GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L03110, doi:10.1029/2003GL018571, 2004 Simulation of extratropical Hurricane Gustav using a coupled atmosphere-ocean-sea spray model William Perrie,1 Xuejuan Ren,1,2 Weiqing Zhang,1,3 and Zhenxia Long1,3 Received 6 September 2003; revised 3 November 2003; accepted 2 January 2004; published 12 February 2004. [1] Numerical simulations of extratropical Hurricane mixing. Storm-induced upwelling influences the mixed Gustav (2002) are performed using the MC2 (Mesoscale layer temperature by pumping deeper colder water and Compressible Community) atmospheric model, coupled to increasing the mixed layer depth. À1 the Princeton Ocean Model (POM), and a sea spray [3] When the wind speed is higher than 15 ms , parameterization. On one hand, the impact of coupling substantial sea spray is produced by breaking waves. POM to MC2 generates sea surface temperature (SST) Andreas and Emanuel [2001] suggest that sea spray cooling, through entrainment mixing at the bottom of the contributions to heat fluxes can provide a net air-sea mixed layer, with the passage of the storm. SST cooling enthalpy transfer in tropical cyclones. In this study, we reduces the sea surface heat fluxes compared to uncoupled use the spray parameterization of Andreas [2003] with the MC2 simulations, which have time-invariant SST. Reduced MC2 atmospheric model, coupled to POM to study extra- heat fluxes lead to reduced storm intensity. On the other tropical Hurricane Gustav. Tropical storms are not our hand, simulation of the heat and mass flux contributions of focus. Gustav was selected to study air-sea interaction sea spray enhances sea surface heat fluxes and slightly during extratropical hurricane development, where barocli- increases maximum storm intensity compared to coupled nicity and midlatitude system interactions are dominant MC2-POM simulations without spray. INDEX TERMS: factors. We show that although the impacts of ocean 0312 Atmospheric Composition and Structure: Air/sea constituent feedbacks are comparatively minor, they are easily seen fluxes (3339, 4504); 3339 Meteorology and Atmospheric in Gustav’s central sea level pressure (SLP) and 10 m Dynamics: Ocean/atmosphere interactions (0312, 4504); 4504 winds U10. Oceanography: Physical: Air/sea interactions (0312). Citation: Perrie, W., X. Ren, W. Zhang, and Z. Long (2004), Simulation of extratropical Hurricane Gustav using a coupled 2. Model Description atmosphere-ocean-sea spray model, Geophys. Res. Lett., 31, L03110, doi:10.1029/2003GL018571. [4] The MC2 model is a state-of-the-art, fully elastic, nonhydrostatic model solving the full Euler equations on a limited-area Cartesian domain. It is able to successfully 1. Introduction simulate midlatitude cyclones [Benoit et al., 1997]. Lateral [2] In tropical cyclones, variability in intensity can orig- boundary and initial conditions are taken from CMC (Ca- inate from the coupling between the storm and the under- nadian Meteorlogical Centre) analysis data. The model lying ocean. This coupling is dominated by momentum and domain is (79.5°W–40.0°W, 24.25°N–56.25°N), using a enthalpy exchanges at the air-sea interface occurring with latitude-longitude projection, 0.25° resolution, 30 vertical the passage of a storm. Tropical cyclones can impact the layers, and 600 s time steps. Over the sea, MC2’s interfacial upper ocean by causing SST cooling by as much as 6°C, as fluxes of momentum, and sensible and latent heat are seen in satellite images [Lin et al., 2003], and related near- calculated using Monin-Obukhov theory. This theory leads inertial currents as large as 2 msÀ1 [Bender and Ginis, to a bulk turbulent flux formulation, based on turbulent 2000]. Because SST cooling can significantly decrease the transfer coefficients for these fluxes, which depend on air-sea heat exchange, it reduces tropical cyclone intensity empirical similarity functions ym and yh and roughness [Schade and Emanuel, 1999; Bender and Ginis, 2000; Chan lengths for wind speed, temperature, and humidity, z0m,z0t, et al., 2001]. Ocean temperature and related mixed layer and z0q. heat and mass budgets are largely controlled by entrainment [5] The MC2 default parameterizations for ym, yh,z0m, mixing at the base of the mixed layer [Jacob et al., 2000]. In z0t, and z0q must be handled with care because Andreas storm conditions, vertical current shear at the mixed layer [2003] developed our bulk spray flux algorithm by sub- base is the dominant factor in determining entrainment tracting estimates of the interfacial heat fluxes from HEXOS measurements of the total heat fluxes. Thus the residual is the spray flux; consequently, his spray algorithm is tuned to a specific turbulent bulk flux algorithm, related to the 1Fisheries & Oceans Canada, Bedford Institute of Oceanography, COARE algorithm [Fairall et al., 1996]. The spray algo- Dartmouth, Canada. rithm is not valid if we use a different bulk flux algorithm. 