Abstract
Abstract. Aerosol and cloud microphysical measurements were collected by a research aircraft during August 2019 over the United Arab Emirates (UAE). The majority of scientific flights targeted summertime convection along the eastern Al Hajar Mountains bordering Oman, while one flight sampled non-orographic clouds over the western UAE near the Saudi Arabian border. In this work, we study the evolution of growing cloud turrets from cloud base (9 ∘C) up to the capping inversion level (−12 ∘C) using coincident cloud particle imagery and particle size distributions from cloud cores under different forcing. Results demonstrate the active role of background dust and pollution as cloud condensation nuclei (CCN) with the onset of their deliquescence in the subcloud region. Subcloud aerosol sizes are shown to extend from submicron to 100 µm sizes, with higher concentrations of ultra-giant CCN (d>10 µm) from local sources closer to the Saudi border, compared with the eastern orographic region where smaller CCN are observed. Despite the presence of ultra-giant CCN from dust and pollution in both regions, an active collision–coalescence (C–C) process is not observed within the limited depths of warm cloud (<1000 m). The state-of-the-art observations presented in this paper can be used to initialize modeling case studies to examine the influence of aerosols on cloud and precipitation processes in the region and to better understand the impacts of hygroscopic cloud seeding on these clouds.
Highlights
Aerosol particles are key components of the atmosphere and have multiscale impacts on Earth’s climate and hydrological cycle, primarily through radiative transfer and precipitation formation
The effect of the orographic dust trapping is evident from the higher passive cavity aerosol spectrometer probe (PCASP) descent concentrations from SF1, which is further east toward the Al Hajar ridge, compared with that of SF4 which has a direct descent over Al Ain
Higher tail concentrations from ultra-giant sizes of 20–50 μm are recorded during SF4 compared with SF1. This is in line with the previous suggestion of higher ultra-giant cloud condensation nuclei (CCN) loading from local pollution and dust aggregation over the southwest compared with the eastern regime (Semeniuk et al, 2015)
Summary
Aerosol particles are key components of the atmosphere and have multiscale impacts on Earth’s climate and hydrological cycle, primarily through radiative transfer and precipitation formation. Our current knowledge is based primarily on observing the influence of background aerosols on the number and size distribution of hydrometeors (Rosenfeld et al, 2008; Freud et al, 2008; Tao et al, 2007; Rosenfeld, 2000; Feingold et al, 1999) Isolating these impacts has been shown to be challenging in polluted and dusty environments where the physiochemical properties of aerosols are continuously altered between large dust particles, fine particle pollution as well as com-. Rosenfeld et al (2001) attributed the reduction in cloud droplet effective radii (re < 14 μm) over the Saharan Desert to the presence of large concentrations of submicron CCN originating from desert mineral dust This is shown to inhibit C–C and warmrain formation, exacerbating a reduction in precipitation over the Saharan region. Representative measurements from two separate flight cases (12 and 19 August 2019) are used to study dominant eastern orographic convection along the Al Hajar Mountains (Branch et al, 2020) and the less frequent southwestern convection associated with the Arabian heat low (AHL) near the Saudi Arabian border (Steinhoff et al, 2018)
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