NCL Home > Documentation > Functions > Regridding, Interpolation

rcm2rgrid

Interpolates data on a curvilinear grid (i.e. RCM, WRF, NARR) to a rectilinear grid.

Prototype

```	function rcm2rgrid (
lat2d [*][*] : numeric,
lon2d [*][*] : numeric,
fi           : numeric,
lat      [*] : numeric,
lon      [*] : numeric,
Option       : numeric
)

return_val  :  numeric
```

Arguments

lat2d

A two-dimensional array that specifies the latitudes locations of fi. Because this array is two-dimensional it is not an associated coordinate variable of fi. The latitude order must be south-to-north.

lon2d

A two-dimensional array that specifies the longitude locations of fi. Because this array is two-dimensional it is not an associated coordinate variable of fi. The longitude order must be west-to-east.

fi

A multi-dimensional array to be interpolated. The rightmost two dimensions (latitude, longitude) are the dimensions to be interpolated.

lat

A one-dimensional array that specifies the latitude coordinates of the regular grid. Must be monotonically increasing.

lon

A one-dimensional array that specifies the longitude coordinates of the regular grid. Must be monotonically increasing.

Option

Reserved for future use. Currently not used. Set to 1.

Return value

A multi-dimensional array of the same size as fi except that the rightmost dimension sizes have been replaced by the sizes of lat and lon respectively. Double if fi is double, otherwise float.

Description

Interpolates RCM (Regional Climate Model), WRF (Weather Research and Forecasting) and NARR (North American Regional Reanalysis) grids to a rectilinear grid. Actually, this function will interpolate most grids that use curvilinear latitude/longitude grids. No extrapolation is performed beyond the range of the input coordinates. Missing values are allowed but ignored.

The weighting method used is simple inverse distance squared. Missing values are allowed but ignored.

The code searches the input curvilinear grid latitudes and longitudes for the four grid points that surround a specified output grid coordinate. Because one or more of these input points could contain missing values (@_FillValue), fewer than four points could be used in the interpolation.

Curvilinear grids which have two-dimensional latitude and longitude coordinate axes present some issues because the coordinates are not necessarily monotonically increasing. The simple search algorithm used by rcm2rgrid is not capable of handling all cases. The result is that, sometimes, there are small gaps in the interpolated grids. Beginning with NCL version 6.0.0, any interior points not interpolated in the initial interpolation pass will be filled using linear interpolation. In some cases, edge points may not be filled.

Use the rcm2rgrid_Wrap function if metadata retention is desired. The interface is identical.

Examples

Example 1

Interpolate to a gaussian T42 (64,128) grid.

```   lat = latGau(64)       ; create gaussian latitudes or read from file
lon = fspan(0,359,128) ; create longitudes or read from file

lat2d = f->xlat                         ; size = (nlat,nlon)
lon2d = f->xlon                         ; size = (nlat,nlon)
x     = f->X

xgrd  = rcm2rgrid(lat2d,lon2d,x,lat,lon,0)

; Use rcm2rgrid_Wrap if metadata retention is desired
; xgrd  = rcm2rgrid_Wrap(lat2d,lon2d,x,lat,lon,0)
```

If x was of size:

```            x(nlat,nlon) ==>  xgrd(64,128)
x(ntim,nlat,nlon) ==>  xgrd(ntim,64,128)
x(ntim,klev,nlat,nlon) ==>  xgrd(ntim,klev,64,128)
```

Example 2

If the latitudes are ordered north-to-south, reorder the coordinates and the associated data.

```   precip = f->precip         ; (time,lat,lon)
lat2d  = f->xlat           ; lat order is S->N
lon2d  = f->xlon

lat2d  = (/ lat2d(::-1,:) /)       ; rearrange values only
precip = (/ precip(:,::-1,:) /)
```