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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  [1] : numeric,  
		dy  [1] : numeric,  
		opt [1] : logical   
	)

	return_val  :  float or double

Arguments

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.

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"

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)