Hemispheric Evolution and Origin of Extra-tropical Cyclone Errors Within NCEP Operational Models Brian A. Colle 1 and Michael Charles 1,2 1 School of Marine and Atmospheric Sciences Stony Brook University – SUNY 2 National Centers for Environmental Prediction

NCEP Automated Cyclone Tracking and Cyclones

Motivation: The Widening Gap Between Tropical and Extra- tropical Cyclone Displacement Errors in the GFS over the Western Atlantic (72 h) GFS Tropical Errors Courtesy of T. Marchok 300 km Error in LFM: Hour 24 in (Silberberg and Bosart 1982) 300 km Error in NGM: Hour 39 in Smith and Mullen (1993)

Motivation A major goal of THORPEX is to better understand and improve predictability, yet we have little knowledge of the cyclone predictability around North America and adjacent oceans in operational models. Question #1: Are cyclone errors in the medium range (GFS model) associated with coherent large- scale flow patterns and cyclone tracks? Question #2: Do relatively large errors over the Pacific can impact errors over the Eastern U.S. 3-5 days later? Question #3: Do re- curving western Pacific TC/ET events diminish downstream predictability in the medium range (3-5 days)?

SLP Errors for “Deep” Cyclones (Global Forecast System model) GFS Central Pressure SLP Abs Error SLP Mean Error Threshold for Deep Cyclones Region SLP E Pacific W. Atl. Cent US NE US E. Pac

Composite of normalized SLP errors over central Pacific starting at hour 30 (>1.5 sdev; 28 cases) Analyzed mean SLP (solid) and 500Z (dashed) also shown) Hour 30 Hour 66 Hour 120

Composite of normalized SLP errors over eastern U.S. starting at hour 102 (>1.5 sdev; 25 cases) Hour 102 Hour 48 Hour 30 Hour 84 Hour 66

Tracks Associated With Cyclone SLP Errors Over Western Atlantic (60-h) Positive Cyclone SLP Error (GFS too weak) Negative Cyclone SLP Error (GFS too strong) Errors are greater than 1.5 standard deviations X marks cyclone position at F60 Negative slp error growth Positive slp error growth

Composite (daily NCEP reanalysis) of the GFS Initialization (500 mb heights) for Cases of 28 Positive and 35 Negative Cyclone SLP Errors (> 0.5 stn deviation) over the Western Atlantic at Hour 96 Positive Cyclone SLP ErrorNegative Cyclone SLP Error The initialization for the positive 96-h errors has a broader ridge extending upstream of the W. Coast, more negative heights to the north, and larger disturbance over the SW U.S.

Positive Cyclone SLP ErrorNegative Cyclone SLP Error Composite of the GFS Initialization (500 mb Z Anomaly from Climo) for Cases of Positive and Negative Cyclone SLP Errors over the Western Atlantic at Hour 96

F96 Cyclone SLP Errors on the East Coast: 48-h After Initialization Time (500 mb Z Anomaly from Climo) Positive Cyclone SLP ErrorNegative Cyclone SLP Error

F96 Cyclone SLP Errors on the East Coast: 72-h After Initialization Time (500 mb Z Anomaly from Climo) Positive Cyclone SLP ErrorNegative Cyclone SLP Error

F96 Cyclone SLP Errors on the East Coast: 96-h After Initialization Time (500 mb Z Anomaly from Climo) Positive Cyclone SLP ErrorNegative Cyclone SLP Error

F96 Cyclone SLP Errors on the East Coast: 96-h After Initialization Time (500 mb heights) Positive Cyclone SLP ErrorNegative Cyclone SLP Error Positive Errors at hour 96 have a more amplified large-scale flow pattern

48-h Cyclone Central Pressure Errors (mb) for Central Pacific CPCP CACA Composite periods with at least two straight days of dominant negative or positive cyclone pressure errors at hour 48 (for at least > 80% of cyclones).

Composite (daily NCEP reanalysis) of SLP Associated with Positive and Negative Cyclone Errors in GFS at Hour 48 over the Central Pacific Positive Cyclone Errors (29 cases)Negative Cyclone Errors (36 cases) Overdeepening occurs for anomalously strong cyclones over the northern Pacific

Positive Cyclone ErrorNegative Cyclone Error Negative Cases have an east-northeast extension of the W. Pacific jet Composite 300 Winds Associated with Positive and Negative Cyclone Errors in GFS at Hour 48 over the Central Pacific

Positive Cyclone ErrorNegative Cyclone Error Positive: More tracks to north-central Pacific Negative: More tracks to the east-central Pacific Negative error growth Positive error growth Cyclone TracksAssociated with Positive and Negative Cyclone Errors in GFS at Hour 48 over the Central Pacific

Positive Cyclone Error Negative Cyclone Error

500 mb Z Anomaly (from climo) for 17 Recurving TC/ET events between Oct-Dec versus 17 random days W. Pacific TC/ET Events t = 0h Random Events Time = 0h is defined as the last time report time by the JTWC L

Hour 72 SLP Errors over Central Pacific Associated With Periods of Recurving Extra-tropical Transition Events over the Western Pacific Green = 17 W Pacific recurving TC/ET events from Red = GFS cyclones too weak Blue = GFS cyclones too strong

GFS SLP displacement and central pressure MAEs versus forecast hour for the TC/ET (solid) and random events (dashed) Cyclone Displacement ErrorsCentral Pressure MAEs E. Pacific W. Atlantic C. Pacific TC/ET Random

Summary GFS East Coast cyclone SLP errors exceed the E. Pacific by day 4. Central U.S. cyclones clearly have the smallest pressure errors in the medium range. The GFS cyclone pressure biases vary by region (underdeepening in the east and overdeepening in the west). The largest cyclone errors over the central Pacific at 30 h are associated with SLP errors over eastern NA by 120 h, and the largest East coast cyclone errors by 102 h are associated with growing Pacific errors in h—rapid downstream development?? Western Atlantic cyclones tend to be underdeveloped over the Gulf Stream, perhaps suggesting physics (e.g., PBL, CP?) errors. This is favored when there is anomalous ridging over the western U.S. and deep trough over the E. Coast. Central Pacific cyclones in the GFS are too deep when there are anomalously strong cyclones over the north Pacific and an eastward extension of the Pacific jet. Why?? W. Pacific re-curving ET/TC events are associated with a wave train across the Pacific, but the downstream cyclone errors not any larger than other cool season events on average, except for perhaps the W. Atlantic.