A study of bimodal droplet size distributions in stratiform clouds

Presented here are the results of studying droplet size distributions in stratiform clouds (St, Sc, As, Ac, Ns) over the USSR obtained with a FSSP-100 (PMS). The bimodal droplet spectra turned out to be common phenomena in stratifonn clouds, and on average, they occupy about 14% of cloud space. Their occurrence practically does not depend on droplet concentration, cloud temperature, height or presence of ice crystals. Classifications of bimodal spectra shapes and their spatial evolution are suggested. Statistical distributions of difference of modal diameters, mode ratio and lengths of cloud volumes with bimodal spectra are presented. The obtained experimental results made it possible to formulate some hypotheses of mechanisms of bimodal spectra formation. The first mechanism is mixing of unimodal spectra with different modal diameters. The formation of droplet spectra with different modal diameters at the same cloud level may be caused by vertical (upward or downward) motions of individual volumes within non-adiabatic cloud layer. Another mechanism is induced by activation of the interstitial condensation nuclei. Analytical calculations and numerical modelling of droplet spectra evolution confirm the suggested assumptions. The formation of bimodal droplet spectra in stratiform clouds is closely connected with turbulence.