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Re: Averaging data for 4 months

Hi Amy,

Ferret has a tool to do the sorts of climatological averages that you are
after directly.

     use climatological_axes
     let climatological_hgt = hgt[gt=MONTH_IRREG@mod]    ! makes a 12
     month climatology
     your answer will be climatological_hgt[L=12:15@ave]

for more details. Be aware that there are minor numerical consequences of
using irregular calendar months vs regular.  In the problem that you describe
I would recommend that you choose MONTH_IRREG or MONTH_REG to match the way in
which your underlying data set was created.

    - steve


Amy Sundquist wrote:

> Hello,
> I am working with gridded monthly 700 mb heights.  The data for all 12
> months is for approximately 49 years (588 months).  I would like to find
> the average 700 mb height for Dec.-Mar. for each year and an average for
> the entire 49 years.  The problem I am having is with incrementing the
> time.
> Since I was having difficulty with this, I decided to increment each month
> individually by 12 months to see where my errors are.  This is what my
> code looks like for the height data:
> Let Dec = hgt[x=-160,y=60,z=700,L=12:582:12,d=2]
> Let Jan = hgt[x=-160,y=60,z=700,L=13:582:12,d=2]
> Let Feb = hgt[x=-160,y=60,z=700,L=14:582:12,d=2]
> Let Mar = hgt[x=-160,y=60,z=700,L=15:582:12,d=2]
> Let Htime = (Dec + Jan + Feb + Mar)/4
> Dec works fine.  It starts in December 1949 and goes through December
> 1995.  When I list Jan, it has the exact same values as Dec. However, when
> I change the time increment in Jan to "L=1:582:12" instead of
> "L=13:582:12" I get the correct heights for Jan starting with January
> 1948, but I don't want to start with month 1, I want to start with month
> 13 (January 1949).
> A similar thing happens for Feb.  I get December values.  But when I
> change the time from "L=14:582:12" to "L=2:582:12", I get January Values
> as I did when I used "L=1:582:12".
> Finally, when I list Mar, I get Dec values for "L=15:582:12" and Jan
> values for "L=3:582:12".
> I can't seem to figure out what is going on here.  There doesn't seem to
> be a pattern on how Ferret is displaying the data.  Is there a simpler way
> to go about averaging 4 months of data for a period of 50 years or so?  If
> anyone has any suggestions, I would greatly appreciate it!
> Thanks,
> Amy Sundquist
> -------------------------------------------
> UC Davis Atmospheric Science Graduate Group

Steve Hankin
NOAA/PMEL, 7600 Sand Point Way NE, Seattle, WA 98115-0070
ph. (206) 526-6080 -- FAX (206) 526-6744

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