Intraseasonal air temperature variability and its trends are sensitive to the definition of daily mean temperature: Germany as a case study

Our previous research on trends in summer day-to-day mean temperature difference in Europe indicates
inhomogeneities to occur at climate stations in Germany. In most cases, they produce an unrealistic decreasing trend. It is
suggested that these discontinuities are caused by the change in daily mean temperature calculation in April 2001, from averaging
the temperature at 7, 14, and two times 21 to averaging all 24-hourly values. Our current analysis demonstrates this inhomogeneity
and uncovers the difference in climatology and trends of summer temperature variability between aforesaid
definitions of daily mean temperature at 13 stations in Germany. It is revealed that the choice of definition is crucial for
seasonal temperature variability while for seasonal mean temperature itself no difference between definitions is noticed.
The first definition [(T₇ + T₁₄ + 2 x T₂₁)/4] shows days after warming warmer and days after cooling colder than if averaging
all 24 observations, which is the reason for overestimated intraseasonal temperature variability before 2001 and for its
artificial decreasing trends.