Droplet spectral dispersion by lateral mixing process in continental deep cumulus clouds

Abstract In this study large-eddy simulations (LES) are used to gain more knowledge on the shell of subsiding air that is frequently observed around cumulus clouds. First, a detailed comparison between observational and numerical results is presented to better validate LES as a tool for studies of microscale phenomena. It is found that horizontal cloud profiles of vertical velocity, humidity, and temperature are in good agreement with observations. They show features similar to the observations, including the presence of the shell of descending air around the cloud. Second, the availability of the complete 3D dataset in LES has been exploited to examine the role of lateral mixing in the exchange of cloud and environmental air. The origin of the subsiding shell is examined by analyzing the individual terms of the vertical momentum equation. Buoyancy is found to be the driving force for this shell, and it is counteracted by the pressure-gradient force. This shows that evaporative cooling at the cloud edge, induced by lateral mixing of cloudy and environmental air, is the responsible mechanism behind the descending shell. For all clouds, and especially the smaller ones, the negative mass flux generated by the subsiding shell is significant. This suggests an important role for lateral mixing throughout the entire cloud layer. The role of the shell in these processes is further explored and described in a conceptual three-layer model of the cloud.

Download Full-text

Analytical Investigation of the Role of Lateral Mixing in the Evolution of Nonprecipitating Cumulus. Part II: Dissolving Stage

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0118.1 ◽

2020 ◽

Vol 77 (3) ◽

pp. 911-924

Author(s):

M. Pinsky ◽

A. Khain

Keyword(s):

Turbulent Mixing ◽

Liquid Water Content ◽

Analytical Investigation ◽

Cloud Environment ◽

Potential Evaporation ◽

Cumulus Clouds ◽

Convective Clouds ◽

Cloud Characteristics ◽

Lateral Mixing

Abstract A minimalistic analytical model allowing analysis of the dissolving stage of nonprecipitating convective clouds is proposed. The model takes into account two mechanisms: turbulent mixing with a dry environment and cloud volume settling. The temporal changes in the spatial structure of a cloud and in its immediate environment in the course of cloud dissolving are analyzed. The comparison of the effects of a temperature increase in the course of cloud descent and mixing with dry surrounding air shows that the descent is a dominating factor determining a decrease in the liquid water content (LWC), while mixing has a stronger effect on the cloud shape. Narrowing/broadening of clouds due to lateral mixing with dry air during cloud dissolving is determined by the potential evaporation parameter proportional to the ratio of the saturation deficit in the cloud environment to LWC. An equation for cloud dissolving time is obtained. After a cloud totally dissolves, it leaves behind an area with humidity exceeding that of the environment. The main parameter determining the dissolving time is the downdraft velocity. It should exceed 50 cm s−1 in order to provide reasonable dissolving time. The turbulent intensity, LWC, and humidity of the environment air also have an impact on dissolving time: the lower the LWC and the humidity of environment air, the faster cloud dissolving is. The simple solution presented in this paper can be useful for evaluation of cloud characteristics at the dissolving stage and can be included in procedures of parameterization of cloud cover formed by nonprecipitating or slightly precipitating cumulus clouds (Cu). Values of the environment humidity and temperature, LWC at cloud top, cloud width, vertical velocity of downdraft, and the turbulent coefficient should be parameters of this parameterization.

Download Full-text

Observational Study of the Entrainment-Mixing Process in Warm Convective Clouds

Journal of the Atmospheric Sciences ◽

10.1175/jas3928.1 ◽

2007 ◽

Vol 64 (6) ◽

pp. 1995-2011 ◽

Cited By ~ 116

Author(s):

Frédéric Burnet ◽

Jean-Louis Brenguier

Keyword(s):

Number Concentration ◽

Mixing Process ◽

Cumulus Clouds ◽

Cloud Droplets ◽

Mixing Processes ◽

Convective Clouds ◽

Droplet Distribution ◽

Homogenization Time ◽

Mean Size ◽

Entrainment Mixing

Thermodynamical and microphysical measurements collected in convective clouds are examined within the frame of the homogeneous/inhomogeneous mixing concept, to determine how entrainment-mixing processes affect cloud droplets, their number concentration, and their mean size. The three selected case studies—one stratocumulus layer and two cumulus clouds—exhibit very different values of the cloud updraft intensity, of the adiabatic droplet mean volume diameter, and of the saturation deficit in the environment, all three parameters that are expected to govern the microphysical response to entrainmentmixing. The results confirm that the observed microphysical features are sensitive to the droplet response time to evaporation and to the turbulent homogenization time scale, as suggested by the inhomogeneous mixing concept. They also reveal that an instrumental artifact due to the heterogeneous spatial droplet distribution may be partly responsible for the observed heterogeneous mixing features. The challenge remains, however, to understand why spatially homogeneous cloud volumes larger than the instrument resolution scale (10 m) are so rarely observed. The analysis of the buoyancy of the cloud and clear air mixtures suggests that dynamical sorting could also be efficient for the selection, among all possible mixing scenarios, of those that minimize the local buoyancy production.

Download Full-text

Homogeneous and Inhomogeneous Mixing in Cumulus Clouds: Dependence on Local Turbulence Structure

Journal of the Atmospheric Sciences ◽

10.1175/2009jas3012.1 ◽

2009 ◽

Vol 66 (12) ◽

pp. 3641-3659 ◽

Cited By ~ 109

Author(s):

Katrin Lehmann ◽

Holger Siebert ◽

Raymond A. Shaw

Keyword(s):

Reaction Time ◽

Time Scale ◽

Inertial Subrange ◽

Observation System ◽

Length Scale ◽

Damkohler Number ◽

Mixing Process ◽

Cumulus Clouds ◽

Damköhler Number ◽

Transition Length

Abstract The helicopter-borne instrument payload known as the Airborne Cloud Turbulence Observation System (ACTOS) was used to study the entrainment and mixing processes in shallow warm cumulus clouds. The characteristics of the mixing process are determined by the Damköhler number, defined as the ratio of the mixing and a thermodynamic reaction time scale. The definition of the reaction time scale is refined by investigating the relationship between the droplet evaporation time and the phase relaxation time. Following arguments of classical turbulence theory, it is concluded that the description of the mixing process through a single Damköhler number is not sufficient and instead the concept of a transition length scale is introduced. The transition length scale separates the inertial subrange into a range of length scales for which mixing between ambient dry and cloudy air is inhomogeneous, and a range for which the mixing is homogeneous. The new concept is tested on the ACTOS dataset. The effect of entrained subsaturated air on the droplet number size distribution is analyzed using mixing diagrams correlating droplet number concentration and droplet size. The data suggest that homogeneous mixing is more likely to occur in the vicinity of the cloud core, whereas inhomogeneous mixing dominates in more diluted cloud regions. Paluch diagrams are used to support this hypothesis. The observations suggest that homogeneous mixing is favored when the transition length scale exceeds approximately 10 cm. Evidence was found that suggests that under certain conditions mixing can lead to enhanced droplet growth such that the largest droplets are found in the most diluted cloud regions.

Download Full-text