Abstract
A horizontal filtering technique is used to decompose the flow into a smoothed (larger-scale) component and a sub-grid scale component. The equation of motion for smoothed flow contains a force which may be interpreted in terms of a forcing effect derived from the motion systems which disappear in the smoothing process. The expression for this forcing (by smoothing out details smaller than about 400 km in the horizontal scale) is evaluated in a synoptic situation with a strong diffluent jet stream over northern Europe. It is found that the force in this kind of flow, which is relatively common over Europe, accelerates the smoothed flow. It is further shown that this force cannot be parametrized with a horizontal frictional force of the Fickian type, or with the more complicated formulations suggested by Smagorinsky (1963) and Leith (1969); in this particular case these three parametrizations all produce decelerating effects that are largely similar.