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Daily departures from monthly means

Greetings -- 

I've got daily values of wind, and I want to calculate the daily departure
from a monthly mean. I calculated the monthly mean by creating an
evenly-spaced monthly time axis and averaging my daily variable:

	def axis/t=15-jan-1979:15-dec-1998/npoints=240 monthlytax
	def grid/x=xax/y=yax/z=zax/t=monthlytax monthlygrd

	let/qui u_monthly = u_daily[gt=monthlygrd@ave]

Here's the variable I want: 

	let/qui u_prime = u_daily - u_monthly

How do I regrid u_monthly back to a daily grid, without linearly
interpolating, so that I subtract the *same* monthly mean value from each
daily value of the wind? Is there a more elegant way to calculate daily
departures from a monthly mean?

I realize that monthly time axes are shifty and so I tried using a
departure from a 30-day running mean instead. This method achieves what I
want more-or-less-precisely on the 15th day of each month, but otherwise
the 30-day mean overlaps two months. (In addition, this definition of
u_prime causes other problems later on in my script which are too
complicated to explain here.)

Any helpful suggestions would be most welcome. 

Jennifer M. Adams

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