Synoptic-scale flow of the free atmosphere in mid-latitudes is generally characterised by the existence of an approximate diagnostic relationship between the mass and flow field and the non-existence of appreciable unbalanced buoyancy wave activity. The prevalence of this quasi-balanced state provides the rationale for two major developments in dynamical meteorology. It has motivated the derivation of simplified mathematical flow systems whose dynamics is balanced, and it has prompted the design of initialisation procedures for primitive equation NWP models that seek to curb the presence or influence of buoyancy waves.
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