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Beware! Time assignment change from older versions of Ferret
Ferret users,
While writing up this posting I found the answer in the mail archives at:
http://ferret.wrc.noaa.gov/Ferret/Mail_Archives/fu_99/tmp/msg00060.html
but thought I'd post it anyway in case anyone else is having similar
trouble --- and as a warning to long-time users who still have work
around that they've done in older versions. Note that the following
only applies to files with reference start times before about the
year 1500, but some commonly used files, such as some Reynolds sst
files, have reference times like this.
I was trying to update some work I did a few years ago, and was
having trouble reproducing the previous work. I finally tracked the
problem to a behavior change in assigning time stamps between v 4.91
(which I was using at the time I did the previous work) and v 5.22
which I'm using now.
Compare the listings below done on the same Reynolds sst file
(downloaded from cdc.noaa.gov) using exactly the same commands but
with different versions of Ferret. The sst listed at each grid
point in time is consistent, but the assigned day-month-year is
different by two days between the two versions of Ferret. For
example (see complete listing and commands below) for identical grid
points:
V 4.91 : V5.22:
01-DEC-1984 00 / 162: 16.81 29-NOV-1984 00 / 162: 16.81
08-DEC-1984 00 / 163: 16.61 06-DEC-1984 00 / 163: 16.61
15-DEC-1984 00 / 164: 15.75 13-DEC-1984 00 / 164: 15.75
This two day change had a fairly significant affect on the results
when performing various averaging! The answer, as I interpret Steve
Hankin's e-mail from the archives, is that they made a change in
version 5.0 to be more consistent with the udunits convention. This
affects files with reference start times before about the year 1500.
The particular Reynolds sst file I was using shows the following in
and ncdump:
double time(time) ;
time:units = "days since 1-1-1 00:00:00" ;
time:long_name = "Time" ;
time:actual_range = 723483., 726465. ;
time:delta_t = "0000-00-07 00:00:00" ;
time:avg_period = "0000-00-07 00:00:00" ;
Now I just need to figure out whether the v 4.91 calendar (Gregorian)
or the v5.0 (udunits) gives me the correct calendar day for the
Reynolds SST datasets so I can combine the data in analyses with
other data sets. I would assume they use udunits. Anyone know?
Anyway, beware!
Lynn
----------Listings using v 4.91----------
NOAA/PMEL TMAP
Program FERRET (GUI)
Version 4.91 - 05/19/98
28-Feb-02 12:10
yes? set data "Reynolds_sst/sst.wkmean.1981-1989.nc"
yes? set region/i=231/j=126
yes? list sst[t=1-dec-1984:1-feb-1985]
Weekly Means of Sea Surface Temperature (degC)
LATITUDE: 35.5N
DATA SET:
/disk04/products/Derived/Reynolds_sst/sst.wkmean.1981-1989.nc
129.5W
231
01-DEC-1984 00 / 162: 16.81
08-DEC-1984 00 / 163: 16.61
15-DEC-1984 00 / 164: 15.75
22-DEC-1984 00 / 165: 15.51
29-DEC-1984 00 / 166: 14.93
05-JAN-1985 00 / 167: 14.98
12-JAN-1985 00 / 168: 14.83
19-JAN-1985 00 / 169: 15.06
26-JAN-1985 00 / 170: 14.35
02-FEB-1985 00 / 171: 14.06
yes? list t[gt=sst,l=1:10]
T
axis TIME
DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
31-OCT-1981 00 / 1: 723483.
07-NOV-1981 00 / 2: 723490.
14-NOV-1981 00 / 3: 723497.
21-NOV-1981 00 / 4: 723504.
28-NOV-1981 00 / 5: 723511.
05-DEC-1981 00 / 6: 723518.
12-DEC-1981 00 / 7: 723525.
19-DEC-1981 00 / 8: 723532.
26-DEC-1981 00 / 9: 723539.
02-JAN-1982 00 / 10: 723546.
yes?
----------Listings using v 5.22----------
NOAA/PMEL TMAP
Program FERRET
Version 5.22 - 08/23/00
28-Feb-02 12:17
yes? set data "Reynolds_sst/sst.wkmean.1981-1989.nc"
yes? set region/i=231/j=126
yes? list sst[t=1-dec-1984:1-feb-1985]
Weekly Means of Sea Surface Temperature (degC)
LATITUDE: 35.5N
DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
129.5W
231
29-NOV-1984 00 / 162: 16.81
06-DEC-1984 00 / 163: 16.61
13-DEC-1984 00 / 164: 15.75
20-DEC-1984 00 / 165: 15.51
27-DEC-1984 00 / 166: 14.93
03-JAN-1985 00 / 167: 14.98
10-JAN-1985 00 / 168: 14.83
17-JAN-1985 00 / 169: 15.06
24-JAN-1985 00 / 170: 14.35
31-JAN-1985 00 / 171: 14.06
yes? list t[gt=sst,l=1:10]
T
axis TIME
DATA SET: Reynolds_sst/sst.wkmean.1981-1989.nc
29-OCT-1981 00 / 1: 723483.
05-NOV-1981 00 / 2: 723490.
12-NOV-1981 00 / 3: 723497.
19-NOV-1981 00 / 4: 723504.
26-NOV-1981 00 / 5: 723511.
03-DEC-1981 00 / 6: 723518.
10-DEC-1981 00 / 7: 723525.
17-DEC-1981 00 / 8: 723532.
24-DEC-1981 00 / 9: 723539.
31-DEC-1981 00 / 10: 723546.
yes?
--------------------------------------------
Lynn deWitt
ldewitt@pfeg.noaa.gov
Pacific Fisheries Environmental Laboratories
1352 Lighthouse Avenue
Pacific Grove, CA 93950-2097
(831)-648-9036
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