gradsf

Computes the gradient of an array that is on a fixed grid using spherical harmonics.

Prototype

	procedure gradsf (
		z    : numeric,          
		gzx  : float or double,  
		gzy  : float or double   
	)

Arguments

array to compute gradient of (input, two or more dimensions, rightmost two dimensions must be nlat x nlon)

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

gradient arrays (output, same dimensions as z, values will be in ascending latitude order)

Description

gradsf computes the gradient given an array z, and returns the results in the arrays gzx and gzy. gradsf operates on a fixed grid. If the input array z is on a gaussian grid, gradsg should be used.

This procedure does not handle missing values, and the input array z must be on a global grid. 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: The underlying Spherepack routines use radians. Thus, for say, temperature (T, units K), the Spherepack units would be dT/dtheta => K/radian. The interface scales these values by the radius of the earth (R=6.37122e06 meters) which is equivalent to one radian: (K/radian)*(one_radian/R)=>(K/meter).

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

See Also

gradsg, igradsf, igradsg, igradsG, igradsF, lderuvg, lderuvf

Examples

Example 1

Given a scalar array T, compute the latitudinal and longitudinal derivatives, and then recompute T. T is on a fixed grid.

  T_grad_lon = T                ; create arrays to hold output, same size and type as input
  T_grad_lat = T                
                                ; this procedure will overwrite
                                ; values in T_grad_lon and T_grad_lat
                                
  gradsf (T, T_grad_lon, T_grad_lat)   
  T_grad_lon@long_name = "longitudinal gradient (derivative)"
  T_grad_lat@long_name = "latitudinal gradient (derivative)"
  igradsf (T_grad_lon, T_grad_lat, T)  

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)