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Re: JFM averages.
Hi Marcelo,
I think that the axis you defined with
define axis/edges/t="16-jan-1950:12":"16-dec-1994:12"/npoints=`45*4` tsea
has a problem. SHOW AXIS indicates a "drift" to your seasonal axis. Here we
examine every 5th year:
yes? show axis/l=1:180:`4*5` tsea
name axis # pts start end
TSEA TIME 180 r 03-MAR-1950 01:40
31-OCT-1994 22:20
T0 = 15-JAN-1901
L T BOX_SIZE TIME_STEP (HOURS)
1> 03-MAR-1950 01:40:00 2187.333 430657.7
21> 27-FEB-1955 20:20:00 2187.333 474404.3
41> 24-FEB-1960 15:00:00 2187.333 518151
61> 20-FEB-1965 09:40:00 2187.333 561897.7
81> 17-FEB-1970 04:20:00 2187.333 605644.3
101> 13-FEB-1975 23:00:00 2187.333 649391
121> 10-FEB-1980 17:40:00 2187.333 693137.7
141> 06-FEB-1985 12:20:00 2187.333 736884.3
161> 03-FEB-1990 07:00:00 2187.333 780631
Here's an off-the-cuff version of the axis -- using 15-FEB as a crude estimate
for the middle of the J-F_M season. Note there is no drift:
yes? define axis/t="15-feb-1950":"15-feb-1995"/npoints=`45*4+1` tsea
yes? show axis/l=1:180:`4*5` tsea
name axis # pts start end
TSEA TIME 181 r 15-FEB-1950 00:00
15-FEB-1995 00:00
T0 = 15-JAN-1901
L T BOX_SIZE TIME_STEP (HOURS)
1> 15-FEB-1950 00:00:00 2191.467 430272
21> 15-FEB-1955 05:20:00 2191.467 474101.3
41> 15-FEB-1960 10:40:00 2191.467 517930.7
61> 14-FEB-1965 16:00:00 2191.467 561760
81> 14-FEB-1970 21:20:00 2191.467 605589.3
101> 15-FEB-1975 02:40:00 2191.467 649418.7
121> 15-FEB-1980 08:00:00 2191.467 693248
141> 14-FEB-1985 13:20:00 2191.467 737077.3
161> 14-FEB-1990 18:40:00 2191.467 780906.7
You can use the DAYS1900 function as shown in
http://ferret.wrc.noaa.gov/Ferret/Documentation/Users_Guide/v50/partII.html#V5.0_DEFINE AXIS$/EDGES
to get really detailed control over the axis definition.
- steve
======================================
Marcelo Barreiro wrote:
> Hi ferreters,
>
> I am computing JFM averages from monthly data of a time series which spans
> 45 years (540 months).
> I did it in two different ways:
>
> (1st)
> define axis/edges/t="16-jan-1950:12":"16-dec-1994:12"/npoints=`45*4` tsea
> let my_var_seas = my_var[gt=tsea@ave]
> let/quiet my_var_JFM = my_var_seas[l=1:180:4]
>
> (2nd)
> let/quiet my_var_J = my_var[l=1:540:12]
> let/quiet my_var_F = my_var[l=2:540:12]
> let/quiet my_var_M = my_var[l=3:540:12]
> let/quiet my_var_JFM= (my_var_J + my_var_F + my_var_M)/3
>
> Results of my_var_JFM are similar but not equal and in cases the
> correlation between the results of different methods is as low as 0.90.
>
> Does anyone know which way is the best?
> Thanks!
>
> Marcelo
>
> ================================================
> Marcelo Barreiro
> Dept. of Oceanography, Physical Section
> College of Geosciences and Maritime Studies
> Texas A&M University
> College Station, TX 77843-3146
> Phone: (979) 845-8216
> Email: marcelo@ocean.tamu.edu
> URL: http://www-ocean.tamu.edu/~marcelo/
> ================================================
--
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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