Abstract
Abstract. From a series of zonal mean global stratospheric tracer measurements sampled in altitude vs. latitude, circulation and mixing patterns are inferred by the inverse solution of the continuity equation. As a first step, the continuity equation is written as a tendency equation, which is numerically integrated over time to predict a later atmospheric state, i.e., mixing ratio and air density. The integration is formally performed by the multiplication of the initially measured atmospheric state vector by a linear prediction operator. Further, the derivative of the predicted atmospheric state with respect to the wind vector components and mixing coefficients is used to find the most likely wind vector components and mixing coefficients which minimize the residual between the predicted atmospheric state and the later measurement of the atmospheric state. Unless multiple tracers are used, this inversion problem is under-determined, and dispersive behavior of the prediction further destabilizes the inversion. Both these problems are addressed by regularization. For this purpose, a first-order smoothness constraint has been chosen. The usefulness of this method is demonstrated by application to various tracer measurements recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). This method aims at a diagnosis of the Brewer–Dobson circulation without involving the concept of the mean age of stratospheric air, and related problems like the stratospheric tape recorder, or intrusions of mesospheric air into the stratosphere.
Highlights
In the context of climate change, possible changes in the intensity of the Brewer–Dobson circulation have become an important research topic
Engel et al (2009), found a weakly significant slow increase in the mean age of stratospheric air. The latter is defined as the mean time lag between the date of the transition of tropospheric air into the stratosphere and the date when the mixing ratio of a monotonically growing tracer was measured in the air volume under investigation, and its increase suggests a deceleration of the Brewer–Dobson circulation
Due to mixing processes, the age of a stratospheric air volume is not unique but characterized by an age spectrum, which has to be considered since the tropospheric growth of SF6 mixing ratios is not strictly linear; some ad hoc assumptions on this spectrum have to be made (Waugh and Hall, 2002)
Summary
In the context of climate change, possible changes in the intensity of the Brewer–Dobson circulation have become an important research topic. Engel et al (2009), found a weakly significant slow increase in the mean age of stratospheric air The latter is defined as the mean time lag between the date of the transition of tropospheric air into the stratosphere and the date when the mixing ratio of a monotonically growing tracer was measured in the air volume under investigation, and its increase suggests a deceleration of the Brewer–Dobson circulation. Due to mixing processes, the age of a stratospheric air volume is not unique but characterized by an age spectrum, which has to be considered since the tropospheric growth of SF6 mixing ratios is not strictly linear; some ad hoc assumptions on this spectrum have to be made (Waugh and Hall, 2002).
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