Factors Affecting Synoptic-Scale Vertical Motions: A Statistical Study Using a Generalized Omega Equation

Abstract

Processes affecting synoptic-scale vertical motions are investigated using as the main tool a generalized omega equation that assumes only hydrostatic balance. This equation and the adiabatic quasigeostrophic (QG) omega equation were solved in a global domain for six synoptic times in February 1979 using spectral truncation T63. The results are portrayed in the form of statistical quantities calculated for four latitude belts in the Northern Hemisphere and Tropics. In the mid- and upper troposphere, the average correlation between the QG and generalized vertical motions is near 0.85 in high latitudes (60°?90°N), roughly 0.7 in midlatitudes (30°?60°N), and slightly over 0.6 in the subtropics (15°?30°N). In these zones, ageostrophic temperature and vorticity advections explain an appreciable part of the nonquasigeostrophic vertical motions. In the Tropics (15°S- 15°N), where the role of diabatic heating is dominant, the QG and generalized solutions correlate only very weakly. In the mid- and high latitudes, the ? components associated with vorticity advection and thermal advection clearly are more important than the other three components calculated with the generalized omega equation, and this also holds for the subtropics above 600 hPa. However, the effect of diabatic heating is far from negligible in midlatitudes and subtropics. The relative influence of friction is largest in high latitudes, but even there it is important only in the lower troposphere. In midlatitudes, vorticity advection and thermal advection are of roughly equal importance at resolution T63. For the large-scale vertical motions resolvable with truncation T21, however, the role of vorticity advection is clearly dominant in the lower and midtroposphere.