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PROGRAM sst_to_obs, oi_sst_to_obs

Overview

There are two gridded SST observation converters in this directory, one for data from PODAAC, and one from NOAA/NCDC. sst_to_obs converts data from PODAAC and has been used by Romain Escudier for regional studies with ROMS. oi_sst_to_obs converts data from NOAA/NCDC and has been used by Fred Castruccio for global studies with POP.

sst_to_obs -- GHRSST to DART observation sequence converter

These routines are designed to convert the GHRSST Level 4 AVHRR_OI Global Blended Sea Surface Temperature Analysis (GDS version 2) from NCEI data distributed by the Physical Oceanography Distributed Active Archive Center. Please remember to cite the data in your publications, specific instructions from PODAAC are available here. This is an example:

National Centers for Environmental Information. 2016. GHRSST Level 4 AVHRR_OI Global Blended Sea Surface Temperature Analysis (GDS version 2) from NCEI. Ver. 2.0. PO.DAAC, CA, USA. Dataset accessed [YYYY-MM-DD] at http://dx.doi.org/10.5067/GHAAO-4BC02.

Many thanks to Romain Escudier (then at Rutgers) who did the bulk of the work and graciously contributed his efforts to the DART project. Romain gave us scripts and source code to download the data from the PODAAC site, subset the global files to a region of interest, and convert that subsetted file to a DART observation sequence file. Those scripts and programs have been only lightly modified to work with the Manhattan version of DART and contain a bit more documentation.

The workflow is usually:

  1. compile the converters by running work/quickbuild.sh in the usual way.
  2. customize the shell_scripts/parameters_SST resource file to specify variables used by the rest of the scripting.
  3. run shell_scripts/get_sst_ftp.sh to download the data from PODAAC.
  4. provide a mask for the desired study area.
  5. run shell_scripts/Prepare_SST.sh to subset the PODAAC data and create the DART observation sequence files. Be aware that the Prepare_SST.sh modifies the shell_scripts/input.nml.template file and generates its own input.nml. work/input.nml is not used.
  6. combine all output files for the region and timeframe of interest into one file using the :doc:`../../../assimilation_code/programs/obs_sequence_tool/obs_sequence_tool`

Example

It is worth describing a small example. If you configure get_sst_ftp.sh to download the last two days of 2010 and then specify the mask to subset for the NorthWestAtlantic (NWA) and run Prepare_SST.sh your directory structure should look like the following:

0[1234] cheyenne6:/<6>obs_converters/SST
.
|-- ObsData
|   `-- SST
|       |-- ncfile
|       |   `-- 2010
|       |       |-- 20101230120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0.nc
|       |       `-- 20101231120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0.nc
|       `-- nwaSST
|           `-- 2010
|               |-- 20101230120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0_NWA.nc
|               `-- 20101231120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0_NWA.nc
|-- oi_sst_to_obs.f90
|-- oi_sst_to_obs.nml
|-- sst_to_obs.f90
|-- sst_to_obs.nml
|-- shell_scripts
|   |-- Prepare_SST.sh
|   |-- functions.sh
|   |-- get_sst_ftp.sh
|   |-- input.nml
|   |-- input.nml.template
|   |-- my_log.txt
|   |-- parameters_SST
|   `-- prepare_SST_file_NWA.sh
|-- masks
|   |-- Mask_NWA-NCDC-L4LRblend-GLOB-v01-fv02_0-AVHRR_OI.nc
|   `-- Mask_NWA120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0.nc
`-- work
    |-- Makefile
    |-- advance_time
    |-- input.nml
    |-- obs_sequence_tool
    |-- oi_sst_to_obs
    |-- preprocess
    |-- quickbuild.sh
    `-- sst_to_obs

The location of the DART observation sequence files is specified by parameter_SST:DIR_OUT_DART. That directory should contain the following two files:

0[1236] cheyenne6:/<6>v2/Err30 > ls -l
'total 7104
-rw-r--r-- 1 thoar p86850054 3626065 Jan 10 11:08 obs_seq.sst.20101230
-rw-r--r-- 1 thoar p86850054 3626065 Jan 10 11:08 obs_seq.sst.20101231

oi_sst_to_obs -- noaa/ncdc to DART observation sequence converter

oi_sst_to_obs is designed to convert the NOAA High-resolution Blended Analysis: Daily Values using AVHRR only data. The global metadata of a typical file is shown here:

