125483688
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...
121492857
Warm-Rain Initiation: An Overview of Microphysical Mechanisms.
Abstract Rain triggering mechanisms are evaluated in three microphysical steps: droplet activation on cloud condensation nuclei, droplet growth by condensation, and droplet growth by coalescence. Although considerable progress has been made since the pioneering work of Squires, crucial questions in each of the above steps remain unresolved: Under what conditions do giant particles trigger rain by acting as coalescence nuclei? What is the contribution of stochastic condensation to the growth of large droplets in regions of entrainment? What are the collection efficiencies for droplet sizes critical to the onset of colaescence growth? Such questions cannot be answered without better observations. Aircraft instruments are becoming available with the potential to measure the very largest particles and cloud droplets at the concentration of raindrops. Recent advances in sampling and analysis techniques have extended observations of cloud microstructure to smaller scales, providing new insight on the growth of ...
121498083
Droplet growth in warm turbulent clouds
In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. not the energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a significant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of different processes to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office
54650206
Turbulent collision-coalescence in maritime shallow convection
Abstract. This paper discusses cloud simulations aiming at quantitative assessment of the effects of cloud turbulence on rain development in shallow ice-free convective clouds. Cloud fields from large-eddy simulations (LES) applying bin microphysics with the collection kernel enhanced by cloud turbulence are compared to those with the standard gravitational collection kernel. Simulations for a range of cloud condensation nuclei (CCN) concentrations are contrasted. Details on how the parameterized turbulent collection kernel is used in LES simulations are presented. Because of the disparity in spatial scales between the bottom-up numerical studies guiding the turbulent kernel development and the top-down LES simulations of cloud dynamics, we address the consequence of the turbulence intermittency in the unresolved range of scales on the mean collection kernel applied in LES. We show that intermittency effects are unlikely to play an important role in the current simulations. Highly-idealized single-cloud simulations are used to illustrate two mechanisms that operate in cloud field simulations. First, the microphysical enhancement leads to earlier formation of drizzle through faster autoconversion of cloud water into drizzle, as suggested by previous studies. Second, more efficient removal of condensed water from cloudy volumes when a turbulent collection kernel is used leads to an increased cloud buoyancy and enables clouds to reach higher levels. This is the dynamical enhancement. Both mechanisms operate in the cloud field simulations. The microphysical enhancement leads to the increased drizzle and rain inside clouds in simulations with high CCN. In low-CCN simulations with significant surface rainfall, dynamical enhancement leads to a larger contribution of deeper clouds to the entire cloud population, and results in a dramatically increased mean surface rain accumulation. These results call for future modeling and observational studies to corroborate the findings.
55164826
PARTICLE-TURBULENCE INTERACTIONS IN ATMOSPHERIC CLOUDS
▪ Abstract Turbulence is ubiquitous in atmospheric clouds, which have enormous turbulence Reynolds numbers owing to the large range of spatial scales present. Indeed, the ratio of energy-containing and dissipative length scales is on the order of 105 for a typical convective cloud, with a corresponding large-eddy Reynolds number on the order of 106 to 107. A characteristic trait of high-Reynolds-number turbulence is strong intermittency in energy dissipation, Lagrangian acceleration, and scalar gradients at small scales. Microscale properties of clouds are determined to a great extent by thermodynamic and fluid-mechanical interactions between droplets and the surrounding air, all of which take place at small spatial scales. Furthermore, these microscale properties of clouds affect the efficiency with which clouds produce rain as well as the nature of their interaction with atmospheric radiation and chemical species. It is expected, therefore, that fine-scale turbulence is of direct importance to the evolu...
128409975
Entrainment in Cumulus Clouds
Abstract Entrainment of dry air into cumulus clouds influences the development of the clouds in a major way. The many aspects of the entrainment process are examined in this paper by critically reviewing the literature from the time when investigations began. It is an interesting time in the evolution of the study of cumulus clouds with the advent of different models and several new instruments. Traditional entraining plume and thermal models that received considerable attention during the early years are being replaced by episodic-type mixing models. Recent observations of the source of entrained air are in part responsible for the new thinking, but the ideas really originate with the suggestion made by Squires more than 40 years age that vertical rather than horizontal mixing causes the dilution of cumuli by the process of penetrative downdrafts.
