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Graham Feingold and Christian J. Grund

Abstract

This paper addresses the feasibility of using mulliwavelength lidar measurements to differentiate both qualitatively and quantitatively between the relative concentrations of hygroscopic and nonhygroscopic aerosol particles. The proposed technique utilizes the fact that hygroscopic particles undergo a size increase and refractive-index change with increasing relative humidity and that different wavelengths respond to these changes in different ways. The lidar wavelengths considered are 0.289, 0.355, 0.532, 0.694, 1.064, and 2.02 µm and the 9–11.5-µm range. It is shown that under certain conditions, a judicious choice of lidar wavelengths can provide a differential backscatter, sufficient to provide information on the size and percentage number concentration of the hygroscopic aerosol and, consequently, cloud condensation nuclei concentration. The presence of a mode of coarse particles (median radius greater than 0.3 µm) produces ambiguous results and limits application of the technique to regions sufficiently distant from coarse mode sources (e.g., in the free troposphere). The authors have identified a pair of wavelengths in the infrared region that provides a clear indication of the existence of these particles. The potential benefits of distinguishing hygroscopic particle concentration from nonhygroscopic particle concentration are great since remote measurement can provide good temporal and spatial coverage of these properties and valuable information for climate monitoring.

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Graham Feingold and Patrick Y. Chuang

Abstract

Decades of cloud microphysical research have not provided conclusive understanding of the physical processes responsible for droplet spectral broadening. Numerous mechanisms have been proposed—for example, entrainment mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing of CCN, and radiative cooling—all of which are likely candidates under select conditions. In this paper it is suggested that variability in the composition of CCN, and in particular, the existence of condensation inhibiting compounds, is another possible candidate. The inferred potential abundance of these amphiphilic film-forming compounds (FFCs) suggests that their effect may be important. Using a cloud parcel model with a simplified treatment of the effect of FFCs, it is shown that modest concentrations of FFCs (on the order of 5% of the total aerosol mass) can have a marked effect on drop growth and can cause significant increases in spectral dispersions. Moreover, it is shown that FFCs may, in some cases, reduce the number concentration of cloud droplets, with implications for cloud-climate feedbacks. This trend is at least in qualitative agreement with results from a recent field campaign.

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Graham Feingold and Andrew J. Heymsfield

Abstract

Motivated by the importance of the effective radius (re) of the droplets to radiative transfer, this paper presents parameterization schemes, which provide a measure of re in stratiform liquid water clouds (in the −13° to +13°C temperature range), for use in general circulation models (GCMs) or mesoscale models. The first scheme developed here is based on theory and numerical calculations of droplet condensational growth, while the second is based on Twomey's analytical approach. Both methods are evaluated against detailed model calculations, and a method for implementing either scheme in general circulation models and remote sensing applications is described. The new parameterization produces accurate (within a few percent) estimates of the effective droplet radius as a function of height, while the cloud optical thickness compares favorably (often to within <10%) with the model calculations. Twomey's scheme gives reasonable estimates of optical thickness, but tends to underestimate the droplet concentration and overestimate the effective radius for typical maritime and continental CCN spectra.

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Adrian A. Hill, Graham Feingold, and Hongli Jiang

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This study uses large-eddy simulation with bin microphysics to investigate the influence of entrainment and mixing on aerosol–cloud interactions in the context of idealized, nocturnal, nondrizzling marine stratocumulus (Sc). Of particular interest are (i) an evaporation–entrainment effect and a sedimentation–entrainment effect that result from increasing aerosol concentrations and (ii) the nature of mixing between clear and cloudy air, where homogeneous and extreme inhomogeneous mixing represent the bounding mixing types. Simulations are performed at low resolution (Δz = 20 m; Δx, y = 40 m) and high resolution (Δz = 10 m; Δx, y = 20 m). It is demonstrated that an increase in aerosol from clean conditions (100 cm−3) to polluted conditions (1000 cm−3) produces both an evaporation–entrainment and a sedimentation–entrainment effect, which couple to cause about a 10% decrease in liquid water path (LWP) when all warm microphysical processes are included. These dynamical effects are insensitive to both the resolutions tested and the mixing assumption. Regardless of resolution, assuming extreme inhomogeneous rather than homogeneous mixing results in a small reduction in cloud-averaged drop number concentration, a small increase in cloud drop effective radius, and ∼1% decrease in cloud optical depth. For the case presented, these small changes play a negligible role when compared to the impact of increasing aerosol and the associated entrainment effects. Finally, it is demonstrated that although increasing resolution causes an increase in LWP and number concentration, the relative sensitivity of cloud optical depth to changes in aerosol is unaffected by resolution.

