Zonal Wind Errors in the Barotropic Model

Zonal wind errors in a series of numerical barotropic forecasts are discussed. A characteristic error is the growth of west winds in middle latitudes and their decay to the south. Momentum transport in the absence of momentum sources and sinks in the barotropic model is suggested as the principal contributor to this type of error. Etridence of nonlinear growth of the zonal wind error suggests that use of past errors as a simple linear correction of forecast zonal profiles is not a promising solution of the problem particularly when no provision is made for stabilization of the longest waves. In an attempt to throw some light on the remaining question of what can be done to improve the barotropic model with respect to zonal wind errors, a series of experimental forecasts is made using a form of the barotropic vorticity equation with a persistent frictional term that provides crudely for momentum sources and sinks. Although the results are: to a considerable degree, inconclusive, they suggest that future efforts should be directed toward the inclusion of friction in a more realistic atmospheric model.