# 6. Regridding¶

Users that wish to interpolate their unipost output to a different grid may do so with the *wgrib2*
utility. The general format for re-gridding to various common projections are outlined in the following
examples.

*Wgrib2* is a versatile program that has the ability to convert grib2 files from one grid to another
for various user-defined grids as well as pre-defined NCEP grids. Complete documentation with examples
of re-gridding for all available grid definitions can be found at:

http://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/new_grid.html

## 6.1. Examples of wgrib2¶

**Example 1: Latitude-Longitude Grid**

*-new_grid latlon lon0:nlon:dlon lat0:nlat:dlat outfile*

Variable |
Description |
---|---|

lon0 |
Longitude of first grid point in degrees |

nlon |
Number of longitudes |

dlon |
Grid resolution in degrees of longitude |

lat0 |
Latitude of first grid point in degrees |

nlat |
Number of latitudes |

dlat |
Grid resolution in degrees of latitude |

**Example 2: Lambert Conic Conformal Grid**

*-new_grid lambert:lov:latin1:latin2 lon0:nx:dx lat0:ny:dy outfile*

Variable |
Description |
---|---|

lov |
Longitude where y axis is parallel to meridian in degrees |

latin1 |
First latitude from pole which cuts the secant cone in degrees |

latin2 |
Second latitude from pole which cuts the secant cone in degrees |

lon0 |
Longitude of the first grid point in degrees |

nx |
Total number of grid points along x |

dx |
Grid cell size in meters in x direction |

lat0 |
Latitude of the first grid point in degrees |

ny |
Total number of grid points along y |

dy |
Grid cell size in meters in y direction |

**Example 3: Polar Stereographic Grid**

*-new_grid nps(or SPS):lov:lad lon0:nx:dx lat0:ny:dy outfile*

Variable |
Description |
---|---|

nps/sps |
North/south polar stereographic |

lov |
Longitude where y axis is parallel to meridian in degrees |

lad |
Latitude where dx and dy are specified |

lon0 |
Longitude of the first grid point in degrees |

nx |
Total number of grid points along x |

dx |
Grid cell distance in meters in x direction at lad |

lat0 |
Latitude of the first grid point in degrees |

ny |
Total number of grid points along y |

dy |
Grid cell distance in meters in y direction at lad |

**Winds**

*-new_grid_winds grid(or earth)*

Variable |
Description |
---|---|

grid |
U-wind goes from grid (i,J) to (i+1,j) |

earth |
U-wind goes eastward, V-wind goes northward |

**Interpolation**

The default interpolation type is bilinear, but it can be set to another type (e.g. neighbor, budget).

*-new_grid_interpolation type*

**Operational Example**

Interpolates to a 0.25 degree latitude-longitude grid using various interpolation types depending on the variable.

```
wgrib2 infile -set_grib_type same -new_grid_winds earth |
-new_grid_interpolation bilinear |
-if ":(CRAIN|CICEP|CFRZR|CSNOW|ICSEV):" -new_grid_interpolation neighbor -fi |
-set_bitmap 1 -set_grib_max_bits 16 |
-if ":(APCP|ACPCP|PRATE|CPRAT):" -set_grib_max_bits 25 -fi |
-if ":(APCP|ACPCP|PRATE|CPRAT|DZDT):" -new_grid_interpolation budget -if |
-new_grid "latlon 0:1440:0.25 90:721:-0.25" outfile
```

**Note:** *wgrib2 is not distributed within the UFS weather application. Users may download and install
from http://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/.*