Experiments in Numerical Forecasting, Using Space-Smoothed Fields

A number of experiments in numerical forecasting using a technique based on the theory developed by Fjörtoft have been performed.

Section A contains the results of making barotropic 500 mb forecasts for 24 and 48 hours where the vorticity is advected in a space-smoothed field held constant in 12 or 24 hours. The results indicate that the forecasts in the mean are slightly less accurate as measured by the correlation coefficient between computed and observed changes. It is pointed out that the root mean square of the computed changes is smaller than for ordinary barotropic forecasts, especially for a large area.

Section B contains the results of an experiment to forecast the thickness field between 1,000 and 500 mb. The forecasts are based on the theory developed by Fjörtoft (1955). The flow at 500 mb has, however, in these forecasts been forecast with the barotropic model. The errors in these forecasts are of the same order of magnitude as in the barotropic forecasts. In the last part it is tried to relate the errors to physical defects of the model, especially the assumption of adiabatic motion.