The Measurement and Parameterization of Effective Radius of Droplets in Warm Stratocumulus Clouds

Martin, Gill; Johnson, D. W.; Spice, A.

doi:10.1175/1520-0469(1994)051<1823:tmapoe>2.0.co;2

Cited by 812 publications

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“…In satellite remote-sensing DSDs are often described using theoretical distributions that fit well with in situ observations, in addition to being mathematically convenient (Deirmendjian, 1964;Tampieri and Tomasi, 1976;Martin et al, 1994;Miles et al, 2000). A popular theoretical DSD is the gamma distribution proposed by Hansen and Travis (1974):…”

Section: Cloud Microphysical and Optical Propertiesmentioning

confidence: 99%

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

et al. 2018

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Abstract. Many passive remote-sensing techniques have been developed to retrieve cloud microphysical properties from satellite-based sensors, with the most common approaches being the bispectral and polarimetric techniques. These two vastly different retrieval techniques have been implemented for a variety of polar-orbiting and geostationary satellite platforms, providing global climatological data sets. Prior instrument comparison studies have shown that there are systematic differences between the droplet size retrieval products (effective radius) of bispectral (e.g., MODIS, Moderate Resolution Imaging Spectroradiometer) and polarimetric (e.g., POLDER, Polarization and Directionality of Earth's Reflectances) instruments. However, intercomparisons of airborne bispectral and polarimetric instruments have yielded results that do not appear to be systematically biased relative to one another. Diagnosing this discrepancy is complicated, because it is often difficult for instrument intercomparison studies to isolate differences between retrieval technique sensitivities and specific instrumental differences such as calibration and atmospheric correction. In addition to these technical differences the polarimetric retrieval is also sensitive to the dispersion of the droplet size distribution (effective variance), which could influence the interpretation of the droplet size retrieval. To avoid these instrument-dependent complications, this study makes use of a cloud remote-sensing retrieval simulator. Created by coupling a large-eddy simulation (LES) cloud model with a 1-D radiative transfer model, the simulator serves as a test bed for understanding differences between bispectral and polarimetric retrievals. With the help of this simulator we can not only compare the two techniques to one another (retrieval intercomparison) but also validate retrievals directly against the LES cloud properties. Using the satellite retrieval simulator, we are able to verify that at high spatial resolution (50 m) the bispectral and polarimetric retrievals are highly correlated with one another within expected observational uncertainties. The relatively small systematic biases at high spatial resolution can be attributed to different sensitivity limitations of the two retrievals. In contrast, a systematic difference between the two retrievals emerges at coarser resolution. This bias largely stems from differences related to sensitivity of the two retrievals to unresolved inhomogeneities in effective variance and optical thickness. The influence of coarse angular resolution is found to increase uncertainty in the polarimetric retrieval but generally maintains a constant mean value.

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Section: Cloud Microphysical and Optical Propertiesmentioning

confidence: 99%

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

et al. 2018

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“…where N a is aerosol number concentrations in the size range 0.05-1.50 mm radius in m À3 [Martin et al, 1994]. This cloud droplet number density is fed to a cloud module with explicit microphysical processes [Lohmann and Roeckner, 1996] to treat the cloud-aerosol interactions.…”

Section: Aerosol Activation and Cloud Formationmentioning

confidence: 99%

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

Gong¹,

et al. 2003

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[1] A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper [Gong et al., 2002].

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“…The sensitivity of cloud droplet concentration to aerosol particle concentration was much lower than that observed by Twomey and Warner (1967) or by Pueschel et al (1986). Martin et al (1994) compared concentrations of cloud droplets (0.5 to 47 pm diameter) in warm st:ratocumulus clouds to concentrations of aerosol particles (0.1 to 3 pm diameter:) in air just below cloud base (Figure 11). At low aerosol particles characteristic of air relatively free of anthropogenic influence the concentration of cloud drops was nearly equal to the concentration of aerosol particles, whereas thlere was a marked fall off at higher aerosol particle concentrations ( 2 400 cm-3) characteristic of anthropogenic influence.…”

Section: "*7----------mentioning

confidence: 90%

“…These distributions are expressed in terms oi the aerosol mass loading, so this was transformed to a number concentration1 by assuming a log-normal size distribution. This was done in order to capitalize on the aircraft data obtained by Martin et al (1994) for boundary layer stratocumulus clouds in several regions of the globe. Martin et al showed that there is a good correlation between the number concentration of aerosols in the sub-cloud layer and the number of cloud drops ( Figure 11).…”

Section: Studies With Climate Modelsmentioning

confidence: 99%

Enhanced Shortwave Cloud Radiative Forcing Due To Anthropogenic Aerosols

Schwartz

Slingo

1996

Clouds, Chemistry and Climate

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Berkshire RG12 2SY C K It has been suggested,, originally by Twomey (SCEP, 1970), that anthropogenic aerosols in the troposphere can influence the microphysical properties of clouds and in turn their reflectivity (albedo), thereby exerting a radiative influence on climate. This chapter presents the theoretical basis for of this so-called indirect forcing and reviews pertinent observational evidence and climate model calculations of its magnitude and geographical distribution. We restrict consideration to liquid-water clouds, in part because these clouds are the principal clouds thought to be influenced by anthropogenic aerosols, and in part because the processes responsible for nucleation of ice clouds are not sufficiently well understood to permit much to be said about any anthropogenic influence. The argument for anthropogenic influence on cloud albedo rests on the premise that aerosol particle number concentrations are substantially increased by industrial emissions. As a consequence, the number concentration of cloud droplets Ned, which is determined by the number concentration of aerosol particles in the precloud air, is also increased. This in turn leads to an enhanced multiple scattering of light within clouds isnd to an increase in the optical depth and albedo of the cloud. In contrast the physical thickness, liquid water content, and liquid water path of the cloud, which are governed to close approximation by large-scale thermodynamics, are considered to be unchanged oir at least not greatly influenced by the increase in cloud droplet concentration. There is evidence that this assumption may not be entirely correct. In particular there is indication that the decrease in drop size may inhibit precipitation development, increasing cloud liquid water content and cloud lifetime, both of which effects would contribute 1 further to reflection of solar radiation by clouds; this phenomenon is also considered here.The present chapter reviews the theory of the indirect forcing phenomenon and estimates of its magnitude, field measurements addressing aspects of this phenomenon, and approaches to describing this phenomenon in climate models. This review is necessarily fairly selective. For more extended accounts see Hobbs (1993), Andreae (1995), andHeintzenberg (1995). THEORETICAL BASIS OF INDIRECT FORCINGAerosol influences on cloud albedo and shortwave forcing. The theoretical basis for a dependence of cloud albedo on cloud droplet number density, and in turn on the number density of the aerosol particles on which cloud droplets form, is outlined by Twomey (1974Twomey ( , 1977a. To relate cloud albedo to optical depth and thus to microphysical properties we employ an analytical expression obtained from the two-stream approximation for the reflectance (albedo) RCT of a nonabsorbing, horizontally homogeneous cloud (e.g., Bohren, 1987): Here g is the asymmetry parameter for single scattering (mean cosine of the scattering angle), approximately 0.85 for cloud droplets of radius much greater than the wavelength of visib...

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The Measurement and Parameterization of Effective Radius of Droplets in Warm Stratocumulus Clouds

Cited by 812 publications

References 0 publications

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

Enhanced Shortwave Cloud Radiative Forcing Due To Anthropogenic Aerosols

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