correct time for GOFS3.1 hindcast from tds data server?
Beaird, Nicholas - 0337 - MITLL
Hello,
I have a question about the timestamp on the GOFS 3.1 GLBy0.08/expt_93.0 hindcast tz3z netcdf files. The data url for the unaggregated file, for example, Feb 06 2020 06:00 (I think) is: 'https://tds.hycom.org/thredds/dodsC/datasets/GLBy0.08/expt_93.0/data/hindcasts/2020/hycom_glby_930_2020020612_t006_ts3z.nc'
My assumption is this is the model output for that time. However when I try to reference the time from the netcdf using the ‘time’ coordinate (long name: Valid Time, units: hours since 2000-01-01 00:00:00) I find that the ‘time’ coordinate is 12 hours ahead of what I expect from the data url. I’ve put a reproducible minimal example of the code I use below. Am I misunderstanding something about the url format? The ‘12_t’ part is mysterious to me and suspiciously like the 12 hours off set I’m finding.
Any help with this would be much appreciated! I’m sure I’m missing something simple about timezones or model run time vs model time, but I can’t figure it out!
import xarray as xr
import pandas as pd
# pick a time
dt = pd.to_datetime('02/06/2020 06:00:00')
# construct url for hycom thredds server:
hycom_org = 'https://tds.hycom.org/thredds/dodsC/datasets/GLBy0.08/expt_93.0/data/hindcasts/'
y = f'{dt.year}'
m = f'{dt.month:02d}'
d = f'{dt.day:02d}'
h = f'{dt.hour:03d}'
hycomTDS = hycom_org + y + '/hycom_glby_930_' + y + m + d + '12_t'+ h +'_ts3z.nc'
# load GOFS hindcast:
ds = xr.open_dataset(hycomTDS, decode_times= False, decode_coords="all")
# get the time in datetime format
units, reference_date = ds.time.attrs['units'].split('since') # 'units since reference time'
ds['time_refd'] = pd.to_datetime(reference_date) + pd.Timedelta( ds.time.data[0] ,units[0])
print('url time:', dt)
print('netcdf time:', ds['time_refd'].data)
>>
url time: 2020-02-06 06:00:00
netcdf time: 2020-02-06T18:00:00.000000000
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Nicholas Beaird, PhD
Technical Staff
Advanced Undersea Systems and Technology
MIT Lincoln Laboratory
Cell: 401-573-9456
