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NCL Graphics: WRF Helicity

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.

In V5.2.0, the wrf_helicity function was added to calculate this quantity.

wrf_helicity_1.ncl: This script shows how to calculate helicity and create a smoothed contour plot over a map.

Dr. Craig Mattocks of the Center for Environmental Modeling for Policy Development, UNC-Chapel Hill contributed this example.