Validation report of the MACC 30-year multi-sensor reanalysis of ozone columns Period 1979-2008

The Ozone Multi-Sensor reanalysis (version 1) provides global ozone column field time series, for the period 1979-2008, on a grid with 1.5 x 1 degree resolution and with a time interval of 6 hours. This detailed data set is produced by assimilating all independent satellite column observation data sets publicly available (12 data sets in total). These satellite retrievals have been calibrated against a "ground-truth" consisting of ozone column measurements from the network of Brewer and Dobson instruments.
In this report, we have compared the MSR fields with individual Brewer-Dobson measurements, and with individual satellite observations. Because the same data set is used to construct the MSR, it should be noted that this is not a fully independent validation.
When comparing the MSR ozone column fields with the full set of Brewer-Dobson groundbased observations the mean fitted offset was found to be smaller than 0.2 DU, and both the trend (0.02 DU/year) and seasonal variation (effective ozone temperature dependence of -0.006 DU/K) were negligible. This consistency check shows that the MSR calibration coefficient are well tuned to reproduce the mean levels of the ozone columns observed from the ground. On average, only small offsets of less than 3% are observed for the individual Brewer or Dobson stations, but a few outliers exist. Because no clear systematic geographical patterns are found, it is reasonable to attribute the few larger offsets to biases of the individual surface instruments.
The Observation-minus-Forecast (OmF) statistics show that the bias of the MSR fields compared to the individual satellite measurements is almost everywhere less than 1%. The RMS standard deviation of about 2-3% is found for data-rich periods (with global satellite coverage in about one day) or 4-5% for data-poor periods (June 1993 -May 1995, SBUV-2 only). Somewhat larger OmF values are observed for large solar zenith angles, close to the dark, unobserved polar cap. Note that the OmF difference contains contributions from the model forecast error, the observation error and the representativity error, and that OmA is smaller than OmF. The MSR error against the individual satellite observations will therefore be smaller than the OmF RMS values quoted above.
Different retrieval algorithms applied to the same instrument produce data sets which may be considered as independent (correlations are small, about 0.2-0.3).