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Application of the CRA Method

Application of the CRA Method. William A. Gallus, Jr. Iowa State University Beth Ebert Center for Australian Weather and Climate Research Bureau of Meteorology. (4) 125 pts to the right and rotated. (5) 125 pts to the right and huge. (3) 125 pts to the right and big. Observed.

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Application of the CRA Method

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  1. Application of the CRA Method William A. Gallus, Jr. Iowa State University Beth Ebert Center for Australian Weather and Climate Research Bureau of Meteorology

  2. (4) 125 pts to the right and rotated (5) 125 pts to the right and huge (3) 125 pts to the right and big Observed (1) 50 pts to the right (2) 200 pts to the right Idealized cases - geometric Which forecast is best?

  3. Traditional verification yields same statistics for cases 1 and 2 forecast

  4. 5th case – traditional verification forecast forecast THE WINNER

  5. 1st case – CRA verification

  6. 2nd case – CRA verification CRA Technique yields similar results with cases 3 and 4

  7. 5th case – CRA verification

  8. RESULTS ARE SENSITIVE TO SEARCH BOX FOR DISPLACEMENTS

  9. Increase of size of rectangle (extra 90 instead of 30 pts) affects results

  10. Further increase from 90 pts to 150 pts does not result in additional change

  11. 1st case vs. 5th case THE WINNER

  12. 1000 km Perturbed cases (1) Shift 24 pts right, 40 pts down "Observed" (2) Shift 12 pts right, 20 pts down, intensity*1.5 Which forecast is better?

  13. 1st case – traditional verification (1) Shift 24 pts right, 40 pts down

  14. 2nd case – traditional verification (2) Shift 12 pts right, 20 pts down, intensity*1.5 THE WINNER

  15. CRA verification Threshold=5 mm/h Case 1 Case 2

  16. CRA verification Threshold=5 mm/h Case 1 Case 2

  17. CRA verification Threshold=5 mm/h Case 1 Case 2

  18. Problem? System far from boundary in Case 1 – small shift – behaves as expected

  19. System closer to boundary yields unexpected results – not all error is displacement

  20. Problem is more serious for smaller system at edge of domain

  21. Central system works well through medium displacements

  22. But…. Larger displacement yields odd results

  23. Summary • CRA requirement for forecast and observed systems to be contiguous may limit some applications • Problems occur for systems near the domain boundaries – not yet clear what causes the problems

  24. Results from separate study using object-oriented techniques to verify ensembles • Both CRA and MODE have been applied to 6-hr forecasts from two 15km 8 member WRF ensembles integrated for60 h for 72 cases • This results in 10 x 16 x 72 = 11,520 evaluations (plots, tables….) from each approach • Results were compared to Clark et al (2008) study

  25. Clark et al. study • Clark et al. (2008) looked at two 8 member WRF ensembles, one using mixed IC/LBC, the other mixed physics/dynamic cores • Spread & skill initially may have been better in mixed physics ensemble vs. IC/LBC one, but spread grew much faster in the IC/LBC one, and it performed better than the mixed physics ensemble at later times (after 30-36 h) in these 120 h integrations.

  26. 0.5 mm 2.5 mm Areas under ROC curves for both ensembles (Clark et al. 2007) Skill initially better in mixed ensemble but IC/LBC becomes better after hour 30-36

  27. Diurnal Cycle Variance continues to grow in IC/LBC ensemble but levels off after hour 30 in mixed ensemble. MSE always worse for mean of mixed ensemble – and performance worsens with time relative to IC/LBC ensemble.

  28. 0.5 mm 2.5 mm Spread Ratio also shows dramatically different behavior with increasing spread in IC/LBC ensemble but little or no growth in mixed ensemble after first 24 hours

  29. Questions: • Do the object parameters from the CRA and MODE techniques show the different behaviors between the Mix and IC/LBC ensembles? • Do the object parameters from the CRA and MODE techniques show an influence from the diurnal trends in observed precipitation?

