Similarities and contrasts in observed aerosol and cloud microphysical characteristics in India and the Arabian Peninsula: the effect on coalescence processes

In-situ cloud microphysical measurements in convective clouds in India, the United Arab Emirates, the Sultanate of Oman, and Saudi Arabia collected over the past five years have shown large spatial and temporal variations in their characteristics. While the concentrations of cloud droplets near cloud base are often fairly similar, precipitation formation processes (especially the condensation-coalescence process) differ substantially from one region to the other and also temporally within the same region. These appear to be linked to temporal and spatial variations in aerosol characteristics and to differences in the thermodynamic structure of the atmosphere.

Observational studies have shown that large nuclei (between 0.8 and 5µm diameter) which are ingested into clouds broaden the cloud droplet spectrum and accelerate the production of raindrops by coalescence. The effect of large cloud condensation nuclei (CCN) on the process and rate of drizzle and rain formation in warm clouds is investigated using the observational data and a one-dimensional cloud parcel model.

Large nuclei are found to be essential for initiating coalescence in polluted environments. Once coalescence has been initiated, the rate at which cloud liquid water is transferred to precipitation-size drops depends on the amount of drizzle that forms in the cloud. Drizzle production is retarded when accumulation mode aerosol concentrations are high, but is promoted when coarse mode aerosol concentrations are high. Giant and ultra-giant CCN (>10 µm diameter) accelerate the onset of coalescence but suppress drizzle production. So while these larger nuclei may enhance the conversion of condensate to precipitation-size drops, the drops may fall out of the cloud before most of the available water has been harvested. These processes are time-dependent and therefore depend on updraft strengths and the depth of cloud below the 0°C level.

Once clouds grow to temperatures colder than 0°C, the presence of drizzle has a large influence on the development of ice processes, which in turn, affects overall precipitation formation, possibly efficiency, and eventually the amount of precipitation that reaches the ground. The implications of these results for precipitation processes in clouds will be discussed.