The Route to Raindrop Formation in a Shallow Cumulus Cloud Simulated by a Lagrangian Cloud Model

AbstractThe mechanism of raindrop formation in a shallow cumulus cloud is investigated using a Lagrangian cloud model (LCM). The analysis is focused on how and under which conditions a cloud droplet grows to a raindrop by tracking the history of individual Lagrangian droplets. It is found that the rapid collisional growth, leading to raindrop formation, is triggered when single droplets with a radius of 20 μm appear in the region near the cloud top, characterized by large liquid water content, strong turbulence, large mean droplet size, broad drop size distribution (DSD), and high supersaturations. Raindrop formation easily occurs when turbulence-induced collision enhancement (TICE) is considered, with or without any extra broadening of the DSD by another mechanism (such as entrainment and mixing). In contrast, when TICE is not considered, raindrop formation is severely delayed if no other broadening mechanism is active. The reason for the difference is clarified by the additional analysis of idealized bo...

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In the extreme situation to which the calculations apply undersaturated air is entrained in a stream, or in blobs, and some droplets of all sizes are completely removed from the condensate spectrum by evaporation, while others do not change in size. This is equivalent to assuming that the time constant for turbulent mixing (ττ) is large relative to that for droplet evaporation (τr), and is thus the antithesis to the homogeneous model utilized by other workers, which assumes implicitly that ττ/τr = 0. 
 
 
 
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abstract: AbstractMicrophysical piggybacking applied previously in shallow convection simulations is employed to highlight microphysical impacts on deep convection. The main idea is to apply two sets of thermodynamic variables, one coupled to the dynamics and driving the simulation and the second one piggybacking the simulation: that is, responding to the simulated flow but not affecting it. The two sets can be driven either by different microphysical schemes or by the same scheme with different scheme parameters. For illustration, two single-moment bulk microphysical schemes are implemented, one with a simple representation of ice processes and the second one with a more comprehensive approach. Each scheme is applied assuming contrasting cloud droplet concentrations, pristine versus polluted, with simulations mimicking dynamical effects of pollution on deep convection. The modeling setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (...

title: Effects of turbulence on warm clouds and precipitation with various aerosol concentrations
abstract: This study investigates the effects of turbulence-induced collision enhancement (TICE) on warm clouds and precipitation by changing the cloud condensation nuclei (CCN) number concentration using a two-dimensional dynamic model with bin microphysics. TICE is determined according to the Taylor microscale Reynolds number and the turbulent dissipation rate. The thermodynamic sounding used in this study is characterized by a warm and humid atmosphere with a capping inversion layer, which is suitable for simulating warm clouds. For all CCN concentrations, TICE slightly reduces the liquid water path during the early stage of cloud development and accelerates the onset of surface precipitation. However, changes in the rainwater path and in the amount of surface precipitation that are caused by TICE depend on the CCN concentrations. For high CCN concentrations, the mean cloud drop number concentration (CDNC) decreases and the mean effective radius increases due to TICE. These changes cause an increase in the amount of surface precipitation. However, for low CCN concentrations, changes in the mean CDNC and in the mean effective radius induced by TICE are small and the amount of surface precipitation decreases slightly due to TICE. A decrease in condensation due to the accelerated coalescence between droplets explains the surface precipitation decrease. In addition, an increase in the CCN concentration can lead to an increase in the amount of surface precipitation, and the relationship between the CCN concentration and the amount of surface precipitation is affected by TICE. It is shown that these results depend on the atmospheric relative humidity.

title: Theoretical Collision Efficiencies of Cloud Droplets at Small Reynolds Numbers
abstract: An outline is presented of a new solution of the hydrodynamic equations for the motion of two spherical drops falling in a viscous fluid at small but non-zero Reynolds numbers. In this solution the flow of the medium is assumed to be governed by a modified form of the Oseen equations, and the boundary conditions at the two drops are simultaneously satisfied approximately. Although the equations allow partially for the effects of fluid inertia, a factor omitted in previous analyses based upon Stokesian hydrodynamics, the method of solution requires use of approximations to a larger extent than Stokesian solutions used by Davis and Sartor and by Hocking and Jonas. Collision efficiencies are presented for larger drops in the range 10–70μ radius. The new results tend to be larger than most previous theoretical computations, especially for drops of comparable size. For small size ratios, for which the coalescence efficiency should he close to unity, the present results appear to be consistent with col...

title: Droplet Spectral Broadening in Marine Stratus
abstract: Broadening of the cloud droplet (diameter 50 μm, i.e., drizzle). Broad droplet spectra were most closely associated with drizzle drops. Both the concentration, C, and slope, k, of the cloud condensation nuclei (CCN) spectra were theoretically found to affect droplet spectral width. For individual cloud parcels a higher C and lower k each contributed to broader droplet spectra. When mixing among cloud parcels with different updrafts was considered, the predictions deviated especially at larger mean droplet diameters. Variations in updraft velocity result in differences in droplet concentrations ...

title: Supersaturation of Water Vapor in Clouds
abstract: A theoretical framework is developed to estimate the supersaturation in liquid, ice, and mixed-phase clouds. An equation describing supersaturation in mixed-phase clouds in general form is considered here. The solution for this equation is obtained for the case of quasi-steady approximation, that is, when particle sizes stay constant. It is shown that the supersaturation asymptotically approaches the quasi-steady supersaturation over time. This creates a basis for the estimation of the supersaturation in clouds from the quasi-steady supersaturation calculations. The quasi-steady supersaturation is a function of the vertical velocity and size distributions of liquid and ice particles, which can be obtained from in situ measurements. It is shown that, in mixed-phase clouds, the evaporating droplets maintain the water vapor pressure close to saturation over water, which enables the analytical estimation of the time of glaciation of mixed-phase clouds. The limitations of the quasi-steady approximation in clouds with different phase composition are considered here. The role of phase relaxation time, as well as the effect of the characteristic time and spatial scales of turbulent fluctuations, are also discussed.