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# wrf_updraft_helicity

Calculates updraft helicity from WRF model output. Available in version 5.2.1 and later.

## Prototype

```	function wrf_updraft_helicity (
ZP      : numeric,
MAPFCT  : numeric,
US      : numeric,
VS      : numeric,
W       : numeric,
dx   : numeric,
dy   : numeric,
opt  : logical
)

return_val  :  float or double
```

## Arguments

ZP

Geopotential (m-2/s-2). An array of at least three dimensions, whose rightmost three dimensions are bottom_top_stag x south_north x west_east.

MAPFCT

The map scale factor on the mass grid. An array of at least two dimensions, whose rightmost two dimensions must be south_north x west_east. If this array is more than two dimensions, they must be the same as zp's leftmost dimensions.

US
VS

U, V wind components (m/s). Arrays of at least three dimensions, whose rightmost three dimensions must be bottom_top x south_north x west_east. The leftmost dimensions must be the same as zp's leftmost dimensions.

W

Z wind component (m/s). An array of the same dimensionality as zp.

dx
dy

Scalars that represent the distance between grid points.

opt

A logical scalar. If set to True, then you can additionally set the attributes uhmnhgt or uhmxhgt. These values will default to 2000 and 5000 if not set by the user.

## Return value

Returns the updraft helicity at each grid point. The return array will have rightmost dimensions of south_north x west_east and the same leftmost dimensions (if any) as zp. The type will be double if any of the input is double, and float otherwise.

## Description

This function calculates updraft helicity from WRF ARW output to detect rotating updrafts.

The formula follows Kain et al., 2008, Wea. and Forecasting, 931-952, but this version has controls for the limits of integration, uhminhgt to uhmxhgt, in m AGL. Kain et al used 2000 to 5000 m. The expected range is 25 to 250 m-2/s-2.

Keith Brewster, CAPS/Univ. of Oklahoma ; March, 2010

The return variable will contain two attributes:

return_val@description = "Updraft Helicity"
return_val@units = "m-2/s-2"

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

```  a = addfile("wrfout.nc","r")

mapfct = wrf_user_getvar(a, "MAPFAC_M", 0)   ; Map projection scale factor
us     = wrf_user_getvar(a,"ua",-1)          ; u averaged to mass points
vs     = wrf_user_getvar(a,"va",-1)          ; v averaged to mass points
w      = wrf_user_getvar(a, "W",-1)          ; vertical velocity
ph     = wrf_user_getvar(a,"PH",-1)
phb    = wrf_user_getvar(a,"PHB",-1)
zp     = ph + phb

;
; Compute updraft helicity. uhmnhgt and uhmxhgt will
; default to 2000 and 5000.
;
uh = wrf_updraft_helicity(zp, mapfct, us, vs, w, dx, dy, 0)
```

Example 2

Same as above example, except change the defaults for uhmnhgt and uhmxhgt:

```  opt         = True
opt@uhmnhgt = 2500.
opt@uhmxhgt = 6000.
uh = wrf_updraft_helicity(zp, mapfct, us, vs, w, dx, dy, opt)
```

For more examples, see the WRF Helicity applications page.