How Drizzle Evaporation Impacts Below-Cloud Turbulent Kinetic Energy in Transition Marine Stratocumulus

 
Poster PDF

Authors

Mark A. Miller — Rutgers University
Zhongyu Kuang — Rutgers University

Category

Warm low clouds, including aerosol interactions

Description

Observations show that most warm marine boundary-layer clouds produce drizzle. Evaporation occurs as this drizzle falls into the sub-cloud layer toward the ocean surface and, in many cases, all this drizzle evaporates before it reaches the ocean surface, while in other cases it reaches the ocean surface, so liquid water is removed from the system. It has been hypothesized that the evaporation of drizzle stabilizes the sub-cloud layer, thereby discouraging turbulent eddies from transporting water vapor and aerosols from the ocean surface into the cloud layer in a process known as decoupling. This decoupling could have the effect of thinning the overlying cloud deck and reducing the supply of sea-salt aerosol, which could stimulate drizzle-out or otherwise effect cellular convection in the marine boundary layer. Unfortunately, the relationship between drizzle evaporation in the sub-cloud layer and the sub-cloud Turbulent Kinetic Energy (TKE) budget is unknown. We use four months of data collected at the new ARM Eastern North Atlantic (ENA) site to explore the relationship between drizzle evaporation and TKE in the sub-cloud layer. In an earlier CAP-MBL study, we analyzed the relationship between normalized cloud layer thickness and the Normalized Drizzle Penetration Depth (NDPD), which we defined as the depth of drizzle detection beneath the optical cloud base, in single-layer stratocumulus. A significant positive correlation between cloud layer thickness and NDPD was indicated and, as expected, deeper clouds were associated with an increased NDPD, but this correlation was non-linear. This observation leads us to wonder whether drizzle evaporation in the sub-cloud layer is a stabilizing process, which has been hypothesized, or whether it actually increases TKE in the sub-cloud layer, and, therefore, reinforces vapor transport into the cloud, especially at night. In this study, we investigate the relationship between sub-cloud TKE, NDPD, and cloud thickness. We compare TKE statistics measured by Doppler lidar in the vicinity of evaporating drizzle identified with the NDPD and cloud layer thickness determined using the ENA Ka-band Doppler radar (KAZR) and laser ceilometer.