122897391
Early Radar Echoes from Ultragiant Aerosol in a Cumulus Congestus: Modeling and Observations
Abstract The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.
120526855
Variability of the Supersaturation in Cumulus Clouds.
Abstract The quasi-steady supersaturation in cumulus clouds from the Cooperative Convective Precipitation Experiment of 1981 are calculated from measurements of vertical wind, cloud droplet size, temperature and pressure. Mean values and spectral characteristics of the supersaturation are presented. The supersaturation values in 147 cloud regions averaged near 0%, with standard deviations in the (10-m average) supersaturation values that ranged from 0.1% in unmixed regions to about 0.4% in regions about 80% diluted by entrainment. The Eulerian variance spectra for vertical wind, integral radius and supersaturation were determined, and a technique is described for estimating the Lagrangian spectra. The Lagrangian characteristic times for supersaturation were estimated to be about 50 s for mixed regions but substantially larger for some regions with strong updrafts. It is argued that the observed variability in supersaturation is comparable to that needed to account for typical dispersions in droplet size s...
121994073
Effects of in‐cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds
SUMMARY Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any artie cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base. An increase of the collision kernel in turbulent surroundings is found to be an important factor in the acceleration of large droplet and raindrop formation. Droplet spectrum formation was found to be affected by three stages of in-cloud droplets’ nucleation: (a) nucleation near the cloud base, forming the primary mode of the droplet spectrum; (b) nucleation within a parcel, where supersaturation exceeds its maximum at the cloud base, this type of nucleation forming the secondary spectral mode; and (c) nucleation within the zone of intensive collisions, when a rapid decrease in drop concentration leads to an increase in supersaturation. It is shown that the secondary mode in the droplet spectrum contributes signie cantly to raindrop formation, therefore the absence of the secondary mode (the single-mode spectrum) can reduce or even inhibit formation of raindrops. The contributions of diffusion and collision growth to drop spectrum formation are compared. Effective collisions are found to start when the effective radius attains about 15 πm. The level where the effective radius attains 15 πm can be considered as the level of the e rst radar echo. This height is shown to crucially depend on cloud dynamics (in particular, on the vertical velocity at the cloud base) and on the concentration of aerosol particles.
9381132
Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 2. Theory and parameterization
The effect of air turbulence on the geometric collision kernel of cloud droplets can be predicted if the effects of air turbulence on two kinematic pair statistics can be modeled. The first is the average radial relative velocity and the second is the radial distribution function (RDF). A survey of the literature shows that no theory is available for predicting the radial relative velocity of finite-inertia sedimenting droplets in a turbulent flow. In this paper, a theory for the radial relative velocity is developed, using a statistical approach assuming that gravitational sedimentation dominates the relative motion of droplets before collision. In the weak-inertia limit, the theory reveals a new term making a positive contribution to the radial relative velocity resulting from a coupling between sedimentation and air turbulence on the motion of finite-inertia droplets. The theory is compared to the direct numerical simulations (DNS) results in part 1, showing a reasonable agreement with the DNS data for bidisperse cloud droplets. For droplets larger than 30??m in radius, a nonlinear drag (NLD) can also be included in the theory in terms of an effective inertial response time and an effective terminal velocity. In addition, an empirical model is developed to quantify the RDF. This, together with the theory for radial relative velocity, provides a parameterization for the turbulent geometric collision kernel. Using this integrated model, we find that turbulence could triple the geometric collision kernel, relative to the stagnant air case, for a droplet pair of 10 and 20??m sedimenting through a cumulus cloud at R?=2?104 and =600?cm2?s?3. For the self-collisions of 20??m droplets, the collision kernel depends sensitively on the flow dissipation rate.
