In summer, there is a general underestimation of the amplitude of the diurnal cycle of temperature and a daytime low-humidity bias. Over recent years, there have been some changes in these biases due to model changes, but they are relatively robust in terms of geographical patterns and annual and diurnal variations.
Turning next to daytime biases in summer, daytime T2m and humidity forecasts for Europe have an overall cold and dry bias. This could be an issue just in the surface layer (up to about 50–100 m) or it could be due to problems in a deeper layer of the atmosphere or both. Figure 7 shows that the model underestimates the difference in temperature and humidity across the surface layer compared to radiosonde observations. This underestimation is particularly pronounced at lower latitudes and contributes to the negative biases there. It means that part of the daytime cool/low humidity bias in summer is likely due to the surface layer in the model being too strongly mixed.
https://www.ecmwf.int/en/newsletter/157/meteorology/addressing-biases-near-surface-forecasts