Entrainment of aerosols and their activation in shallow cumulus clouds - Large-Eddy Simulations with an Embedded Lagrangian Particle Model

浅积云在许多方面影响着气候。一个主要的影响是地外短波辐射的反射（反照率），这是由云滴的数量和光谱分布调节的，因此，气溶胶的数量。然而，只有活化的气溶胶才能作为云滴的凝结核。气溶胶的主要活化发生在云底，是由于上升的空气团的绝热冷却。对于云底以上的激活（二次激活）存在两种解释：(i)气溶胶的激活，它们太小而无法在云底激活，以及（ii）气溶胶的横向夹带进入云及其随后在云的过饱和环境中激活。当前项目的总体目标是通过应用嵌入拉格朗日粒子模型（LPM）的大涡模拟（LES）来参数化云物理，从而更好地理解导致浅积云中气溶胶活化的过程，其中拉格朗日粒子（所谓的超级液滴）代表一定数量的具有相同特征（例如，液滴大小，气溶胶质量）的真实液滴/气溶胶。云物理的拉格朗日视角允许跟踪单个液滴/气溶胶并直接计算夹带速率。这是必需的，并将用于量化在单个云的生命周期中，在云集合中（即依赖于云的特征，如云高度）或在不同的背景气溶胶条件下（海洋和大陆），不同的激活过程如何对云滴的数量做出贡献。

Shallow cumulus clouds are affecting the climate in many ways. One main impact is the reflection of extraterrestrial shortwave radiation (albedo), which is regulated by the amount and spectral distribution of cloud droplets and, therefore, the amount of aerosols. However, only activated aerosols can serve as condensation nuclei for cloud droplets. Primary activation of aerosols takes place at the cloud base by the adiabatic cooling of rising air parcels. For the activation above cloud base (secondary activation) two explanations exist: (i) the activation of aerosols, which are too small to be activated at cloud base, and (ii) the lateral entrainment of aerosols into the cloud and their subsequent activation in the supersaturated environment of the cloud. The general objective of the current project is to obtain an improved understanding of the processes which lead to the activation of aerosols in shallow cumulus by applying large-eddy simulations (LES) with an embedded Lagrangian particle model (LPM) to parameterize cloud physics, in which Lagrangian particles (so-called super-droplets) represent a certain number of real droplets/aerosols with same features (e. g., droplet size, aerosol mass). This Lagrangian perspective of cloud physics allows tracking of individual droplets/aerosols and the direct calculation of entrainment rates. This is required and will be used to quantify how the different processes of activation contribute to the number of cloud droplets during thelife-cycle of an individual cloud, in a cloud ensemble (i. e., dependencies on cloud features like cloud height) or under different background aerosol conditions (maritime and continental).