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| Titel |
How to average logarithmic retrievals? |
| VerfasserIn |
B. Funke, T. Clarmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 5, no. 4 ; Nr. 5, no. 4 (2012-04-25), S.831-841 |
| Datensatznummer |
250002781
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| Publikation (Nr.) |
 copernicus.org/amt-5-831-2012.pdf |
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| Zusammenfassung |
| Calculation of mean trace gas contributions from profiles obtained by
retrievals of the logarithm of the abundance rather than retrievals of the
abundance itself are prone to biases. By means of a system simulator, biases
of linear versus logarithmic averaging were evaluated for both maximum
likelihood and maximum a priori retrievals, for various signal to noise
ratios and atmospheric variabilities. These biases can easily reach ten
percent or more. As a rule of thumb we found for maximum likelihood
retrievals that linear averaging better represents the true mean value in
cases of large local natural variability and high signal to noise ratios,
while for small local natural variability logarithmic averaging often is
superior. In the case of maximum a posteriori retrievals, the mean is
dominated by the a priori information used in the retrievals and the method
of averaging is of minor concern. For larger natural variabilities, the
appropriateness of the one or the other method of averaging depends on the
particular case because the various biasing mechanisms partly compensate in
an unpredictable manner. This complication arises mainly because of the fact
that in logarithmic retrievals the weight of the prior information depends on
abundance of the gas itself. No simple rule was found on which kind of
averaging is superior, and instead of suggesting simple recipes we cannot do
much more than to create awareness of the traps related with averaging of
mixing ratios obtained from logarithmic retrievals. |
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