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Seoung-Soo Lee
,
Graham Feingold
, and
Patrick Y. Chuang

Abstract

This study examines the role of aerosol in mediating interactions between a warm trade cumulus cloud system and the environment that spawns it. Numerical simulations of the observed and well-studied Rain in Cumulus over the Ocean (RICO) field experiment are performed. The results draw on simulations of 34-h duration so as to avoid conclusions based on transients. Simulations show that, on average, aerosol-perturbed clouds are initially deeper and more vigorous but that after about 14 h there is a reversal in this trend, and unperturbed clouds deepen relative to the perturbed clouds. Differences in cloud depth are about 100 m, and differences in vertical velocity variance are about 30%. After about 20 h, most cloud fields are statistically similar with the exception of rain rate and optical depth, which are lower and higher, respectively, in the high-aerosol conditions. By sampling the model output at various points in the cloud system evolution, the mechanisms responsible for the initial differences and then convergence of most of the cloud field properties are addressed. Sensitivity tests indicate that responses are driven primarily by temperature profiles, rather than by humidity profiles, and that the general trend to homogenization of the bulk cloud field properties is robust for different forcings. Finally, the paper shows that even transient aerosol perturbations may endure beyond the duration of the perturbation itself, provided they persist long enough. Short-duration aerosol perturbations are unlikely to have much influence on the system.

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Bjorn Stevens
,
Graham Feingold
,
William R. Cotton
, and
Robert L. Walko

Abstract

A set of 500 simulated trajectories and a simple parcel model are used to (i) evaluate the performance of a large eddy simulation model coupled to a detailed representation of the droplet spectrum (the LES-BM model) and (ii) gain insight into the microphysical structure of numerically simulated nonprecipitating stratocumulus. The LES-BM model reasonably reproduces many observed features of stratocumulus. The largest sources of error appear to be associated with limited vertical resolution, the neglect of gas kinetic effects and the inability of the model to properly represent mixing across cloud interfacial boundaries. The first two problems have simple remedies; for instance, a condensation–nucleation scheme is derived that includes gas–kinetic effects thus obviating the second problem. The third source of error poses a more vexing, and as yet unsolved, problem for models of the class described herein.

Trajectories timescales are analyzed and in-cloud residence times are found to be, in the mean, on the order of the large eddy turnover time. In addition, it is shown that the length of time trajectories spend near cloud top may be an important factor in the droplet growth equation for a certain favored subset of trajectories. An analysis of the adiabatic trajectory data also indicates that (i) values of diameter dispersion are a factor of 2 to 5 smaller than commonly observed; (ii) simulated values of the dispersion in number concentration are found to be explainable solely on the basis of trajectories having different updraft velocities; (iii) diameter dispersions are not found to be equal to a third of number dispersions, nor did they relate simply to the dispersion in the cloud-base updraft velocity.

Problems with coupling one- and two-dimensional models to detailed representations of the droplet spectrum are discussed. In the case of the former, the lack of an explicit representation of turbulent eddies requires that the coupling between the microphysics and the dynamics be parameterized. In the case of the latter, boundary layer eddies are represented, thus allowing for a more reasonable coupling between turbulence and microphysics. However, the resolved eddies have a different structure than their three-dimensional counterparts, one consequence of which is that timescales of in-cloud circulations are found to be shorter and have less variability.

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Fabian Hoffmann
,
Franziska Glassmeier
,
Takanobu Yamaguchi
, and
Graham Feingold

Abstract

Stratocumulus clouds constitute one of the largest negative climate forcings in the global radiation budget. This forcing is determined, inter alia, by the cloud liquid water path (LWP), which we analyze using a combination of Gaussian process emulation and mixed-layer theory. For nocturnal, nonprecipitating stratocumuli, we show that LWP steady states constitute an equilibrium primarily between radiative cooling and entrainment warming and drying. These steady states are approached from lower LWPs due to reduced entrainment, while higher LWPs are depleted by stronger entrainment. An analytical solution for the LWP steady state reveals not only the environmental conditions in which a stratocumulus cloud can be maintained, but also distinct analytical properties of the entrainment velocity that are required for a stable LWP steady state that opposes perturbations. In particular, the results highlight the importance of an entrainment velocity that increases strictly monotonically with the LWP if stratocumuli are to attain a stable LWP steady state. This is demonstrated through analysis of two commonly used mixed-layer entrainment parameterizations.

