Turbulence and Droplet Clustering in Shallow Cumulus: The Effects of Aerosols and Cloud Height

Aerosol-cloud interactions are complex including direct effects, indirect effects, and evaporation-entrainment feedback mechanisms that lead to modifications in cloud lifetime, cloud albedo, and precipitation as well as the resulting effects on climate. With most cloud-aerosol interactions focused on the previously stated phenomena, few studies have focused explicitly on how aerosols can effect turbulence within a cloud, especially cloud edge entrainment which impacts cloud lifetime and size. Along with turbulence, little research has been conducted outside the lab on droplet clustering within clouds and its relationships with turbulence. Turbulence interacts with cloud droplets through preferential concentration, with droplets being preferentially concentrated into regions of lower vorticity, leaving regions of higher vorticity relatively free of droplets. This project aims to use droplet clustering to gain a better understanding on how clustering can be used to map turbulence within a cumulus cloud and how aerosol number concentration effects droplet clustering, keeping in mind that droplet clustering is a tool not only for analyzing turbulence but can also have implications relating to precipitation formation through collision and coalescence.

Aerosol-cloud relationships are derived from warm continental cumuli subjected to various levels of anthropogenic influence sampled during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) by the Center for interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Drop size distributions, cloud liquid water content (LWC), and a time stamp for when each cloud droplet was encountered were measured using the Artium Flight phase-Doppler interferometer (PDI). These data along with other meteorological observations are used to investigate turbulence and droplet spacing within the cumuli. The pair correlation function (PCF) is used to identify the scale of preferential concentration, with more clustering signifying a more turbulent environment and vice versa. The time stamp from the PDI allows droplets to be calculated down to 10^-6 meter scale.

Preliminary results using four complete days of data with 81 non-precipitating cloud penetrations (minimum 300 m in length) organized into two flights of low pollution data (L1, L2) and two flights of high pollution data (H1, H2) show a more turbulent environment near cloud edge as compared to the cloud center for all four cases, with lower polluted clouds showing more droplet clustering for both cloud edge and center. The mean inter-arrival time (MIT) of cloud droplets is shown to be larger at cloud edge vs cloud center with higher polluted clouds having a larger range of MIT for both cloud center and edge compared to the lower polluted clouds. MIT also shows an exponential decrease as a function of Cloud Drop Number Concentration (CDNC). Droplet clustering will also be compared at cloud edge vs cloud center as a function of height using a single cloud in which multiple flight passes were made at different altitudes ranging from 806-3381 meters. MIT will also be analyzed as a function of altitude.