The role of small aerosols in the microphysics and precipitation of deep maritime cumulus clouds

Description

Some observational evidence such as bi-modal size distributions, comparatively high concentrations of supercooled drops at upper levels, high concentrations of small ice crystals in cloud anvils leading to high optical depth, as well as lightning that can take place in the eye walls of hurricanes, indicate that the traditional view of microphysics of deep maritime clouds requires, possibly, some revisions. Numerical simulations using the spectral microphysics Hebrew University cloud model show that these observed features can be attributed to existence in the atmosphere of small aerosols with diameters less than about 0.05 microns in the cloud condensational nuclei (CCN) size spectra. Intense drop collisions below and around the freezing level lead to a dramatic decrease in droplet concentration during ascent in convective updrafts. Being accompanied by intense vertical velocity, this decrease in the drop concentration leads to a dramatic increase in supersaturation, activation of the smallest CCN aerosols, and production of new droplets several kilometers above cloud base. The increase in supersaturation and in-cloud nucleation during ascent can also be partly related to a decrease in droplet concentration by riming at cold temperatures. Successive growth of these droplets leads to formation of supercooled water and significant ice crystal concentrations aloft. The role of giant CCN is reconsidered. The synergetic effect of the smallest CCN and giant CCN in production of small supercooled water and ice crystals in cloud anvils is analyzed. Significant effects from small aerosols on various microphysical parameters of clouds such as precipitation, cloud updrafts, and radar reflectivity were found. The possible role of these small aerosols in formation of conditions favorable for lightning in deep maritime clouds is discussed. As far as we are aware, this is the first study in the literature to chart the diverse impacts on the physics and dynamics of maritime deep convection from such soluble aerosols smaller than 0.05 microns.