The Maintenance Mechanisms of Transient Large-Scale Variations in the Atmosphere. Part 1: Potential Temperature and Moisture

Abstract

The budget equations for the transient variance of a scalar s (s′2¯) and its squared time mean (s2¯) are derived for a hydrostatic fluid. They are discussed and illustrated with two alternative box diagrams. The 10 yr rawinsonde global data of Oort are used in presenting and discussing the observed atmospheric variance budgets of potential temperature and moisture in the DJF season, excluding year-to-year variations. The results indicate that the daily variances (θ′2¯ and q′2¯) are maintained largely by the downgradient heat and moisture horizontal transient eddy (TE) fluxes which are strong in the midlatitude baroclinic zones (storm tracks) in both hemispheres. The flux convergence (advection of existing variance by the time-mean wind) is small and so the production must be compensated by the damping net effect of upgradient vertical potential temperature fluxes, third order flux divergences and time correlations between eddies and forcing. In contrast, the budgets of the squared means (θ′2¯ and q2¯) are governed by the time-mean sources with compensating large advections by the mean wind and a smaller smoothing effeect from the transient eddies.