Free access
Bjorn Stevens
,
William R. Cotton
,
Graham Feingold
, and
Chin-Hoh Moeng

Abstract

Large-eddy simulations that incorporate a size-resolving representation of cloud water are used to study the effect of heavy drizzle on PBL structure. Simulated surface precipitation rates average about 1 mm day−1. Heavily drizzling simulations are compared to nondrizzling simulations under two nocturnal PBL regimes—one primarily driven by buoyancy and the other driven equally by buoyancy and shear. Drizzle implies a net latent heating in the cloud that leads to sharp reductions in both entrainment and the production of turbulent kinetic energy by buoyancy (particularly in downdrafts). Drizzle, which evaporates below cloud base, promotes a cooler and moister subcloud layer that further inhibits deep mixing. The cooling and moistening is in quantitative agreement with some observations and is shown to favor the formation of cumuli rising out of the subcloud layer. The cumuli, which are local in space and time, are responsible for most of the heat and moisture transport. They also appear to generate a larger-scale circulation that differs dramatically from the regularity typically found in nonprecipitating stratocumulus. Time-averaged turbulent fluxes of heat and moisture increase in the presence of precipitation, suggesting that drizzle (and drizzle-induced stratification) should not necessarily be taken as a sign of decoupling. Because drizzle primarily affects the vertical distribution of buoyancy, shear production of turbulent kinetic energy mitigates some of the effects described above. Based on large-eddy simulation the authors hypothesize that shallow, well-mixed, radiatively driven stratocumulus cannot persist in the presence of heavy drizzle. In accord with some simpler models, the simulated case with heavy precipitation promotes a reduction in both liquid-water path and entrainment. However, the simulations suggest that time-integrated cloud fraction may increase as a result of drizzle because thinner precipitating clouds may persist longer if the boundary layer does not deepen as rapidly. These somewhat more complicated dynamics have important implications for a number of hypotheses suggesting that changes in aerosol concentrations, when metabolized by stratocumulus, have a significant effect on climate.

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Ian B. Glenn
,
Graham Feingold
,
Jake J. Gristey
, and
Takanobu Yamaguchi

Abstract

The indirect radiative effect of aerosol variability on shallow cumulus clouds is realized in nature with considerable concurrent meteorological variability. Large-eddy simulations constrained by observations at a continental site in Oklahoma are performed to represent the variability of different meteorological states on days with different aerosol conditions. The total radiative effect of this natural covariation between aerosol and other meteorological drivers of total cloud amount and albedo is quantified. The changes to these bulk quantities are used to understand the response of the cloud radiative effect to aerosol–cloud interactions (ACI) in the context of concurrent processes, as opposed to attempting to untangle the effect of individual processes on a case-by-case basis. Mutual information (MI) analysis suggests that meteorological variability masks the strength of the relationship between cloud drop number concentration and the cloud radiative effect. This is shown to be mostly due to variation in solar zenith angle and cloud field horizontal heterogeneity masking the relationship between cloud drop number and cloud albedo. By combining MI and more traditional differential analyses, a framework to identify important modes of covariation between aerosol, clouds, and meteorological conditions is developed. This shows that accounting for solar zenith angle variation and implementing an albedo bias correction increases the detectability of the radiative effects of ACI in simulations of shallow cumulus.

Open access
Shelby Frisch
,
Matthew Shupe
,
Irina Djalalova
,
Graham Feingold
, and
Michael Poellot

Abstract

In situ samples of cloud droplets by aircraft in Oklahoma in 1997, the Surface Heat Budget of the Arctic Ocean (SHEBA)/First ISCCP Regional Experiment (FIRE)-Arctic Cloud Experiment (ACE) in 1998, and various other locations around the world were used to evaluate a ground-based remote sensing technique for retrieving profiles of cloud droplet effective radius. The technique is based on vertically pointing measurements from high-sensitivity millimeter-wavelength radar and produces height-resolved estimates of cloud particle effective radius.

