Gridded NetCDF formats¶
Ocean circulation and meteorological models output their results in a number of different grid formats. The goal of GNOME is to work with the native output for each model, and to do appropriate interpolation on the native grid. This assures fidelity to the original model, particularly near the boundaries, which are critical to oil spill applications.
Standards Compliant netCDF:¶
GNOME is designed to be able to directly read netcdf files that conform to the following standards:
- The CF Metadata Conventions
These specify ways of storing data, standard names, etc, and are suitable for rectangular gridded model results.
- The Unstructured grid (UGRID) conventions
These specify additions to the CF conventions for model results on unstructured grids, in particular triangle meshes used in many finite element and finite volume models.
- The Staggered (SGRID) conventions
These specify additions to the CF conventions for model results on quadrilateral curvilinear grids (e.g. Arakawa grids) used in many finite difference and hybrid models.
If your input files conform to these conventions, and use the standard names that GNOME is expecting, they should “just work”. In particular, GNOME has been known to work with the output from the following models:
ROMS
POM
HYCOM
FVCOM
ADCRIC
NOAA NAM
NOAA GFS
NEMO
If your files are compliant with the above standards and don’t work with GNOME, it is a bug (or missing feature), and we would appreciate your letting us know via posting an issue on the PyGNOME gitHub project
Standard Names expected by GNOME:¶
It is best to use the CF standard names with files used with GNOME. However, there are a lot of files in the wild that do not use the standard names, so GNOME will also look for a certain set of common variable names used for various parameters.
Name Mapping:¶
grid_temperature
Default Names: water_t, temp
CF Standard Names: sea_water_temperature, sea_surface_temperature
grid_salinity
Default Names: salt
CF Standard Names: sea_water_salinity, sea_surface_salinity
grid_sediment
Default Names: sand_06
CF Standard Names:
ice_concentration
Default Names: ice_fraction, aice
CF Standard Names: sea_ice_area_fraction
bathymetry
Default Names: h
CF Standard Names: depth
grid_current
Default Names for u: u, U, water_u, curr_ucmp, u_surface, u_sur
Default Names for v: v, V, water_v, curr_vcmp, v_surface, v_sur
Default Names for w: w, W
CF Standard Names for u: eastward_sea_water_velocity, surface_eastward_sea_water_velocity
CF Standard Names for v: northward_sea_water_velocity, surface_northward_sea_water_velocity
CF Standard Names for w: upward_sea_water_velocity
grid_wind
Default Names for u: air_u, Air_U, air_ucmp, wind_u, u-component_of_wind_height_above_ground, u10
Default Names for v: air_v, Air_V, air_vcmp, wind_v, v-component_of_wind_height_above_ground, v10
CF Standard Names for u: eastward_wind, eastward wind
CF Standard Names for v: northward_wind, northward wind
ice_velocity
Default Names for u: ice_u, uice
Default Names for v: ice_v, vice
CF Standard Names for u: eastward_sea_ice_velocity
CF Standard Names for v: northward_sea_ice_velocity
Legacy netcdf¶
Early versions of GNOME supported netcdf before the above standards had been developed. As a result, there is a legacy “GNOME format” which is still supported by PyGNOME. It is better to use the above standards, but this should work.
GNOME can read in NetCDF files for rectangular, curvilinear, and triangular grids. This section includes examples of the three formats currently in use and some descriptions of the required information. Please note that the NetCDF formats described here are presently undergoing revision to conform to the newly forming Climate & Forecast unstructured grid data model, to be adopted in future releases of GNOME.
NetCDF Rectangular Grid¶
Below is an example of the regular grid format for NetCDF files. The global attribute grid_type = REGULAR is the default. Time units can be hours, minutes, seconds, or days. A separate map will be needed in order to set a spill.
