5A.3 Medium-range extratropical cyclone errors and their origin within the NCEP GFS model

Tuesday, 2 June 2009: 2:00 PM
Grand Ballroom East (DoubleTree Hotel & EMC - Downtown, Omaha)
Brian A. Colle, Stony Brook University - SUNY, Stony Brook, NY ; and M. Charles

A major goal of THORPEX has been to better understand and improve the predictability of significant weather (e.g., extratropical cyclones) downstream from the Pacific to North America. The objective of this presentation is to verify a full spectrum of forecast cyclones over several years in the medium range (3-5 day forecast) NCEP GFS, and relate the cyclone predictability across North America and adjacent oceans to different large-scale flow regimes. Using an automated cyclone-tracking algorithm, the cyclone central pressure and track errors to day 5 have been quantified for all cyclones in the NCEP GFS for the 2002-2007 cool seasons (October to March). For 8 regions from the central Pacific to the western Atlantic, a time series of the errors was constructed for the 5-cool season period. Composites using the GFS analyses were generated for periods with large (> 1.5 standard dev) cyclone sea-level pressure (SLP) errors at hours 96-120 over the western Atlantic in order to note the origin and flow patterns associated with these large error events.

By days 4-5, the cyclone central pressures errors in the GFS over the Northeast U.S. and western Atlantic become similar in magnitude to the eastern Pacific. The positive errors tend to occur during a high-amplitude wave pattern, with ridging over the western U.S. and positive PV anomalies moving southward from the relatively data spare south-central Canada. A composite of cyclone events with large errors over the eastern Pacific early in the forecast (24-36h) shows the propagation of relatively large SLP errors eastward across Canada to the western Atlantic by days 3-5. The origin of these rapid East Coast errors is being investigated using a cyclone relative composite, and it is hypothesized that uncertainties in convective heating in the models play an important role. Cyclone overdeepening in the central Pacific is favored during an anomalously strong jet and Aleutian low, while more mobile PV anomalies away from the main jet tend to be more underdeepened in the medium range.

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