In Situ Measurements of Cloud and Aerosol Microphysical Properties in Summertime Convective Clouds over Eastern United Arab Emirates

Abstract

Aircraft observations were conducted over the eastern mountainous areas of the United Arab Emirates (UAE) in September 2017 to characterize the microphysical properties of diurnal convective clouds. Aerosol particle and cloud condensation nuclei (CCN) measurements indicate that the air mass had a continental nature, resulting in high cloud droplet concentrations of 600-800 cm⁻³. Two case studies were undertaken to obtain the vertical profiles of hydrometeors up to the cloud top. The ice particle number concentrations in the updraft core were a few particles L⁻¹, which is similar to the primary ice nucleating particle (INP) number concentrations estimated from immersion freezing of high concentration dust particles in the convectively mixed boundary layer. The ice particle number concentrations were several tens of particles L⁻¹ outside the updraft core in the upper levels. INP measurements and the observed cloud microphysical structure suggest that drizzles, frozen via the immersion freezing nucleation of dust particles, formed graupel embryos and ice particles, with concentrations of one to two orders of magnitude greater than those of the primary INPs, which may be generated via secondary ice production and/or ice particle accumulation from primary ice nucleation in clouds with relatively warm cloud top temperatures (approximately −12°C).