In-situ observations of cloud microphysics over Arabian Sea during dust transport events

The unique in situ measurements of clouds and precipitation within the shallow and deep cumulus over the north-eastern Arabian Sea region during the Indian monsoon are illustrated in this study with a focus on droplet spectral parameters. The observational period showed a significant incursion of Arabian dust and the presence of giant cloud condensation nuclei (GCCN), modifying the cloud and precipitation spectral properties. Warm rain microphysics supported the mixed-phase development in these clouds and exhibited hydrometeors of snow, graupel and large aggregates as part of ice processes. Cloud base droplet number concentration is about 142 ± 79 cm⁻³ which is one third of the cloud condensation nuclei (CCN) number concentration at 0.2% supersaturation. A rapid broadening of droplet size distribution (DSD) near to the cloud base was noted in contrast to polluted continental clouds. Relationship between the relative dispersion ( ϵ ; the ratio of DSD spectral width ( σ ) to mean radius ( r m )) and liquid water adiabatic fraction (AF) indicates that the entrainment effect has increased relative dispersion significantly (2-3 times larger) in these clouds. Effective radius ( r eff ) is found to be proportional to mean volume radius ( r v ) with a proportionality constant ( β ) that varies between 1.0-1.6, depending on the spectral dispersion parameter. Drop size distributions for the small cloud droplets with size range 2-50 μ m and the large drizzle drops (or ice hydrometeors) with size range 100-6400 μ m are parameterized using the gamma function distributions useful for large-scale cloud models.