DIAGNOSIS AND PREDICTION OF TROPICAL CYCLONE BEHAVIOUR IN THE AUSTRALIAN REGION AND THE SOUTH ATLANTIC :

SOME COMPARISONS BETWEEN THE TROPICAL TRANSITION OF CATARINA (SA) AND DONALD (AR)

NOEL DAVIDSON, ALEX PEZZA#, LUKE GARDE# AND GRAHAM MILLS

Centre for Australian Weather and Climate Research

# The University of Melbourne, Earth Sciences

1. Subtropical Cyclones Catarina (S. Atlantic) and Donald (Aust. Reg.)

Large-scale Environment (LSE)

Internal Structure and Evolution

Climatological Aspects

2. TC-LAPS Initialization and Prediction

Components: Large-scale Environment, Vortex Specification, High-resolution Analysis, Initialization, High-resolution Prediction, Verification

3. Initialization and Prediction for Catarina (preliminary results)

The Vortex Specification Dilemma!!

Initial Conditions; Vortex Structure; Prediction

4. Summary

5. Possible Future Work?

CONTENT

Track forecast quite skilful

Intensity forecast quite skilful to 48 hours, and then “apparently” doesn’t get the intensification (or does it?).

VMAX fluctuates between 55 and 70 knots throughout, even though CP lies between ~ 995-1000 hPa !!!!!???

W-E XSECT of VCMP at 48 hours: Intense, mesoscale, warm inner-core embedded in a synoptic-scale, cold-cored trough (Structure OK??)

Q1: Do we know the structure of the inner-core at any time?

(when, depth, RMW?)

Q2: Is the Dvorak Method applicable here?

Q3: When and how does the mesoscale inner-core form?

Q4: Why and how does the model maintain this beautiful large-scale and inner-core structure? (TT)

Dare and Davidson, 2004: Tropical Cyclone Characteristics in the Australian Region. MWR.

Forecast issues:

Track,

Genesis, (>>>>> TT)

Intensification/Decay,

ET,

Landfall

Points of Origin

Final Decay Locations

Locations with Minimum Central Pressure

Data for Initialization and Validation

nearly NON-EXISTENT!!

Cannot validate structure and quantify forecast skill much beyond track. So, validate over the US…..

SUB-TROPICAL CYCLONES (MILLS)

• A cold-cored low, developing from the upper troposphere

• Can move westwards large distances

• Circularly symmetric at low levels

• “Weak” circulations near centre

• Strong winds on poleward side at large radii

• Surface low equatorward of sub-tropical ridge

(Simpson, 1952)

• Often termed a Kona Low if it occurs off the Hawaiian coast.

***

28/2/2001 23:30UTC GMS 5 IR

ex-TC Abigail pre-Donald TC Paula

High Latitude Influences: Large Amplitude, Long Waves at Upper Levels

Subtropical Cyclone Donald

The “DUCK”

TC Donald – Not Named

March 2001

Donald produced large rain totals at Landfall; Catarina produced much less rainfall??

Catarina and Donald Similarities: Large-Scale Environment

• Blocking

• Split Jet

• STJ to N

• Moisture from NE

DONALDCATARINA

Similarities: Structure

Phase Diagrams for Catarina and Donald (Hart, 2003):

Medium-to-Deep Cold-Core transitioning to

SHALLOW, SYMMETRIC WARM CORE

Tracks start (UTC)– finish times (UTC):

ERA40:02/03/01 (1800) – 08/03/01 (1800)Δt = 6hrs

NCEP:02/03/01 (1200) – 08/03/01 (1800)Δt = 6hrs

NCEP2:03/03/01 (1200) – 08/03/01 (1800)Δt = 6hrs

Meso LAPS:03/03/01 (2300) – 08/03/01 (1100)Δt = 12hrs

LAPS (Simulation Results) :04/03/01 (0000) – 08/03/01 (0000)Δt = 6hrs

Similarities: MOTION?

Donald: Internal Structure Change

Lagrangian (System-following)

Diagnostics from ERA40

Time-Height Series 00UTC 4 March – 00UTC 9 March, 6-hourly

Vorticity, Omega, Theta-E over a 300 km radius circle.

e

Similarities: Structure Change

Moisture Advection, Surface Fluxes – Budget?

Catarina: Internal Structure Change

Lagrangian (System-following)

Diagnostics from GASP Analyses

Time-Height Series

00UTC 23 March – 00UTC 28 March , 6-hourly

Vorticity, Omega, Theta-E over a 300 km radius circle.

e

Similarities: Structure Change

=>Hypothesis: Inhibition, BL Moistening, Release Inhibition, Active Convection, RI

=> Hypothesis:

Initially, a weak low-level cyclonic circulation is present.

