an investigation of cool season extratropical cyclone forecast errors within operational models...
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An Investigation of Cool Season
Extratropical Cyclone Forecast Errors
Within Operational Models
Brian A. Colle1 and Michael Charles1,2
1 School of Marine and Atmospheric Sciences Stony Brook University – SUNY
2 National Centers for Environmental Prediction
Motivation
• Complete a long-term (5-year) cyclone verification of the operational NCEP GFS and NAM models (several years since the last objective evaluation).
• What synoptic flow patterns are associated with particular cyclone errors in operational models?
• What is the impact of using the NCEP Short-Range Ensemble Forecast (SREF) system for cyclone prediction?
Automated Cyclone Verification using NCEP Tracking algorithm (Marchok 2002)
fcst - 960 mb
obs - 955 mbfcst - 982 mb
obs - 982 mb fcst - 982 mb
obs - 984 mb
http://www.emc.ncep.noaa.gov/gmb/tpm/emchurr/tcgen/
Data• Cyclone Events (Oct-Mar 2002-2007)
- GFS (0-120 hr every 6 h) at 80 km grid spacing- Eta/NAM (0-60 h every 6-h) at 40-80 km grid
spacing- Same cyclones and times were used to compare
models. Interpolate to common 80-km grid
• SREF (Oct-Mar 2004-2007)- 15 members (5 Eta-KF, 5 Eta-BM, 5 RSM)
- ~40 km grid spacing (212 grid)
- Available at 09 & 21 UTC ( 63 h forecast)
- Included 6 SREF WRF members for 2006-2007
NAM-GFS analysis cyclone central
pressures for 2002-2007 cool seasons
Central and Eastern N Amer
Western N Amer
PacificW. Atlantic
W. Atlantic
Central and Eastern N Amer
Western N AmerPacific
Mean Analysis SLP Error as compared to surface observations for cyclones within 500 km of station
Mean Abs ErrorMean Error
Cyclone SLP Abs Error versus Fhour By Region
NAM GFSE. Pacific
E. Pacific
Cent N.A.
W. Atlantic
W. N.A.
CP 12
3
4
5
6CA
Western Atlantic Cyclone Position Errors (42-60 h)
NAM GFS
All Cyclones
Deep (1.5 stnd
dev) Cyclones
GFS SLP MAE 12 UTC versus 18 UTC Runs
GFS cyclone displacement error (km) for 12 UTC versus 18 UTC Runs
E. Pacific
W. Atlantic
Cent N.A.
18 UTC
12 UTC
2002-2006 GFS Atlantic hurricane track error
X
X
X
X
X
X
X
X1995-1998 GFS Atlantic hurricane track error
*
*
GFS extra-tropical 1989-90 (Mullen and Smith 1993)
Cyclone SLP Abs Error in mb (48 h)
NAM GFSE. Pacific
E. Pacific
E. US. and W Atl
E. US. and W Atl
Central U.S.
Central U.S.
GFS Median Range Mean Absolute SLP Error
(970 mb)(990 mb)(992 mb)(975 mb)(973 mb)
ALL CYCLONESDEEP CYCLONES
(> 1.5 stnd dev)
E. Pacific
W. Atlantic
E. N.A.
Cent N.A.
W. N.A.
E. Pacific
W. Atlantic
E. N.A.
W. N.A.
Cent N.A.
GFS Median Range Mean SLP Error (Deep storms)
(970 mb)(990 mb)(992 mb)(975 mb)(973 mb)
E. PacificW. Atlantic
E. N.A.
W. N.A.
Cent N.A.
GFS Large Error Cyclone Events for 48-h (Regions 5,6)
GFS negative SLP error (1.5 std dev > mean error, or < -5.1 mb)
GFS positive SLP error (1.5 std dev > mean error, or > 4.5 mb)
Model pressure tendency error (mb/6h)
SREF and GFS/NAM
Displacement Error
(W. Atlantic)
SREF and GFS/NAM Central Pressures Mean Absolute Error (W. Atl)
GFS
SREF
NAM
GFS
SREF
NAM
Rank Histogram of cyclone central
pressure and best member likelihood
percentage (W. Atlantic)
RSMEta-KFEta-BM
Hours 33-45
Conclusions• GFS analysis for cyclones is significantly better than the
NAM or NARR, especially over the oceanic regions. The NAM and NARR cyclones are too weak on average.
• GFS cyclone forecasts have more skill than the NAM in all regions. NAM cyclones are too weak over the E. Pacific on average.
• By hour 84, W. Atlantic errors become comparable to the E. Pacific and the errors become greater than E. Pacific for deeper cyclones.
• The 09/21z SREF has larger cyclone MAEs than the deterministic GFS and slightly less than NAM. The SREF pressures tends to be overdispersed in many locations.
• Some model cyclone biases for the eastern U.S. may favor specific storm tracks.
• Large cyclone errors over the E. Pacific can impact the western Atlantic 2-3 days later.
Cyclone Identification (NCEP Approach)
SLP field from grib file
Locate grid point with lowest SLP
Try to find 2mb closed isobar
Mask out cyclone and repeat
21
3 4
Same used at NCEP - http://www.emc.ncep.noaa.gov/gmb/tpm/emchurr/tcgen/