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# wrf_avo

Calculates absolute vorticity from WRF model output.

## Prototype

```	function wrf_avo (
u       : numeric,
v       : numeric,
msfu    : numeric,
msfv    : numeric,
msfm    : numeric,
cor     : numeric,
dx   : numeric,
dy   : numeric,
opt  : integer
)

return_val  :  float or double
```

## Arguments

u

X-wind component. An array whose rightmost three dimensions are bottom_top x south_north x west_east_stag.

v

Y-wind component. An array whose rightmost three dimensions are bottom_top x south_north_stag x west_east, and whose leftmost dimensions are the same as u's.

msfu

Map scale factor on u-grid. An array whose rightmost two dimensions are the same as u's. If it contains additional leftmost dimensions, they must be the same as the u and v arrays.

msfv

Map scale factor on v-grid. An array with the same number of dimensions as msfu, whose rightmost two dimensions are the same as v's. If it contains additional leftmost dimensions, they must be the same as the u and v arrays.

msfm

Map scale factor on mass grid. An array with the same number of dimensions as msfu and msfv, whose rightmost two dimensions are south_north x west_east. If it contains additional leftmost dimensions, they must be the same as the u and v arrays.

cor

Coriolis sine latitude term. An array of the same dimensionality as msfm.

dx

A scalar representing the grid spacing in X.

dy

A scalar representing the grid spacing in Y.

opt

An integer option, not in use yet. Set to 0.

## Return value

Absolute vorticity at each grid point. The rightmost three dimensions will be bottom_top x south_north x west_east, and the leftmost dimensions, if any, will be the same as u's. The array will contain the same named dimensions as u and v; otherwise, the rightmost two dimensions will be named "south_north" and "west_east". The type will be double if any of the input is double, and float otherwise.

## Description

This function returns absolute vorticity, which is the sum of the relative vorticity at each grid point and the Coriolis parameter at the latitude.

The return variable will contain two attributes:

return_val@description = "Absolute Vorticity"
return_val@units = "10-5 s-1"

See the full list of WRF functions.

## Examples

Note: for WRF variable names and their definitions, you can easily check them by using "ncl_filedump":

```  ncl_filedump wrfout_d01_2000-01-24_12:00:00.nc
```

Example 1

``` nc_file = addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")
U    = nc_file->U
V    = nc_file->V
MSFU = nc_file->MAPFAC_U
MSFV = nc_file->MAPFAC_V
MSFM = nc_file->MAPFAC_M
COR  = nc_file->F

DX = nc_file@DX
DY = nc_file@DY

avo = wrf_avo( U, V, MSFU, MSFV, MSFM, COR, DX, DY, 0)
```
Example 2

The function wrf_user_getvar (available in the \$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl script) can also be used to calculate many diagnostics in one step.

```  load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"