Period of Inhibition to Convection by LSE occurs

With sustained Moisture Advection + Surface Fluxes, PBL moistens

Period of Inhibition passes

Active, Deep Convection develops in a much moister PBL

Convection occurs in a low wind shear, cyclonic environment

Intensification ensues

SOUTH ATLANTIC INDEX (1979-2004)

Pezza and Simmonds, 2008Annals of the NY Academy of Science

ANYAS, in press

•Usually positive

•Slight positive trend

•High peak March 2004

TT

South Atlantic Index (SAI): normalized lagged difference (12 days) between high latitude blocking index and minimum midlatitude shear. (Pezza et al 2008, IJC, under review).

Some Historical Perspectives

TRENDS OVER THE CATARINA BOX (ERA40)

Pezza et al., 2008IJOC, under review

TC-LAPS - Operational from 1999 Davidson and Weber, 2000, MWR

TC-LAPS has five basic components – all are critical : 1. Data assimilation (Bourke, Steinle, Kepert, Tingwell, …) to

establish the storm’s large-scale environment (LSE) and outer structure. (0.3750X29L,~550S-550N, 700E-1800E)

2. Vortex specification (Weber and Davidson; Holland vortex) to

construct the inner core circulation and asymmetries consistent with the estimated size, intensity and past motion, and then to re-locate the circulation to its observed position.

3. High-resolution (HR) analysis, (Steinle, Mills, Davidson) with

appropriate observational errors, length scales and quality control tolerances, to merge the intense vortex into the LSE. (0.150X29L, 270Lat-Long)

4. Initialisation with diabatic, dynamical nudging, (Davidson and

Puri) to balance the vortex using the model’s dynamics, and to re-define the vertical motion field to be consistent with the satellite imagery.

5. High-resolution prediction (Puri et al., 1998) with the generalised

LAPS forecast model, which contains high order numerics and sophisticated physical parameterisations.

VERIFICATION OF TC-LAPS : Australian Region 2004-2005

Forecast model initialized and verified against AR Operational Track Data t = 00

hrs t = 24

hrs t = 48

hrs t = 72

hrs Mean Direct Position Error (kms)

14.7 148.9 245.1 361.6

Mean Abs Central Pressure Error (hPa)

6.1 13.0 17.6 21.6

Number of forecasts 62 56 43 31

0- 100

100-200

200-300

300-400

400-500

500-600

600-700

No of 48 hour errors in 100km bins

7

9

13

11

2

0

1

Quite competitive for track and intensity by any measure. Major Errors : At 48 hours , ~ 1 BUSTS (> 500km).

Related to (a) Initial Vortex Structure, and/or (b) Analysis and Forecasts of the Large Scale Environment (DA).

Systematic Errors : Westward and Poleward Bias . Underestimation of Intensity

(Central Pressure). Unusual Behaviour around Coast.

OPERATIONAL TC-LAPS

VORTEX SPECIFICATION

Construction of Symmetric Vortex

• INPUT: size (ROCI), intensity (Pc), past motion (0, -6, -12 hour locations);

and LSE (Large Scale Environment) from DA

• Surface Pressure Distribution: Analytical form

(Fujita, Holland, Chan and Williams), merge with LSE using ROCI.

• Assume thermal structure at storm centre is moist adiabat from surface (Tc)

• Pc, Tc => Zc (geopotential height at centre of storm)

• Interpolate between Zc and Zlse using an empirical structure function and

Storm Size to obtain Z

• Compute winds from Z using Gradient Wind Equation

=> Symmetric Vortex

• NO SECONDARY CIRCULATION

• LACKS BALANCE between MASS and WIND FIELDS

Nudging Initialization t=-24 t=-18 t=-12 t=-6 t=0 HRGenSI HRGenSI HRGenSI HRGenSI |--------------------|--------------------|--------------------|---------------------|---48 hours.. |LAPS Forc with NI towards HR GenSI anals………………….| - Forecast |…...^ GMS ^……....^ GMS ^.…….....^ GMS ^.…....^ GMS ^….| |Deep Convection only: Rationale:

DA builds analysed rotational flows to construct LSE and outer structure of storm. VS constructs Synthetic Vortex and merges it with LSE and outer structure from A NI creates ‘mesoscale balance’ and local, “estimated” divergent flows, consistent with cloud imagery, while preserving rotational flows. (Insert empirical convective heating profiles in regions of satellite-observed deep convection. Model tries to remove inserted heat sources with adiabatic cooling by ascent => ascent develops where deep convective clouds are observed)

GMS Assimilation and Initialisation

GMS CTT Data 2002020111

400hPa initialised omega

Initialised analysis and 48 hour forecast of 950hPa windfield

After initialization, if all goes to plan, the initial condition contains a vortex with :

• An accurate analysis of the large-scale environment

• A primary circulation which is consistent with estimated characteristics of the TC (location, past motion, intensity).

• Model-balanced primary and secondary circulations.

• A vertical motion field consistent with satellite imagery,

• A structure “consistent” with its environment.

