Innovative methods for tropical cyclone genesistrack prediction

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Innovative methods for tropical cyclone
genesis/track prediction

• T. N. Venkatesh
• Flosolver Unit
• National Aerospace Laboratories
• Bangalore, INDIA
• tnv_at_flosolver.nal.res.in

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Outline

• Introduction Flosolver Lab
• Problem of Tropical cyclone (TC) genesis/track
  prediction
• TC genesis
• Vortex merger theory
• Prediction method
• TC track
• Effect of new boundary layer
• Conclusion

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Flosolver Lab

• 1980s denial regime in-house
  supercomputer development
• Indias first parallel computer in 1986
• Six generations

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Flosolver Lab

• NAL / Flosolver parallel computer for fluid
  dynamics
• Atmospheric modelling for nearly two decades

• TC genesis PhD problem
• Track simulations part of NMITLI project

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Tropical cyclones

• Of both scientific and practical interest
• Track, intensity prediction
• Genesis
• Storm surge
• Accurate track forecasts have considerable
  societal value.
• Genesis prediction, could help in advanced
  warning

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TC Genesis Grays conditions

• Warm sea waters ( gt 27 degrees)
• Weak vertical shear of wind
• Latitude greater than 5 degrees
• Conditions suitable for moist convection

Necessary but not sufficient What is the critical
factor ?
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Earlier theories CISK

• Conditional Instability of the Second Kind
• Charney 1964
• Growth at realistic length and time scales
• Short wavelength cutoff
• Energy source

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Earlier theories WISHE

• Air Sea interaction
• Emanuel, 1986
• Integral view of moisture/heating
• Finite amplitude nature
• Energy source

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Vortex merger theory(PhD Thesis T. N.
Venkatesh, IISc, April 2003)

• Stage 2 This larger vortex increases in
  strength due to the air-sea interaction mechanism

• Stage 1 Mid -level mesoscale vortices
  interact. If this interaction results in merger,
  the second stage is reached

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Numerical simulations

• Stage 1 2 D vortex patch studies
• Critical distance for merger of regular
  configurations of vortex patches
• These occur at length and time scales relevant to
  atmospheric vortices

Vortex blob method of Beale and Majda Also
Second order moment model of Melander etal
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Two patches
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Two patches
Critical distance 3.2
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Three patches
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Three patches
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Four patches
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Four patches
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Critical distance for merger
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Stage 2

• Axisymmetric model
• Clouds
• Boundary layer
• ...
• Mid-level vortices decay, but a deep vortex which
  extends down to the boundary layer amplifies

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Observational Evidence from IR Images

• Merger of MCVs prior to TC formation using
  satellite images and observed wind fields

23 October 1999
28 October 1999
Also, evidence exists from aircraft observations
in the Pacific Ritchie et al and Simpson et al
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Prediction method

• Prediction True test of a theory
• Identify MCVs, integrate
• Simpler approach Merger index

• R_cg Average distance of the systems
  from centroid
• L Average radius of the systems
• r(n) Critical radius of merger

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Vortex merger index

• Calculated from satellite IR images
• From the CIMSS website (3 hour intervals)
• Studies in the Bay of Bengal
• Real-time tests since October 2002
• Can give advance warning for formation by about
  48 hours
• Geophysical Research Letters, Vol 31, L04105,
  February 2004
• Four seasons Eight events
• 6 lead to TC formation
• 2 False alarms (depressions formed)

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The merger index
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Test cases
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Recent seasons
False alarm
More analysis required
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Possible use of additional data from
Megha-Tropiques

• Mesoscale structures (MCSs, MCVs)
• Validate theory
• Earlier detection of MCS/MCVs
• Velocity fields ?

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Track prediction
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NMITLI project on Mesoscale modelling for
monsoon related predictions

• NAL, IISc, TIFR team Option A software
• Development of a new prediction code to be run
  efficiently on NMITLI hardware
• Reengineered NCMRF T-80 code forms the backbone
  for the present model
• Written in Fortran 90
• New boundary layer module
• New radiation module
• Grid clustering

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NMITLI Code Version 1

• Operational on Flosolver MK6
• Rewritten in Fortran 90
• Seed code NCMRWF/NCEP GCM T-80
• Incorporates new physics modules
• Boundary layer
• Radiation
• Engineered software
• Code length reduced
• Nanjundiah Sinha, Current Science, 1999

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New boundary layer scaling at low winds

• Tropics characterized by convection at low winds
• Monin-Obukhov not applicable
• Usual fix Gustiness parameter (Hack et al, 1993)
• New parameterization in NMITLI code for weakly
  forced convection
• Heat-flux scaling for weakly forced turbulent
  convection in the atmosphere
• K. G. Rao and R. Narasimha, JFM 2005
• Based on data from MONTBLEX-90 (Narasimha, Sikka
  and Prabhu 1997) and BLX-83 (Stull 1994)

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Weakly forced convection
Drag is linear in wind speed
MONTBLEX-90(Jodhpur, India)
BLX-83(Chickasha, USA)
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Weakly forced convection
Heat flux given by free convection
MONTBLEX-90(Jodhpur, India)
BLX-83(Chickasha, USA)
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Implementation of new parameterization

• Weakly forced convection
• Drag is a linear function of wind speed
• Heat flux is independent of wind speed
• Define matching velocity -Vm
• V gt Vm use M-O estimates
• V lt Vm use Heat flux scaling
• Match at Vm
• Integrated into the NMITLI GCM and tested
• Values of Vm 1, 3, 5 m/s

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Low resolution 80 Modes Old BL

• Observed
• Simulated

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Low resolution 80 Modes New BL

• Observed
• Simulated

Vm 5 m/s
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Higher resolution 120 Modes Old BL

• Observed
• Simulated

Grid 512x256
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Higher resolution120 Modes New BL

• Observed
• Simulated

Grid 512x256
Vm 3 m/s
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Higher resolution120 Modes New BL

• Observed
• Simulated

Grid 512x256
Vm 5 m/s
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Orissa Supercyclone 1999Track errors
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Track improvement Preliminary analysis

• Surface force on the TC due to the PBL computed
• Within a radius of 8 grid lengths (approximately
  640 km) from centre of TC
• Total torque

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Preliminary analysis Stress fields
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Surface force on TC
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Possible use of additional data from
Megha-Tropiques

• Surface fluxes
• Heat
• Moisture
• Accurate fixing of the Initial position of the
  Tropical Cyclone

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Concluding remarks

• Tropical Cyclone genesis
• New prediction method
• Results are encouraging
• Further work is necessary
• Tropical Cyclone track
• Use of a new boundary layer scaling improves
  track simulation significantly
• Additional data from the Megha-Tropiques
  satellite would help in refining these schemes

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Cyclone 03B, 2003Track errors
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Radiation Module

• Long wave - new code based on work of Varghese
  etal
• Valid from surface to 100 km
• Accurate near the surface
• Integrated into NMITLI Code Version 1
• CPU time
• Being optimized
• Look up table

Varghese, A. V. Murthy and R. Narasimha, JAS
2003