School of Meteorology (Defense)

Investigating dynamics of error growth in ECMWF forecast busts

Sam Lillo
OU School of Meteorology

02 May 2014, 10:00 AM

National Weather Center, Room 5820
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Researchers at the European Centre for Medium Range Weather Forecasts (ECMWF) have noted a number of events when their global forecast model has experienced large errors around day-6 over Europe (Rodwell et al. 2013). These events are known as dropouts, or busts. Using the ERA-interim forecast as a fixed model, Rodwell et al. identified 584 cases in a 22-year period that fit their criteria for a bust. The mean initial conditions for the bust cases included a trough over the Rockies and favorable conditions for organized convection to the east of the trough. Rodwell proposed that mesoscale convective systems slowed the eastward movement of the trough and thus errors in the treatment of convection in the model resulted in errors in the large-scale features. This upscale growth of errors appeared to be amplified by bifurcations in the atmospheric evolution leading to a blocking regime over Europe in the verification.

In this study, these regimes were examined through an EOF analysis of the northern hemisphere 500hPa heights during the 6-day forecast period. The cases were clustered according to their behavior in the first EOF, revealing distinctly different pattern changes. Further analysis of each cluster found the leading two EOFs were characterized by two distinct Rossby wave trains extending from North America to Europe. This pattern is consistent with a low wavenumber Rossby train reflecting over Greenland and a higher wavenumber Rossby train trapped in the Atlantic waveguide. Analysis of the waveguide revealed increased definition, and extension toward Europe during bust cases. In addition to the EOF analysis, a maximum in bust frequency during September-October was investigated in the context of tropical cyclone influences over the Atlantic. Recurving tropical storms through the central Atlantic were common during the cases. Overall, these results suggest that the busts are occurring during large-scale pattern transitions and high wave activity across the Atlantic, possibly influenced by organized convection.

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