Simulation of aerosol effects on precipitation from green-ocean, smoky and pyro-clouds using a spectral microphysics cloud model with a precise calculation of supersaturation and diffusion growth

It is a usual practice in cloud models which detailed microphysics to perform nucleation of cloud condensational nuclei (CCN) just after dynamical time steps, when supersaturation in the updrafts attains its maximum values. This leads to an overestimation of the droplet concentration especially in clouds growing in a dirty air, for instance, in the zones of biomass burning. Besides, the droplet concentration turns out to be highly dependent on model time steps. At the same time microphysical processes take place simultaneously, so that the diffusion growth does not allow supersaturation to reach too large values. To avoid the uncertainty, a new method of the supersaturation calculation has been developed. According to this method the supersaturation values with respect to water and ice are calculated analytically by solving equations, in which both dynamic tendency and diffusion growth are taken into account simultaneously. This scheme is much less sensitive to the model time step than the previous one. To decrease numerical broadening the new scheme of remapping has been developed that conserves first (concentration), third (mass) and sixth (radar reflectivity) moments of the droplet and ice size distributions. These schemes have been implemented into a spectral (bin) microphysics cloud model of the Hebrew University of Jerusalem. The approach has been used for simulation of deep maritime and continental clouds, as well as pyro-clouds observed in the Amazon region during the SMOCC experiment. Results show that the new approach allows reproduction of observed droplet size distributions. Dynamics and microphysics of green-ocean, smoky and pyro-clouds is analyzed. It is shown that aerosols lead to a delay in the precipitation formation and to a decrease in precipitation efficiency (defined as a ratio of precipitation amount to mass of hydrometeors formed). At the same time, accumulated rain amount from smoky and pyro-clouds can be quite significant. Mechanisms leading to the precipitation formation from clouds with very high droplet concentrations are discussed.