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Microsoft PowerPoint - cawcr_wks

VIEWS: 5 PAGES: 32

									        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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