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Constantin Pontikis - One of the best experts on this subject based on the ideXlab platform.
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Warm cloud droplet Effective Radius parameterizations and their use in general circulation models
2000Co-Authors: Constantin Pontikis, E. M. Hicks, N. MichalonAbstract:An accurate theoretical expression of the cloud droplet Effective Radius has been used in order to infer small scale and cloud scale Effective Radius expressions that take into account the cloud character (continental or maritime) and the cloud type (layer cloud or convective cloud). These expressions have been validated by means of observational data collected during different field experiments and have further been used in order to obtain pertinent Effective Radius parameterizations for use in General Circulation Models (GCMs). These and other existing Effective Radius parameterizations have been implemented in the French atmospheric community GCM, ARPEGE-IFS-2, in order to establish global Effective Radius distributions. The latter have been compared to the Effective Radius distributions obtained on a global scale by using experimental ISCCP observations.
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Sensitivity of a GCM to changes in the droplet Effective Radius parameterization
Geophysical Research Letters, 1997Co-Authors: Philippe Dandin, Constantin Pontikis, Elizabeth HicksAbstract:The French Community Climate model (ARPEGE-climat) has been used in relation to four different warm cloud droplet Effective Radius parameterizations in order to simulate the January and July climatologies. Two of these parameterizations depend upon the cloud character (maritime or continental). The comparison between the results of the different simulations reveals that the model is very sensitive to the Effective Radius parameterization change. This change induces differences up to 30Wm−2 in the global values of the radiative fluxes at the top of the atmosphere and at the surface level. Important differences are also induced in the zonally averaged short-wave cloud radiative forcing, cloudiness and precipitation. At smaller scales, non negligible precipitation and surface temperature changes are also observed. The comparison between the simulation results obtained by using model versions containing the above parameterizations reveals that two of them considerably improve the agreement between model and climatological cloudiness, thus allowing a pertinent choice of the most appropriate cloud droplet Effective Radius parameterization.
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Parameterization of the droplet Effective Radius of warm layer clouds
Geophysical Research Letters, 1996Co-Authors: Constantin PontikisAbstract:Two pertinent parameterizations representing respectively the cloud droplet Effective Radius of maritime and continental warm layer clouds are presented. They are obtained by assuming that warm layer clouds are adiabatic. In order to derive these parameterizations, a theoretical expression of the mean adiabatic cloud droplet Effective Radius is first developed. This expression depends upon the cloud droplet concentration, the liquid water path and a parameter A that relates linearly the liquid water content to the distance above cloud base. Secondly, an analytic expression of the adiabatic liquid water content is derived and reveals that A depends exclusively upon the cloud base characteristics. By using climatological cloud base values to determine the A value as well as two “standard” droplet concentrations representing respectively droplet concentrations in maritime and continental clouds, one obtains parameterized expressions of the droplet Effective Radius that depend exclusively upon the liquid water path. The maritime parameterization is validated by using observational data.
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Contribution to the droplet Effective Radius parameterization of warm convective clouds: Clear air entrainment effects
Geophysical Research Letters, 1995Co-Authors: Constantin PontikisAbstract:The effects of clear air entrainment and mixing on the cloud droplet Effective Radius are investigated. Approximate expressions of the droplet Effective Radius corresponding to extreme scenarios of mixing and secondary droplet activation are derived. They depend upon the liquid water path as well as upon the cloud droplet Effective Radius and liquid water path of the corresponding adiabatic cloud. The validity of these expressions is verified by using field experiment data. A parameterization of the mean droplet Effective Radius for tropical warm convective clouds is deduced from the above expressions and is further validated with the same field experiment data.
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The Influence of Clear Air Entrainment on the Droplet Effective Radius of Warm Maritime Convective Clouds
Journal of the Atmospheric Sciences, 1993Co-Authors: Constantin Pontikis, Elizabeth HicksAbstract:Abstract The influence of clear air entrainment on the droplet Effective Radius of cloudy air parcels is investigated theoretically and experimentally by using data collected in 16 warm maritime tropical cumuli during the Joint Hawaii Warm Rain Project (1985). The theoretical study consists of calculations of the droplet spectrum, droplet Effective Radius, and liquid water content performed by an entraining cloud parcel model for different entrainment-mixing scenarios. The numerical simulation results are interpreted by means of an analytic equation of the droplet Effective Radius expressed as a function of both the liquid water content and the droplet concentration. In the experimental study, the behavior of the Effective Radius is examined at all scales as a function of the liquid water content, used as a dilution degree indicator. At a given cloud level, in the absence of secondary droplet activation, the Effective Radius of the droplet spectrum of small-scale parcels (10-Hz data) is roughly independen...
