Using the Absence of Profiler Reflectivity to Study Stratocumulus-Topped Marine Boundary Layer Processes

Stratocumulus (Sc) clouds occur frequently over the cold waters of the southeastern Pacific Ocean. Data collected during two Pan American Climate Study (PACS) research cruises in the tropical east Pacific illuminate many aspects of this Sc-topped marine boundary layer (MBL). Here the focus is on understanding gaps in detectable wind-profiler reflectivities during two boreal fall cruises. After rigorous quality control that included applying the Riddle threshold of minimum signal-to-noise ratio (SNR) detectability, there are many instances with no measureable atmospheric signals through a depth of up to several hundred meters, often lasting for an hour or more. Rain gauge data from the fall 2004 cruise are used to calibrate the profiler, which allows SNR to be converted to both equivalent reflectivity and the structure-function parameter of the index of refraction, C_n². Profiles of C_n² statistics from the two profiler modes (resolutions) highlight the wide range of C_n² during a 24-hour period and bound the atmosphere's C_n² when low-mode gaps are not mirrored in the high-mode data. Considering the gaps in terms of C_n² allows them to be understood as indications of reduced “top-down” buoyancy processes and/or reduced turbulent intensity, both of which have been demonstrated by previous researchers to be associated with decoupling within the Sc-topped MBL. A decoupling index calculated from surface and ceilometer data confirms that decoupled conditions were common, and that the MBL was coupled when gaps in profiler reflectivity were unlikely. Further study of data from other cruises may lead to a method of using profiler reflectivity as an indicator of decoupled conditions.