Broadening of Cloud Droplet Spectra in a Lagrangian Modeling Methodology: Chemistry and Size Distributions of Aerosols with Eddy Hopping Mechanism

Recently, the particle-based Lagrangian method has become more common for the understanding of cloud physical processes (Shima et al., 2009; Andrejczuk et al., 2010; Riechelmann et al., 2012; Hoffman et al., 2015, 2017; Arabas et al., 2015; Grabowski et al., 2017). In general, there are two main approaches to study cloud microphysics: the first is the bulk parameterization, and the second is the explicit bin-resolving method. In addition to the traditional Eulerian methodology for simulating dynamics and continuous thermodynamics, Lagrangian particles referred to as “Super-droplets” can represent condensation and collision processes.

This study investigates the activation of aerosol served as cloud condensation nuclei (CCN) in the Lagrangian Cloud Model coupled with Large Eddy Simulation (LCM-LES), see, e.g., Riechelmann et al. (2012) or Hoffmann et al. (2017). Empirical parameterizations of water activity in large-scale models have been of interest for decades. The two key parameters of droplet activation are the dry aerosol properties and integral CCN activity spectra depending on a saturated critical point. The motivation of this study is to use the Lagrangian methodology to study the impact of different chemical compositions and size distributions on the spectrum of cloud droplets. Until now, the LCM-LES has been used to investigate the activity of aerosol as CCN by explicitly solving the Kohler equation, but only sodium chloride and limitations of the size spectrum (Hoffmann et al., 2015). However, the ammonium sulfate in the categories of soluble particles is also essential for activation of CCN. In addition to this, it has been shown from various field campaigns that insoluble aerosols (e.g., organics) are a significant fraction of the aerosol mass both in urban and remote areas (Jimenez et al., 2009) and can affect the ability of mixed particles to act as CCN (Raymonds and Pandis, 2002; Kumar et al., 2003; Fofie et al., 2018). It was done by using a convenient parameterization of the soluble fraction and CCN activation. (Khvorostyanov and Curry, 2007). Consequently, in the LCM-LES different aerosol properties play a critical role in broadening of droplet spectra.

But, even the high-resolution LCM-LES coupled LES is not well suited for the consideration of the small-scale supersaturation fluctuations because the edge of the cloud is often too thin to be well resolved by LES, which is only able to resolve structures larger than its grid spacing. To represent the true CCN process in the LCM-LES, we have implemented eddy hopping method, which affects the diffusional growth of cloud droplets (Grabowski and Abade, 2017). The turbulent fluctuations of supersaturation processes are essential for activation of CCN, accelerating the formation of raindrops through the collision-coalescence process. The formation of cloud droplets from aerosol particles is highly affected by turbulent motion around the edge of the cloud because turbulent perturbation at the microscale determines local dynamics and the CCN activation.

With the help of Eddy hopping mechanism, the LCM-LES can incorporate the variety of chemical compositions to act as CCN. As expected, our pilot investigation shows that the fluctuation of supersaturation can result in a broadening of the spectrum depending on the chemical compositions of dry aerosols.

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