Can multiple collections of NetCDF files be aggregated using NcML?

Question

I've got 40 NetCDF files that all constitute one simulation from an ocean model. The time series of data for each variable is split into multiple files, so that the directory listing looks like this:

$ ls ./output/*.nc
1_hvel.nc           3_hvel.nc           5_hvel.nc           7_hvel.nc
1_MeanPeriod.nc     3_MeanPeriod.nc     5_MeanPeriod.nc     7_MeanPeriod.nc
1_PeakPeriod.nc     3_PeakPeriod.nc     5_PeakPeriod.nc     7_PeakPeriod.nc
1_pwd.nc            3_pwd.nc            5_pwd.nc            7_pwd.nc
1_SigWaveHeight.nc  3_SigWaveHeight.nc  5_SigWaveHeight.nc  7_SigWaveHeight.nc
1_WaterLevel.nc     3_WaterLevel.nc     5_WaterLevel.nc     7_WaterLevel.nc
2_hvel.nc           4_hvel.nc           6_hvel.nc           8_hvel.nc
2_MeanPeriod.nc     4_MeanPeriod.nc     6_MeanPeriod.nc     8_MeanPeriod.nc
2_PeakPeriod.nc     4_PeakPeriod.nc     6_PeakPeriod.nc     8_PeakPeriod.nc
2_pwd.nc            4_pwd.nc            6_pwd.nc            8_pwd.nc
2_SigWaveHeight.nc  4_SigWaveHeight.nc  6_SigWaveHeight.nc  8_SigWaveHeight.nc
2_WaterLevel.nc     4_WaterLevel.nc     6_WaterLevel.nc     8_WaterLevel.nc

Is it possible to aggregate each variable along the time dimension and then join all these aggregations together?

Answer 1

Yes, you can nest aggregations using NcML, so you can use a type union aggregation around several type join_existing aggregations on time.

If the join_existing aggregations on time turn out to have different time bases, you can just rename time variable and dimension to time2 on one of the aggregations and then change the attributes of those variables to use the newly named coordinates.

An example should make this clearer:

<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
 <attribute name="title" type="string" value="SELFE - Ike - Ultralite - Variable Drag - Waves - 3D"/>
 <attribute name="institution" type="string"
  value="Virginia Institute of Marine Science -- http://web.vims.edu/physical/"/>
 <attribute name="source" type="string"
  value="SELFE run  (version v.vvv  compiled yyyy-mm-dd) for Hurricane Ike on the Ultralight mesh using variable drag, wind waves from WWM and 3D depth-averaged physics"/>
 <attribute name="history" type="string" value="ncml aggregation of .../whole_domain/ datafiles"/>
 <attribute name="references" type="string"
  value="testbed.sura.org:/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Ike/3D_varied_roughness_windstress_with_wave/Inundation_Model_Metadata_Template_v3_for_3D_windstress_sv_with_wave_Rita.docx"/>
 <variable name="selfe_mesh" shape="" type="int">
  <attribute name="cf_role" value="mesh_topology"/>
  <attribute name="topology_dimension" type="int" value="2"/>
  <attribute name="node_coordinates" value="x y"/>
  <attribute name="face_node_connectivity" value="ele"/>
 </variable>
 <variable name="time">
  <attribute name="units" value="seconds since 2005-09-18 00:00:00"/>
  <attribute name="standard_name" value="time"/>
 </variable>
 <variable name="ele" shape="nface nele" type="int">
  <attribute name="cf_role" value="face_node_connnectivity"/>
  <attribute name="start_index" type="short" value="1"/>
 </variable>
 <variable name="x" shape="node" type="float">
  <attribute name="units" value="degrees_east"/>
 </variable>
 <variable name="y" shape="node" type="float">
  <attribute name="units" value="degrees_north"/>
 </variable>
 <variable name="depth" shape="node" type="float">
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="m"/>
  <attribute name="standard_name" value="depth_below_geoid"/>
  <remove type="attribute" name="positive"/>
 </variable>
 <variable name="sigma" shape="sigma" type="float">
  <attribute name="long_name" value="S coordinates at whole levels"/>
  <attribute name="units" value="non-dimensional"/>
  <attribute name="positive" value="up"/>
 </variable>
 <variable name="Cs" shape="sigma" type="float">
  <attribute name="long_name" value="Function C(s) at whole levels"/>
  <attribute name="units" value="non-dimensional"/>
  <attribute name="positive" value="up"/>
 </variable>
 <variable name="elev" shape="time node" type="float">
  <attribute name="long_name" value="free_surface_elevation"/>
  <attribute name="standard_name" value="sea_surface_height_above_geoid"/>
  <attribute name="missing_value" type="float" value="-999.00"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="m"/>
 </variable>
 <variable name="wwm_1" shape="time node" type="float">
  <attribute name="long_name" value="significant_wave_height"/>
  <attribute name="standard_name" value="sea_surface_wave_significant_height"/>
  <attribute name="missing_value" type="float" value="-999.00"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="m"/>
 </variable>
 <variable name="wwm_3" shape="time node" type="float">
  <attribute name="long_name" value="mean_period"/>
  <attribute name="standard_name" value="sea_surface_wave_period_at_variance_spectral_density_mean"/>
  <attribute name="missing_value" type="float" value="-999.00"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="s"/>
 </variable>
 <variable name="wwm_9" shape="time node" type="float">
  <attribute name="long_name" value="peak_period"/>
  <attribute name="standard_name"
   value="sea_surface_wave_period_at_variance_spectral_density_maximum"/>
  <attribute name="missing_value" type="float" value="-999.00"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="s"/>
 </variable>
 <variable name="wwm_15" shape="time node" type="float">
  <attribute name="long_name" value="peak_direction"/>
  <attribute name="standard_name"
   value="sea_surface_wave_from_direction_at_variance_spectral_density_maximum"/>
  <attribute name="missing_value" type="float" value="-999.00"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="degrees_east"/>
 </variable>
 <variable name="u" shape="time2 sigma node" type="float">
  <attribute name="standard_name" value="barotropic_eastward_sea_water_velocity"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="m/s"/>
 </variable>
 <variable name="v" shape="time2 sigma node" type="float">
  <attribute name="standard_name" value="barotropic_northward_sea_water_velocity"/>
  <attribute name="mesh" value="selfe_mesh"/>
  <attribute name="location" value="node"/>
  <attribute name="coordinates" value="y x"/>
  <attribute name="units" value="m/s"/>
 </variable>
 <aggregation type="union">
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_WaterLevel\.nc$"/>
   </aggregation>
  </netcdf>
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_SigWaveHeight\.nc$"/>
   </aggregation>
  </netcdf>
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_MeanPeriod\.nc$"/>
   </aggregation>
  </netcdf>
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_PeakPeriod\.nc$"/>
   </aggregation>
  </netcdf>
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_pwd\.nc$"/>
   </aggregation>
  </netcdf>
  <netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
   <dimension name="time2" orgName="time"/>
   <variable name="time2" orgName="time"/>
   <aggregation dimName="time" type="joinExisting">
    <scan
     location="/data/ftp/upload/Inundation/vims/selfe_tropical/runs/Rita/3D_varied_roughness_windstress_with_wave/output/"
     regExp=".*[0-9]{1}_hvel\.nc$"/>
   </aggregation>
  </netcdf>
 </aggregation>
</netcdf>