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# 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

z

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"
begin
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:

1 : error in the specification of nlat
2 : error in the specification of nlon
4 : error in the specification of N (jer only)