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The bias (an estimate of systematic error) is used as the measure of trueness (Table 3). The targets regarding trueness are stated so that the bias (average of O-B over a certain period) should be close to zero for all measured variables (sections 2.1 and 2.2). The trueness is assessed for all the temporal intervals considered in the tool (section 4): 6-hourly, daily and, in the future, monthly. Also, a 5-day moving average (Alert) of the absolute value of daily calculated O-B (Table 9) needs to be calculated daily for all observed variables and compared against the prescribed thresholds (Table 6). This is used as one of the main performance indicators on the daily monitoring activities.
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Both bias and standard deviation contribute to the overall measurement uncertainty. The root mean square error (rmse) is a common metric used to measure accuracy, and it is applied to upper-air observations in the WDQMS web tool (section 6.2) based on the quantitative information for the two vertical layers (mean and standard deviation of O-B departures over Trop and Stra, see section 2.2) provided by the NWP reports (see Table 5).
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Note that quality indicators are applied only to the measured quantities whose O-B departures are available in the NWP monitoring reports, i.e. the ones whose model equivalent is available from the NWP assimilation system (see sections 2.1 and 2.2). Therefore, if the O-B departures are missing because the model background is not calculated in a particular NWP assimilation (e.g., not all centres compute O-B departures for observations they do not use in assimilation) the quality indicator will not be calculated and the station will not show up on the quality map. This is why some stations appear in the availability map, but not in the quality map.
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