Time Threshold Diagnostics

Abstract

A new method, named time threshold diagnostics (TTD), is developed for use in discussing long-term global tracer transport as the result of atmospheric motions. The method works for analysis of a large number of parcels' trajectories in numerical tracer experiments. This method considers the motion of every air parcel passing through a given surface and focuses on periods between its passages through the surface. It selects passages that have periods longer than the specified time threshold. Aggregation of all the selected passages makes effective flux of the parcels, which should contribute to the transport through the surface for time scales over the time threshold. The TTD is capable of probing approximately boundaries between mixing regions in the atmosphere and assessing the substantial mass transport through the boundaries.In order to check its advantage, the TTD was applied to the investigation of trajectories of a large number of parcels in the troposphere and lower stratosphere in northern hemisphere winter, simulated in a general circulation model. Meridional effective mass flux through each equal-latitude surface was obtained for a variety of specified time thresholds. Each mid-latitude shows a local minimum in a meridional distribution of the meridional effective flux integrated for the upper troposphere for time thresholds over two days; this suggests that meridional transport for time scales over two days is suppressed in mid-latitudes in the upper troposphere. Stream functions of zonally averaged meridional circulation obtained from net effective meridional fluxes for these time scales show a one-cell circulation in each hemisphere.