NCEP Operational Global Cyclone Tracking and Verification System

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NCEP Operational Global Cyclone Tracking and
Verification System

• Guang Ping Lou, Geoff DiMego, Tim Marchok,
• Binbin Zhou, Mike Charles
• NCEP/EMC/MMB
• EMC seminar Nov. 4, 2008

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1. The Ideas

• What are needed
• A global cyclone tracking system. Existing
  tracker is confined to North America and adjacent
  oceanic basins.
• A real time cyclone track verification system
  that gives feedback to forecasters and modelers.

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1. The Ideas (cont.)

• Challenges
• In order to do track verification, the truth is
  needed. In the tropical case, NHC puts out storm
  vitals, the best observed tracks as the truth.
  However, there is none-existence in the
  extra-tropical regions. Operationally, it is
  laborious at best, and almost impossible to
  manually identify all cyclone tracks in the
  middle latitudes.

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1. The Ideas (cont.)

• Solution
• Utilization of existing resources to create
  truth for verification. That is, model analysis
  tracks are used as the truth, which are
  embedded to the track output files. Stringing
  them together will form analysis tracks.
• Schematic Diagram

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1. The Ideas (cont. )

• Advantages
• 1. The system is objective
• 2. It is a completely automated system and no
  human intervention is needed from identifying
  both analysis and forecast tracks to
  verification, plotting and web displaying
• 3. Provide a quick look at how well the
  forecasts are against the analysis tracks
• 4. It can be used to generate retro-run tracks
  and verification easily.
• Disadvantages
• 1. The analysis tracks are often short lived
  due to weakening and observation data ingest in
  each synoptic time
• 2. The tracks can only be verified against
  model self analysis because matching difficulties
  against another model analysis.

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2. Tracking Algorithm

• Single-pass Barnes analysis scheme is used to
  locate the cyclone center.
• Barnes analysis equation For a variable F, the
  Barnes analysis B, at a point g,
• 
  
• 
  where
• Where dn is the distance from a data point n to
  the grid point g, and re is the e-folding radius.
  Typically an re value of 75 km for models with a
  grid resolution finer than 1.25 degree and 150 km
  for those with coarser resolution.

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2. Tracking Algorithm (cont.)

• Tracking parameters are mslp, 700 and 850hpa
  relative vorticity, and geopotential height and
  winds at 700 and 850hpa. The Barnes analyses of
  these 7 parameters determine the tropical cyclone
  center. For middle latitude cyclones, only mslp
  is used to determine the center.
• To minimize the iterations, after one parameter
  center is located, the other four parameters are
  restricted to smaller domains.
• For next forecast center position, an
  extrapolation of previous locations is computed
  and current storm advection according to winds is
  calculated.
• Other criteria are applied to the searched
  position so that it is indeed a cyclone center.
  Such as vicinity of previous position pressure
  gradient, etc.

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2. Tracking Algorithm (Cont.)

• Phase space The 3 diagnostics from Robert Hart's
  cyclone phase space. The three parameters used to
  described the general structure of cyclones are
  the lower-tropospheric thermal asymmetry
  (parameter B), the lower-tropospheric thermal
  wind and the upper-tropospheric thermal wind
  (cold versus warm core).
• Cyclone thermal symmetry parameter B
• Where h 1 for northern hemisphere and h -1
  for southern hemisphere.

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2. Tracking Algorithm (Continue)

• Lower-tropospheric thermal wind
• Upper-troposheric thermal wind
• Where
• d the distance between the geopotential extrema

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3. Differences Between Old Regional and New
Global Trackers

• Old regional tracker
• Include only CONUS, Atlantic basin, West Pacific
  basin, East Pacific basin, and Alaska, mainly
  north America-centric
• Single CPU does all processing. Regional search
  needs as much as 3.5 hrs wall clock.
• Storm ID has no continuity.

• New global tracker
• From -9090, 0360, lat lon, truly global
  coverage
• Multitasking schemes to have as many tasks as
  number of ensemble members. Global search
  finishes within 20 minutes wall clock. Users have
  much earlier access to tracks.
• New storm ID is named with initial date,
  lead-time in forecast and lat/lon position where
  storm was first identified.

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3. Old Regional And New Global (Cont.)

• 4. Regular atcf text output with limited
  parameters.

• 4. In addition to regular output, added phase
  parameters Positions for the 3 diagnostics from
  Bob Hart's cyclone phase space. Mean max values
  of relative vorticity near the storm at 850
  700. Direction of model storm motion, Translation
  speed of model storm, The pressure of the last
  closed isobar and the radius of that closed
  isobar.
• 5. TIGGE web format output. (THORPEX
  Interactive Grand Global Ensemble)

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4. The Steps

• Tracking
• Operational tracker is invoked whenever model
  forecast data
• arrive at NCO. The forecast models include GFS,
  NAM, CMC,
• UKMO, ECMWF, NOGAPS, and ensemble forecasts such
  as
• NCEP ensemble GEFS and SREF, CMC and ECMWF.
  Cyclone
• tracks are plotted and graphics are pushed to a
  web server.
• They are also archived in ATCF, MySQL format and
  TIGGE
• format.
• http//www.emc.ncep.noaa.gov/mmb/gplou/emchurr/ver
  ify/

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4. Steps (cont.)

