A diagnostic study of the long‐term budget of momentum of atmospheric large scale motion over the British Isles

Abstract

AbstractDifferent terms in the equations of zonal and meridional motion for large scale flow in the atmosphere have been evaluated using British aerological data for the period 1974‐76; the effect of motion on scales smaller than a few hundred kilometres on large scale flow is evaluated as the residual term in these equations.In the long‐term mean there is horizontal advection of zonal momentum into the area of the British Isles. This advection, which is characteristic of both the time‐mean flow and transient disturbances and takes place mainly in the zonal plane, is to a large extent compensated for by Ferrel‐type ageostrophic meridional circulation. The momentum budget in the meridional direction reflects the high degree of geostrophic balance in the time‐mean zonal flow in the free atmosphere.The mean residual forces in the atmosphere below 700mb are used to infer the vertical sub‐grid scale flux of momentum by assuming that the residual forces at these levels are due to vertical eddy stresses (i.e. stresses involving the vertical velocity component) only, and by evaluating the stress at the earth's surface with the aid of the geostrophic drag law. The results indicate that the annual mean vertical flux of zonal momentum above about 900mb is downwards but relatively small; the corresponding flux of meridional momentum is relatively large and upwards. Mesoscale circulations in connection with fronts are suggested as a possible explanation of the latter phenomenon which, however, may also partly arise in calculations as the result of the poor vertical resolution of the routine upper wind observations in the atmospheric boundary layer.The residual force in the upper troposphere is very variable. However, consistent with an earlier analysis of the kinetic energy budget using the same data, it exhibits the ‘negative viscosity’ phenomenon, implying acceleration of the large scale flow by motion on scales smaller than the average distance between aerological stations.