123711104
A Warm Rain Microphysics Parameterization that Includes the Effect of Turbulence
Abstract A warm rain parameterization has been developed by solving the stochastic collection equation with the use of turbulent collision kernels. The resulting parameterizations for the processes of autoconversion, accretion, and self-collection are functions of the turbulent intensity of the flow and are applicable to turbulent cloud conditions ranging in dissipation rates of turbulent kinetic energy from 100 to 1500 cm2 s−3. Turbulence has a significant effect on the acceleration of the drop size distribution and can reduce the time to the formation of raindrops. When the stochastic collection equation is solved with the gravitational collision kernel for an initial distribution with a liquid water content of 1 g m−3 and 240 drops cm−3 with a mean volume radius of 10 μm, the amount of mass that is transferred to drop sizes greater than 40 μm in radius after 20 min is 0.9% of the total mass. When the stochastic collection equation is solved with a turbulent collision kernel for collector drops in the r...
51772966
The role of air turbulence in warm rain initiation
Quantitative parameterization of turbulent collision of cloud droplets represents a major unsolved problem in cloud physics. Here a hybrid direct simulation tool is used specifically to quantify the turbulent enhancement of the gravitational collision-coalescence. Simulation results show that air turbulence can enhance the collision kernel by an average factor of about 2, and the observed trends are supported by scaling arguments. An impact study using the most realistic collection kernel suggests that cloud turbulence can significantly reduce the time for warm rain initiation. Areas for further development of the hybrid simulation and the impact study are indicated. Copyright © 2009 Royal Meteorological Society
55752153
Giant Sea-Salt Aerosols and Warm Rain Formation in Marine Stratocumulus
Abstract The concentrations and sizes of smaller aerosols (radius smaller than 0.5 μm) in the marine atmosphere vary owing to natural and anthropogenic factors. The concentrations and sizes of giant and ultragiant aerosols vary primarily due to wind-speed-dependent wave breaking. In climate models the formation of warm rain from marine stratocumulus clouds is usually parameterized based on the drops that form on the smaller aerosols. The present process study, using a stochastic Monte Carlo cloud model, shows that the variability of giant sea-salt aerosols and the variability of smaller aerosol cloud condensation nuclei are equally important in determining precipitation flux in marine stratocumulus. This strongly suggests that the effects of giant sea-salt aerosols should be included in the parameterization of warm rain formation in climate and other large-scale models. The above results are based on highly detailed calculations of droplet growth in an idealized marine stratocumulus cloud; the authors bel...
129304930
The potential impacts of pollution on a nondrizzling stratus deck : Does aerosol number matter more than type?
[1] In this paper results from a cloud-resolving model that can efficiently examine the impact of aerosol on nondrizzling stratus clouds will be shown. Because the model tracks aerosol and cloud droplets in a Lagrangian framework, it does not suffer from numerical errors associated with advection, and unlike most Eulerian approaches, the method can track cloud boundaries as they move across a grid cell. After illustrating the capability of the model to reproduce various observed cloud statistics such as the cloud water mixing ratio and the mean cloud droplet radius from the DYCOMS-II field program, the ability of the model to assess the impact of changes in aerosol number and composition on a stratus deck will be highlighted. Specifically, by using activation curves appropriate for soluble, insoluble, or a mixture of both types of aerosol and for certain extreme aerosol regimes, i.e., a majority of the aerosol are hydrophobic carbon aerosol, limiting situations were examined to bound their impact on clouds. However, though these situations may be somewhat extreme, they could occasionally occur in the atmosphere, e.g., an oceanic stratus field downwind of a large ship or an urban area. Not unexpectedly, results from these simulations support previous ship track observations that for increasing aerosol numbers, cloud droplet number concentrations increase, whereas cloud droplet radii decrease. However, these simulations also suggest that the correlation between cloud droplet number concentration and aerosol number concentration may be not only a function of aerosol number concentration but also aerosol types and/or cloud dynamics.
