On the Reliability of Surface Observations and the Pitfalls of Verification Against Own Analyses

ABSTRACT Near‐surface observations can suffer from significant representativeness errors, especially for Numerical Weather Prediction (NWP) at lower resolution in global applications. Therefore, in Data Assimilation (DA), many operational centers have long been reluctant to assimilate them (e.g., the European Center for Medium‐range Weather Forecast, ECMWF, started assimilating all 6‐h screen‐level temperature reports only in 2024). For forecast verification, some studies advocate that we should not rely on them and use only verification against our own near‐surface analyses. At Environment and Climate Change Canada (ECCC), both temperature and humidity observations from SYNOPs have been assimilated in our global NWP system for more than two decades and, in June 2024, METARs have been added following some positive impacts found only when comparing forecasts against near‐surface observations. To shed light on the impact of the assimilation of screen‐level observations, in this study we present an evaluation of the impact of removing the assimilation of all screen‐level temperature and humidity observations using various verification references: the NWP forecasts were evaluated against radiosondes and surface observations, independent (ECMWF) analysis, our own analysis and surface analysis. Results show that, despite the lack of a proper estimation of representativeness errors in the DA approach, the assimilation of screen‐level temperature and humidity leads to forecast improvements that can be detected from the verification against independent measurement sources, here radiosondes and ECMWF upper‐air analyses. Verification against own analyses, for both upper‐air and screen‐level variables, led instead to opposite and misleading conclusions. In fact, the removal of assimilated screen‐level temperature and humidity measurements renders the NWP forecast more similar to the own analysis, therefore leading to better scores but detachment from the observed world.