• b) Getting the analysis tracks
• Model analysis ATCF data are picked out from the
  forecast tracks.
• Search nearby cyclone center in the next synoptic
  time. The criteria for recognizing as the same
  cyclone are 3.5 by 4.5 lat and lon in middle to
  low latitudes 4.5 by 5.5 lat and lon in high
  latitudes. Set aside preliminary storm tracks.
• Search for cyclones that are weakened and then
  re-appeared within 12 hours in the vicinity
  areas.
• Another iteration that string fragmented analysis
  storm tracks to form a unique track that will
  span the cyclogenesis to die out.

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4. Steps (cont.)

• c) Matching forecasts with analysis tracks
• Separate each forecast track into independent
  files.
• Search for analysis cyclone track that matches
  the forecast track.
• Combine the forecast and analysis tracks into one
  file. Operationally, previous 10 days cyclones
  are lined up for verification. Climatologically,
  uniquely paired cyclones are stored in a monthly
  database. This database is also converted to
  MySQL format.
• d) Verification.

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5. What Is Running in NCO Operations

• Currently there are 6 single models and 4
  ensemble track runs are in operations. Trackers
  run in different time and various frequencies
  depending on the model. Starting time is
  immediately after the model data become
  available. All tracker runs are carried out by
  NCO.
• After each tracker run, a cron job starts to
  produce forecast track plots, analysis track
  plots, track verification and plots, web display.
  
• GFS Tracker runs 4 times a day at 00Z, 06Z, 12Z,
  18Z.
• NAM 4 times a day at 00Z, 06Z, 12Z, 18Z.
• ECMWF 2 times a day at 00Z, 12Z.
• UKMET 2 times a day at 00Z, 12Z.
• CMC 2 times a day at 00Z, 12Z.
• NOGAPS 2 times a day at 00Z, 12Z.
• GEFS 4 times a day at 00Z, 06Z, 12Z, 18Z. 21
  members.
• SREF 4 times a day at 03Z, 09Z, 15Z, 21Z. 21
  members.
• CENS 2 times a day at 00Z,12Z. 17 members.
• EENS 1 time a day at12Z. 50 members.

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6. Examples
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6. Examples (cont.)
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Forecasts at 12Z more accurate than at 00Z?
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6. Examples (global)
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6. Examples (global)
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7. Verification and Schemes

• Position error and bias
• OB1-3 Observed positions
• FC Forecast position
• DX Error in the East-West direction
• DY Error in the North-South direction
• AT Error in the Along Track direction
• CT Error in the Cross Track direction
• DXXo-Xf DX errors are positive if the
  forecast position lies westward of obs
• DY-(Yo-Yf) DY errors are positive if the
  forecast position lies poleward of obs
• PESQT(DXDXDYDY) Position error

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7. Verification and Schemes (cont.)

• Following conventional tropical storm
  verification procedure, cyclone tracks are
  arranged in two separate decks
• Adeck, contains past 10 days forecast tracks.
• Bdeck, holds analysis tracks that match the
  forecasts.
• Verification is done for both track position
  errors and intensity errors, RMSE, x, y biases,
  amongst other stats.
• In the examples shown previous slides, track
  errors are
• calculated as displayed next.

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Average Forecast Position Errors for Tracks in 10
Day Span
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Average Forecast x,y Track Bias
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8. Climatology of Cyclone Tracks

• Monthly statistics of forecast position error,
  x-bias, and y-bias
• Seasonally and yearly statistics

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8. Climatology of Cyclone Tracks (cont.)
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12 Month Mean Position Error
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12 Month x-bias
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12 Month y-bias
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9. Summary and Discussion

• Established a new automated cyclone track
  verification system
• Replacing regional tracker with a new global
  cyclone tracker
• Shortened track delivery time from 3.5 hours to
  within 30 minutes for ensemble tracks
• Created tracks database in ATCF text, MySQL,
  TIGGE formats, and increased content of the
  output files
• Tracks climate verification
• Upgraded web display capability
• 
  
  
  

1. Tracking forecast cyclone tracks 2.
Seeking out analysis tracks 3. Matching
forecast with analysis tracks 4.
Verification 5. Plotting and pushing plots to
server
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9. Summary and Discussion (cont.)

• ECMWF model is most accurate overall in
  forecasting the cyclone position
• GFS fairs very well within 48 hours forecasts,
  but overtaken by CMC and ECMWF models after that.
• NAM has largest position errors. Most of it comes
  from moving too fast for forecasting cyclones.
• Position errors in other models mainly come from
  north-south displacement
• But GFS is less consistent in forecasting
  east-west position tends to oscillate between
  too fast and too slow.