Cloud Nuclei Spectra and Updrafts beneath Convective Cloud Bases in the High Plains

Abstract

Air samples and updraft speeds were obtained simultaneously beneath the bases of convective clouds in northeastern Colorado and in South Dakota. Air samples were examined on the ground in a thermal diffusion nuclei chamber in the supersaturation range of 0.1?0.3% and the nuclei spectra determined. Using Twomey's technique, the maximum supersaturation was estimated, thereby estimating the number of cloud nuclei activated. The updraft speeds were plotted against the concentration of activated cloud nuclei for varying storm intensities. The results indicate the dominate variable, in identifying storm intensity, is updraft strength, with 4?5 m sec?1 being the cutoff point between thunderstorm and hailstorms. The data gave no clear indication that cloud nuclei concentrations played a significant role in identifying storm intensity. Since N ? V3k/(2k+4), where V is updraft strength, N number of activated nuclei and the values of k obtained from the analyzed air samples ranged from 0.4 to 2.9, it was determined that in the case of High Plains convection the resulting relation between activated nuclei and updraft strength is N ? V0.25 to V0.89.