Weather and Climate Systems

Investigating dynamics of error growth in ECMWF forecast busts

Sam Lillo
OU School of Meteorology

23 April 2014, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The European Centre for Medium Range Weather Forecasts (ECMWF) has noted a number of events when their global forecast model has experienced large errors (busts) around day-6 over Europe (Rodwell et al. 2013). Using the ERA-interim forecast as a fixed model, they 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, which mimics the NAO loading pattern. This revealed distinctly different pattern changes. Further analysis of each cluster found the leading two EOFs to suggest two distinct Rossby wave trains extending from North America to Europe. This may represent low wavenumbers reflecting over Greenland and higher wavenumbers trapped in the Atlantic wave guide. In addition, a maximum in bust frequency during September-October was investigated for the influence of tropical cyclones in the Atlantic. Recurving tropical storms through the central Atlantic were common during the cases. The 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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