The influence of helicity on numerically simulated convective storms

A three-dimensional numerical cloud model is used to investigate the influence of storm-relative environmental helicity (SREH) on convective storm structure and evolution, with a particular emphasis on the identification of ambient shear profiles that are conductive to the development of long-lived, strongly rotating storms. The results demonstrate that storms forming in environments characterized by large SREH are longer-lived than those in less helical surroundings. Further, it appears that the storm-relative winds in the layer 0-3 km must, on average, exceed 10 m s^-1 over most of the lifetime of a convective event to obtain supercell storms. The correlation coefficient between w and ζ based on linear theory is found to be a significantly better predictor of net updraft rotation than the bulk Richardson number (BRN) or the BRN shear, and slightly between than the 0-3-km SREH. Computed using the storm-relative winds, the NHD shows little ability to predict storm rotation (i.e., maximum w-ζ correlation and maximum vertical vorticity), because it neglects the magnitudes of the vorticity and storm-relative wind vectors. Histograms of the disturbance NHD show a distinct bias toward positive values near unity for supercell storms, indicating an extraction of helicity from the mean flow by the disturbance, and only a slight bias for multicell storms. -from Authors