Observational constraints on entrainment and the entrainment interface layer in stratocumulus

Abstract

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 tends 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 demonstrates 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.

Physics of Stratocumulus Top (POST): turbulent mixing across capping inversion

“…POST was a research campaign held in the vicinity of Monterey Bay in July and August 2008 (Gerber et al, 2010;Carman et al, 2012;. High-resolution in situ measurements with a Center for Interdisciplinary RemotelyPiloted Aircraft Studies (CIRPAS) Twin Otter research aircraft were focused on a detailed study of processes occurring at the interface between the STBL and the FT.…”

“…Data from in situ measurements (e.g., Caughey et al, 1982;Nicholls, 1989;Lenschow et al, 2000;De Roode and Wang, 2007) and the results of numerical simulations (e.g., Moeng et al, 2005;Yamaguchi and Randall, 2008) clearly indicate that the top of Sc is located below the capping inversion and does not directly interact with the FT above. The thickness of the EIL, related to the distance between the cloud top and inversion, varies from a few meters to a few tens of meters (e.g., Haman et al, 2007;Kurowski et al, 2009;Katzwinkel et al, 2012), and differs for different Sc cases (Carman et al, 2012).…”

Physics of Stratocumulus Top (POST): turbulence characteristics

“…Recently, airborne measurements of fine spatial resolution (at the centimeter scale for some parameters), aimed at providing a better understanding of the EIL, were performed in the course of Physics of Stratocumulus Top (POST) field campaign (Gerber et al, 2010Carman et al, 2012). A large data set was collected from sampling the marine stratocumulus top during porpoising (flying with a rising and falling motion) across the EIL and is freely available for analysis (see http://www.eol.ucar.edu/projects/post/).…”

Aerosol characteristics in the entrainment interface layer in relation to the marine boundary layer and free troposphere

“…Three vertical layers were defined with respect to the cloud layer including the subcloud (SUB) layer, EIL, and FT. The vertical bounds of the EIL are defined based on considerations of buoyancy and turbulence, similar to past studies (Carman et al, 2012). An example from FASE research flight 7 (F07) on 1 August 2016 illustrates the criteria used to determine the vertical boundaries of the EIL, STBL, and FT (Fig.…”

“…This layer exhibits strong gradients in thermodynamic and dynamic properties. Although numerous airborne and modeling studies have attempted to increase our understanding about the thermodynamic and dynamic nature of the EIL (e.g., Caughey et al, 1982;Moeng et al, 2005;Haman et al, 2007;Wang et al, 2008;Carman et al, 2012;Katzwinkel et al, 2012;Gerber et al, 2013;Malinowski et al, 2013;Jen-La Plante et al, 2016), aerosol characteristics in this thin layer have not been studied in detail.…”