Aerosol-cloud interactions over Istanbul, Turkey and central Saudi Arabia

In recent advertent and inadvertent weather modification studies, a considerable effort has been made to understand the impact of varying aerosol properties and concentration on cloud properties. Significant uncertainties exist with aerosol-cloud interactions for which complex microphysical processes link the aerosol and cloud properties. Under almost all environmental conditions, increased aerosol concentrations within polluted air masses will enhance cloud droplet concentration relative to that in unperturbed regions. The interaction between dust particles and clouds are significant, yet the conditions in which dust particles become cloud condensation nuclei (CCN) are uncertain. In order to quantify this aerosol effect on clouds and precipitation, a field campaign was launched in central Saudi Arabia as part of a Precipitation Enhancement Feasibility Study funded by the Presidency of Meteorology and Environment in the Kingdom of Saudi Arabia.

Measurements of aerosol size distributions, CCN concentrations and cloud droplet size distributions were conducted from an instrumented aircraft in central Saudi Arabia in December 2006 through April 2007 and December 2007 through April 2008. The aerosol size distributions presented in this work were measured with three instruments. The aerosol out-of-cloud measurements were made by the Texas A&M University Differential Mobility Analyzer (DMA) (0.0124 μm to 0.38774μm) combined with the Passive Cavity Aerosol Spectrometer Probe (0.1μm to 3.0μm) and the Forward Scattering Spectrometer Probe (FSSP) (3.0μm to 47.0μm). CCN concentrations were measured using the Droplet Measurement Technologies (DMT) CCN counter at 0.2% to 1.2% supersaturation. Cloud droplet Mean Volume Diameter (MVD) and cloud droplet concentration were measured using the FSSP (3.0μm to 47.0μm).

The results of the December 2006 through April 2007 campaign suggest that:

1. In the winter aerosol layers are stratified and ‘stacked'. The calculated monthly CCN concentration exhibits a clear seasonal cycle with a significant deepening in the aerosol boundary layer in early spring.

2. When dust aerosol is present in greater concentrations in early spring a decrease in the cloud droplet concentration is measured. Although very speculative, this observation suggests that in continental environments dust particles may enhance precipitation formation by providing giant CCN that form large droplets with high collection efficiencies.

3. The interaction between dust particles and clouds are significant, yet the conditions in which dust particles become CCN and cloud droplets are still uncertain.

Data from the 2007/2008 campaign will be presented in the form of aerosol size distributions and drop size distributions (DSD) profiles. A companion paper presented by the second author will present numerical simulations using these data as run by the WRF model.