Although most meteorological radars lack the sensitivity to detect small cloud droplets, millimeter-wavelength cloud radars provide opportunities for remotely monitoring the properties of nonprecipitating clouds. These high-sensitivity radars reveal detailed reflectivity structure of most clouds that are within several kilometers range. In order to turn reflectivity into usable microphysical quantities, relationships between the measured quantities and the desired quantities must be developed. This can be done through theoretical analysis, modeling, or empirical measurements. Then the uncertainty of each procedure must be determined in order to know which ones to use. In this study, two related techniques are examined for the retrieval of the effective radius. One method uses both radar reflectivity and integrated liquid water through the clouds obtained from a microwave radiometer; the second uses the radar reflectivity and an assumption that continental stratus clouds have a concentration of 200 drops per cubic centimeter and marine stratus 100 cm−3. Using in situ measurements of marine and continental stratus, the error analysis herein shows that the error in these techniques would be about 15%. In comparing the techniques with in situ aircraft measurements of effective radius, it is found that the radar radiometer retrieval was not quite as good as the technique using radar reflectivity alone. The radar reflectivity alone gave a 13% standard deviation with the in situ comparison, while the radar–radiometer retrieval gave a 19% standard deviation.

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Wayne M. Angevine
,
Joseph Olson
,
Jake J. Gristey
,
Ian Glenn
,
Graham Feingold
, and
David D. Turner

Abstract

Proper behavior of physics parameterizations in numerical models at grid sizes of order 1 km is a topic of current research. Modifications to parameterization schemes to accommodate varying grid sizes are termed “scale aware.” The general problem of grids on which a physical process is partially resolved is called the “gray zone” or “terra incognita.” Here we examine features of the Mellor–Yamada–Nakanishi–Niino (MYNN) boundary layer scheme with eddy diffusivity and mass flux (EDMF) that were intended to provide scale awareness, as implemented in WRF, version 4.1. Scale awareness is provided by reducing the intensity of nonlocal components of the vertical mixing in the scheme as the grid size decreases. However, we find that the scale-aware features cause poorer performance in our tests on a 600-m grid. The resolved circulations on the 600-m grid have different temporal and spatial scales than are found in large-eddy simulations of the same cases, for reasons that are well understood theoretically and are described in the literature. The circulations [model convectively induced secondary circulations (M-CISCs)] depend on the grid size and on details of the model numerics. We conclude that scale awareness should be based on effective resolution, and not on grid size, and that the gray-zone problem for boundary layer turbulence and shallow cumulus cannot be solved simply by reducing the intensity of the parameterization. Parameterizations with different characteristics may lead to different conclusions.

Free access
Jake J. Gristey
,
Graham Feingold
,
Ian B. Glenn
,
K. Sebastian Schmidt
, and
Hong Chen

Abstract

This study examines shallow cumulus cloud fields and their surface shortwave radiative effects using large-eddy simulation (LES) along with observations across multiple days at the Atmospheric Radiation Measurement Southern Great Plains atmospheric observatory. Pronounced differences are found between probability density functions (PDFs) of downwelling surface solar irradiance derived from observations and LES one-dimensional (1D) online radiation calculations. The shape of the observed PDF is bimodal, which is only reproduced by offline three-dimensional (3D) radiative transfer calculations, demonstrating PDF bimodality as a 3D radiative signature of continental shallow cumuli. Local differences between 3D and 1D radiative transfer calculations of downwelling surface solar irradiance are, on average, larger than 150 W m−2 on one afternoon. The differences are substantially reduced when spatially averaged over the LES domain and temporally averaged over the diurnal cycle, but systematic 3D biases ranging from 2 to 8 W m−2 persist across different days. Covariations between the domain-averaged surface irradiance, framed as a surface cloud radiative effect, and the simulated cloud fraction are found to follow a consistent diurnal relationship, often exhibiting hysteresis. In contrast, observations show highly variable behavior. By subsampling the LES domain, it is shown that this is due to the limited sampling density of inherently 3D observations. These findings help to define observational requirements for detecting such relationships, provide valuable insight for evaluating weather and climate models against surface observations as they push to ever higher resolutions, and have important implications for future assessments of solar renewable energy potential.

