Drizzle formation in stratocumulus clouds: effects of turbulent mixing

Abstract

Abstract. The mechanism of drizzle formation in shallow stratocumulus clouds and the effect of turbulent mixing on this process are investigated. A Lagrangian-Eularian model of the cloud-topped boundary layer is used to simulate the cloud measured during flight RF07 of the DYCOMS-II field experiment. The model contains ∼ 2000 air parcels that are advected in a turbulence-like velocity field. In the model all microphysical processes are described for each Lagrangian air volume, and turbulent mixing between the parcels is also taken into account. It was found that the first large drops form in air volumes that are closest to adiabatic and characterized by high humidity, extended residence near cloud top, and maximum values of liquid water content, allowing the formation of drops as a result of efficient collisions. The first large drops form near cloud top and initiate drizzle formation in the cloud. Drizzle is developed only when turbulent mixing of parcels is included in the model. Without mixing, the cloud structure is extremely inhomogeneous and the few large drops that do form in the cloud evaporate during their sedimentation. It was found that turbulent mixing can delay the process of drizzle initiation but is essential for the further development of drizzle in the cloud.

Effects of environment forcing on marine boundary layer cloud‐drizzle processes

“…The hypothesis for the wind shear effect is that vertical wind shear helps the production of TKE and enhances drizzle formation near cloud top. This hypothesis is in general agreement with a recent model study by Magaritz-Ronen et al [2016]. Physical explanation of the hypothesis is straightforward.…”

“…In their model simulations, the first drizzle-sized drops (termed "luck parcels") form near the cloud top where the humidity is high, LWC values are at a maximum and the cloud parcels reside long enough in the cloud to allow the formation of drizzle drops as a result of efficient collisions. Although the coalescence between cloud particles was not included in Magaritz-Ronen et al [2016], their results are indicative and support our hypothesis. Inspired by Feingold et al [1996] and Magaritz-Ronen et al [2016] and through an integrative analysis of the ground-based observations, we attempt to explain the mechanisms that lead to the differences in drizzling status for the periods A, B, and C. During the periods A and B, several large particles form near the top of the cloud layer and start falling toward the cloud base.…”

“…Pinsky et al [2007] found that turbulence can increase the collision efficiency by a factor of 4 at high flow dissipation rate. More recently, Magaritz-Ronen et al [2016] used a Lagrangian-Eulerian model to simulate the effects of turbulent mixing on drizzle formation in stratocumulus clouds and found that drizzle develops only when turbulent mixing of parcels is included in the model. In this study, we investigate the impact of the turbulent mixing on the cloud-to-drizzle process using available measurements at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) site in the Azores.…”

“…Unlike previous observation/parameterization [Nicholls, 1987;Baker, 1993;Austin et al, 1995] or model [Feingold et al, 1996;Magaritz-Ronen et al, 2016] studies, we attempt to assess the roles of vertical wind shear and boundary layer instability (buoyancy) in the drizzle initiation processes and investigate how the precipitation patterns respond to these two forcings using ground-based observations. The manuscript is organized as follows.…”

Physical Interpretation of Mixing Diagrams

“…In the study, we use the results of simulations of Sc observed during research flight RF07 in DYCOMS‐II field experiment described in detail by Magaritz‐Ronen, Pinsky, and Khain () and Magaritz‐Ronen, Khain, and Pinsky (). Sc measured during this flight was 500 m thick and capped by a strong inversion layer at 850 m. Light drizzle was observed at the surface.…”