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Fabian Hoffmann, Takanobu Yamaguchi, and Graham Feingold

Abstract

Although small-scale turbulent mixing at cloud edge has substantial effects on the microphysics of clouds, most models do not represent these processes explicitly, or parameterize them rather crudely. This study presents a first use of the linear eddy model (LEM) to represent unresolved turbulent mixing at the subgrid scale (SGS) of large-eddy simulations (LESs) with a coupled Lagrangian cloud model (LCM). The method utilizes Lagrangian particles to provide the trajectory of air masses within LES grid boxes, while the LEM is used to redistribute these air masses among the Lagrangian particles based on the local features of turbulence, allowing for the appropriate representation of inhomogeneous to homogeneous SGS mixing. The new approach has the salutary effect of mitigating spurious supersaturations. At low turbulence intensities, as found in the early stages of an idealized bubble cloud simulation, cloud-edge SGS mixing tends to be inhomogeneous and the new approach is shown to be essential for the production of raindrop embryos. At higher turbulence intensities, as found in a field of shallow cumulus, SGS mixing tends to be more homogeneous and the new approach does not significantly alter the results, indicating that a grid spacing of 20 m may be sufficient to resolve all relevant scales of inhomogeneous mixing. In both cases, droplet in-cloud residence times are important for the production of precipitation embryos in the absence of small-scale inhomogeneous mixing, either naturally due to strong turbulence or artificially as a result of coarse resolution or by not using the LEM as an SGS model.

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Shalva Tzivion (Tzitzvashvili), Graham Feingold, and Zev Levin

Abstract

The evolution of raindrop spectra with altitude through collisional collection/breakup sedimentation and evaporation is presented. Two-moment treatment of sedimentation and evaporation is developed to complement Part I (Feingold et al.) of this series. We have obtained an accurate, stable numerical scheme for evaporation that enables the investigation of the effect of evaporation on spectra subject to entrainment of strongly subsaturated air (including ventilation). The method includes provision for treatment of the variation of the sub/supersaturation within a time step in a dynamical framework. Results confirm that steady-state raindrop spectra are characterized by a bimodal or trimodal structure that becomes evident shortly after evolution commences. After sufficient evolution, peaks become clearly defined at 0.25 mm and 0.8 mm and further evolution with altitude affects only the relative magnitude of these peaks. It is shown that the evaporation process is not only dependent on the subsaturation of ambient air but is also strongly dependent on the shape of the drop spectrum. Evaporation tends to increase the number of the smallest raindrops (≤ 0.1 mm) at the expense of the larger drops but does not modify the position of the peaks. The effect of drop spectral evolution on radar reflectivity (Z) and scavenging (Λ) profiles is studied.

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Hongli Jiang, Graham Feingold, and Armin Sorooshian

Abstract

Large-eddy simulations of warm, trade wind cumulus clouds are conducted for a range of aerosol conditions with a focus on precipitating clouds. Individual clouds are tracked over the course of their lifetimes. Precipitation rate decreases progressively as aerosol increases. For larger, precipitating clouds, the polluted clouds have longer lifetimes because of precipitation suppression. For clean aerosol conditions, there is good agreement between the average model precipitation rate and that calculated based on observed radar reflectivity Z and precipitation rate R relationships. Precipitation rate can be expressed as a power-law function of liquid water path (LWP) and Nd, to reasonable accuracy. The respective powers for LWP and Nd are of similar magnitude compared to those based on observational studies of stratocumulus clouds. The time-integrated precipitation rate represented by a power-law function of LWP, Nd, and cloud lifetime is much more reliably predicted than is R expressed in terms of LWP and Nd alone. The precipitation susceptibility (So = −dlnR/dlnNd) that quantifies the sensitivity of precipitation to changes in Nd depends strongly on LWP and exhibits nonmonotonic behavior with a maximum at intermediate LWP values. The relationship between So and precipitation efficiency is explored and the importance of including dependence on Nd in the latter is highlighted. The results provide trade cumulus cloud population statistics, as well as relationships between microphysical/macrophysical properties and precipitation, that are amenable for use in larger-scale models.

