Notes on the quality of in situ data.

The recent increase in the amount and type of climate measurements has improved our knowledge of the Earth system and the quality of the reanalysis data. Nevertheless, the huge amount of high-resolution data, in time and space, requires a rigorous quality assurance system able to identify issues in the data and reduce the negative effect of wrong or low-quality data that may have an impact the data analysis. In the frame of Copernicus Climate Change Service (C3S), several datasets covering different essential climate variables are provided: these datasets are made available by networks and international measurement programs, running at the global scale, and may include added-value products elaborated upon them. The effort spent within the activities implementing the C3S in situ component allowed the identification of corrupted or anomalous data, despite routine quality checks applied by the data providers. This communication will discuss the lesson learned during the data-checking of a few in situ datasets available in the CDS, with a few examples also used to interact with the data providers and improve both the source data and C3S products.