cpc unified precipitation climatology, regime …...15 april 2010 5th naefs workshop d collins 2 •...
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CPC Unified Precipitation Climatology,
Regime-dependant bias-correction and
Forecast Verification
Dan Collins and David Unger
Climate Prediction Center
NCEP/NOAA
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• Station-based 0.5x0.5 degree gridded precipitation analysis
– ¼ degree resolution available over the U.S.
– Land-only
– Merging with satellite data expected this year to produce global analysis
• 1979 to 2010 (present)
– Daily resolution.
• Model bias-correction occurs on 5 and 7 day
accumulated precipitation forecasts.
CPC Unified Precipitation dataset application for NAEFS forecasts
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• Need full distribution climatology as opposed to simply above and below normal category threshold values
– Allows determination of extremes.
– Non-normal distribution.
– Difficult to parameterize the distribution for all locations fitting same type of distribution everywhere
• Obstacles
– Discontinuous variable with many zeros
– Some grid points with few positive values at all
– Limited independent data to sample
• Solution:
– Non-parametric distribution
– To compensate for sampling errors, distribution is smoothed using kernel distribution function
– Width of kernel proportional to standard deviation of the observations
Development of a climatology using CPC Unified Precipitation dataset
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Climatology derived from precipitation observations as sample of true climatology distribution
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Climatology probability of precipitation used to determine locations that are dry normally
Climatologically dry areas are only forecast to be near-normal or above-normal;Never should be below-normal
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• Minimum accurately measurable and predictable precipitation chosen.
– Using 1 mm as minimum measurable precipitation, but model may have little or no skill below 2-3 mm
– Log(P) < 0 are separated for distribution
• Remaining measurable amounts used to create conditional distribution for measurable precipitation forecast and observation.
• Log precipitation used for both observational analysis and modeldata.
– Decreases skewness; often too much
– Errors are considered proportional to precipitation amount
Discontinuity at zero precipitation
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• Bias correction on log (precipitation) at multiple threshold values from 1 mm to ~150 mm (6 inches)
• Bias is dependent on precipitation amount
– Allows bias correction of probability of precipitationat the 1 mm threshold; however, model
representation of low precipitation amounts lack skill.
Regime-dependent Bias-correction
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• Each precipitation event affects nearest thresholds– Threshold values are pushed towards neighboring values.– Change is related to closeness of next threshold creating a
rippling or slinky effect when a thresholds move– Green forecast move thresholds up, Red moves them down,
Blue members have no effect
Regime-dependent Bias-correction
Obs
10 member ensemble forecasts
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Week-2 Precip
1 mm threshold bias estimate for April 2009
Wet bias over
relatively dry, high
latitude regions
1 mm used as “zero”
threshold in
forecasts
Limited ability to
bias correct due to
zero measurable
precipitation
discontinuity & poor
model
representation
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Week-2 Precip
20 & 33 mm threshold bias estimate for April
Wet bias widespread but especially along
Pacific & Atlantic Coasts and much of
Eastern N. America
Dry biases appear related to individual
extreme wet events
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• Because CPC Unified precipitation analysis is currently higher resolution ( ½ degree) than model data (1 degree)
– Model forecasts are “downscaled” by the bias-correction process.
• Appears to be one of more skillful forecast tools,
especially for week-2, when skill is low.
6-10 day and Week-2 CPC Precipitation Forecasts
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25 mm or 1 inch threshold probability of exceedence
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3-Category forecasts for above (left) and below (right) normal week-2 precipitation
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Week-2 Precipitation Heidke SS
DJF 2009-2010 & JJA 2009
0.17 HSS over Mexico during this winter
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3-Category (Above, Below and Near-Normal) Rank Probability Skill Score
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Above-Normalbias ratio
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3-Category (Above, Below and Near-Normal) Rank Probability Skill Score (Top) & Bias in above-
normal forecast frequency
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CPC Week-2 NAEFS Temperature:
Winter Verification Heidke Skill Score
-50
-40-30
-20-10
010
2030
4050
6070
8090
100
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Official temperature forecast
verification, Heidke Skill Score
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Official Week-2 precipitation forecast verification, Heidke Skill
Score
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Week-2 Temp Verification:
Tercile skill scoresHSS
0
0.05
0.1
0.15
0.2
0.25
0.3
U.S. 48 STATES CANADA MEXICO
CMCE/NCEP
NCEP
RPSS
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
U.S. 48
STATES
CANADA MEXICO ALASKA
CMCE/NCEP
NCEP
CMCE/NCEP calibrated
• No appreciable change in the Heidke skill score (HSS) with the addition of CMC ensemble to the NCEP ensemble
• Significant improvement in the probabilistic skill determined by a 3-category rank probability skill score (RPSS) and continuous RPSS.
• PDF calibration increases probabilistic skill
CRPSS
-0.1
-0.05
0
0.05
0.1
0.15
0.2
U.S. 48
STATES
CANADA MEXICO
CMCE/NCEP
NCEP
CMCE/NCEP calibrated
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Week-2 Temperature:
Reliability of Probabilities
North America
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Observed frequency
Fo
recast
freq
uen
cy
Perfect reliability
CMC & NCEP
Climatology
NCEP only
• Reliability improves with addition of CMC ensemble to NCEP alone
• Reliability closer to climatology
• Climatology forecast reliability indicates November through April colder than climatology
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The North American Ensemble
Forecast System (NAEFS)