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wrf_helicity

Calculates storm relative helicity from WRF model output.

Available in version 5.2.0 and later.

Prototype

	function wrf_helicity (
		UA      : numeric,  
		VA      : numeric,  
		Z       : numeric,  
		TER     : numeric,  
		top [1] : numeric   
	)

	return_val [dimsizes(TER)] :  float or double

Arguments

UA
VA

U,V components. Arrays of the same dimensionality whose rightmost three dimensions are bottom_top x south_north x west_east.

Geopotential height in [m] with the same dimensionality as UA and VA. It must be on the ARW WRF unstaggered grid. The rightmost three dimensions must be level bottom_top x lat (south_north) x lon (west_east).

TER

Terrain height. An array with the same leftmost two dimensions as UA and VA, and rightmost two dimensions are south_north x west_east. Units must be meters [m].

top

A scalar value that represents the height of layer below which helicity is calculated (meters above ground level).

Return value

Returns the storm relative helicity at each grid point. The multi-dimensional array will contain the same size and named dimensions as TER. If TER does not contain any named dimensions, 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 calculates storm relative helicity from WRF ARW output. SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear "boundaries" between non-tornadic and significant tornadic supercells.

For more information:

http://w1.spc.woc.noaa.gov/exper/mesoanalysis/help/help_srh1.html

The return variable will contain two attributes:

return_val@description = "Storm Relative 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

  ua     = wrf_user_getvar(a,"ua",4)
  va     = wrf_user_getvar(a,"va",4)
  geopt  = wrf_user_getvar(a,"z",4)
  ter    = wrf_user_getvar(a,"ter",4)

  ua1    = ua(::-1,:,:)
  va1    = va(::-1,:,:)
  geopt1 = geopt(::-1,:,:)

  sreh = wrf_helicity(ua1, va1, geopt1, ter, 3000.)