Examination of the the dynamics of drizzle-cells observed during DYCOMS-II

Airborne radar observations collected during the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) project show evidence of mesoscale organization on a scale of several (4-10) times the depth of the boundary layer in unbroken marine stratocumulus. These observations were collected using the Wyoming Cloud Radar (WCR), which was deployed on the rear ramp of the NSF/NCAR C-130 aircraft in a dual-beam downward-looking configuration thereby allowing the radar to observe a near-vertical plane below the flight level. The dual-beam configuration of the WCR allows the horizontal velocity component parallel to the aircraft flight track to be retrieved in addition to the vertical Doppler velocity through the use of dual-Doppler analysis.

Mesoscale organization is readily apparent in both the reflectivity and horizontal velocity fields recorded by the WCR. In the reflectivity field, the mesoscale organization is evident in a series of high-reflectivity cores separated by lower-reflectivity boundaries, while in the horizontal velocity field the organization is evident as a series of anti-correlated velocity variations between the cloud and subcloud layers. Due to the apparent relation between the mesoscale circulations and the appearance of high-reflectivity cores, we refer to these features as ‘drizzle-cells'.

Perhaps the most interesting aspect of the drizzle cells is that the highest reflectivities are found in the updraft-branch of the mesoscale circulation implied by the variations in horizontal velocity (the updraft-branch of the circulation cannot be observed directly in the vertical Doppler velocity due to the combined effects of contribution from particle fallspeeds and correlations between particle fallspeeds and vertical air motions). The co-location of high-reflectivity with regions of upward air motion suggests that the evaporation of drizzle, rather than driving downdrafts and divergent outflow and increasing stability in the subcloud layer as would be expected in cold-pool type models, the evaporation of drizzle occurs primarily in upward moving air.

That most drizzle occurs in and around the updraft-branch of the mesoscale circulation has implications for both the dynamics and the microphysics of the stratocumulus layer. Since a significant fraction of the precipitation falls into upward moving air, the drizzle evaporates more slowly and closer to cloud base. Thus, while the evaporation of drizzle continues to act as a brake on the circulation, this effect is reduced due to the change in the evaporation profile. The evaporation also results in lower cloud base heights and increased liquid water contents in the neighborhood of the updraft-branch. Since bulk parameterizations of the precipitation rate such as those of Pawlowska and Brenguier (2003) and vanZanten et al. (2005) suggest that the precipitation rate depends on the cloud liquid water content to the ~3rd power, processes that locally enhance the liquid water content are likely to result in enhanced precipitation both locally and overall (note that the aforementioned parameterizations are based on leg- or flight-averaged data, thus the response to small scale fluctuations in cloud liquid water content may differ significantly from the bulk prediction).

Pawlowska, H. and J.-L. Brenguier, 2003: An observational study of drizzle formation in stratocumulus clouds for general circulation model (GCM) parameterization. J. Geophys. Res. -Atmos., 108, 8630.

vanZanten, M. C., B. Stevens, D. H. Lenschow, and G. Vali, 2005: Observations of drizzle in nocturnal marine stratocumulus. J. Atmos. Sci., 62, 88-106.