Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds

Abstract

A cloud rift is characterized as a large-scale, persistent area of broken, low-reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of California was investigated using an instrumented aircraft to compare the aerosol, cloud microphysical, and thermodynamic properties in the rift with those of the surrounding solid stratocumulus deck. The microphysical characteristics in the solid stratocumulus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Furthermore, cloud condensation nuclei (CCN) concentrations were found to be about 3 times greater in the solid-cloud area compared with those in the rift. Although drizzle was observed near cloud top in parts of the solid stratocumulus cloud, the largest drizzle rates were associated with the broken clouds within the rift area and with extremely large effective droplet sizes retrieved from satellite data. Minimal thermodynamic differences between the rift and solid cloud deck were observed. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud-rift boundary is presented. This mesoscale circulation may provide a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill. A review of results from previous studies indicates similar microphysical characteristics in rift features sampled serendipitously. These observations indicate that cloud rifts are depleted of aerosols through the cleansing associated with drizzle and are a manifestation of natural processes occurring in marine stratocumulus.

Observations of the boundary layer, cloud, and aerosol variability in the southeast Pacific near-coastal marine stratocumulus during VOCALS-REx

“…Compared with the BL depth west of Point Alpha over the remote ocean, the BL at Point Alpha was about 200-400 m shallower and more wellmixed . The BL depth was about 300 m higher than that over the coastal region in the NE Pacific (Stevens et al, 2003;Sharon et al, 2006). The surface wind and the surface fluxes at Point Alpha were weaker than those over the remote ocean and also weaker than the observations in the NE Pacific during DYCOMS-II.…”

Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects

“…In many cases, observations show pockets of open cells (POCs) or cloud rifts 1 forming within and surrounded by otherwise overcast sheets of Sc (e.g., Stevens et al, 2005;Sharon et al, 2006). POCs and cloud rifts are prominent examples of Sc with open MCC characteristics and contribute considerably to the cloud coverage of open cellular clouds.…”

Limited-are a modelling of stratocumulus over South-Eastern Pacific

“…The open-cell structures are called Pockets of Open Cells (POCs) or rifts (Sharon et al, 2006). Their origin is not fully understood, but significant differences in aerosol and cloud microphysical properties between POCs and the surrounding clouds are typically observed Petters et al, 2006;Sharon et al, 2006;Wood et al, 2008Wood et al, , 2011a.…”

“…Their origin is not fully understood, but significant differences in aerosol and cloud microphysical properties between POCs and the surrounding clouds are typically observed Petters et al, 2006;Sharon et al, 2006;Wood et al, 2008Wood et al, , 2011a. VanZanten and Stevens (2005), Sharon et al (2006), and Wood et al (2011a) found that POCs are characterized by enhanced drizzle, although drizzle itself seem insufficient for transition from closed-to open-cell circulations Wood et al (2011a).…”