The GEWEX Water Vapor Assessment: Overview and Introduction to Results and Recommendations

Abstract

To date, a large variety of water vapour data records from satellite and reanalysis are available. It is key to understand the quality and uncertainty of these data records in order to fully exploit these records and to avoid data being employed incorrectly or misinterpreted. Therefore, it is important to inform users on accuracy and limitations of these data records based on consistent inter-comparisons carried out in the framework of international assessments. Addressing this challenge is the major objective of the Global Water and Energy Exchanges (GEWEX) water vapor assessment (G-VAP) which was initiated by the GEWEX Data and Assessments Panel (GDAP).Here, an overview of G-VAP objectives and an introduction to the results from G-VAP's first phase are given. After this overview, a summary of available data records on water vapour and closely related variables and a short introduction to the utilized methods are presented. The results from inter-comparisons, homogeneity testing and inter-comparison of trend estimates, achieved within G-VAP's first phase are summarized. The conclusions on future research directions for the wider community and for G-VAP's next phase are outlined and recommendations have been formulated.For instance, a key recommendation is the need for recalibration and improved inter-calibration of radiance data records and subsequent reprocessing in order to increase stability and to provide uncertainty estimates. This need became evident from a general disagreement in trend estimates (e.g., trends in TCWV ranging from −1.51 ± 0.17 kg/m 2 /decade to 1.22 ± 0.16 kg/m 2 /decade) and the presence of break points on global and regional scale. It will be a future activity of G-VAP to reassess the stability of updated or new data records and to assess consistency, i.e., the closeness of data records given their uncertainty estimates. aimed at the consolidation of the G-VAP strategy and the technical implementation. Results from these workshops and feedback from GDAP were used to establish the G-VAP assessment plan. G-VAP organises annual workshops to provide opportunities for (i) presenting state-of-the-art of water vapour retrievals and products, (ii) discussion of results from inter-comparisons exercises and quantifying the sources of observed differences, and (iii) providing recommendations for future directions and roadmap. It was consensus at the G-VAP workshops to continue G-VAP beyond the finalisation of the World Climate Research Programme (WCRP) report on G-VAP, with support from GDAP. Details of the future of G-VAP were discussed and agreed upon at the 7th workshop in October 2017. The assessment plan and minutes from the G-VAP workshops are available at http://gewex-vap.org/?page_id=19 and partly workshop summaries have been published in the GEWEX News (see http://www.gewex.org/resources/gewex-news/).Prior to analysis, an inventory of available satellite and reanalysis data records has been carried out and made available online at http://gewex-vap.org/?pageid=13. A subset of these data rec...

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“…Therefore, in this study, we have used ERA‐Interim reanalysis IWV data extracted at the site locations. The drawbacks of this approach are representativeness differences between the two IWV data sets at a number of sites (Bock & Parracho, 2019) and the possible presence of inhomogeneities in the reference ERA‐Interim data set as well (Ning et al, 2016; Schröder et al, 2016; Schröder et al, 2019). Representativeness differences might result in seasonality and trends in the difference time series, while remaining inhomogeneities in ERA‐Interim, even after 4‐D variational bias correction in the data assimilation, might be responsible for trends in the differences time series.…”

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