Requested from RT ticket (Binyu, https://rt.rap.ucar.edu/rt/Ticket/Display.html?id=97459)

Describe the New Feature

The sample file of RT-Ticket 97459 has (x,y) dimensions for coordinate variables (lat/lon/time) and MET's NCCF library does not support 2 dimensional time variable. The 2D time variable does not make sense for the gridded data. But it could be used for the point observation data (sparse point data). It would be nice to support 2D time variable for the point2grid.

Background assessment from helpdesk ticket:

From John:
Binyu,

I'm adding my response here and will then reassign this ticket to Howard Soh to see if he has any more ideas.

I downloaded your file to our anonymous ftp site:

ftp://ftp.rap.ucar.edu/incoming/irap/met_help/wang_data_20201116/VOLCAT_HIMAWARI-8_FLDK_s2020296_173000_v300250_VCB_w167_FLDK_b2020295_204000_g001.nc

I'm able to replicate the same error you're getting:

terminate called after throwing an instance of 'netCDF::exceptions::NcInvalidCoords'

After some testing I see that this is caused by the time variable having 2 dimensions: int time(y, x) ;

When reading 2-D gridded data, the code expects the time variable to be 1 dimensional. A file may contain multiple timesteps and a coordinate variable defines what those timesteps are. To sidestep this issue, I just removed the "time(y, x)" variable entirely. But then MET isn't able to read the projection info:

ERROR : NcCfFile::read_netcdf_grid() -> Couldn't figure out projection from information in netCDF file.

Looking closely at the latitude and longitude variables, I see that this really is not gridded data at all. Instead, it's a dense set of pixels, but it doesn't appear that the lat/lon's are evenly spaced or follow any sort of projection (e.g. Lambert Conformal, Mercator, PolarStereographic, and so on).

Since this is a dense collection of individual points, you'll still need to run the Point2Grid tool to place them onto a real grid. And that's the question to Howard. Is there anyway that Binyu could format this NetCDF file that would enable it to be read easily by the Point2Grid tool?

From Howard:
The point2grid handles a CF complaint NetCDF which has

1. 2D variables with lat/lon dimensions and the time variable with 1 value OR
2. 3D variables with time/lat/lon dimensions and the time variable with 1 dimension.

The point2grid should be enhanced to support the 2-dimensional time variable (if the enhancement is approved).

The sample NetCDF has two time offsets: 1603387817 and 1603387833 ("seconds since 1970-01-01 00:00:00"). One possible way without changing MET applications is generating two files (one file with time offset 1603387817 and the other file with time offset 1603387817).

netcdf1:
int time(1);
time = 1603387817
fill the missing value for the time 1603387833

netcdf2:
int time(1);
time = 1603387833
fill the missing value for the time 1603387817

Acceptance Testing

point2grid should process the file at /d3/personal/hsoh/data/RT-97459//VOLCAT_HIMAWARI-8_FLDK_s2020296_173000_v300250_VCB_w167_FLDK_b2020295_204000_g001.nc

Time Estimate

1 or 2 days

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Relevant Deadlines

NONE.

Funding Source

2791541

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