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How homogeneous and isotropic is stratospheric mixing? Comparison of CRISTA‐1 observations with transport studies based on the Chemical Lagrangian Model of the Stratosphere (CLaMS)
Author(s) -
Konopka P.,
Spang R.,
Günther G.,
Müller R.,
Mckenna D. S.,
Offermann D.,
Riese M.
Publication year - 2005
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.04.47
Subject(s) - isotropy , mixing (physics) , ellipse , physics , stratosphere , lagrangian analysis , crista , turbulence , length scale , mechanics , atmospheric sciences , lagrangian , optics , chemistry , mitochondrion , mathematical physics , astronomy , biochemistry , quantum mechanics
The Chemical Lagrangian Model of the Stratosphere (CLaMS) is used for the interpretation of N 2 O observed during the CRISTA‐1 experiment in early November 1994. By comparing CRISTA data with CLaMS simulations, the impact of the large‐scale horizontal deformations on mixing is studied. Using the probability density function technique (PDF) quantifying the statistics of N 2 O variability, the critical deformation γc was inferred that triggers the mixing algorithm in CLaMS. The critical deformation γc measures the ratio between the major and minor axes of the ellipse resulting from the stretching of a circle surrounding a given Lagrangian air parcel, i.e. only deformations stronger than γc are relevant for mixing in CLaMS. The PDF derived from CRISTA observations at 700 K and on horizontal scales of the order of 200 km is characterized by a Gaussian core and non‐Gaussian tails indicating filamentary structures typical for 2D turbulence. The PDFs obtained from CLaMS simulations strongly depend on γc but only weakly on the horizontal resolution r 0 that was varied between 45 and 200 km. The choice γc=0.8 in the model best reproduces the observed PDF. This implies that the large‐scale isentropic transport leads to scale collapse and subsequent mixing in those parts of the flow where on a time scale ≈12 hours and a spatial scale ≈200 km the flow stretches a circle to an ellipse with the ratio between the major and minor axes exceeding 5. Owing to the spatial resolution of the CRISTA instrument that smooths out the non‐Gaussian tails, the elongation rate ≈5 estimates only the lower bound of the critical deformation. Furthermore, our simulations show that air masses of low N 2 O amounts observed by CRISTA between 20 ° and 40 ° S are fragments of the polar vortex that have been peeled from the vortex edge. The history of these fragments can be divided into two phases: formation and mixing of filaments at the vortex edge where γ>γc and pure advection of the remnants of such filaments into midlatitudes in flow regions with γ<γc. Here, the lifetime of such remnants may exceed two weeks due to negligible mixing in these parts of the flow. Copyright © 2005 Royal Meteorological Society.