Open access
Mikael K. Witte
,
Patrick Y. Chuang
,
Orlando Ayala
,
Lian-Ping Wang
, and
Graham Feingold

Abstract

Two case studies of marine stratocumulus (one nocturnal and drizzling, the other daytime and nonprecipitating) are simulated by the UCLA large-eddy simulation model with bin microphysics for comparison with aircraft in situ observations. A high-bin-resolution variant of the microphysics is implemented for closer comparison with cloud drop size distribution (DSD) observations and a turbulent collision–coalescence kernel to evaluate the role of turbulence on drizzle formation. Simulations agree well with observational constraints, reproducing observed thermodynamic profiles (i.e., liquid water potential temperature and total moisture mixing ratio) as well as liquid water path. Cloud drop number concentration and liquid water content profiles also agree well insofar as the thermodynamic profiles match observations, but there are significant differences in DSD shape among simulations that cause discrepancies in higher-order moments such as sedimentation flux, especially as a function of bin resolution. Counterintuitively, high-bin-resolution simulations produce broader DSDs than standard resolution for both cases. Examination of several metrics of DSD width and percentile drop sizes shows that various discrepancies of model output with respect to the observations can be attributed to specific microphysical processes: condensation spuriously creates DSDs that are too wide as measured by standard deviation, which leads to collisional production of too many large drops. The turbulent kernel has the greatest impact on the low-bin-resolution simulation of the drizzling case, which exhibits greater surface precipitation accumulation and broader DSDs than the control (quiescent kernel) simulations. Turbulence effects on precipitation formation cannot be definitively evaluated using bin microphysics until the artificial condensation broadening issue has been addressed.

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Graham Feingold
,
William R. Cotton
,
Sonia M. Kreidenweis
, and
Janel T. Davis

Abstract

The impact of giant and ultragiant cloud condensation nuclei (>5-μm radius) on drizzle formation in stratocumuli is investigated within a number of modeling frameworks. These include a simple box model of collection, a trajectory ensemble model (comprising an ensemble of Lagrangian parcel models), a 2D eddy-resolving model, and a 3D large-eddy simulation model. Observed concentrations of giant cloud condensation nuclei (GCCN) over the ocean at ambient conditions indicate that 20-μm radius haze particles exist in concentrations of between 10−4 and 10−2 cm−3, depending on ambient wind speed and seastate. It is shown that these concentrations are sufficient to move a nonprecipitating stratocumulus into a precipitating state at typical cloud condensation nucleus (CCN) concentrations of 50 to 250 cm−3, with higher concentrations of GCCN being required at higher CCN concentrations. However, at lower CCN concentrations, drizzle is often active anyway and the addition of GCCN has little impact. At high CCN concentrations, drizzle development is slow and GCCN have the greatest potential for enhancing the collection process. Thus, although drizzle production decreases with increasing CCN concentration, the relative impact of GCCN increases with increasing CCN concentration. It is also shown that in the absence of GCCN, a shift in the modal radius of the CCN distribution to larger sizes suppresses drizzle because larger modal radii enable the activation of larger droplet number concentrations. Finally, calculations of the impact of GCCN on cloud optical properties are performed over a range of parameter space. Results indicate that the presence of GCCN moderates the effect of CCN on optical properties quite significantly. In the absence of GCCN, an increase in CCN from 50 to 150 cm−3 results in a threefold increase in albedo; when GCCN exist at a concentration of 10−3 cm−3, the increase in albedo is only twofold. Thus the variable presence of GCCN represents yet another uncertainty in estimating the influence of anthropogenic activity on climate.

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