NetCDF MacintoshHD:Desktop Folder:test {
dimensions:
lat = 16 ;
lon = 20 ;
time = UNLIMITED ; (85 currently)
variables:
double lat(lat) ;
lat:long_name = "Latitude" ;
lat:units = "degrees_north" ;
lat:point_spacing = "even" ;
double lon(lon) ;
lon:long_name = "Longitude" ;
lon:units = "degrees_east" ;
lon:point_spacing = "even" ;
double time(time) ;
time:long_name = "Valid Time" ;
time:units = "minutes since 1999-11-25 00:00:00" ;
float water_u(time, lat, lon) ;
water_u:long_name = "Eastward Water Velocity" ;
water_u:units = "m/s" ;
water_u:_FillValue = -9.9999e+32f ;
water_u:scale_factor = 1.f ;
water_u:add_offset = 0.f ;
float water_v(time, lat, lon) ;
water_v:long_name = "Northward Water Velocity" ;
water_v:units = "m/s" ;
water_v:_FillValue = -9.9999e+32f ;
water_v:scale_factor = 1.f ;
water_v:add_offset = 0.f ;
global attributes:
:grid_type = "REGULAR" ;
data:
lat = 51.144606, 51.234386, 51.324167, 51.413944, 51.503722, 51.5935,
51.683275, 51.77305, 51.862825, 51.952594, 52.042364, 52.132133, 52.2219,
52.311664, 52.401425, 52.491186 ;
lon = 2.3155722, 2.4583139, 2.6010833, 2.743875, 2.8866917, 3.0295306,
3.1723917, 3.3152694, 3.4581667, 3.6010833, 3.7440139, 3.8869583,
4.0299167, 4.1728861, 4.3158667, 4.4588583, 4.6018583, 4.7448639,
4.887875, 5.0308917 ;
time = 7020, 7080, 7140, 7200, 7260, 7320, 7380, 7440, 7500, 7560, 7620,
7680, 7740, 7800, 7860, 7920, 7980, 8040, 8100, 8160, 8220, 8280, 8340,
8400, 8460, 8520, 8580, 8640, 8700, 8760, 8820, 8880, 8940, 9000, 9060,
9120, 9180, 9240, 9300, 9360, 9420, 9480, 9540, 9600, 9660, 9720, 9780,
9840, 9900, 9960, 10020, 10080, 10140, 10200, 10260, 10320, 10380, 10440,
10500, 10560, 10620, 10680, 10740, 10800, 10860, 10920, 10980, 11040,
11100, 11160, 11220, 11280, 11340, 11400, 11460, 11520, 11580, 11640,
11700, 11760, 11820, 11880, 11940, 12000, 12060 ;
NetCDF Curvilinear Grid¶
Below is an example of the curvilinear format for NetCDF files. The global attribute grid_type = CURVILINEAR is required (the default is grid_type = REGULAR). In addition to x and y, there are several other dimension name options for latitude and longitude. The dimension names only need to start with X, Y or LAT, LON to be recognized. The variable names must appear as shown. The velocities can be short, float, or double precision numbers. Time units can be hours, minutes, seconds, or days. The land-mask is required if you want to use the grid boundary as the shoreline: 0 is land, 1 is water. If no map is available, the mask is used to identify land points (land = 0, water = 1) and a boundary map is created. The first sigma value is used, though currently GNOME is being extended to handle 3-D currents. The topology can be saved out the first time and reloaded.
netcdf 20040726_11z_HAZMAT {
dimensions:
x = 73 ;
y = 163 ;
sigma = 3 ; optional
time = UNLIMITED ; (12 currently)
variables:
float time(time) ;
time:long_name = "Time" ;
time:base_date = 2004, 1, 1, 0 ;
time:units = "days since 2004-01-01 0:00:00 00:00" ;
time:standard_name = "time" ;
float lon(y, x) ;
lon:long_name = "Longitude" ;
lon:units = "degrees_east" ;
lon:standard_name = "longitude" ;
float lat(y, x) ;
lat:long_name = "Latitude" ;
lat:units = "degrees_north" ;
lat:standard_name = "latitude" ;
float mask(y, x) ;
mask:long_name = "Land Mask" ;
mask:units = "nondimensional" ;
float depth(y, x) ; optional
depth:long_name = "Bathymetry" ;
depth:units = "meters" ;
depth:positive = "down" ;
depth:standard_name = "depth" ;
float sigma(sigma) ; optional
sigma:long_name = "Sigma Stretched Vertical Coordinate at Nodes" ;
sigma:units = "sigma_level" ;
sigma:positive = "down" ;
sigma:standard_name = "ocean_sigma_coordinate" ;
sigma:formula_terms = "sigma: sigma eta: zeta depth: depth" ;
float u(time, sigma, y, x) ;
u:long_name = "Eastward Water Velocity" ;
u:units = "m/s" ;
u:missing_value = -99999.f ;
u:_FillValue = -99999.f ;
u:standard_name = "eastward_sea_water_velocity" ;
float v(time, sigma, y, x) ;
v:long_name = "Northward Water Velocity" ;
v:units = "m/s" ;
v:missing_value = -99999.f ;
v:_FillValue = -99999.f ;
v:standard_name = "northward_sea_water_velocity" ;
global attributes:
:file_type = "Full_Grid" ;
:Conventions = "COARDS" ;
:grid_type = "curvilinear" ;
:z_type = "sigma" ;
:model = "POM" ;
:title = "Forecast: wind+tide+river" ;
data:
time = 208.4688, 208.4792, 208.4896, 208.5, 208.5104, 208.5208, 208.5312,
208.5417, 208.5521, 208.5625, 208.5729, 208.5833,,;
sigma = 0, .5, 1.;
}
NetCDF Triangular Grid¶
Example – Triangular Grid Format with Velocities on the Nodes