119365198
The super-droplet method for the numerical simulation of clouds and precipitation: a particle-based and probabilistic microphysics model coupled with a non-hydrostatic model
A novel, particle-based, probabilistic approach for the simulation of cloud microphysics is proposed, which is named the super-droplet method (SDM). This method enables the accurate simulation of cloud microphysics with a less demanding cost in computation. SDM is applied to a warm-cloud system, which incorporates sedimentation, condensation/evaporation and stochastic coalescence. The methodology to couple super-droplets and a non-hydrostatic model is also developed. It is confirmed that the result of our Monte Carlo scheme for the stochastic coalescence of super-droplets agrees fairly well with the solutions of the stochastic coalescence equation. The behaviour of the model is evaluated using a simple test problem, that of a shallow maritime cumulus formation initiated by a warm bubble. Possible extensions of SDM are briefly discussed. A theoretical analysis suggests that the computational cost of SDM becomes lower than the spectral (bin) method when the number of attributes—the variables that identify the state of each super-droplet—becomes larger than some critical value, which we estimate to be in the range . Copyright © 2009 Royal Meteorological Society
85556655
Investigation of droplet dynamics in a convective cloud using a Lagrangian cloud model
AbstractA precipitating convective cloud is simulated successfully using the Lagrangian cloud model, in which the flow field is simulated by large eddy simulation and the droplets are treated 
as Lagrangian particles, and the results are analyzed to investigate precipitation initiation and to examine the parameterization of cloud microphysics. It is found that raindrops appear initially near the cloud top, in which strong turbulence and broadened droplet spectrum are induced by the entrainment of dry air, but high liquid–water mixing ratio is maintained within cloud parts because of insufficient mixing. Statistical analysis of the downward vertical velocity of a droplet W reveals that the transition from cloud droplets to raindrops occurs in the range 20 μm < r < 100 μm, while the variation of W depends on turbulence as well as the droplet radius r. The general pattern of the raindrop size distribution is found to be consistent with the Marshall–Palmer distribution. The precipitation flux can be underestimated substantially, if the terminal velocity $$w_{\text{s}}$$ws is used instead of W, but it is not sensitive to the choice of the critical droplet radius dividing cloud drops and raindrops. It is also found that precipitation starts earlier and becomes stronger if the effect of turbulence is included in the collection kernel.
121512288
PALM - A large-eddy simulation model performing on massively parallel computers
An existing code of a large-eddy simulation (LES) model for the study of turbulent processes in the atmospheric and oceanic boundary layer has been completely recoded for use on massively parallel systems with distributed memory. Parallelization is achieved by two-dimensional domain decomposition and communication is realized by the message passing interface (MPI). Periodic boundary conditions, which are used in both horizontal directions, helped to minimize the parallelization effort. The performance of the new PArallelized LES Model (PALM) is excellent on SGI/Cray-T3E systems and an almost linear speed-up is achieved up to very large numbers of processors. Parallelization strategy and model performance is discussed and validation experiments as well as future applications are presented.
54530853
The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives
Abstract. In this paper we present the current version of the Parallelized Large-Eddy Simulation Model (PALM) whose core has been developed at the Institute of Meteorology and Climatology at Leibniz Universitat Hannover (Germany). PALM is a Fortran 95-based code with some Fortran 2003 extensions and has been applied for the simulation of a variety of atmospheric and oceanic boundary layers for more than 15 years. PALM is optimized for use on massively parallel computer architectures and was recently ported to general-purpose graphics processing units. In the present paper we give a detailed description of the current version of the model and its features, such as an embedded Lagrangian cloud model and the possibility to use Cartesian topography. Moreover, we discuss recent model developments and future perspectives for LES applications.
122092920
Comparison of Raindrop Size Distributions Measured by Radar Wind Profiler and by Airplane
Abstract Wind profilers are radars that operate in the VHF and UHF hands and are designed for detecting the weak echoes reflected by the optically clear atmosphere. An unexpected application of wind profilers has been the revival of an old method of estimating drop size distributions in rain from the Doppler spectrum of the received signal. Originally attempted with radars operating at microwave frequency, the method showed early promise but was seriously limited in application because of the crucial sensitivity of the estimated drop sizes to the vertical air velocity, a quantity generally unknown and, at that time, unmeasurable. Profilers have solved this problem through their ability to measure, under appropriate conditions, both air motions and drop motions. This paper compares the drop sizes measured by a UHF profiler at two altitudes in a shower with those measured simultaneously by an instrumented airplane. The agreement is satisfactory, lending support to this new application of wind profilers.
