Abstract
Forecast errors exhibit the characteristics of approximations in simulating dynamical and physical processes in models. The models are very complex and hence it is not always possible to identify the approximations responsible for any particular error pattern in forecasts. A comparison between the models' forecast performances can be valuable in isolating the causes of error patterns. Here a comparison of forecast errors in the AFGL and COLA models is made with the intent of identifying the causes of forecast errors. The two models are based on identical approximations in simulating the dynamical processes and only minor differences in parameterizations of the physical processes. Nine ten-day forecasts are made to study the error characteristics in the two models. The errors in the 500 mb geopotential height are negative in tropics and positive in extratropics. The temperatures at 850 mb are colder than observed in tropics and warmer than observed in extratropics. At 150 mb the temperatures are warmer than observed in tropics and colder than observed in extratropics. These qualitative error characteristics are not only common to these two models, but are also common in the NMC, GFDL, and ECMWF forecast models. The difference in the error structure between the two models is the magnitude of the error in tropics. The tropical error in the AFGL model is larger than than in the COLA model. (GRA)