Below is an example of the triangular grid format for NetCDF files with velocities on the nodes. The global attribute grid_type = TRIANGULAR is required (the default is grid_type = REGULAR). The first depth value is used. Time units can be hours, minutes, seconds, or days. A map will be created using the boundary data. The topology can be saved out the first time and reloaded. The NetCDF header description for finite element model:
NetCDF MacintoshHD:Desktop Folder:testFile {
dimensions:
node = 7258 ;
nele = 13044 ; not currently used
nbnd = 1476 ;
nbi = 4 ;
sigma = 11 ; optional
time = UNLIMITED ; (12 currently)
variables:
short bnd(nbnd, nbi) ;
bnd:long_name = "Boundary Segment Node List" ;
bnd:units = "index_start_1" ;
float time(time) ;
time:long_name = "Time" ;
time:units = "days since 2003-01-00 0:00:00 00:00" ;
time:base_date = 2003, 1, 0, 0 ;
float lon(node) ;
lon:long_name = "Longitude" ;
lon:units = "degrees_east" ;
float lat(node) ;
lat:long_name = "Latitude" ;
lat:units = "degrees_north" ;
float sigma(sigma) ; optional
sigma:long_name = "Stretched Vertical Coordinate" ;
sigma:units = "sigma_level" ;
sigma:positive = "down" ;
float u(time, sigma, node) ;
u:long_name = "Eastward Water Velocity" ;
u:units = "m/s" ;
u:missing_value = -99999.f ;
u:_FillValue = -99999.f ;
float v(time, sigma, node) ;
v:long_name = "Northward Water Velocity" ;
v:units = "m/s" ;
v:missing_value = -99999.f ;
v:_FillValue = -99999.f ;
global attributes:
:file_type = "FEM" ;
:Conventions = "COARDS" ;
:grid_type = "Triangular" ;
data:
time = 26.95833, 27, 27.04167, 27.08333, 27.125, 27.16667, 27.20833, 27.25,
27.29167, 27.33333, 27.375, 27.41667 ;
sigma = 1, 0.9807215, 0.9306101, 0.83061, 0.6807215, 0.5, 0.3192785,
0.1693899, 0.06938996, 0.01927857, 0 ;
}
Notes:
1. The boundary list is an array of dimension bnd(nbnd, 4). It consists of node numbers of the line segments, with a digit to indicate which land or island the segment is a part of, and a digit to indicate whether a boundary is land or water:
node1 node2 island land/water (0/1)
1 2 0 0 1 is usually the continent and outer water BC
2 5 0 0
5 23 0 1
…
3568 1 0 1 The last segment joins up with the first.
551 552 1 0 next island
552 567 1 0
…
677 551 1 0
789 388 2 0
… next island, etc.
2. Only the first sigma level is used, although GNOME is currently being extended to handle 3-D currents.
1.2.2.3.2 Example – Triangular Grid Format with Velocities on the Triangles
Following is an example of the triangular grid format for NetCDF files with velocities on the triangles. The global attribute grid_type = TRIANGULAR is required (the default is grid_type = REGULAR). The first depth value is used. Time units can be hours, minutes, seconds, or days. A map will be created using the boundary data. The topology must be included in the file.
netcdf FVCOM_example {
dimensions:
node = 32649 ;
nele = 60213 ;
nbnd = 5099 ;
nbi = 4 ;
time = UNLIMITED ; // (1 currently)
three = 3 ;
variables:
int bnd(nbnd, nbi) ;
float time(time) ;
time:units = "days since 1978-11-17 00:00:00 0:00" ;
time:long_name = "time" ;
time:time_zone = "UTC" ;
time:format = "modified julian day (MJD)" ;
float lon(node) ;
float lat(node) ;
float u(time, nele) ;
u:units = "meters s-1" ;
u:long_name = "Eastward Water Velocity" ;
u:grid = "fvcom_grid" ;
u:type = "data" ;
float v(time, nele) ;
v:units = "meters s-1" ;
v:long_name = "Northward Water Velocity" ;
v:grid = "fvcom_grid" ;
v:type = "data" ;
int nbe(three, nele) ;
int nv(three, nele) ;
// global attributes:
:grid_type = "Triangular" ;
data:
time = 11452 ;
}
Notes:
The boundary list is an array of dimension bnd(nbnd, 4), same as above.
The triangle vertices are contained in nv and the neighboring triangles in nbe.
Data in Multiple NetCDF Files: When Your NetCDF Files Start To Get Too Big
Note
This approach is deprecated – in current versions, you can simply pass the list of filenames to PyGNOME. [How is it done in WebGNOME?]
Longer simulations require more model data, and that can cause problems with putting the entire time-series into one data file. GNOME allows you to break the time-series into separate files using a master file to identify all the pieces of the time-series in order. This also makes possible using a series of nowcasts and forecasts strung together to make a times-series. This technique worked well during the 2002 T/V Prestige incident in Spain.
First create a text master file with the list of file path-names (relative to the GNOME directory) in order. Next supply the full path name if the files are not in the same directory as GNOME, or in a subdirectory. The file will also need a header line, “NetCDF Files”. When you go to load the currents in GNOME, load your master file (e.g.,
Example 1 – Filename: MyMasterFileEx.txt
NetCDF Files
[FILE] :day1.nc
[FILE] :day2.nc
[FILE] :day3.nc
[FILE] :day4.nc
[FILE] :day5.nc
[FILE] :day6.nc