Lost Coordinate Variables and Attributes

The use of NCL built-in functions strips away the original data's meta data such as coordinate variables and attributes.

There are several ways you can restore this information

Use a copy function

Some useful functions for copying attributes include: copyatt , copy_VarCoords , copy_VarAtts.

Each of these functions has a slightly different effect, so please see the documentation to determine which function is best for you. To access in a script, be sure to load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"

    load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
    begin
       f    = addfile ("dummy.nc", "r")                 ; pointer to file
       x    = f->X                                      ; read in data
       xZon = dim_avg (x)                               ; zonal average
       copyatt(xZon, x)                                 ; copy meta data
       xZon@long_name = x@long_name + ": Zonal Average" ; change attribute
    end

Use a Wrapper function

Dennis Shea has created numerous wrappers to many popular NCL functions. Retrieval and reassignment of the meta data is done within the wrapper.

    load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
    begin
       f    = addfile ("dummy.nc", "r")  
       x    = f->X 
       xZon = dim_avg_Wrap(x)
    end

Use DIMENSION REDUCTION/collapsing

This can be very useful but does require some understanding of NCL dimensioning and variable-to-variable transfer.

    begin
       f    = addfile ("dummyI.nc", "r")  
       x    = f->X 
       xZon = x(:,:,:,0)          ; xZon is 3D: "x" is collapsed
       xZon = dim_avg (x)         ; compute zonal average
    end

The statement "xZon = x(:,:,:,0)" copies the "time", "lev", "lat" coordinate variables and all attributes from x to xZon. At this point, xZon is merely a subset of x. IMPORTANTLY, the values at the lon subscript corresponding to 0 will also be copied. Since NCL knows it is collapsing the longitude array, it automatically creates an additional variable attribute to let the user know this. e.g. "xZon@lon = lon(0)". This is very useful if you are doing this on purpose, but since this attribute is an artifact of the "trick" we are employing, we just delete it, so it doesn't confuse future or present users of the data.