Use of long‐lived tracer observations to examine transport characteristics in the lower stratosphere

Abstract

The Halogen Occultation Experiment (HALOE) CH4 data were used in a modified Lagrangian‐mean (MLM) analysis. The implemented MLM analysis is advantageous in that it provides a unique method for capturing tracer distributions and identifying so‐called transport barriers (i.e., regions where quasi‐horizontal transport is inhibited) and mixing regions, knowing only the mixing ratios of long‐lived tracers. This analysis can be used with any long‐lived tracer provided that adequate spatial coverage can be achieved either instantaneously or over a reasonably short period of time (i.e., characteristic time over which diabatic processes responsible for cross‐isentropic transport are small or negligible, ∼7–10 days in the lower stratosphere). Previous authors have demonstrated the use of this technique with data from limb‐sounding instruments which can provide near‐hemispheric coverage in a day. We show how this technique can be implemented with HALOE occultation data. Because the solar occultation sampling pattern of HALOE requires ∼1 month to achieve near‐hemispheric coverage, synoptic distributions of CH4 are reconstructed using the UK Meteorological Office potential vorticity (PV) distributions, exploiting the tracer's correlation with PV. In previous studies, authors have used PV for filling voids in tracer distributions. It is concluded that an MLM cross section of CH4 can be constructed from the HALOE occultation data, provided that a suitable synoptic distribution of PV can be obtained for the desired period. To illustrate the feasibility and usefulness of applying the MLM technique to HALOE reconstructed CH4 data, transport characteristics in the lower stratosphere were investigated using CH4 data for two different regimes (polar and subtropical) at times when airborne in situ data were available. In this manner we were able to compare information available with respect to transport from the MLM equivalent length analyses with that provided by aircraft tracer‐tracer correlations. The very good agreement between ER‐2 and HALOE data observed in CH4‐O3 correlations and in CH4 and O3 near‐coincident comparisons increased our confidence in using the HALOE data in the MLM equivalent length analyses. Unlike the satellite data, the ER‐2 in situ measurements cannot be used in the MLM technique. Instead, correlations between the mixing ratios of various tracer constituents have been used extensively in the past to provide insight into the relative roles of chemistry and transport in the lower stratosphere. However, separating the effects of chemistry and transport and accurately interpreting tracer‐tracer correlations is often difficult. We show that associating an aircraft tracer‐tracer correlation with a coincident HALOE MLM equivalent length analysis may provide for a less ambiguous evaluation of the correlation in the sense that it provides information on transport processes in physical space. Also, there are obvious implications for use of MLM equivalent length analyses in regions/time frames when aircraft data are not available.