Issue with fieldAnonaly applied on daily data

[pascaloettli]

I think the fieldAnomaly function is not correct for daily dataset.

# Generating daily dates between 1983-01-01 and 1984-12-31
# with a leap year (1984)

x <- as.POSIXlt(seq.Date(as.Date("1983-01-01"), as.Date("1984-12-31"), by="day"))

level="daily"
if(level=="daily"){levs=unique(x$yday)}

levs_lookup=vector("list", length(levs))
names(levs_lookup) <- levs
for(i in 1:length(levs)){
  if(level=="daily"){levs_lookup[[i]] <- which(x$yday == names(levs_lookup[i]))}
}

Here is the problem. For the same index (60), we get different dates:

j <- 60
x[levs_lookup[[j]]]
# [1] "1983-03-01 UTC" "1984-02-29 UTC"

And here, only December 31st, 1984

j <- 366
x[levs_lookup[[j]]]
# [1] "1984-12-31 UTC"

[marchtaylor]

Thanks for your comment. This could be solved by using text strings for days:

x <- as.POSIXlt(seq.Date(as.Date("1983-01-01"), as.Date("1984-12-31"), by="day"))

level="daily"
if(level=="daily"){levs=levs=unique(format(x, format="%m%d"))}

levs_lookup=vector("list", length(levs))
names(levs_lookup) <- levs
for(i in 1:length(levs)){
  if(level=="daily"){levs_lookup[[i]] <- which(format(x, format="%m%d") == names(levs_lookup[i]))}
}

j <- 60
x[levs_lookup[[j]]]
# [1] "1983-03-01 UTC" "1984-03-01 UTC"

j <- 366
x[levs_lookup[[j]]]
# [1] "1984-02-29 UTC"

But, I think I may still leave the function as julian day and specify this more clearly in the function description.

Cheers

[marchtaylor]

I've updated the function and added some additional examples. There does indeed seem to be some issues with using julian day (or else I have made a mistake with my example). I would have thought that subtracting the julian day means would bring the anomalies closer to zero, but in fact the month-date combination proved better (see last example).

[marchtaylor]

My example was a bit incorrect. I've provided a more straightforward example containing a yearly signal and a monthly one based on julian day. The function can now correctly remove means by month, date, or julian day