# 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 [1] : numeric, dy [1] : numeric, opt [1] : 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"

Questions on this function should be sent to wrfhelp@ucar.edu.

## See Also

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.