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Numerical Simulations of the Genesis of Hurricane Diana (1984)

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Numerical Simulations of the Genesis of Hurricane Diana (1984) 1100 MONTHLY WEATHER REVIEW VOLUME 130 Numerical Simulations of the Genesis of Hurricane Diana (1984). Part II: Sensitivity of Track and Intensity Prediction CHRISTOPHER DAVIS National Center for Atmospheric Research,* Boulder, Colorado LANCE F. B OSART University at Albany, State University of New York, Albany, New York (Manuscript received 14 June 2001, in ®nal form 6 November 2001) ABSTRACT The authors examine numerous simulations that probe the dynamics governing the intensi®cation and track of Tropical Cyclone Diana (1984) simulated in Part I. The development process is fundamentally dependent on a preexisting upper-tropospheric trough±ridge couplet. This couplet focuses mesoscale vertical motion that, in turn, produces lower-tropospheric potential vorticity (PV) anomalies, which form the seed for the tropical storm. Removal of this trough±ridge couplet from the initial conditions eliminates cyclogenesis. The simulated rate of development of Diana in the prehurricane phase depends principally on choices of cumulus parameterization, boundary layer treatment, sea surface temperature, and grid spacing. Simulations with cumulus schemes that allow more grid-scale precipitation on the 9-km grid exhibit unrealistic grid-scale over- turning and slower intensi®cation, primarily due to production of cyclonic vorticity anomalies at large radii. Use of an innermost nest with 3-km grid spacing, without a cumulus scheme, generally produces the intensi®cation that agreed best with observations. Improvement over the control simulation stems from the emergence of convective downdrafts and a vertical motion spectrum that is less biased toward ascent. Consistent with recent work by Braun and Tao, the medium-range forecast model (MRF) planetary boundary layer (PBL) scheme produces a PBL that is too dry and too deep as winds intensify toward hurricane strength. Use of the Burk±Thompson scheme leads to excessive intensi®cation with a 9-km grid spacing. Manual analysis of surface data produces a sea surface temperature (SST) ®eld 18±28C warmer than is obtained from operational analysis. The warmer SSTs result in a storm that is about 27 hPa deeper after 60 h of integration. Storm track depends primarily on synoptic-scale structure at upper levels. Cumulus schemes that allow more grid-scale overturning enhance the anticyclonic out¯ow aloft. The out¯ow deforms the tropopause, building an anticyclone poleward of the storm and facilitating cutoff low formation equatorward of the storm. Using PV attribution, it is shown that these upper-level changes are responsible for an enhanced easterly steering ¯ow and more westward storm track. Later initialization allows a better analysis of trough fracture, particularly the cutoff low and this also leads to a more westward storm track. Overall, despite the presence of a well-de®ned baroclinic precursor, the large sensitivity of track and intensity prediction to variations in model physics and initial conditions suggests that the development of Diana pushes the current limits of predictability. 1. Introduction cyclone along a quasi-stationary front that had pene- trated to remarkably low latitudes for early September. a. Review of Part I A cold-core upper-tropospheric trough is important for The development of Hurricane Diana (1984) is no- initiating baroclinic development, the precursor of trop- table for the important role of synoptic-scale, baroclinic ical cyclogenesis. precursors. As shown in Bosart and Bartlo (1991, here- The modeling study of Davis and Bosart (2001, here- after BB), the storm begins as a subtropical baroclinic after DB) captures the transformation of the baroclinic disturbance into a warm-core mesoscale vortex within the Pennsylvania State University±National Center for * The National Center for Atmospheric Research is sponsored by Atmospheric Research ®fth-generation Mesoscale Mod- the National Science Foundation. el (MM5). In the simulation, weak baroclinic devel- opment begins as a cold-core upper-tropospheric trough Corresponding author address: Christopher A. Davis, National moves off the Florida coast. Low-level warm advection, Center for Atmospheric Research, P.O.Box 3000, Boulder, CO 80307. induced mainly by the upper-level disturbance initiates E-mail: [email protected] widespread precipitation and latent heating poleward of q 2002 American Meteorological Society Unauthenticated | Downloaded 09/24/21 09:07 PM UTC MAY 2002 DAVIS AND BOSART 1101 a quasi-stationary surface front. The heating produces mental shear (e.g., Gray 1982; DeMaria 1996), the me- numerous low-level positive potential vorticity (PV) soscale distribution of precipitation (e.g., Krishnamurti anomalies. A