In the first step, they are removing a 21-day, 31-year running mean trend. I am wondering if you know any possible reason for the selection of this particular numbers. The 31-year seems to be related with global warming. What might be the meaning of the 21-day?
I discussed this with Iwan Supit and this what we think: the 21 day running mean results in 10 monthly values. The running mean is used to remove the daily variation but to keep to better preserve some of the inter monthly variation. for example temperature follows a sine curve over the yea. The yearly “movement” is better described by 120 points than by 12. However this is a trade off computational wise if you use 120 values in stead of 12.
let me know if you need further explanation.