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

Abstract

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 process. 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 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 μm and the large drizzle drops (or ice hydrometeors) with size range 100-6400 μm are parameterized using the gamma function distributions useful for large-scale cloud models.