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The Good, the Bad, and the Ugly:

The Good, the Bad, and the Ugly:. Ron McTaggart-Cowan, John Gyakum, and Lance Bosart. Numerical Prediction of Hurricane Juan. Outline. Case Background Operational Forecasts and NWP Improved NWP Guidance Summary/Discussion. Case Background.

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The Good, the Bad, and the Ugly:

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  1. The Good, the Bad, and the Ugly: Ron McTaggart-Cowan, John Gyakum, and Lance Bosart Numerical Prediction of Hurricane Juan

  2. Outline • Case Background • Operational Forecasts and NWP • Improved NWP Guidance • Summary/Discussion

  3. Case Background • Depression crosses Atlantic 14-20 September with weak convection • Isabel’s outflow suppresses devel-opment from 21-24 September • During 22-24 September, genesis occurred along an extended front to the northeast of the Lesser Antilles

  4. Case Background NHC Best Track • Tropical transition completed by 25th • Accelerates and develops strong asymmetry by 0000 UTC 29th • Landfall near Halifax at 0300 UTC 29th as a Category 2 Hurricane

  5. Case Background 0000 UTC 28 September Still near maximum intensity (obtained at 1800 UTC 27th) Minimum SLP 970 hPa with est. 90 kt winds

  6. Case Background 0600 UTC 28 September Moving W-N-westward and weakening slightly Minimum SLP 972 hPa with est. 85 kt winds

  7. Case Background 1200 UTC 28 September Moving W-N-westward and developing strongly asymmetric structures Minimum SLP 970 hPa with est. 90 kt winds

  8. Case Background 1800 UTC 28 September Moving northward and accelerating rapidly Minimum SLP 972 hPa with est. 80 kt winds

  9. Case Background 0000 UTC 29 September Accelerating rapidly towards Halifax (landfall 3h later) Minimum SLP 974 hPa with est. 70-85 kt winds

  10. Forecasting and NWP Operational forecasting for Juan was provided primarily by NHC and CHC Track forecast errors were below the 5 yr mean for all lead times (NHC) Intensity forecast errors were larger than average, but still reasonable

  11. Forecasting and NWP Forecasters at both the CHC and the NHC issued excellent forecasts Lead times of over 24h were given for winds and rain associated with Juan

  12. Forecasting and NWP • Forecasters noted a bifurcation in model guidance from the 0000 UTC 28th run • Common features at mid-levels are used to produce a good forecast

  13. Forecasting and NWP Over the past 5 years, the major American, British, and European models have all been upgraded to deal with tropical features … So what caused this potentially dangerous split in the guidance?

  14. Forecasting and NWP * Quality is defined subjectively based on 24h surface forecasts from the 0000 UTC 28th run

  15. Forecasting and NWP Initialization GFS Sat Image satellite GFS SLP and surface winds (kts) at 0000 UTC 28th Composite vapour/IR at 0000 UTC 28th

  16. Forecasting and NWP 24h Forecast GFS Sat Image satellite GFS SLP and surface winds (kts) at 0000 UTC 29th Quickscat winds at 2300 UTC 28th

  17. Forecasting and NWP Initialization Eta Sat Image satellite Eta SLP and surface winds (kts) at 0000 UTC 28th Composite vapour/IR at 0000 UTC 28th

  18. Forecasting and NWP 24h Forecast Eta Sat Image satellite Eta SLP and surface winds (kts) at 0000 UTC 29th Quickscat winds at 2300 UTC 28th

  19. Forecasting and NWP Initialization GFDL Sat Image Not Available

  20. Forecasting and NWP 24h Forecast GFDL Sat Image Not Available

  21. Forecasting and NWP Initialization UK Sat Image satellite UKMet SLP and surface winds (kts) at 0000 UTC 28th Composite vapour/IR at 0000 UTC 28th

