Statistical analysis of the precision of the Match method

rlehmann [ at ]


The Match method quantifies chemical ozone loss in the polar stratosphere.The basic idea consists in calculating the forward trajectory of an air parcelthat has been probed by an ozone measurement (e.g., by an ozonesonde orsatellite instrument) and finding a second ozone measurement close to thistrajectory. Such an event is called a "match". A rate of chemical ozonedestruction can be obtained by a statistical analysis of several tens of suchmatch events.Information on the uncertainty of the calculated rate can be inferred from thescatter of the ozone mixing ratio difference (second measurement minus firstmeasurement) associated with individual matches. A standard analysis wouldassume that the errors of these differences are statistically independent.However, this assumption may be violated because different matches can share acommon ozone measurement, so that the errors associated with these match eventsbecome statistically dependent. Taking this effect into account, we present ananalysis of the uncertainty of the final Match result. It has been applied toMatch data from the Arctic winters 1995, 1996, 2000, and 2003. For theseozonesonde Match studies the effect of the error correlation on the uncertaintyestimates is rather small: compared to a standard error analysis, theuncertainty estimates increase by 15% on average. However, the effect may bemore pronounced for typical satellite Match analyses: for an Antarctic satelliteMatch study (2003), the uncertainty estimates increase by 60% on average.The analysis showed that the random errors of the ozone measurements and the"net match errors", which result from a displacement of the second ozonemeasurement of a match from the required position, are of similar magnitude.This demonstrates that the criteria for accepting a match (maximum trajectoryduration, match radius, spread of trajectory clusters etc.) ensure that, giventhe unavoidable ozone-measurement errors, the magnitude of the net match errorsis adequate. The estimate of the random errors of the ozonesonde measurementsagrees well with laboratory results.

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Lehmann, R. , von der Gathen, P. , Rex, M. and Streibel, M. (2005): Statistical analysis of the precision of the Match method , Atmospheric chemistry and physics, 5 , pp. 2713-2727 .

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