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ADVANCING the UNDERSTANDING OF TROPICAL CYCLONES through NUMERICAL MODELLING Noel Davidson, Mai Nguyen, Yimin Ma, Yi Xiao, Marie-Dominique Leroux, Hongyan Zhu and Lili Liu CAWCR, Centre for Australian Weather and Climate Research A Partnership between CSIRO and the Bureau of Meteorology Acknowledgments: John McBride, Kevin Tory, Jeff Kepert, Richard Dare, Peter Steinle and Luke Garde Tropical Cyclone Characteristics in the Australian Region TC behaviour and forecast issues: Points of Origin • Track, • Genesis, • Intensification/RI/Decay, • Structure Change (size, etc), Points with Min. CP • ET • Landfall!!! (Dare and Davidson, 2004, MWR) Points of Final Decay Limitations on (Numerical) Prediction of Structure and Intensity • Horizontal and vertical resolution • Current paucity of observations to define the intense inner-core structure (need for VS or bogus) • Lack of the required assimilation techniques to initialize an intense inner-core • Rapid dynamical error amplification due to barotropic and convective instabilities • (Over)Sensitivity of prediction to parameterizations of moist processes. These deficiencies are compounded by lack of understanding of: • Environmental influences on intensity, • Vortex structure and structure change, • Internal structure change, and • Coupling between the atmosphere and the ocean. Many problems to solve, but good reasons to be optimistic!! Specific points of discussion will include: • Initialization for TC Prediction – current (TC-LAPS) and planned (ACCESS-TC). • Preliminary results from the Australian Community Climate Earth System Simulator (ACCESS) to TC prediction. ACCESS-TC (Y. Xiao) • Influence of LSE, in particular Planetary Rossby Waves, on the behaviour of TCs. • Importance of Accurate Initial Vortex Structure (CP, VMAX, RMW, R34) for TC prediction, (implications for rainfall, storm surge, etc….) (Y. Ma, M-D. Leroux) • Internal Structure Change during RI. (M. Nguyen) • Extratropical Transition. (L.Liu, H. Zhu, L.Garde) • Summary and Recommendations for Future Work. TC Initialization: After initialization, if all goes well, the initial condition should contain a vortex with: • An accurate analysis of the large-scale environment (LSE) • A primary circulation which is consistent with the OBS and estimated characteristics of the TC: location, past motion, intensity AND structure (CP,VMAX, RMW, R34, ROCI). • Model-balanced primary and secondary circulations. • A vertical motion field consistent with observed convective asymmetries, • A structure “consistent” with its environment. (Right size and intensity) Unfortunately, everything rarely goes to plan!!! TCLAPS: Dynamical Nudging to Analyses with TC Vortex Specification; Ascent field defined by Convective Heat Sources from IR Imagery. STORM STRUCTURES OF NW PACIFIC TCs (Actual and Synthetic Cloud Imagery from L.Rikus and Z. Sun,2004) NW Pacific Storms : Podul, Fengshen, Fung-Wong, Rammasun, Utor Podul Fengshen Fung-Wong Rammasun Utor + Storm structures are reasonably well predicted + Some mean skill at 48 hour intensity compared to CLIPER for the Australian Region!! (Knaff and Sampson, 2008, AMOJ). + Similar encouraging results for ACCESS-TC (Rikus, 2009, personal communication) M-D Leroux: Hybrid NWP System LSE from ACCESS-G, ACCESS-T Storm Structure and Prediction of RI from TC-LAPS (0.050, L29) TY Sinlaku, Hagupit, Jangmi during TCS08 ACCESS-TC OPERATIONS, RESEARCH and CLIMATE APPLICATIONS (Yi Xiao, Les Logan, Noel Davidson, Harry Weber and ACCESS-WEP Groups) Evolve TC-LAPS to ACCESS_TC Anticipating improvements in prediction due to: • Good performance of UKMO’s operational track forecasts (Heming, 2008) • Good forecasts of the LSE from ACCESS_G, _R, _T (Puri et al., 2008; Tingwell et al., 2008) • Application of 4DVAR to TC Initialization Methodology • New VS that fits observed structures (CP, VMAX, RMW, R34, ROCI) • 5 cycles of 4DVAR over 24-hours to initialize the primary and secondary circulations (demonstrated, but still some issues for some cases!!) • Improved (LSE + VS + Initialization) => Improved Track, Structure and Intensity? Resolution • 12km => 4km, 2km • 50 