55340899
A new method for large-eddy simulations of clouds with Lagrangian droplets including the effects of turbulent collision
In this paper, a new Lagrangian cloud model (LCM) is introduced in which the flow field is simulated by large-eddy simulation, and the droplets are treated as Lagrangian particles responding to the simulated flow field. In order to handle the extremely large number of droplets within a cloud, the concept of a super-droplet, which represents a large number of real droplets of the same size, is introduced, and the number of contributing real droplets is called the weighting factor. A novel method is developed to realize the collision/coalescence of droplets, in which the consequent variation of the droplet spectrum is represented in terms of the modification of the radius and weighting factor of super-droplets, while keeping the number of super-droplets unchanged. Using an idealized single cloud and trade wind cumuli, the LCM is shown to reproduce the general features of shallow cumulus clouds in agreement with traditional bulk models. The droplet spectrum simulated by the LCM, using collision kernels with and without the effects of turbulence, also shows a pattern consistent with the spectral bin model. Furthermore, the sensitivity of the LCM to two model parameters, the time step and the number of super-droplets, is examined.
122034580
A NEW ASPECT OF COALESCENCE THEORY
Abstract The process of growth by coalescence is examined from the viewpoint of a discrete rather than a continuous accretion process. It is concluded that, for drops beginning growth at twice the volume of their neighbors, random fluctuations in the times taken for different drops to effect captures can lead to the formation of a complete raindrop spectrum in times shorter than that required for growth to raindrop size by the continuous growth process. Under typical conditions, drops of 23-microns radius can form in the order of 5 minutes in a cloud of droplets of 10 microns, while smaller numbers of much larger drops can be expected in reasonable times. The latter are likely to be important in chain-reaction theories.
55611519
Collection/aggregation algorithms in Lagrangian cloud microphysical models: Rigorous evaluation in box model simulations
Abstract. Recently, several Lagrangian microphysical models have been developed which use a large number of (computational) particles to represent a cloud. In particular, the collision process leading to coalescence of cloud droplets or aggregation of ice crystals is implemented differently in various models. Three existing implementations are reviewed and extended, and their performance is evaluated by a comparison with well-established analytical and bin model solutions. In this first step of rigorous evaluation, box model simulations, with collection/aggregation being the only process considered, have been performed for the three well-known kernels of Golovin, Long and Hall. Besides numerical parameters, like the time step and the number of simulation particles (SIPs) used, the details of how the initial SIP ensemble is created from a prescribed analytically defined size distribution is crucial for the performance of the algorithms. Using a constant weight technique, as done in previous studies, greatly underestimates the quality of the algorithms. Using better initialisation techniques considerably reduces the number of required SIPs to obtain realistic results. From the box model results, recommendations for the collection/aggregation implementation in higher dimensional model setups are derived. Suitable algorithms are equally relevant to treating the warm rain process and aggregation in cirrus.
34455375
Rain in Shallow Cumulus Over the Ocean: The RICO Campaign
Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...
55561168
Effective radius and droplet spectral width from in-situ aircraft observations in trade-wind cumuli during RICO
Received 19 March 2009; accepted 28 April 2009; published 3 June 2009. [1] This paper presents statistics of cloud microphysical properties of shallow tropical cumuli observed by a research aircraft during RICO field campaign. Cloud properties are derived from 10 Hz (about 10 m spatial distance) Fast-FSSP data in four different flights. The motivation comes from similar analyses of either aircraft data from stratocumulus clouds or remote-sensing data of tropical cumuli. In the lowest few hundred meters, the standard deviation of the droplet size distribution sr and the relative dispersion, the ratio of sr and the mean radius, are similar to stratocumulus clouds, but they are significantly larger in the upper half of the cloud field depth. The frequency distribution of the effective radius is significantly narrower than in the remote-sensing observations in the middle and upper third of the cloud field. These results can be used in parameterizations and validations of cloud microphysics in numerical models of various complexity. Citation: Arabas, S., H. Pawlowska, and W. W. Grabowski (2009), Effective radius and droplet spectral width from insitu aircraft observations in trade-wind cumuli during RICO, Geophys. Res. Lett., 36, L11803, doi:10.1029/2009GL038257.
