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Published March 2006 | Published
Journal Article Open

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


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.

Additional Information

© 2006 American Meteorological Society Manuscript received 20 April 2004, in final form 28 July 2005 This research was supported under NSF Grant ATM-9902416. The efforts of Dr. Phil Durkee in directing the aircraft to the rift area studied are greatly appreciated. Hal Maring is thanked for his valuable assistance with the interpretation of the new particle production. Additionally, we wish to thank Kirk Ayers for assistance with analyzing the satellite data. The satellite analyses were supported by NOAA Cooperative Agreement NA00AABRG0330 under the PACS Program and the Environmental Sciences Division of the U.S. Department of Energy through the Interagency Cooperative Agreement DE-AI02-97ER62341.

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