  30. Diurnal signal not pronounced, only weak hint of IC/LBC tendency to have increasing spread with time – and only in MODE results Rain Rate Standard Deviation (in.) – mean usually around .5 inch Mix-MODE IC/LBC-MODE Mix-CRA Wet times in blue IC/LBC-CRA 06 12 18 24 30 36 42 48 54 60 Forecast Hour

  31. Standard Deviation of Rain Volume (km3) – MODE values multiplied by 10 (mean ~ 1) No diurnal signal, hard to see different trends between 2 ensembles Mix-MODE IC/LBC-CRA Mix-CRA IC/LBC-MODE 06 12 18 24 30 36 42 48 54 60

  32. CRA results show both ensembles with growing spread, and IC/LBC having faster growth Areal Coverage Standard Deviation (number of points above .25 inch) – Mean ~ 800 pts IC/LBC-CRA Mix-CRA Mix-MODE IC/LBC-MODE 06 12 18 24 30 36 42 48 54 60

  33. No clear diurnal signal, both CRA & MODE show max in 24-48 h Mix-MODE IC/LBC-MODE Mix-CRA IC/LBC-CRA 06 12 18 24 30 36 42 48 54 60

  34. Mix-CRA IC/LBC-MODE Mix-MODE IC/LBC-CRA No diurnal signal, no obvious differences in behavior of Mix and IC/LBC 06 12 18 24 30 36 42 48 54 60

  35. Other questions: • Is the mean of the ensemble’s distribution of object-based parameters a good forecast (better than ensemble mean put into CRA/MODE)? • Does an increase in spread imply less predictability? • How should a forecaster handle a case where only a subset of members show an object? These questions have been examined using CRA results

  36. Mix Ensemble – in general, slight positive bias in rain rate, with Probability Matching forecast slightly less intense than mean of rates from members (PM usually better but not by much). Only during 06-18 period does observed rate not fall within forecasted range. wet mean PM dry 06 12 18 24 30 36 42 48 54 60 wet IC/LBC – usually too dry with rain rate (at all hours except 06-18), Probability Matching forecast exhibits much more variable behavior, again its performance is comparable to mean of rates of members mean PM dry 06 12 18 24 30 36 42 48 54 60

  37. Notice that at all times, the observed rain rate falls within the range of values from the full 16 member ensemble – indicating potential value for forecasting

  38. Mix Ensemble – clear diurnal signal, usually too much rain volume except at times of observed peak, when it is too small. Probability Matching equal in skill to mean of member volumes wet mean PM dry 06 12 18 24 30 36 42 48 54 60 wet mean IC/LBC Ensemble – also clear diurnal signal, less volume than Mix ensemble, Probability Matching usually a little wetter but generally comparable to mean of members PM dry 06 12 18 24 30 36 42 48 54 60

  39. NOTE: Even with all 16 members, there are still times when observed volume does NOT fall within range of predictions --- not enough spread (indicated with red bar) Mix Mix-PM Mix IC/LBC IC/LBC-PM IC/LBC Mix IC/LBC 06 12 18 24 30 36 42 48 54 60

  40. Percentage of times the observed value fell within the min/max of the ensemble Rate - Mix Volume-Mix Volume-IC/LBC Rate-IC/LBC IC/LBC Areal Coverage Mix

  41. Skill (MAE) as a function of spread (> 1.5*SD cases vs < .5*SD cases) CRA applied to Mix Ensemble (IC/LBC similar) Area/1000 big SD Vol big SD Rate*10 low SD Rate*10 big SD Vol low SD Area/1000 low SD 06 12 18 24 30 36 42 48 54 60

  42. It thus appears that total system rain volume and total system areal coverage of rainfall show a clear signal for better skill when spread is smaller • Rain rate does not show such a clear signal – (especially when 4 bins of SDs are examined). • Perhaps average rain rate for systems is not as big a problem in the forecasts as areal coverage (and thus volume)? Seems to be ~5-10% error for rate, 10-20% for volume, 10-20% for area

  43. Summary • Ensemble spread behavior for object-oriented parameters may not behave like traditional ensemble measures • Some similarities but some differences also in output from CRA vs MODE • Some suggestion that ensembles may give useful information on probability of systems having a particular size, intensity, volume

  44. Acknowledgments • Thanks to Eric (and others?) for organizing the workshop • Thanks to John Halley-Gotway and Randy Bullock for help with MODE runs for ensemble work, and Adam Clark for precip forecast output • Partial support for the work was provided by NSF Grant ATM-0537043

  45. Gulf near-boundary system with increased rainfall

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