:Conventions = "CF-1.5" ;
:title = "NOAA High-resolution Blended Analysis: Daily Values using AVHRR only" ;
:institution = "NOAA/NCDC" ;
:source = "NOAA/NCDC  ftp://eclipse.ncdc.noaa.gov/pub/OI-daily-v2/" ;
:comment = "Reynolds, et al., 2007:
     Daily High-Resolution-Blended Analyses for Sea Surface Temperature.
     J. Climate, 20, 5473-5496.
     Climatology is based on 1971-2000 OI.v2 SST,
     Satellite data: Navy NOAA17 NOAA18 AVHRR, Ice data: NCEP ice." ;
:history = "Thu Aug 24 13:46:51 2017: ncatted -O -a References,global,d,, sst.day.mean.2004.v2.nc\n",
        "Version 1.0" ;
:references = "https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.highres.html" ;
:dataset_title = "NOAA Daily Optimum Interpolation Sea Surface Temperature" ;

The workflow is usually:

  1. compile the converters by running work/quickbuild.sh in the usual way.
  2. download the desired data.
  3. customize the work/input.nml file.
  4. run work/oi_sst_to_obs to create a single DART observation sequence file.
  5. combine all output files for the region and timeframe of interest into one file using the :doc:`../../../assimilation_code/programs/obs_sequence_tool/obs_sequence_tool`

sst_to_obs namelist

This namelist is read from the file input.nml. Namelists start with an ampersand '&' and terminate with a slash '/'. Character strings that contain a '/' must be enclosed in quotes to prevent them from prematurely terminating the namelist.

&sst_to_obs_nml
   sst_netcdf_file     = '1234567.nc'
   sst_netcdf_filelist = 'sst_to_obs_filelist'
   sst_out_file        = 'obs_seq.sst'
   subsample_intv      = 1
   sst_rep_error       = 0.3
   debug               = .false.
   /
Contents Type Description
sst_netcdf_file character(len=256) Name of the (usually subsetted) netcdf data file. This may be a relative or absolute filename. If you run the scripts 'as is', this will be something like: ../ObsData/SST/nwaSST/2010/20101231120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0_NWA.nc
sst_netcdf_filelist character(len=256) Name of the file that contains a list of (usually subsetted) data files, one per line. You may not specify both sst_netcdf_file AND sst_netcdf_filelist. One of them must be empty.
sst_out_file character(len=256) Name of the output observation sequence file.
subsample_intv integer It is possible to 'thin' the observations. subsample_intv allows one to take every Nth observation.
sst_rep_error real In DART the observation error variance can be thought of as having two components, an instrument error and a representativeness error. In sst_to_obs the instrument error is specified in the netCDF file by the variable analysis_error. The representativeness error is specified by sst_rep_error, which is specified as a standard deviation. These two values are added together and squared and used as the observation error variance. Note: This algorithm maintains backwards compatibility, but is technically not the right way to combine these two quantities. If they both specified variance, adding them together and then taking the square root would correctly specify a standard deviation. Variances add, standard deviations do not. Since the true observation error variance (in general) is not known, we are content to live with an algorithm that produces useful observation error variances. If your research comes to a more definitive conclusion, please let us know.
debug logical Print extra information during the sst_to_obs execution.

oi_sst_to_obs namelist

This namelist is read from the file input.nml. Namelists start with an ampersand '&' and terminate with a slash '/'. Character strings that contain a '/' must be enclosed in quotes to prevent them from prematurely terminating the namelist.

&oi_sst_to_obs_nml
   input_file       = '1234567.nc'
   output_file_base = 'obs_seq.sst'
   subsample_intv   = 1
   sst_error_std    = 0.3
   debug            = .false.
   /
Contents Type Description
input_file character(len=256) Name of the input netcdf data file. This may be a relative or absolute filename. If you run the scripts 'as is', this will be something like: ../ObsData/SST/nwaSST/2010/20101231120000-NCEI-L4_GHRSST-SSTblend-AVHRR_OI-GLOB-v02.0-fv02.0_NWA.nc
output_file_base character(len=256) Partial filename for the output file. The date and time are appended to output_file_base to construct a unique filename reflecting the time of the observations in the file.
subsample_intv integer It is possible to 'thin' the observations. subsample_intv allows one to take every Nth observation.
sst_error_std real This is the total observation error standard deviation.
debug logical Print extra information during the oi_sst_to_obs execution.

Decisions you might need to make

See the general discussion in the :doc:`../../../guide/creating-obs-seq-real` page about what options are available for the things you need to specify. These include setting a time, specifying an expected error, setting a location, and an observation type.

Known Bugs

I do not believe sst_to_obs will work correctly if given multiple files in sst_netcdf_filelist. The number of observation used to declare the length of the output observation sequence is based on a single file ... yet seems to be used by many. I have not tested this configuration, since the scripting does not use the sst_netcdf_filelist mechanism.