122159164
A Detailed Microphysical Model Within a Two-Dimensional Dynamic Framework: Model Description and Preliminary Results
Abstract A two-dimensional anelastic cloud model which incorporates detailed treatments of the water and ice phase is presented. The liquid phase processes considered include condensation, quasi-stochastic coalescence, fallout and breakup, while the ice phase processes include diffusional and accretional growth of ice particles. Results of two cloud simulations are presented. The first case assumes an atmosphere with maritime cloud condensation nuclei (CCN) activation characteristics and considers the warm rain processes only. It was found that with the appearance of precipitation, the model-predicted supersaturations within updraft regions often reach values larger than 5% with respect to water. The second case assumes an atmosphere with continental CCN characteristics and includes the ice phase processes leading to the formation of graupel. The results of the second case illustrate the importance of cloud vertical motions in transporting ice particles from preferential nucleation regions in the upper po...
123035620
A Cumulus Cloud Microphysics Parameterization for Cloud-Resolving Models
AbstractA microphysical parameterization for shallow cumulus and boundary layer stratocumulus clouds has been developed. Similar to the Khairoutdinov and Kogan parameterization for stratocumulus clouds, the new parameterization is based on an explicit microphysical large-eddy simulation (LES) model as a data source and benchmark for comparison. The predictions of the bulk model using the new parameterization were tested in simulations of shallow cumulus and boundary layer stratocumulus clouds; in both cases the new parameterization matched the predictions of the explicit microphysics LES quite accurately. These results show the importance of the choice of the dataset in parameterization development and the need for it to be balanced by realistic dynamic conditions. The strong sensitivity to representation of rain evaporation is also demonstrated. Accurate formulation of this process, tuned for the case of cumulus convection, has substantially improved precision of rain production.
55214237
New Observations of Precipitation Initiation in Warm Cumulus Clouds
Abstract The mechanism responsible for formation of rain in warm clouds has been debated for over six decades. Here, the authors analyze new measurements of shallow cumulus made with a phase Doppler interferometer during the Rain in Cumulus over the Ocean (RICO) experiment. These observations show that drops sufficiently large (>55-μm diameter) to initiate precipitation (termed collision–coalescence initiators or CCIs) are found preferentially at cloud top, tend to cluster with each other, and are found in environments that are thermodynamically, dynamically, and microphysically distinct from those of smaller drops. The CCI environments exhibit cloud spectra that are shifted to larger sizes, with enhanced broadening toward larger drop sizes. Increased entrainment is also associated with CCIs, suggesting that it is an important process in CCI production. A simple model combining inhomogeneous mixing and condensation is inadequate to explain these observations. It is hypothesized that CCIs are produced in c...
119911800
The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixing
In this, the first of two related papers, we present calculations of the growth of a population of condensate droplets rising above cloud base within small cumuli which are entraining undersaturated environmental air. It is assumed, on the basis of dimensional arguments and laboratory experiments on entrainment, conducted within a cloud droplet evolution tunnel, that this mixing process is inhomogeneous. 
 
 
 
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. 
 
 
 
The calculations based on the extreme inhomogeneous model produce spectral shapes which agree well with those reported in cumulus by Warner (1969a) and indicate that a small proportion of the droplets can grow several times faster through the condensate spectrum than classical theory predicts.
