NCL Home > Documentation > Functions > Spherical harmonic routines

lapsg

Computes the Laplacian using spherical harmonics, given a scalar z on a gaussian grid.

Prototype

	procedure lapsg (
		z     : numeric,          
		zlap  : float or double   
	)

Arguments

scalar function array (input, two or more dimensions, last two dimensions must be nlat x nlon)

input values must be in ascending latitude order
input array must be on a global grid

zlap

Laplacian array (output, same dimensions as z, values will be in ascending latitude order)

Description

lapsg computes the Laplacian z and returns it in the array zlap. lapsg operates on a gaussian grid.

This procedure does not handle missing values. If any missing values are encountered in a particular 2D input grid, then all of the values in the corresponding output grid will be set to the missing value defined by the output grid's _FillValue attribute.

Note: For the arrays whose last two dimensions are nlat x nlon, the rest of the dimensions (if any) are collectively referred to as N. If the input/output arrays are just two dimensions, then N can either be considered equal to 1 or nothing at all.

Arrays which have dimensions N x nlat x nlon should not include the cyclic (wraparound) points when invoking the procedures and functions which use spherical harmonics (Spherepack).

For example, if an array x has dimensions nlat = 64 and nlon = 129, where the "129" represents the cyclic points, then the user should pass the data to the procedure/function via:

    z = sample ( x([...],:,0:nlon-2) )  ; does not include cyclic points

If the input array z is on a fixed grid, lapsf should be used. Also, note that lapsg is the procedural version of lapsG.

Examples

Example 1

Read Z (on a gaussian grid) from a netCDF file and compute the Laplacian:

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
begin
   a = addfile("/cgd/cas/shea/MURPHYS/ATMOS/80.nc","r")
   z200 = a->Z(0,{189.},:,:)			; z200 is dimensioned nlat x nlon
   printVarSummary(z200)
   lapl = z200					; preallocate space for the returned array
   lapsg(z200,lapl)
end

Errors

If jer or ker is equal to: