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 Central Pressure Mean Absolute Errors in mb (18-36 h)

GFS

NAM

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

Cyclone Central Pressure Mean Error in mb (48-60 hrs)

GFS

NAM

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.

Cyclone Displacement Error in km (48-h)

NAM GFS

E. Pacific

E. Pacific

W. Atlantic

W. Atlantic

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.

96-h GFS forecast (12z 16 Jan 2004)

Hour 72

Hour 30

Hour 96

Random Error Days Large Error E. Pac (>1.5 std dev)

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

51-h SREF (valid 2006011500)

XXX

EKF

EBM

RSM

NAMGFS

OBS-10 to 0 mb 0 to 10 mb 10 to 20 mb

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.

EXTRA SLIDES

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/

Cool Season Cyclone Numbers per 2.5o grid