2 Department of Atmospheric Sciences, Nanjing University, Nanjing, PR Therefore we ignore MC2’s default z0m,z0t,z0q, ym, China. 3 yh parameterizations and implement Andreas’ [2003] Department of Engineering Mathematics, Dalhousie University, parameterizations. Total momentum t , latent H ,and Halifax, Canada. T L,T sensible Hs,T fluxes, constituting the boundary conditions Copyright 2004 by the American Geophysical Union. at the lowest model level, are obtained by simply adding the 0094-8276/04/2003GL018571$05.00 corresponding bulk interfacial (t,HL,Hs) and spray fluxes L03110 1of4 L03110 PERRIE ET AL.: SIMULATION OF EXTRATROPICAL HURRICANE GUSTAV L03110 ated, embedded within a southwesterly flow ahead of a mid- level trough centered on New England. Dominated by bar- oclinic processes, it intensified rapidly over 28oC Gulf Stream waters and began to merge with the associated midlatitude low. Gustav became a hurricane by 12 UTC, with maximum winds of 85 kt (43 msÀ1) near 18 UTC. It made landfall over Cape Breton with 80-kt winds, achieving minimum central sea level pressure (SLP) at 06 UTC 12 September. It transi- tioned to an extratropical cyclone, and turning north, made a second landfall over Newfoundland. Figure 1. (a) Simulations of Gustav’s storm track from 18 4. Upper Ocean Impacts UTC 10 September to 12 UTC 13 September: Control MC2 (—6), MC2-POM (– –), MC2-POM-spray (—&), and [9] Figure 1a presents simulations of Gustav’s storm NHC analysis (—.). Six-hourly intervals shown. Surface track in comparison with the NHC (National Hurricane currents at 48 h in the simulations (! msÀ1), and SST (°C) Center) analysis. The model integration is from 18 UTC at 48 h, minus the initial SST, from MC2-POM-spray 10 September to 12 UTC 13 September. Gustav moves model. (b) Difference in SST (°C) distributions of the MC2- slowly during the initial portion of the simulation, and POM-spray minus MC2-POM simulations at 48 h. Storm accelerates during its extratropical phase as it moves past location is .. Buoy station 44011 is È. Nova Scotia. Compared to the NHC analysis, the simulation from MC2-POM-spray is slightly better than that of MC2- POM, which in turn is better than the uncoupled MC2. [10] Results for surface currents and SST are also shown (tsp,QL,sp,QS,sp), following Andreas and Emanuel [2001] and [Andreas, 2003], in Figures 1a and 1b. By 48 h in the MC2-POM-spray simulation, which is late in Gustav’s life cycle, widespread SST cooling has occurred on both sides of the storm track. t ¼ t þ t ð1Þ T sp A maximum of 8°C cooling occurs along a narrow area to the right of the storm track because of rightward bias of the wind structure. This can be verified with satellite SST data H H Q 2 L;T ¼ L þ L;sp ð Þ from The Johns Hopkins University http://fermi.jhuapl.edu/ avhrr). Surface-currents induced by Gustav’s winds are also presented in Figure 1a: These achieve maximum speeds to Hs;T ¼ Hs þ QS;sp ð3Þ the right of the storm track. Maximum differences in surface currents and SST between MC2-POM-spray and MC2- 1 [6] The POM model [Mellor, 1998] is implemented for POM simulations, are 0.1 msÀ and 0.3°C. the ocean domain (82°W–40°W, 20°N–57.5°N) on a [11] Figures 2a–2c show time–depth section profiles for latitude-longitude projection, with 0.1/6° horizontal resolu- ocean temperature To, horizontal U, and vertical w currents, tion, and 23 vertical layers, of which 8 are in the upper 80 m. Lateral prescribed barotropic transports are used to keep the Gulf Stream in the right position. Initial and boundary conditions for temperature and salinity are taken from monthly Naval Oceanographic Office Generalized Digital Environmental Model (GDEM) data [Bender and Ginis, 2000]. The model spin-up involves integration for one year, using monthly mean wind stress, heat flux, and fresh water flux data from the National Centers for Environmental Prediction (NCEP). The integration is continued for a second year using NCEP data for a given storm to provide a realistic pre-storm ocean representation. [7] The coupled model system exchanges information between the atmosphere and ocean at every coupling time step. Wind stress, sensible and latent heat fluxes, radiative flux and fresh water flux, as computed from MC2 (includ- ing sea spray), are transferred to POM, which is then integrated for two time-steps. The new POM-produced SST is then passed to MC2, which is then integrated forward for another five time-steps (50 minutes). Figure 2. Time-depth sections of (a) ocean temperature To (°C), (b) horizontal current speed U (msÀ1), and (c) vertical velocity w (msÀ1). Location (65.34°W, 41.33°N) is the 3. Extratropical Hurricane Gustav (2002) position (Figure 1a) of greatest SST cooling. At 21 UTC 11 [8] Gustav was designated a tropical storm by 12 UTC on September, this location achieves its shortest distance to the 10 September, north of the Bahamas.