LOOKING BEYOND THE FIRST TWO MOMENTS IN ENSEMBLEBASED PROBABILISTIC FORECASTS

1
LOOKING BEYOND THE FIRST TWO MOMENTS IN
ENSEMBLE-BASED PROBABILISTIC FORECASTS

• Zoltan Toth
• Environmental Modeling Center
• NOAA/NWS/NCEP,
• Yuejian Zhu, Dingchen Hou(1), Ervin Zsoter(2)
• (1) SAIC at NCEP/EMC, Washington, US
  (www.emc.ncep.noaa.gov)
• (2) Hungarian Meteorological Service
• http//wwwt.emc.ncep.noaa.gov/gmb/ens/index.html

2
OUTLINE / SUMMARY

• FORECAST ATTRIBUTES
• Reliability - Can be statistically corrected
• Resolution - Can be improved, but not
  corrected statistically
• VARIATIONS IN FORECAST UNCERTAINTY
• Probability associated with mode of fcst
  distribution varies
• Ensemble can capture variations in fcst
  uncertainty
• BI- AND MULTIMODALITY
• Composite distributions according to their
  max/min
• Details in ensemble pdf seem to correspond with
  distribution of verifying observs.

3
ATTRIBUTES OF FORECASTS
RELIABILITY Lack of systematic error (No
conditional bias) CAN BE statistically
corrected (assuming stationary processes)
RESOLUTION Different fcsts precede
different observations CANNOT be
statistically corrected - INTRINSIC VALUE OF FCST
SYSTEM
For perfectly reliable fcsts, resolution
ensemble spread spread in observations
gt Perfect predictability only 0 100
probabilities used, and always correct No
predictability No matter what we forecast,
climate distribution is observed
4
(No Transcript)
5
PROBABILISTIC FORECASTING Based on SINGLE
FORECAST One integration with an NWP model,
combined with past verification statistics

• Does not contain all forecast information
• Not best estimate for future evolution of system
• UNCERTAINTY CAPTURED IN TIME AVERATE SENSE -
• NO ESTIMATE OF CASE DEPENDENT VARIATIONS IN FCST
  UNCERTAINTY

6

• INITIAL CONDITION RELATED ERRORS
• Sample initial errors
• Run ensemble of forecasts
• Can flow dependent variations in forecast
  uncertainty be captured?
• May be difficult or impossible to reproduce with
  statistical methods

7
Full ensemble pdf
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
144 hr forecast
Poorly predictable large scale wave Eastern
Pacific Western US
Highly predictable small scale wave Eastern US
Verification
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)