15.1 RADAR OBSERVATIONS OF VORTEX-RING ENTRAINMENT PATTERNS IN CUMULUS CLOUDS

The representation of cumulus clouds in weather or climate models is challenging since they occur in a broad range of vertical and horizontal scales (e.g., Lopez 1977; Wielicki and Welch 1986), and the circulations generally are sub-gridscale (e.g., Khairoutdinov et al. 2008). Thus a number of Cu parameterizations have been developed to represent the effect of sub-gridscale convection on precipitation and the vertical profile of resolved variables. These have focused on deep convection, but more recently they have also addressed shallow convection (e.g., Deng et al. 2003; Bretherton et al. 2004). Such parameterizations make assumptions about the turbulent mixing of Cu clouds with the environment (Siebesma and Cuijpers 1995). Entrainment mechanisms of Cu clouds have long been studied. Blyth (1993) describes Cu clouds as shedding thermals, with entrainment occurring near the ascending cloud-top and with mixed parcels descending around the edges of the thermal updraft core. This suggests that the source level of much of the diluted air measured at some level generally is at a higher level. The Large Eddy Simulation (LES) study by Heus et al. (2008) however shows that the Lagrangian tracking of air parcels reveals little cloud top entrainment: particle trajectories extracted from the LES runs clearly indicate that the main source of entrainment is lateral. Lateral entrainment yields characteristic horizontal variations of thermodynamic and moisture variables across the cloud edge. These characteristic variations have been documented by means of composite in situ data collected in a large number of aircraft penetrations through Cu and their immediate environment (Rodts et al. 2003; Wang et al. 2009; Wang and Geerts 2010). Aside from lateral entrainment at any level, much entrainment also appears to result from a cloud-scale cloud-top vortex-ring (toroidal) circulation. Evidence for such circulation includes modeling simulations (Klaassen and Clark1985; Grabowski and Clark1993; Zhao and Austin 2005), observational studies using aircraft data (MacPherson and Isaac 1977; Blyth et al. 1988; Jonas 1990; Blyth et al. 2005), airborne radar data analyses (Damiani et al. 2006), trace gas analyses (Stith 1992) and tank experiments (Woodward 1959; Sanchez et al.1989; Johari1992). This toroidal circulation is important because it affects basic Cu characteristics, i.e. entrainment and detrainment, aerosol evolution, drop size distribution, hydrometeor recycling, and precipitation development. The toroidal motion can also dynamically affect the maximum height of Cu turrets, as the associated dynamic pressure deficit can oppose the suppressing effect of a dynamic pressure high resulting from the interaction between an updraft and the ambient vertical wind shear (Damiani and Vali 2007). The radar observations by Damiani et al. (2006) and Damiani and Vali (2007) improved our understanding of vortex-ring circulations at cloud top, but they only showed the results from select case studies. The present study examines the generality of Cu cloud-top circulation patterns by compositing Doppler reflectivity and vertical velocity data from a large number of vertical transects collected in aircraft penetration of Cu clouds. The selection is unbiased as it includes all penetrations, between the Cu entry and exit points, subject only to a minimum cloud width and relative isolation from adjacent clouds. The objective is to show the characteristic reflectivity and vertical velocity structure in Cu, and stratify this as a function cloud age. The data sources and analysis method are introduced in Section 2. The results are presented in Section 3, and their implications are discussed in Section 4. The main results are summarized in Section 5.