A Comparison of Observed and Model Energy Balance for an Extratropical Cyclone System

Abstract

Eddy kinetic energy budgets are presented for both moist and dry 48 h forecasts and corresponding observations of a developing, winter extratropical cyclone. Forecasts are from the Drexel-NCAR Limited Area Mesoscale Prediction System initialized at 1200 GMT 9 January 1975. Observations consist of North American rawinsonde data objectively analyzed to the model grid. The energetics quantities are calculated for both model and observed data on a domain including most of the contiguous United States and southern Canada for times corresponding to the model initialization and the 12, 24, 36 and 48 h forecasts. Results show that the eddy kinetic energy content is dominated by the jet stream development for both model and observed data, with maxima flanking the trough at approximately 300 mb. The area-averaged, vertically-integrated eddy kinetic energy content is approximately 25% greater for the model initialization than the observations. The excess is localized in the jet maximum west of the trough, where geostrophic first-guess values are retained during initialization in place of missing winds. The increased kinetic energy west of the trough may be a factor in the excessive upper-air wave development predicted in the 24 h forecast and in the southeastward displacement of the cyclone system in the latter half of the forecast. Despite the larger initial eddy kinetic energy content, both models predict only a doubling of the integrated value, whereas the observed value tripled. The observed increase in eddy kinetic energy content primarily results from an upper-tropospheric surplus of the source due to horizontal flux convergence over the sinks, generation and dissipation. The inability of the models to produce the observed increase in eddy kinetic energy is consistent with the underprediction of the horizontal flux convergence. The predicted flux values are affected by model boundary errors which are expected to propagate into the domain at the rate of 20 to 30° longitude per day. This implies the influence of boundary errors on the flux maximum associated with the western jet core as early as the 24 h forecast. Generation by cross-contour flow tends to be negatively correlated with the horizontal flux convergence, but also is affected by latent heat release. Latent heat release is found to enhance positive generation in both the observed and moist model results as compared to the dry model results.