  22. Forecasting and NWP 24h Forecast UK Sat Image satellite UKMet SLP and surface winds (kts) at 0000 UTC 29th Quickscat winds at 2300 UTC 28th

  23. Forecasting and NWP Initialization Navy Sat Image Not Available

  24. Forecasting and NWP 24h Forecast Navy Sat Image Not Available

  25. Forecasting and NWP Initialization GEM Sat Image satellite GEM SLP and 1000-500 hPa thickness 0000 UTC 28th Composite vapour/IR at 0000 UTC 28th

  26. Forecasting and NWP 24h Forecast GEM Sat Image satellite GEM SLP and 1000-500 hPa thickness 0000 UTC 29th Quickscat winds at 2300 UTC 28th

  27. Forecasting and NWP Initialization MC2 Sat Image satellite MC2 SLP and 1000-500 hPa thickness 0000 UTC 28th Composite vapour/IR at 0000 UTC 28th

  28. Forecasting and NWP 24h Forecast MC2 Sat Image satellite MC2 SLP and 1000-500 hPa thickness 0000 UTC 29th Quickscat winds at 2300 UTC 28th

  29. Forecasting and NWP So what happened? … Forecast models initialized without a strong enough vortex failed to develop or maintain a closed circulation and favoured the western trough as the weather-maker

  30. Forecasting and NWP Weak initial vortex without troposphere-deep PV column Weak poten-tial stability ahead of trough

  31. Forecasting and NWP PV tower tilts downshear as development occurs over the Bay of Fundy Surface circu-lation tracks westward

  32. Forecasting and NWP Bogusing methodologies are very different at the various operational centres most complexity least

  33. Forecasting and NWP Who produced “Good” guidance for Hurricane Juan? most complexity least

  34. Forecasting and NWP GFDL vortex initialization Vortex in GFS initial fields is removed using filters and optimal interpolation New vortex is grown in model Full treatment of SST evolution and interaction Strong, well-balanced vortex in initial fields

  35. Forecasting and NWP UKMO vortex initialization Manual intervention when satellite or in-situ data is significantly different from first guess field Synthetic wind observa-tions are added Analysis cycle balances mass field to produce a realistic vortex

  36. Forecasting and NWP US Navy vortex initialization Vortex generation based on balanced hurricane vortex of Kurihara Fully-developed vortex is inserted in HPC-analyzed position

  37. Juan? Forecasting and NWP NCEP vortex initialization Vortex removal is made using multiple filter passes Hurricane compon-ent moved to HPC-analyzed position Eta adds bogus winds (for now)

  38. Improved Guidance Dense observations to the N/W/S of the storm over the continent mean that upstream analysis should be good Quality of forecast depends on the strength/coherence of the initial vortex Can we improve a bad forecast with a better initial vortex alone?

  39. Improved Guidance McGill vortex initialization Insert a fully-developed Kurihara-type vortex at 0000 UTC 28th Rerun 48h forecast without any other modifications to the initial or boundary conditions (supplied by GEM) Both 12km and 3km domains are rerun

  40. Improved Guidance satellite satellite Modified initial conditions at 0000 UTC 28th September

  41. Improved Guidance satellite satellite Modified forecast SLP and winds fields valid 0000 UTC 29th September

  42. Improved Guidance Initial PV tower is well-defined throughout the troposphere Convective instability over the continent is not changed

  43. Improved Guidance Vortex tube remains upright and wraps potential instability into the circulation Much improv-ed track and structure

  44. Improved Guidance How much could have been done in real time at high resolution?

  45. Summary The operational forecasts for Hurricane Juan were of very high quality Operational models without a realistically- bogused vortex gave poor sfc guidance Synthesis of a strong (Category 2) vortex in the initial conditions of one of the “bust” models results in a much-improved forecast

  46. Discussion The NWP for Hurricane Juan reduces to an initial value problem involving the tropical vortex itself: A realistically-strong, balanced synthetic vortex was ALL that was required for accurate numerical prediction of Juan in real time

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