  The study of chaotic mixing is important for its potential to improve our
understanding of fluid systems. Contour advection simulations provide a good
model of the phenomenon by tracking the evolution of one or more contours or
isolines of a trace substance to a high level of precision. The most accurate
method of validating an advected contour is to divide the tracer concentration
into discrete ranges and perform a maximum likelihood classification, a method
that we term, "isoline retrieval." Conditional probabilities generated as a
result provide excellent error characterization.
  In this study, a water vapour isoline of 0.001 mass-mixing-ratio is advected
over five days in the upper troposphere and compared with high-resolution AMSU
(Advanced Microwave Sounding Unit) satellite retrievals. The goal is to find
the same fine-scale, chaotic mixing in the isoline retrievals as seen in the
advection simulations. Some of the filaments generated by the simulations show
up in the conditional probabilities as areas of reduced probability. By
rescaling the probabilities, the filaments may be revealed in the isoline
retrievals proper with little effect on the overall accuracy. Limitations
imposed by the specific context, i.e. water-vapour retrieved with AMSU in the
upper troposphere, are discussed. Nonetheless, isoline retrieval is shown to be
a highly effective technique for atmospheric sounding, showing good agreement
with 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
