Why are changes in Tmean not always the average of changes in Tmin and Tmax?
A. Whether or not the change in mean monthly mean temperature is the average of the changes in mean monthly Tmin and Tmax depends on how the particular GCM calculates its daily Tmean, Tmin and Tmax. In HadCM2, for example, the daily Tmean is the average temperature over 24 hours (the average of 48 half hourly calculations), whereas daily Tmin is the lowest temperature reached in a day and Tmax is the highest temperature reached in a day. Clearly, in this case unless there is a perfect sine curve in the diurnal temperature cycle then daily Tmean need not be the average of daily Tmin and daily Tmax. Since the monthly and 30-year mean monthly values are derived from the daily temperatures, this explains why changes in Tmean may not equal the average of changes in Tmin and Tmax. How close Tmean changes are to the average of Tmin and Tmax changes depends on how the model calculates these daily quantities and on the shape of the diurnal cycle. Changes in 30-year mean diurnal temperature rang
