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The dynamic structure of a small trade wind cumulus (Cu) is analyzed using a novel approach. Cu developing in a shear-free environment is simulated by 10-m-resolution LES model with spectral bin microphysics. The aim is to clarify the dynamical nature of cloud updraft zone (CUZ) including entrainment and mixing in growing Cu. The validity of concept stating that a cloud at developing state can be represented by a parcel or a jet is tested. To investigate dynamical entrainment in CUZ performed by motions with scales larger than the turbulence scales, the modeled fields of air velocity were filtered by wavelet filter that separated convective motions from turbulent ones. Two types of objects in developing cloud were investigated: small volume ascending at maximal velocity (point parcel) and CUZ. It was found that the point parcel representing the upper part of cloud core is adiabatic. The motion of the air in this parcel ascending from cloud base determines cloud-top height. The top-hat (i.e., averaged) values of updraft velocity and adiabatic fraction in CUZ are substantially lower than those in the point parcel. Evaluation of the terms in the dynamical equation typically used in 1D cloud parcel models show that this equation can be applied for calculation of vertical velocities at the developing stage of small Cu, at least up to the heights of the inversion layer. Dynamically, the CUZ of developing cloud resembles the starting plume with the tail of nonstationary jet. Both the top-hat vertical velocity and buoyancy acceleration linearly increase with the height, at least up to the inversion layer. An important finding is that lateral entrainment of convective (nonturbulent) nature has a little effect on the top-hat CUZ velocity and cannot explain the vertical changes of conservative variables qt and θl. In contrast, entrained air lifting inside CUZ substantially decreases top-hat liquid water content and its adiabatic fraction. Possible reasons of these effects are discussed. Significance Statement: (i) The study improves the understanding of the effects of lateral entrainment and mixing. (ii) The study shows the dominating role of the convective-scale motions in cloud microphysics and dynamics. (iii) The comparison of results of 10-m-resolution large-eddy simulations with a simple cloud model allows evaluating validity of current schemes of convective parameterization. [ABSTRACT FROM AUTHOR]

Shallow convective clouds are important players in Earth's energy budget and hydrological cycle, and are abundant in the tropical and subtropical belts. They greatly contribute to the uncertainty in climate predictions due to their unresolved, complex processes that include coupling between the dynamics and microphysics. Analysis of cloud structure can be simplified by considering cloud motions as a combination of moist adiabatic motions like adiabatic updrafts and turbulent motions leading to deviation from adiabaticity. In this work, we study the sizes and occurrence of adiabatic regions in shallow cumulus clouds during their growth and mature stages, and use the adiabatic fraction (AF) as a continuous metric to describe cloud processes and properties from the core to the edge. To do so, we simulate isolated trade wind cumulus clouds of different sizes using the System of Atmospheric Modeling (SAM) model in high resolution (10 m) with the Hebrew University spectral bin microphysics (SBM). The fine features in the clouds' dynamics and microphysics, including small near-adiabatic volumes and a thin transition zone at the edge of the cloud (∼20–40 m in width), are captured. The AF is shown to be an efficient measure for analyzing cloud properties and key processes determining the droplet-size distribution formation and shape during the cloud evolution. Physical processes governing the properties of droplet size distributions at different cloud regions (e.g., core, edge) are analyzed in relation to AF. Significance Statement: 1) This study investigates the evolution of cumulus clouds (Cu) using a 10-m-resolution LES model with spectral bin microphysics. 2) The study improves the understanding of the mutual effects of adiabatic updrafts and lateral entrainment and mixing. 3) The study demonstrates the existence of an adiabatic core in nonprecipitating Cu. 4) Shapes of the droplet size distributions are closely related to the adiabatic fraction values. 5) Utilization of high resolution reveals the existence of physically significant small features in the cloud structure, such as a narrow cloud interface zone and small adiabatic volumes. [ABSTRACT FROM AUTHOR]