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Shouping Wang, Qing Wang, and Graham Feingold

Abstract

Condensation and turbulent liquid water transport in stratocumulus clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play essential roles in defining the cloud structure. This work aims at understanding this dynamical–microphysical interaction and providing information necessary for parameterizations of the ensemble mean condensation rate and turbulent fluxes of liquid water variables in a coupled turbulence–microphysics model. The approach is to simulate nonprecipitating stratocumulus clouds with a coupled large eddy simulation and an explicit bin-microphysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration, and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensation in turbulent updrafts and reduces evaporation in the downdrafts. Turbulent liquid water flux results from a close balance between turbulence dynamics and microphysical processes. Consequently, the flux can be parameterized in terms of the common diffusive downgradient formulation, fluxes of conservative thermodynamic variables, the turbulence mixing timescale, and the condensation timescale, which is determined by the droplet spectrum. The results also suggest that the condensation timescale regulates the turbulence fields, as does the number concentration, because it affects the condensation fluctuation, which is highly correlated with the turbulence vertical motion. A saturation adjustment cloud model, which diagnoses liquid water content at its equilibrium level, instantly condenses (evaporates) all available water vapor (liquid water) surplus. Consequently, there is likely to be a systematic difference between the turbulence field resolved with this type of model and that with a supersaturation-based cloud scheme for which a finite condensation timescale applies.

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Takanobu Yamaguchi, W. Alan Brewer, and Graham Feingold

Abstract

Numerically modeled turbulence simulated by the Advanced Research Weather Research and Forecasting Model (ARW) is evaluated with turbulence measurements from NOAA’s high-resolution Doppler lidar on the NOAA Research Vessel Ronald H. Brown during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study—Regional Experiment (VOCALS-Rex) field program. A nonprecipitating nocturnal marine stratocumulus case is examined, and a nudging technique is applied to allow turbulence to spin up and come into a statistically stationary state with the initial observed cloud field. This “stationary” state is then used as the initial condition for the subsequent free-running simulation. The comparison shows that the modeled turbulence is consistently weaker than that observed. For the same resolution, the turbulence becomes stronger, especially for the horizontal component, as the length of the horizontal domain increases from 6.4 to 25.6 km. Analysis of the power spectral density shows that, even for the largest domain, the horizontal component of the turbulence is limited by the upper limit of the domain size; supporting evidence from past studies is provided. Results suggest that convergence is expected for (i) energy spectra of turbulence with a sufficiently large domain and (ii) liquid water path with an adequately large domain and fine resolution. Additional tests are performed by changing momentum advection and turning off subgrid-scale diffusion. These exhibit more significant changes in turbulence characteristics compared to the sensitivity to domain size and resolution, suggesting that the model behavior is essentially established by the configuration of the model dynamics and physics and that the simulation only gradually improves when domain size and resolution are increased.

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Huiwen Xue, Graham Feingold, and Bjorn Stevens

Abstract

This study investigates the effects of aerosol on clouds, precipitation, and the organization of trade wind cumuli using large eddy simulations (LES). Results show that for this shallow-cumulus-under-stratocumulus case, cloud fraction increases with increasing aerosol as the aerosol number mixing ratio increases from 25 (domain-averaged surface precipitation rate ∼0.65 mm day−1) to 100 mg−1 (negligible surface precipitation). Further increases in aerosol result in a reduction in cloud fraction. It is suggested that opposing influences of aerosol-induced suppression of precipitation and aerosol-induced enhancement of evaporation are responsible for this nonmonotonic behavior.

Under clean conditions (25 mg−1), drizzle is shown to initiate and maintain mesoscale organization of cumulus convection. Precipitation induces downdrafts and cold pool outflow as the cumulus cell develops. At the surface, the center of the cell is characterized by a divergence field, while the edges of the cell are zones of convergence. Convergence drives the formation and development of new cloud cells, leading to a mesoscale open cellular structure. These zones of new cloud formation generate new precipitation zones that continue to reinforce the cellular structure. For simulations with an aerosol concentration of 100 mg−1 the cloud fields do not show any cellular organization. On average, no evidence is found for aerosol effects on the lifetime of these clouds, suggesting that cloud fraction response to changes in aerosol is tied to the frequency of convection and/or cloud size.

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