levels => 70 levels Assimilation of New Data Sources with 4DVAR • How to initialize a 920 hPa storm using “conventional” obs data? Grand Challenge!! ACCESS_TC: 12km 4DVAR, no VS 12km 4DVAR with VS 4km from 12km 4DVAR with VS OBSERVED FORECASTED Typhoon Nuri Base time 00UTC, 20080819 Global Initial Condition for ALL 0.3750, 50L: t = 0, t = 48, t = 72 0.1100, 50L: t = 48, t = 72 0.0400, 50L: t = 48, t = 72 Improvements in BOTH Intensity and Track with Increasing resolution. (Why??) (Resolution dependence of prediction of track, intensity, recurvature, landfall, ET?) Rapid Acceleration during Intensification for Eastward-moving Storms. Obsvd and Fcst Tracks and Intensities for Non-intensifying (left) and Intensifying forecasts. 60 hour forecasts. Weakening trend in observations. Rapid Acceleration during Intensification for Eastward-moving Storms. 24-hour forecast of 500hPa Flow from non- intensifying and intensifying forecasts. Lower panel is the difference plot. Note: Background westerly stream (favourable for a downstream response) Spuriously strong ridge development to East in intensifying forecast. Thought Experiment: REVERSE SITUATION? Rapid Intensification: Environmental Influences (Davidson, Nguyen, Reeder) Dynamical Influence of Planetary Rossby Waves in Changing the Environment of Storms 200hPa PV for Opal at one day intervals commencing 0000 UTC 30 September 1995. The ellipses mark the location of the leading synoptic-scale amplification at each time. ‘X’ marks Opal’s location at analysis time. RI occurred during the time between the right center and bottom left panels (3 - 4 October) ALSO: Katrina, Larry, Hamish, others …. Theoretical Behavior of Planetary Rossby Waves The dispersion relation for Planetary Rossby Waves (PRWs) in a meridionally- varying background zonal flow, U(y), can be written as: ω = kU – kβ* / (k2 + l2) (1) where ω is frequency, U is the background zonal flow, k is the zonal wavenumber (k = 2π/Lx, Lx is zonal wavelength), l is the meridional wavenumber (l = 2π/Ly, Ly is meridional wavelength), and β* = β - d2U/dy2 , the meridional gradient in the absolute vorticity of the background flow, with β the beta-plane parameter. The zonal phase and group propagation speeds are: Cp = ω /k = U – β*/(k2 + l2) (2) and Cg = ∂ω/∂k = Cp + 2k2 β* /(k2 + l2)2 (3) For Intensification we need Cg >> Cp so that we can (a) rapidly change the storm environment to reduce the wind shear, (b) modulate the ascent field within the storm, and (c) delay interaction with high wind shear zones associated with the upper trough to the west. Large β* (Jet Structures) Small k, l, ….. Large Amplitude, Long Waves (Lx, Ly) Time mean 200 hPa streamlines for the period 20 September to 5 October 1995. (Background mean flow for Opal). Time-longitude section for the 40 – 45N band of 200 hPa stream function anomaly for the period 20 September to 10 October 1995. The longitudinal span is 180 – 330 E. The ‘T’s and ‘R’s indicate where the troughs and ridges are amplifying. Hypothesis: Sustained BL Fluxes and moisture advection from the SE during the brief period of convective inhibition assw the PRW, moistens the BL. When the environmental inhibition passes, deeper, more active convection (colder clouds: c/w Dvorak) develops in the low wind shear, increasingly-cyclonic low level environment. => Rapid Intensification Initialization of Vortex Structure for Hurricane Prediction: (Ma and Davidson, 2009) I: Observed and Synthetic Vortex Structures II: Impact on Prediction of Track, Intensity and Structure BONNIE 98 KATRINA 05 LILI 96 LARGE NATURAL VARIABILITY IN STORM STRUCTURE: ( VMAX, CP, RMW, R64(4), R50(4), R34(4), ROCI ) What determines the variability? Is storm structure important for the evolution of the storm? Correct prediction of CP and Vmax does not imply correct prediction of structure. Visualize the differences in rainfall and storm surge associated with different structures. TC-LAPS: Structure Prediction: Mean Vt and Estimates for Bonnie, Ivan, Katrina at t = 0, t=48 of (CP, VMAX,RMW, R34, ROCI; QC MDeM EXBT DS; 5km, 29L) Implications for Track, Intensity, Structure, Rainfall, Storm Surge? Marie-D Leroux (M-France) RI for TY