Jean S writes at Climate Audit:
He was able to replicate (visually perfectly) the smooths of MBH9x thereby showing that the smooths involved padding with the instrumental data. The filter used by UC was the zero-phase Butterworth filter (an IIR filter), which has been Mann’s favourite since at least 2003. However, there was something else that I felt was odd: UC’s emulation required a very long (100 samples or so) additional zero padding. So about two years ago, I decided to take an additional look at the topic with UC.
Indeed, after digitalizing Mann’s smooths we discovered that UC’s emulation was very, very good but not perfect. After a long research, and countless hours of experimenting (I won’t bore you with the details), we managed to figure out the “filter” used by Mann before Mann (2004)-era. Mann had made his own version of the Hamming filter (windowing method, an FIR filter)! Instead of using any kind of usual estimate for the filter order, which is usually estimated from the transition bandwidth (see, e.g., Mitra: Digital Signal Processing) and has typically the length of a few dozen coefficients at maximum, he used the filter length equal to the length of the signal to be filtered! As Mann’s PCA was apparently just a “modern” convention, this must be a “modern” filter design. Anyhow, no digital signal processing expert I consulted about the matter had ever seen anything like that.
JeanS adds in a comment:
the main issue here is not which filters/smoothers are “appropriate”, but the fact Mann was using a method unknown to anyone else. This made it practically impossible to replicate his smoothings, and later, to definitely show beyond the reasonable doubt that he indeed used the trick (i.e., padded with the instrumental data).
Bold mine. Read more here: http://climateaudit.org/2014/08/29/mannomatic-smoothing-technical-details/
I suppose a man who designates himself as a Nobel Laureate in court proceedings, even after being chastised by the IPCC, only to have to retract the court claims later, would have no moral issues at all with making a special version of an established filter to suit his own special purposes.