The study of chaotic mixing is important for its potential to improve ourunderstanding of fluid systems. Contour advection simulations provide a goodmodel of the phenomenon by tracking the evolution of one or more contours orisolines of a trace substance to a high level of precision. The most accuratemethod of validating an advected contour is to divide the tracer concentrationinto discrete ranges and perform a maximum likelihood classification, a methodthat we term, "isoline retrieval." Conditional probabilities generated as aresult provide excellent error characterization. In this study, a water vapour isoline of 0.001 mass-mixing-ratio is advectedover five days in the upper troposphere and compared with high-resolution AMSU(Advanced Microwave Sounding Unit) satellite retrievals. The goal is to findthe same fine-scale, chaotic mixing in the isoline retrievals as seen in theadvection simulations. Some of the filaments generated by the simulations showup in the conditional probabilities as areas of reduced probability. Byrescaling the probabilities, the filaments may be revealed in the isolineretrievals proper with little effect on the overall accuracy. Limitationsimposed by the specific context, i.e. water-vapour retrieved with AMSU in theupper troposphere, are discussed. Nonetheless, isoline retrieval is shown to bea highly effective technique for atmospheric sounding, showing good agreementwith both ECMWF (European Centre for Medium-range Weather Forecasts)assimilation data and radiosonde measurements. Software for isoline retrieval can be found at: http://isoret.sourceforge.net
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