Analysis of aerosol-cloud interactions and their implications for precipitation formation using aircraft observations over the United Arab Emirates

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 science flights targeted summertime convection along the eastern 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 sub-cloud region. Sub-cloud 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 to the eastern orographic region where smaller size 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 modelling case studies to study 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.

Comment on acp-2021-200

“…The capability of the Learjet to conduct rapid maneuvers and swift ascents/descents was key for sampling the short-lived UAE clouds while complying with local Air Traffic Control (ATC) restrictions. The aircraft was equipped with state-of-the-art cloud physics instrumentation used on multiple platforms throughout various airborne campaigns (Lawson et al, 2019). The instruments deployed for the UAE campaign included a Condensation Particle Counter (CPC), Passive Cavity Aerosol Spectrometer Probe (PCASP), multiple optical array probes (OAPs) and scattering probes (see Table 1).…”

“…Hence, assessing the impact of aerosols on cloud microphysics and precipitation generation has been a longstanding research area with many processes yet to be fully understood (Morrison et al, 2020;Flossmann et al, 2019;Solomon et al, 2007;Squires, 1958;Rosenfeld, 1999). 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 particularly 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 complex aggregates depending on both regional and local forcing (Abuelgasim and Farahat, 2020;Filioglou et al, 2020;Semeniuk et al, 2014).…”

“…Dust introduces large particles with diameters exceeding 2 µm which could serve as giant CCN (GCCN) (Jensen and Nugent, 2017;Yin et al, 2000;Jensen and Lee, 2008). This alludes to more effective droplet collection during the collision-coalescence (C-C) process.…”

“…NaCl) results in their deliquesce at 74% RH (Wise et al, 2007). Similarly, compounds from anthropogenic pollution (sulfates) interact with mineral dust to produce internally-mixed aggregates which deliquesce at 80% RH (Semeniuk et al, 2015).…”

“…The aircraft was equipped with state-of-the-art cloud physics instrumentation (see Sect. 4) to sample summertime convective clouds over the UAE (Lawson et al, 2019). Here, we use in situ aerosol and cloud microphysical measurements from the SPEC UAE campaign to assess the role of background aerosols on natural precipitation formation.…”