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More details on the estimation of these parameters are given in the Report on Updated KPIs (D4, e.g. section 2.1).
Scope of the document
This document provides relevant information on requirements and gaps for retrievals of atmospheric water vapour contents based on data from four different sources:
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The consistency between the Water Vapour THP ICDR product, described in Section 1.2, and the corresponding TCDR product is checked by a test designed to detect certain type of differences between the ICDR and the TCDR [D4, section 3]. The relative differences between the monthly mean observed data and a reference data set are computed on a global latitude-height grid, for both the ICDR and the TCDR. These relative differences are globally averaged (properly area weighted) and vertically averaged (in 0-4 km, 4-8 km, and 8-12 km layers).
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For the UTH ICDR, a reference dataset of the equivalent UTH was derived using the NWP SAF radiance simulator, which constitutes an interface to the radiative transfer model of the NWP SAF RTTOV. RTTOV estimates the brightness temperature based on ERA-Interim input, which is then converted to UTH as is done for the microwave sounder observations, as described in reference document [D3]. KPIs were defined in terms of deviations of the TCDR UTH from the ERA-Interim reanalysis reference dataset, by the methods described in reference document [D4, section 3 and 4.2.5]. Reference performance targets were set by determining the UTH limits of the 2.5 and 97.5 percentiles of the differences between the UTH TCDR and the ERA-Interim derived reference data set. For any of the satellites, no more than 2.5% of the TCDR data was found to have a UTH value more than 0.47% below the reference UTH, and no more than 2.5% of the TCDR UTH data was more than 0.55% higher than the reference UTH. These limits were taken as the KPI values (see Table 2.2).
In 2019, when the ERA_Interim reanalysis dataset became unavailable, new KPIs were redefined for the MetOp-A and MetOp-B satellites by determining the UTH limits of the 2.5 and 97.5 percentiles of the differences between the UTH ICDR and the ERA-5 derived reference data set during the year 2018. This was done in the same way as had been done previously with the TCDR UTH and ERA-Interim reanalysis differences, as described and specified in document [D4, section 4.2.5]. For either of the satellites MetOp-A and MetOp-B, no more than 2.5% of the ICDR data was found to have a UTH value less than 0.40% above the reference UTH, and no more than 2.5% of the ICDR UTH data was more than 1.57% higher than the reference UTH. These limits were taken as the updated KPI values for the MetOp satellites against which to measure performance for data from 01/01/2019 onwards.
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