dominant PV anomaly forms near the up- et al. 1998), presence of concentric eyewalls (e.g., Wil- shear (western) extremity of the lower-tropospheric loughby et al. 1982), dissipative heating in the core frontal zone, ampli®ed further through latent heating. (Bister and Emanuel 1997; Zhang and Altshuler 1999), The incipient vortex sweeps surrounding PV anomalies angular momentum ¯uxes resulting from interactions around itself, growing further by eddy vorticity ¯uxes with midlatitude upper-level troughs (e.g., Riehl 1954; as the anomalies are axisymmetrized. The transition to Molinari et al. 1998), and storm-induced sea surface warm core occurs in about 8 h, from 1000 UTC 8 Sep- temperature changes (e.g., Schade and Emanuel 1999). tember to 1800 UTC 8 September. A 12-h period of In addition, numerical simulations show considerable quiescence follows during which the boundary layer and sensitivity to a number of factors that are entirely spe- lower troposphere near the storm moisten. Renewed ci®c to models themselves as contrasted with the above deepening in response to enhanced ¯uxes of water vapor factors, which are all believed to be true sensitivities in from the ocean then occurs and the simulated storm nature. A key sensitivity is horizontal grid spacing. No- reaches hurricane intensity by 1800 UTC 9 September. table improvement of intensity prediction occurs as the In the present paper, we examine the robustness of grid spacing decreases to roughly the radius of maxi- the mechanism advanced in DB of the transformation mum wind (RMW) or less (Walsh and Watterson 1997). of the baroclinic disturbance into Tropical Storm Diana Further sensitivity has been reported as the grid spacing and eventually into Hurricane Diana. Dynamics of the is reduced below 10 km, such that simulations with initial development, featuring warm-core transforma- resolutions of about 5 km are able to capture eyewall tion, are examined using different treatments of physical dynamics and better resolve storm out¯ow (Tripoli processes (cloud physics, cumulus parameterization), 1992; Liu et al. 1997; LeMarshall and Leslie 1999; variations in horizontal grid spacing (27 km, 9 km, and Braun and Tao 2000). 3 km), different sea surface temperature (SST) analyses, The effect of varying model grid spacing and physical and different initial conditions. Furthermore, we inves- parameterizations in ®ner-scale simulations (,10 km tigate the effect of these variations on the intensi®cation grid spacing) of tropical cyclone formation has not been of the storm to hurricane strength and on the track of investigated extensively. Based on simulations of me- Tropical Cyclone Diana. We will use the behavior of soscale convective systems, we expect considerable sen- simulations with perturbed physical parameterizations, sitivity of tropical cyclone genesis to changes in grid initial conditions, and SST, and the extensive body of spacing between roughly 2 and 20 km (Molinari and existing knowledge regarding sensitivities in numerical Dudek 1992) due to the lack of appropriate separation simulations of tropical cyclones to obtain a more com- between resolved and parameterized scales of motion. plete understanding of the essential processes involved In many simulations, an initial vortex is imposed, pre- in the development of Diana. cluding any chance of simulating the earliest stages of tropical cyclogenesis and investigating the dynamics of the process and the related sensitivities, both natural and b. Review of known sensitivities in numerical numerical. Much of the motivation for the present paper simulations is to address this issue. 1) INTENSIFICATION 2) TRACK A large body of research focuses on the sensitivity of the numerical treatment of air±sea heat and water In recent years there have been numerous extensions vapor exchange in mature hurricanes. Based on work of the basic concept of tropical cyclone motion resulting by Emanuel (1995, 1999) the intensi®cation rate of hur- from beta gyres, the circulation around tropospheric PV ricanes seems to depend on thermodynamic properties anomalies arising from meridional advection of plane- of the large-scale environment and the details of the air± tary vorticity. More generally, it is recognized that any sea exchange under the core of the storm. In particular, azimuthal wavenumber one asymmetry in PV gives rise a key parameter is the ratio of the exchange coef®cients to a ventilation ¯ow, which can effectively steer the of enthalpy and momentum; the larger this ratio, the tropical cyclone. In general, the largest gradients of PV faster the storm can intensify. Comparing simulations are located at the tropopause and therefore, perturbation of Hurricane Bob (1991) using four different planetary of these gradients can yield important ¯ow anomalies. boundary layer (PBL) schemes and ®ve permutations Studies by
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