Unfortunately, everything rarely goes to plan!!

Additionally, the circulation may not fit all conventional obs

So…..

….. Design a new Vortex, constructed from estimates of CP, VMAX, RMW, R34, ROCI

OR

….. Use 4DVAR to construct vortex from standard observations.

INITIALIZATION OUTCOMES

TC STRUCTURE NW Pacific Storms : Podul, Fengshen, Fung-Wong, Rammasun, Utor

Figure 1 : Actual satellite cloud imagery from GMS (top panels), and (lower panels) corresponding synthetic cloud imagery (Rikus and Sun, 2004) from operational TC-LAPS for TCs Podul (30 hour forecast), Fengshen (18 hour), Fung-Wong (12 hour), Rammasun (18 hour), Utor (36 hour)

**Good OBS NETWORK + sophisticated High-Resolution Modelling System = well-defined LS Environment + consistent, well-defined Vortex Structure

=> Some skill in Intensity Forecasts!! BUT BUSTS TOO!!

1. System Options:

NCEP, GASP Initial Conditions

Vortex Specification or No Bogus (which Vortex Structure?)

Nudging Initialization or Cold Start

Analysed or Forecast BCS

Satellite-based Convective Heating Profiles (not yet)

2. Experiments (so far):

A. GASP Initial Conditions, Cold Start

B. GASP Initial Condition, Nudging

C. Vortex Specification, Cold Start

D. Vortex Specification, Nudging

E. Revised Vortex Specification, Nudging

F. Application of Satellite-based Convective Heating Profiles

PREDICTION EXPERIMENTS FOR CATARINA

Catarina: Vortex Structure: The Vortex Specification Dilemma!!

Catarina Structure:

W-E XSECT of VCMP

24, 25, 26, 27 March

Horiz and Vert Structure:

VMAX at outer radii?

Cold-cored till 26,27?

Base time 00UTC, 20040325: The Vortex Specification Dilemma!!

Sensitivity to Initial Conditions

NCEP REANAL ABCS GASP, ABCS VS, FBCS

GASP ICs. Forecasts from base times 00UTC, 21, 22, 23, 24, 25 26 March

21 22 23

24 25 26

W-E CROSS SECTIONS OF V COMPONENT FOR CATARINA

(00UTC 25 March 2004)

Global Analysis Full Vortex Specification (VS) VS in Lower Troposphere

Basetime 00UTC, 20040325: CS, NG, Full VS, “Half” VS

…….MORE WORK

Sensitivity to Initialization. (base time 00utc, 25 March)

1. Catarina and Donald had similar structures and evolution.

2. Both developed warm-cored structures from cold-cored systems in which a large-scale midlatitude upper trough and subtropical jet enveloped the low-level circulation. Catarina underwent Extratropical to Tropical Transition (TT), Donald underwent partial TT.

3. Diagnostics suggests that TT may be associated with environmental descent, boundary layer moistening (advection and surface fluxes), and eventually, as the inhibition to ascent passes, development of deep, active convection, in a low wind shear, cyclonic environment.

4. Track prediction for Catarina, after “the loop”, is quite encouraging, but intensity prediction is so far from TC-LAPS, only fair. (although intensity and structure estimates are uncertain?)

5. Prediction of the pre-hurricane stage and TT should not be conducted with existing vortex specification methods. The imposed TC structures are too different from the pre-hurricane structures!! (Physical Initialization, Krishnamurti et al., or 4DVAR may be the preferred pathway).

6. Some forecasts may benefit from a revised form of Vortex Specification for Subtropical Cyclones – but is this the best way?. (It may improve understanding).

SUMMARY OF MAIN POINTS

• Do we understand how the LSE influences the internal structure? 1. Modulation of ascent and environmental vorticity. 2. Lorenz Energy Conversions suggest dramatic changes occurred ~ 22/23 March when Storm Track looped.

(Veiga, Pezza, Simmonds, Silva Dias, GRL, 2008?)

• How well do we predict these (rare) phenomena? (Intercomparisons between basins)

• What do we need to predict these phenomena?

• Need to know more about the internal structure of subtropical cyclones and how the structure evolves. (RMW, Vertical Structure)

• Would a revised form of Vortex Specification be useful for prediction of subtropical cyclones? (May improve understanding?)

• Assimilation of additional data at higher resolution?

• Application of imposed convective heating profiles during initialization to improve intensity, and possibly track prediction?

• Physical Initialization, 4DVAR?

POSSIBLE FUTURE WORK?

SOUTH ATLANTIC HURRICANE ‘CATARINA’

Pezza and Simmonds, 2008Annals of the NY Academy of Science,

ANYAS, in press

Pezza and Simmonds, 2005

SST minus 1000 hPa temp. 20-28 March 2004

< SST used in NWP Expts. OK??

05/3/2001 23:30UTC GMS 5 IR

ex-TC Abigail Donald