Sinlaku during TCS08. Eyewall Cycles, Multiple wind maxima at radii from 20km to 200km!! Marked environmental flow changes. Weak steering Influence of β-propagation for a storm with large RMW in an environmental with weak steering. Small inner-cores intensify more rapidly then large inner-cores???? Large Initial inner-core better reproduces observed structure change??? Vortex Structure Issues: Define VS by the set of structure parameters (CP, VMAX, RMW, R34) • What determines structure variability? • Correct prediction of CP and Vmax does not imply correct prediction of structure (RMW, R34). • Differences in structure will (obviously) generate different patterns of wind, rain and storm surge. • Forecast model is capable of forecasting structure and intensity!!! • Forecast model preserves initial structure characteristics, so accurate initial structures (CP, VMAX, RMW, R34) are important. • For the same large-scale forcing, vortex can sometimes respond in different ways depending on structure. (ie, different pathways to the same outcome) • Require Initial Vortex Structure to be correct!!! Mai NGUYEN: Rapid Intensification: Inner-Core Processes: Internal Structure Change during RI 1000 33 980 31 960 Pmin (hpa) 29 940 27 920 25 900 880 23 0 6 12 18 24 30 36 42 48 54 60 66 72 270 272 274 276 278 FCST hour 6 dVm SDM 5 4 3 2 1 0 0 6 12 18 24 30 36 42 48 54 60 66 -1 -2 18 S1 16 S2 S1 14 S2 12 Symmetric phase 10 8 Ring structure 6 4 2 0 0 50 100 150 200 250 -2 18 A1 16 A2 14 A1 A2 12 10 Asymmetric phase 8 Monopole structure 6 4 2 0 0 50 100 150 200 250 -2 Nguyen et al., 2008, 2009: Two phase structure cycles S → A Rapid Transition: Mechanism (ask Mai) A → S Slow Transition: Mechanism (ask Mai) Internal Structure Change during Rapid Intensification How to deal with VHTs, Eyewall Cycles, Multiple Wind Maxima, Moisture Analysis during assimilation and initialization??? Extratropical Transition: Lili Liu, Hongyan Zhu, Noel Davidson Fig.3. 200hPa and 850hPa wind field of Hurricane Maria Pre-ET (00UTC Sep. 03rd 2005) and Post-ET (00UTC Sep. 06th 2005). (a) 200hPa wind field Pre-ET (b) 200hPa wind field Post-ET (c) 850hPa wind field Pre-ET (d) 850hPa wind field Post-ET. The dark dot is the position of hurricane and the line is axis of cross section in Fig. 7. Fig.9 time-longitude series of Stream Function Anomaly at 45N on 250hPa level (a) Hurricane Michael (b) Hurricane Wilma (c) Hurricane Maria (d) Hurricane Rita. The arrow is the propagation direction of trough/ridge wave train and the black dot is the position of the hurricane in ET time. Fig.10: Dry, No-Initial-Vortex Simulation of MSLP for Maria. (a) Base time 00UTC 3 Sep 2005. (b) , (c), (d) are 24-, 48- and 72- hour MSLP simulations. The black dot is the position of Hurricane Maria at the valid simulation time. Top panels: Lagrangian time series over a 350-km circle of vorticty, vertical motion and theta-e from high-resolution, full physics simulation. (Evolution of Storm) Lower Panels: As above, but for coarse-resolution, dry, no-initial-vortex simulation. (Evolution of Environment) HYPOTHESIS: --------------- FUTURE PLANS Modelling: • ACCESS TC Forecast System for Operations, Climate and Research. New Vortex Specification (fits structure estimates) Verification and validation 4DVAR (NOPP/ONR Award on Rapid Intensification) LSE, (Prediction of PRWs) Initialization of accurate primary and secondary circulations, Balance conditions (non-geostrophic coupling?), Covariance modelling (flow dependent), Moisture Initialization New data sources (how to initialize a 920 hPa storm!) Diagnosis of Special Events/Forecast Failures. • Coupled Land-Ocean-Atmosphere Assimilation and Prediction System (GB, PS) FUTURE PLANS TC Behaviour • Formation • Understanding and Prediction of RI (NOPP/ONR Award) • ET and TT • Boundary Layer Structure and Surface Exchange Processes • Size and Structure Change • LANDFALL (Structure, Rainfall, etc…CLOK) <= Longer Term Objective Climate • TCs and Climate Change • Idealized Simulations Fig.7. Vertical cross sections of PV and ascent for Pre-ET (00UTC Sep.03rd 2005) and Post ET (00UTC Sep.06th 2005) Maria.
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