Intercomparison of GNSS-RO Quality Control Checks in NWP. Part I: Survey of Methods

While understanding that quality control (QC) processes crucially affect the statistics of the observations compared to short-range numerical weather prediction (NWP) forecasts and the subsequent analyses, we also realize the fact that many centers implement entirely different QC methods from each other in operations. Here, we catalog the QC methods of Global Navigation Satellite System (GNSS) radio occultation (RO) observations used for data assimilation (DA) by NWP centers. The QC methods are categorized into four groups: preliminary checks which often refer to the metadata of the observed profile, background checks where the observation is compared to a short-range forecast, superrefraction checks which attempt to look for the presence of superrefraction in the profile, and miscellaneous checks such as running a one-dimensional variational scheme or using the variational QC method. This survey is the first published catalog of QC checks used in GNSS-RO observations. It also sets up the second part of this paper, where various methods for diagnosing superrefraction are compared.