a result, redistribution of vorticity along the radial a cloud-resolving, coupled hurricane–ocean modelling direction could be expected (Schubert et al., 1999; Kossin system, the realistic intensity change of Rita as well as the and Schubert, 2001) and the radial profile of vorticity outside response of the upper ocean in the GOM is reproduced. the eyewall was broadened (see black solid line in Figure 6). The spatial distribution and the magnitude of SST cooling According to the beta-skirt axisymmetrization hypothesis are similar to those observed. Moreover, the formation of a

Copyright c 2011 Royal Meteorological Society Q. J. R. Meteorol. Soc. 137: 2149–2156 (2011) Intensity Forecast Experiment of Hurricane Rita (2005) 2155

(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 2005. Shadings are from −1.5to−4◦C with interval of 0.5◦C. secondary eyewall during the rapid weakening phase of Rita Acknowledgements is also well simulated. The importance of coupling to an eddy-resolving ocean model and the high resolution of the The first author is supported by NCAR/ASP graduate student atmospheric model for the intensity forecast of Hurricane visiting program during his stay at NCAR. Special thanks go Rita is examined through a set of numerical experiments in to Alan Wallcraft and Henry Winterbottom for discussing a coherent context. the HYCOM model, and to Jimy Dudhia and Wei Wang The coupling with the ocean model is essential for for discussing the WRF model. The authors are also very eliminating the erroneous intensification that occurred in grateful to the NCAR/MMM hurricane group for their the uncoupled experiments, since Rita caused extensive scientific insight and comments on the experimental design SST cooling over the area between the Loop Current and and results. This research is supported by the National Key Project for Basic Research of China under the grant the Warm-Core Eddy. However, with lower horizontal 2009CB421500, National Natural Science Foundation of resolution of the atmospheric model (∼4 km), the China with the grants 40828005 and 40921160382, and the fully coupled experiment (CPLCTRL) has little skill in National Special Funding Project for Meteorology of China capturing the observed rapid intensity change. 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