Systematic errors in the baroclinic waves of the ECMWF model

The space-time spectral analysis technique is used to investigate the forecast errors of the baroclinic waves in the ECMWF forecast model. The study concentrates on two groups of disturbances, the medium-frequency baroclinic waves (MFBW: wavenumber 4 to 7, period 3.6 to 10.3 days) and the high-frequency baroclinic waves (HFBW: wavenumber 5 to 9, period 2.4 to 3.5 days).

Both mid-latitude wave types suffer a considerable weakening in the early stages of the forecast. The geographical distribution of this error shows the largest decrease over North America and the North Atlantic. A close connection between the forecast changes in the baroclinic waves and the time-mean state is indicated by the decrease of the baroclinicity of the mean flow over North America. In the Pacific region, an increased baroclinicity seems to be responsible for increased wave activity in the high-frequency range.

An increase of the SW to NE phase tilt of the MFBW’s at low latitudes seems to be due to problems in the interaction of deep baroclinic waves with the tropical flow. In the vertical, the forward tilt of the temperature waves increases during forecast.