NSF-BSF: Quantitative Evaluation of Aerosol Impacts on the Microphysical Composition, Electrification and Radiative Forcing of Deep Tropical Convective Clouds

Atmospheric aerosol particles range in size from a few nanometers to tens of micrometers and have both natural and anthropogenic sources. Aerosol particles act as cloud condensation nuclei (CCN), and therefore changes in aerosol particle abundance can alter the cloud drop concentrations and subsequently cloud reflectivity and lifetime as well as associated precipitation.

Tropical deep convective clouds (DCC) are important to Earth’s global energy balance and hydrologic cycle and can induce high impact weather with significant precipitation and lightning. However, the effects of aerosol particles on DCC properties, including the onset and amount of precipitation, vertical development, electrification and associated lightning, and the extent and lifetime of associated high altitude ice clouds, remain poorly quantified.

In this project, the effect of aerosol particles on such weather and climate related phenomena is studied, addressing a major open question of whether and how human-induced aerosol particle pollution has altered weather phenomena (e.g., potentially moving tropical showers to thunderstorms), and climate. Global climate models are being used to inform socioeconomic policy decisions around the world, but the processes investigated in this project are practically absent from these models.

Therefore, the outcome of this study could have broad impacts on our understanding of the anthropogenic aerosol impacts on climate predictions and in turn upon policies being developed for mitigation and adaptation. The project will train graduate and undergraduate students through international collaboration in the fundamental physics and chemistry of the atmosphere and developing their technical skills in the analysis of large multivariable datasets, 3-dimensional computer models of the atmosphere, and satellite remote sensing technology.

Tropical marine regions are likely most sensitive to the additional input of aerosol particles which act CCN. Ultrafine aerosol particles (UAP) < 40 nm in size are not typically considered CCN, but they may in fact nucleate cloud droplets in DCC. Observations of UAP, specifically, and CCN, are lacking in such clean regions needed to test hypotheses about the aerosol impacts on deep convective clouds and associated effects on climate.

To address these limitations, this joint NSF-BSF project between University of Washington (UW), The Hebrew University Jerusalem Israel (HUJI), and collaborators, leverages i) remote sensing of DCC microphysics to constrain CCN and UAP impacting individual convective events, ii) analysis of 15-years of global tropical lightning stroke fields guided by chemical transport model predictions of UAP and CCN, and iii) cloud resolving modelling of key domains constrained by the satellite remote sensing insights, lightning observations, and chemical transport model predictions of UAP and CCN. The project will evaluate how CCN, including UAP, perturb the microphysics of tropical deep convective clouds and to what extent such perturbations affect lightning and cloud radiative effects, and provide a test of the hypothesis that increases in CCN and UAP since preindustrial time, due to human activities associated with fuel combustion, have induced a positive radiative forcing (warming) on climate through deep convective clouds.

Such a climate forcing would be in opposition to the negative radiative forcing in which CCN increases affects low cloud albedo and lifetime and would alter our understanding of climate sensitivity.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.