A Numerical Method for Predicting the Perturbations of the Middle Latitude Westerlies

It is shown that the large-scale quasi-stationary disturbances of the middle-latitude westerlies are produced by the forced ascent of the westerly current over the continental land masses. Friction is found to have an important modifying effect on the motion. Using these results a numerical method is devised for predicting the height profile of the 500 mb pressure surface at a fixed latitude. The method involves the use of the notion of an ≥equivalentbarotropic atmosphere≤ and of the geostrophic approximation. Six actual forecasts are made for a period of one day and the results compared with observation. The accuracy obtained is thought to justify incorporation of the method into day-to-day forecast procedures.