Aircraft and radar observations of mesoscale cellular circulations in marine Stratocumulus

Marine Stratocumulus have been the subject of several major field campaigns conducted over the past two+ decades. During the recent Dynamics and Chemistry of Marine Stratus (DYCOMS-II) project the Wyoming Cloud Radar (WCR) was deployed on the NCAR C-130 aircraft in a dual-beam, downward-looking configuration. This configuration allows the along-track component of horizontal velocity to be retrieved in addition to the reflectivity and vertical Doppler velocity that are also recorded by the WCR.

It is generally recognized that drizzle and entrainment exert critical, intertwined roles in determining the characteristics and evolution of the stratocumulus-topped boundary layer, yet neither of these processes is well understood. Several authors have suggested that positive feedbacks may exist between circulations within the boundary layer and entrainment. The vertical-plane velocity fields derived from the WCR data collected during DYCOMS-II presents a unique opportunity to examine circulations within the boundary layer since, in the presence of sufficient drizzle, it is possible to retrieve a component of horizontal velocities below-cloud as well as in the cloud layer. Our analysis has focused mainly on RF07, the DYCOMS-II flight with the most extensive drizzle, which allows velocities to be obtained through most of the boundary layer.

Circulations with a scale of ~4km (5x) the depth of the boundary-layer are readily apparent in the horizontal velocity. These circulations, which are also easily visible by eye in the reflectivity pattern, are characterized by out-of-phase horizontal velocities in the cloud and subcloud layers with a maximum (anti-) correlation between in-cloud and sub-cloud horizontal velocities at a scale of ~2km (1/2 of the scale of the circulation). The highest reflectivities and cloud-top heights are found near regions of divergence (convergence) in the horizontal velocities near cloud-top (the surface). Regions of convergence (divergence) near cloud-top (the surface) correspond to low reflectivities and cloud tops. The pattern of convergence/divergence at cloud-top/base suggests the existence of weak mesoscale updrafts which are not readily apparent in the vertical Doppler velocity (due to masking of the updraft by drizzle, and due to the relative width of the updraft regions compared to the depth of the cloud layer). The co-location of high-reflectivity and updrafts inferred from the convergence/divergence pattern suggests that the formation of drizzle is relatively rapid, and that the evaporation of drizzle in the subcloud layer occurs primarily in updrafts as opposed to driving downdrafts.

The spacing/frequency of entrainment events suggests that entrainment is associated with scales comparable to the depth of the boundary layer, rather than with the ~4km circulations described above.