The structure and mesoscale organization of precipitatingstratocumulusVERICA SAVIC-JOVCIC∗and BJORN STEVENSDepartment of Atmospheric and Oceanic Sciences,University of California Los Angeles, Los Angeles CA, USAJournal of the Atmospheric SciencesSubmitted March 12, 2007AbstractLarge-eddy simulations are used to explore the structure and mesoscale organization of precipitatingstratocumulus. The simulations incorporate a simple, two-moment, bulk representation of microphysi-cal processes, which by varying specified droplet concentrations allows for comparisons of simulationsthat do and do not develop precipitation. The boundary layer is represented over a large (25.5 by 25.5km) horizontal domain using a relatively fine mesh, thereby allowing for the development of mesoscalecirculations while retaining an explicit representation of cloud radiative, dynamical and microphysi-cal interactions, on scales much smaller than the dominant eddy scale. Initial conditions are based onmeasurements made as part of the second dynamics and chemistry of marine stratocumulus field study(DYCOMS-II).The simulations show that precipitation is accompanied by sharp reductions in cloudiness and changesin flow topology. A cloud albedo of near 75% in the non-precipitating simulation is reduced to less than35% in the precipitating case. The circulation transitions from a well mixed, stationary stratocumuluslayer with closed-cellular cloud planforms to a stationary cumulus-coupled layer, with incipient open-cellular cloud planforms and sustained domain-averaged precipitation rates near 1 mm day−1. Thedrizzling simulations embody many other features of observed precipitating stratocumulus, includingelevated cloud tops in regions of precipitation and locally higher values of sub-cloud equivalent potentialtemperature. The latter is shown to result from the tendency for precipitating simulations to developgreater thermodynamic gradients in the sub-cloud layer as well as mesoscale circulations which locateregions of upward motion in the vicinity of precipitating cells.∗Corresponding Author Address: Verica Savic-Jovcic, UCLA Atmos & Ocean Sci, Box 951565, 7229 MSB, Los Angeles, CA90095-1565. Email: [email protected]. IntroductionDrizzle is a diabatic process that affects the stratocumulus-topped boundary layer (STBL) and, thus, mayplay a role in setting Earth’s radiative balance. Simple models suggest that drizzle alters the cloud albedo byaffecting the cloud fraction (Albrecht 1989) and thickness (Pincus and Baker 1994). From a more dynamicalpoint of view, drizzle provides a link between cloud microphysical processes and boundary-layer circulations(Paluch and Lenschow 1991; Stevens et al. 1998).The tendency of the STBL to precipitate – at times significantly – is by now well established (Brost et al.1982a,b; Nicholls 1984). Recent field campaigns, DYCOMS-II and EPIC (Stevens et al. 2003; Brethertonet al. 2004, respectively), provide the clearest picture yet that drizzle is prevalent, long-lasting and locallyintense (vanZanten et al. 2005; Comstock et al. 2005). These data further suggest that drizzle is associatedwith changes in PBL structure and cloud planforms. For instance, Paluch and Lenschow (1991) show thattemperature and moisture are correlated on scales commensurate with the PBL depth, but anticorrelatedon the mesoscale in the presence of drizzle; vanZanten et al. (2005) diagnose pools of elevated equiva-lent potential temperature, θe, within the sub-cloud layer of the regions associated with drizzle; Comstocket al. (2005) report higher horizontal variability in thermodynamic properties during drizzling events, whileStevens et al. (2005b) associate drizzle with open-cellular organization of clouds embedded into the other-wise closed-cellular stratocumulus, which they call POCs (Pockets of Open Cells). Similarly, Sharon et al.(2006) suggest that drizzle erodes stratocumulus decks and promotes patchy, broken clouds with cellularstructure on the order of 10-20 km, which they call rifts.Theoretical work recognizes drizzle as an additional diabatic forcing and strives to understand the mech-anisms by which it reorganizes the PBL circulations. Large-eddy simulations (LES) by Stevens et al. (1998)suggest that drizzle promotes cumulus-type circulations by increasing the buoyancy of downdrafts and there-fore stabilizing the STBL, thus promoting decoupling and less deepening of the STBL. However, the smalldomain of their simulations raises questions about the reliability of their results – which essentially samplea single cumulus element in the decoupled regime. As they also note, the coarse vertical grid used in theirstudy raises further questions about the fidelity of their simulations. Nonetheless, subsequent studies arriveat similar conclusions, although they debate the details of how precipitation interacts with and stabilizesthe circulation (e.g. Ackerman et al. 2004). However, because the subsequent studies have also been forrelatively small computational domains, they are also subject to the concerns that the statistics of the floware insufficiently sampled to draw reliable conclusions, and that the small domain inappropriately filtersmesoscale circulations thought to be associated with drizzle.In this study, we revisit the question of how drizzle affects the evolution of the SBTL. Specifically,we ask: what is the structure of the drizzling STBL in terms of cloud and circulation organization? Towhat degree does LES capture the observed characteristics of the drizzling STBL? What is the extent ofthe agreement between the previous modeling work and ours? And, what generates the pools of elevatedθein the drizzling STBL? We address these questions by using a much larger, yet finer in the vertical,computational mesh than previous studies. The simulation domain of 25.5 × 25.5 × 1.5 km proves to belarge enough to resolve numerous precipitating clouds that organize into loose networks reminiscent of openand closed cells, yet small enough to be computationally manageable. The initial conditions and forcingsfor the simulations are based on the second research flight (RF02) of DYCOMS-II (the second dynamicsand chemistry of marine stratocumulus field study) (vanZanten and Stevens 2005) as summarized for thepurposes of a case study by the GCSS (GEWEX [Global Energy and Water Cycle EXperiment] CloudSystem Study) boundary layer working group (Ackerman et al. 2007).This paper is organized as