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# lapvg

Computes the vector Laplacian using spherical harmonics, given a vector quantity (u,v) on a gaussian grid.

## Prototype

```	procedure lapvg (
u     : numeric,
v     : numeric,
ulap  : float or double,
vlap  : float or double
)
```

## Arguments

u
v

vector field arrays (input, two or more dimensions, last two dimensions must be nlat x nlon)

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

ulap
vlap

vector Laplacian arrays (output, same dimensions as u, v, values will be in ascending latitude order)

## Description

Given vector field u and v, lapvg computes the Laplacian vector and returns the results in the arrays ulap and vlap. lapvg operates on a gaussian grid. If the input arrays u and v are on a fixed grid, lapvf should be used.

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 grids will be set to the missing value defined by the output grids' _FillValue attributes.

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
```

## Examples

Example 1

Read u and v (on a gaussian grid) from a netCDF file and compute the vector Laplacian:

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
begin
v200 = a->V(0,{189.},:,:)			; v200 is dimensioned nlat x nlon
u200 = a->U(0,{189.},:,:)			; u200 is dimensioned nlat x nlon
printVarSummary(v200)
lapl_v = v200				; preallocate space for the returned array
lapl_u = u200				; preallocate space for the returned array
lapvg(u200,v200,lapl_u,lapl_v)
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)