Cloud-Top Entrainment in Stratocumulus Clouds

Abstract

It has been considered that the unevenness of cloud top and cloud base has a great influence on solar and atmospheric radiation. Thus, to understand the cloud base structure of a widespread stratocumulus cloud, observations of stereoscopic photographs were made in which the optical axis of two wide-lens cameras was pointed upwards in a vertical direction. The observations were carried out in the summer and fall of 1986 and 1987. A set of stereophotographs was processed by an image analyzer and the edge lines of unevenness of the cloud base were obtained. Using the edge lines of unevenness of a pair of stereophotographs, 100 to 200 corresponding points were selected on the edge lines and the heights of points were obtained. As a result, it was recognized that the stratocumulus clouds analyzed have an unevenness of 300 m or more at the cloud base. Furthermore, a correlation between the height of cloud base and brightness of cloud base using a set of stereophotographs was obtained. Using the correlation, the cloud base structure, that is, the variation in cloud base height, was analyzed. As a result, it was demonstrated that the relationship between the size and depth of unevenness of the stratocumulus clouds resembled a hemisphere when the wind velocity was weak and wind shear was not strong up to cloud base height. It was recognized, however, that when the wind velocity and wind shear were strong up to the cloud base height, the unevenness at the cloud base changed from a hemispherical shape to a flattened shape.

DNS and LES for Simulating Stratocumulus: Better Together

“…We refer to this class of expressions as entrainment rate equations. These equations allow us to quantify the contributions from various phenomena to w e (Mellado, ), and they recover the jump relations used in mixed‐layer models when the mixed‐layer model approximations to the actual solutions are substituted into them (Stevens, ).…”

Object‐Oriented Identification of Coherent Structures in Large Eddy Simulations: Importance of Downdrafts in Stratocumulus

“…that can be useful to validate a multiplume parameterization (e.g., Neggers et al, ; Sušelj et al, ) or justify the use of a bulk parameterization. The use of passive tracers above the cloud top helps tracking the onset of downdraft coherent structures by informing about processes of buoyancy reversal instability. Several mechanisms have been highlighted to explain this triggering such as radiative cooling (Lilly, ), evaporative cooling (Kuo & Schubert, ), wind shear (Mellado, ) or turbulent mixing (Yamaguchi & Randall, ). Statistics of downdrafts provided by this framework could be used to infer a parameterization of downdraft initiation. The diurnal cycle of updraft and downdraft contributions to boundary layer fluxes suggests that future parameterization development needs to include source and sinks of heat linked to cloud‐radiation interactions.…”

Competing Effects of Droplet Sedimentation and Wind Shear on Entrainment in Stratocumulus

“…According to equation , these values correspond to a bulk sedimentation velocity of u sed ≃13 mm/s and u sed ≃30.5 mm/s. The exponential factor in equation explains why these values are larger than the sedimentation velocities obtained for single droplets, which are typically on the order of 3–12 mm/s (e.g., Mellado, ).…”

“…As the first nondimensional parameter, we consider the sedimentation number where U 0 =( B 0 λ ) 1/3 is a reference radiative velocity scale. In this definition, λ is the extinction length scale, which characterizes the depth over which the radiative flux divergence concentrates, and is the reference buoyancy flux that is associated with the reference longwave radiative cooling R 0 (de Lozar & Mellado, ; Mellado, ). In the definition of B 0 , and T c are the specific heat capacity and temperature of cloudy air, respectively.…”