53596095
Entrainment, Mixing, and Microphysics in Trade-Wind Cumulus
The vertical evolution of microphysics in trade-wind cumuli (Cu) observed from the NCAR C-130 research aircraft during one flight of the RICO (Rain in Cumulus Over the Ocean) study is analyzed. Conditional sampling of > 200 Cu traversed on this flight is used to chose Cu for which the aircraft penetrated single and growing Cu turrets about 250-m below cloud top where maximum LWC is often found and where radar has often observed initial stages of precipitation. The vertical evolution of the sampled set of Cu was assumed to follow Lagrangian behavior. The entrainment rate, entrained parcel scales, mixing mechanisms, and effects on the droplet size distribution are measured and evaluated. A parcel model is applied over the 1100-m maximum Cu height of the traverses to determine the relationship between the observed large number of small droplets and the fewer ultra-giant sea-salt nuclei (UGN) in order to assess the role of these nuclei in evolving the size spectrum and in causing a growing “drizzle tail”. New insight on these topics is obtained by using the PVM (Particle Volume Monitor) probe to measure incloud microphysics with 10-cm resolution.The results include the following: Entrainment causes primarily dilution of the drops without significant size changes, thus either extreme inhomogeneous mixing or more likely homogeneous mixing resulting from mixing with cool and humid entrained air take place. The entrained parcels are surprisingly small following lognormal behavior and decaying rapidly upon entering the Cu, as a result super-adiabatic drops are not evident. The entrained parcels are consistent with the Bragg-scattering “mantle echo” often observed by radar in small Cu. The FSSP (Forward Scattering Spectrometer Probe) droplet spectra are nearly constant with height. These “self-preserving” spectra are a result of an approximate balance between dilution by entrainment of droplets originating at cloud base, droplet activation on entrained CCN (cloud condensation nuclei), and detrainment and coalescence losses. Sea-salt nuclei follow Woodcock’s wind dependence, and are shown with the parcel model to play an important role in forming the observed drizzle that increases with cloud height. Accretion is the dominant coalescence mechanism near cloud top in these Cu.
129724307
Linear relation between convective cloud drop number concentration and depth for rain initiation
[1] Coalescence of cloud droplets is essential for the production of small raindrops at a given vertical distance above the cloud base (Dp). The rate of droplet coalescence is determined mainly by droplet size, spectrum width and concentrations. The droplet condensational growth is determined by the number of activated CCN (Na) and height above cloud base. Here we show that when the droplet mean volume radius, rv, exceeds ∼13 μm, or when droplet effective radius (re) exceeds ∼14 μm, considerable precipitation mass (>0.03 g kg−1) is likely to be present in growing convective clouds. This is because the rate of droplet coalescence is proportional to ∼rv5 which practically implies the existence of a threshold rv above which efficient warm rain formation can occur, and also because the vertical profile of rv, even in diluted clouds, nearly follows the theoretical adiabatic condensational growth curve. The small observed deviations are mainly caused by deviations from purely inhomogeneous mixing which cause partial droplet evaporation. Consequently, Dp must theoretically change nearly linearly with Na. This is confirmed here observationally, where increasing Na by 100 per milligram (≈cm3 at cloud base) of air, resulted in an increase of ∼280 m in Dpfor both Israeli and Indian deep convective clouds. This means that in highly polluted clouds or where strong cloud-base updrafts occur, clouds have to grow well above the freezing level, even in tropical atmosphere, before precipitation forms either by warm or by mixed-phase processes.
120571309
Turbulence effects on warm‐rain autoconversion in precipitating shallow convection
A parametrization of the rain formation in warm clouds is developed which includes the effects of turbulence on the collision rate of droplets in a cloud. It is shown that already moderate turbulence with dissipation rates of 400 cm2s−3 can lead to a significant speed-up of the rain formation corresponding to an increase in the autoconversion rate by a factor of 4–6 depending on the size of the droplets. Large-eddy simulations of trade wind cumuli also produce a significant enhancement of rain formation through turbulence effects on cloud microphysics. In small cumulus clouds, the highest turbulent dissipation rates occur near cloud-top, which is also the decisive region of the cloud for the initial rain formation. Copyright © 2010 Royal Meteorological Society
53330610
Untangling Microphysical Impacts on Deep Convection Applying a Novel Modeling Methodology
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 (...
121655141
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.
120782849
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...
124257507
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 ...
18778820
Supersaturation of Water Vapor in Clouds
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.