Atmospheric motions in clouds and cloud surroundings have a wide range of scales, from several kilometers to centimeters. These motions have different impacts on cloud dynamics and microphysics. Larger-scale motions (hereafter referred to as convective motions) are responsible for mass transport over distances comparable with cloud scale, while motions of smaller scales (hereafter referred to as turbulent motions) are stochastic and responsible for mixing and cloud dilution. This distinction substantially simplifies the analysis of dynamic and microphysical processes in clouds. The present research is Part I of the study aimed at describing the method for separating the motion scale into a convective component and a turbulent component. An idealized flow is constructed, which is a sum of an initially prescribed field of the convective velocity with updrafts in the cloud core and downdrafts outside the core, and a stochastic turbulent velocity field obeying the turbulent properties, including the −5/3 law and the 2/3 structure function law. A wavelet method is developed allowing separation of the velocity field into the convective and turbulent components, with parameter values being in a good agreement with those prescribed initially. The efficiency of the method is demonstrated by an example of a vertical velocity field of a cumulus cloud simulated using the System for Atmospheric Modeling (SAM) with bin microphysics and resolution of 10 m. It is shown that vertical velocity in clouds indeed can be represented as a sum of convective velocity (forming zone of cloud updrafts and subsiding shell) and a stochastic velocity obeying laws of homogeneous and isotropic turbulence. [ABSTRACT FROM AUTHOR]

This study summarizes and generalizes findings from recent studies, focusing on the connection between Cu dynamic properties, such as velocity field, entrainment/detrainment, and cloud microphysical properties, such as cloud dilution rate and droplet size distribution parameters. Special attention is paid to the mechanisms of cloud-surrounding interactions. In particular, we focus on numerical and analytical derivations from the results of 10-m-resolution Large Eddy Simulations (LES) with spectral bin microphysics and statistical analysis of the motion of passive tracers. We used wavelet filtration to separate the cloud's dynamic and microphysical fields into turbulent and convective ones. The main parameters of cloud turbulence and convective motions were evaluated. Turbulence was shown to form an interface zone of a few tens of meters between the cloud and the surrounding air. Convection-scale motions are responsible for dynamic and microphysical properties' formation in the cloud interior. The special role of the vortex ring (toroidal vortex, TV) arising in the upper part of developing clouds is stressed. This TV is responsible for dynamic and microphysical cloud structure formation. It determines the cloud's size, internal dynamics, and ascent velocity of the cloud top. It is demonstrated numerically and analytically that the TV-related cloud circulation leads to a mean adiabatic fraction of 0.4–0.5. The close relationship between this value and the shapes of the size distribution functions is demonstrated. The TV determines the width of the cloud core and disappears as soon as the core becomes diluted. Knowledge of the effects TV has on cloud microphysics and dynamics allows us to propose parameterization of the main dynamic and microphysical properties of small Cu using sounding data and aerosol concentrations. a) Turbulence forms a narrow interface zone along the cloud's edges and is not responsible for the dilution of the cloud body. b) The entrainment and detrainment in growing Cu are closely related to a convective-scale ring vortex (toroidal vortex, TV). The air circulation related to the TV is the main reason of cloud body dilution leading to the decrease in the adiabatic fraction. c) The mechanisms of entrainment related to the TV resolve "cloud top-liquid water paradox" and explain formation of relatively high cloud top under strong average cloud dilution. d) One of the main deficiencies of most convective parameterizations is the assumptions of cloud horizontal homogeneity and the coincidence of the altitudes of maximum entrainment and maximum vertical velocity. Due to existence of the TV, the entrainment level is located below the level of maximum velocity. The altitude of the maximum entrainment ascends together with the ascent of TV. e) The TV controls microphysical properties like adiabatic fraction (AF). There is a high correlation of properties of droplet size distributions and the value of AF. f) Considerations are given for a physically based simple parameterization of small Cu, based on the connection between cloud dynamics and microphysics. [ABSTRACT FROM AUTHOR]