The effects of giant CCN and Ice Nuclei on clouds and precipitation: A case study from the Saudi Arabia program for the assessment of rainfall augmentation

One of the most important challenges in current cloud physics research is to evaluate and quantify the impact of different types of aerosols on the formation of precipitation. The eastern Mediterranean and Arabian Penisula area is an excellent natural laboratory for studying the interactions between clouds and aerosols because this region is affected by a wide variety of aerosol sources. These include air masses from different sources containing anthropogenic pollution from eastern and western Europe; marine aerosols and biogenic material from various land and marine sources; aged smoke particles from biomass burning in Africa; and mineral dust particles from the North African and Arabian deserts and from local sources.

In this study, data from ground and airborne measurements collected during the rainfall enhancement assessment project in Saudi Arabia were used in and compared with cloud model simulations in order to investigate the effects of air pollution and mineral dust particles, typically present in that region, on cloud development and precipitation formation processes. On 9 April 2007 the region of interest was affected by a dust storm that caused high Aerosol Optical Depth values of about 0.9. The airborne measurements of the aerosol and cloud droplet properties as well as the atmospheric sounding were used as initial conditions in simulations of microphysical cloud processes using the Weather Research and Forecasting (WRF) model coupled with a new bin microphysics scheme and the Tel Aviv University 2D numerical cloud model (TAU-2D).

Preliminary TAU-2D simulations show that adding giant cloud condensation nuclei (GCCN) to conditions with heavy polluted aerosol concentrations (above 1000 cm-3) results in earlier initiation of precipitation of about 10 minutes, but the contribution of these particles to the simulated total precipitation is not always positive. In some cases, adding GCCN caused large raindrops to be formed early in the warm regions of the cloud, which significantly reduced the later production of graupel particles by freezing. This resulted in a negative effect on the total amount of ice particles falling out of the cloud, therefore suppressing the total precipitation by about 10%.

Further results on the contribution of ice nuclei to precipitation within the same environmental conditions as well as comparisons with WRF simulations and the airborne measurements will be provided in the presentation.