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Bias Adjusted Precipitation Scores Fedor Mesinger NOAA/Environmental Modeling Center and Earth System Science Interdisciplinary Center (ESSIC), Univ. Maryland, College Park, MD VX-Intercompare Meeting Boulder, 20 February 2007. Most popular “traditional statistics”: ETS, Bias
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Bias Adjusted Precipitation ScoresFedor MesingerNOAA/Environmental Modeling CenterandEarth System Science Interdisciplinary Center(ESSIC), Univ. Maryland, College Park, MDVX-Intercompare MeetingBoulder, 20 February 2007
Most popular “traditional statistics”: ETS, Bias Problem: what does the ETS tell us ?
“The higher the value, the better the model skill is for the particular threshold” (a recent MWR paper)
Example: Three models, ETS, Bias, 12 months, “Western Nest” Is thegreenmodel loosing to red because of a bias penalty?
J12.6 17th Prob. Stat. Atmos. Sci.; 20th WAF/16th NWP (Seattle AMS, Jan. ‘04) BIAS NORMALIZED PRECIPITATION SCORES Fedor Mesinger1 and Keith Brill2 1NCEP/EMC and UCAR, Camp Springs, MD 2NCEP/HPC, Camp Springs, MD
Two methods of the adjustment for bias(“Normalized” not the best idea) • dHdF method: Assume incremental • change in hits per incremental change in • bias is proportional to the “unhit” area, O-H • Objective: obtain ETS adjusted to unit bias, • to show the model’s accuracy in placing precipitation • (The idea of the adjustment to unit bias to arrive at placement accuracy: • Shuman 1980, NOAA/NWS Office Note) 2. Odds Ratio method: different objective
Forecast, Hits, and Observed (F, H, O) area, or number of model grid boxes:
dHdF method, assumption: can be solved; a function H(F) obtained that satisfies the three requirements:
Number of hits H -> 0 for F -> 0; • The function H(F) satisfies the known value of H for the model’s F, the pair denoted by Fb, Hb, and, • H(F) -> O as F increases
Bias adjusted eq. threats West Eta GFS NMM
A downside: if Hb is close to Fb, or to O, it can happen that dH/dF > 1 for F -> 0 Physically unrealistic ! Reasonableness requirement:
“dHdM” method: Assume as F is increased by dF, ratio of the infinitesimal increase in H, dH, and that in false alarms dM=dF-dH, is proportional to the yet unhit area:
One obtains ( Lambertw, or ProductLog in Mathematica, is the inverse function of )
H(F) now satisfies the additional requirement: dH/dF never > 1
dHdF method H=O H=F H(F) Fb,Hb
dHdM method H=O H=F H(F) Fb,Hb
Results for the two “focus cases”, dHdM method (Acknowledgements: John Halley Gotway, data;Dušan Jović, code and plots)
5/13 Case dHdM wrf2caps wrf4ncar wrf4ncep
6/01 Case dHdM wrf2caps wrf4ncar wrf4ncep
Impact, in relative terms, for the two cases is small, because the biases of the three models are so similar !
5/25 Case dHdM wrf2caps wrf4ncar wrf4ncep
Comment: Scores would have generally been higher had the verification been done on grid squares greater than ~4 km This would have amounted to a poor-person’s version of “fuzzy” methods !