Observational constraints on entrainment and the entrainment interface layer in stratocumulus

Abstract

Aircraft sampling of the stratocumulus-topped boundary layer (STBL) during the Physics of Stratocumulus Top (POST) experiment was primarily achieved using sawtooth flight patterns, during which the atmospheric layer 100 m above and below cloud top was sampled at a frequency of once every 2 min. The large data set that resulted from each of the 16 flights document the complex structure and variability of this interfacial region in a variety of conditions. In this study, we first describe some properties of the entrainment interface layer (EIL), where strong gradients in turbulent kinetic energy (TKE), potential temperature and moisture can be found. We find that defining the EIL by the first two properties tend to yield similar results, but that moisture can be a misleading tracer of the EIL. These results are consistent with studies using large-eddy simulations. We next utilize the POST data to shed light on and constrain processes relevant to entrainment, a key process in the evolution of the STBL that to-date is not well-represented even by high resolution models. We define "entrainment efficiency" as the ratio of the TKE consumed by entrainment to that generated within the STBL (primarily by cloud-top cooling). We find values for the entrainment efficiency that vary by 1.5 orders of magnitude, which is even greater than the one order magnitude that previous modeling results have suggested. Our analysis also demonstrate that the entrainment efficiency depends on the strength of the stratification of the EIL, but not on the TKE in the cloud top region. The relationships between entrainment efficiency and other STBL parameters serve as novel observational contraints for simulations of entrainment in such systems

Marine Boundary Layer Cloud Observations in the Azores

“…Interestingly, the averaged stratocumulus top is sometimes within the inversion layer. This agrees well with recent in situ observations (e.g., Carman et al 2012), although the measurements have a great variability (not depicted here) due to intraseasonal variations and various air mass intrusions. Another noticeable finding is the very frequent occurrence (higher than 80%) of inversion and transition layers in the MBL throughout the field campaign (Fig.…”

“…Although this situation can still satisfy the stability criterion (Kuo and Schubert 1988;MacVean and Mason 1990) since Δr is usually also negative, most values fall within the range generally accepted for the criterion, and only a few soundings are clearly stable. Such persistence under unstable conditions has been observed in other studies (e.g., Faloona et al 2005;Carman et al 2012). Yamaguchi and Randall (2008) explain this behavior by the weakness of the feedback in stratocumulus.…”

“…8d). This is much deeper than most observations (e.g., Carman et al 2012) and simulations (e.g., Stevens et al 1999). Nevertheless, this derived quantity is likely influenced by the smoothing of the data by the sounding software, as well as by wetting, which would have mainly caused an overestimate of the layer top (see Caldwell et al 2005).…”

CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

“…The high radiative temperatures and large albedo contrasts with the underlying ocean provide substantial global cooling that can be increased by the advection and injection of anthropogenic cloud condensation nuclei (CCN), which increase cloud droplet concentrations N c (albedo; first IAE; Twomey 1977a) and decrease droplet sizes (lifetime; second IAE; Albrecht 1989). Hudson et al (2010a, hereafter H10) broke the conventional wisdom of maximum S , 0.3% (100.3% RH) in stratus clouds (Hudson 1983;Hoppel et al 1986;Leaitch et al 1996;Roberts et al 2006;Hegg et al 2009) by comparing below-cloud CCN spectra with nearby N c during the Physics of Stratocumulus Tops (POST) project (Carman et al 2012). Hudson et al (2010a, hereafter H10) broke the conventional wisdom of maximum S , 0.3% (100.3% RH) in stratus clouds (Hudson 1983;Hoppel et al 1986;Leaitch et al 1996;Roberts et al 2006;Hegg et al 2009) by comparing below-cloud CCN spectra with nearby N c during the Physics of Stratocumulus Tops (POST) project (Carman et al 2012).…”

Estimating collision–coalescence rates from in situ observations of marine stratocumulus

“…We use observations from the Physics of Stratocumulus Top (POST) flight campaign, which took place during July and August 2008 off the central California coast. Flights were conducted by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft with dynamic, thermodynamic and microphysics instrumentation (details available in Carman et al , 2012 and Gerber et al , 2013). The flight plans were designed to follow approximately a Lagrangian air mass, while sampling the interface between the stratocumulus layer and the free troposphere in the vertical direction (Malinowski et al , 2013).…”