The Influence of Helicity on Numerically Simulated Convective Storms

Abstract

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 conducive to the development of long-lived, strongly rotating storms. Eleven numerical simulations are made in which the depth and turning angle of the ambient vertical shear vector are varied systematically while maintaining a constant magnitude of the shear in the shear layer. In this manner, an attempt is made to isolate the effects of different environmental Felicities on storm morphology and show that the SREH and bulk Richardson number, rather than the mean shear in the low levels, determine the rotational characteristics and morphology of deep convection. 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 vertical vorticity? and vertical velocity w, which (according to linear theory of dry convection) should be proportional to the product of the normalized helicity density, NHD (i.e., relative helicity), and a function involving the storm-relative wind speed, has the largest peak values (in time) in those simulated storms exhibiting large SREH and strong storm-relative winds in the low levels. Even when the vorticity is predominantly streamwise in the storm-relative framework, giving a normalized helicity density near unity (as is the case in many of these simulations), significant updraft rotation and large w??correlation coefficients do not develop and persist unless the storm-relative winds are sufficiently strong. The correlation coefficient between w and ? 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 better than the 0-3-km SREH. Both the theoretical correlation coefficient and the SREH are based on the motion of the initial storm after its initially rapid growth. Linear theory also predicts correctly the relative locations of the buoyancy, vertical velocity, and vertical vorticity extrema within the storms after allowance is made for the effects of vertical advection. In predicting the maximum vertical vorticity both above and below 1.14 km, rather than the actual w and ?correlation, the 0?3-km SREH performs slightly worse than the BRN. The correlation coefficient, SREH, and BRN all do a credible job of predicting storm type. Thus, it is recommended that operational forecasters use the BRN to predict storm type because it is independent of storm motion, and the SREH to characterize the rotational properties of storms once their motions can be established. Finally, the ability of the NHD to characterize storm type and rotational properties is examined. Computed using the storm-relative winds, the NHD shows little ability to predict storm rotation (i.e., maximum w-?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.