Anning Cheng - One of the best experts on this subject based on the ideXlab platform.
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estimate of satellite derived cloud optical thickness and Effective Radius errors and their effect on computed domain averaged irradiances
Journal of Geophysical Research, 2006Co-Authors: Seiji Kato, Laura M. Hinkelman, Anning ChengAbstract:[1] The process of retrieving cloud optical thickness and Effective Radius from radiances measured by satellite instruments is simulated to determine the error in both the retrieved properties and in the irradiances computed with them. The radiances at 0.64 μm and 3.7 μm are computed for three cloud fields (stratus, stratocumulus, and cumulus) generated by large eddy simulation models. When overcast pixels are assumed and the horizontal flux is neglected in the retrieval process, the error in the domain-averaged retrieved optical thickness from nadir is 1% to −32% (1% to −27%) and the error in the retrieved Effective Radius is 0% to 67% (0% to 63%) for the solar zenith angle of 30° (50°). Using the radiance averaged over a 1 km size pixel also introduces error in the optical thickness because of the nonlinear relation between the reflected radiance and optical thickness. Both optical thickness and Effective Radius errors increase with increasing horizontal inhomogeneity. When the 0.64 μm albedo is computed with the independent column approximation using retrieved properties from nadir (oblique) view for a solar zenith angle of 50°, the error is −0.3% to 14% (−5% to −30%) relative to the albedo from 3-D radiative transfer computations with the true cloud properties. The albedo error occurs even though the radiance at one angle is forced to agree because a plane parallel cloud with a single value of optical thickness and Effective Radius cannot consistently match the radiance angular distribution. In addition, the error in the retrieved cloud properties contributes to the albedo error. When albedos computed with cloud properties derived from nadir and oblique views are averaged, the albedo error can partially cancel. The absolute error in the narrowband 0.64 μm (3.7 μm) albedo averaged over a 1° × 1° domain is less than 1.5% (0.6%), 5.0% (4.1%), and 7.1% (11%) in order of increasing inhomogeneity, when albedos computed with cloud properties derived from viewing zenith angles between 0° and 60° are averaged and when the solar zenith angle is between 10° and 50°. When the solar zenith angle is 70°, the error increases to up to +24% (+37%) for all three scenes.
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Estimate of satellite‐derived cloud optical thickness and Effective Radius errors and their effect on computed domain‐averaged irradiances
Journal of Geophysical Research, 2006Co-Authors: Seiji Kato, Laura M. Hinkelman, Anning ChengAbstract:[1] The process of retrieving cloud optical thickness and Effective Radius from radiances measured by satellite instruments is simulated to determine the error in both the retrieved properties and in the irradiances computed with them. The radiances at 0.64 μm and 3.7 μm are computed for three cloud fields (stratus, stratocumulus, and cumulus) generated by large eddy simulation models. When overcast pixels are assumed and the horizontal flux is neglected in the retrieval process, the error in the domain-averaged retrieved optical thickness from nadir is 1% to −32% (1% to −27%) and the error in the retrieved Effective Radius is 0% to 67% (0% to 63%) for the solar zenith angle of 30° (50°). Using the radiance averaged over a 1 km size pixel also introduces error in the optical thickness because of the nonlinear relation between the reflected radiance and optical thickness. Both optical thickness and Effective Radius errors increase with increasing horizontal inhomogeneity. When the 0.64 μm albedo is computed with the independent column approximation using retrieved properties from nadir (oblique) view for a solar zenith angle of 50°, the error is −0.3% to 14% (−5% to −30%) relative to the albedo from 3-D radiative transfer computations with the true cloud properties. The albedo error occurs even though the radiance at one angle is forced to agree because a plane parallel cloud with a single value of optical thickness and Effective Radius cannot consistently match the radiance angular distribution. In addition, the error in the retrieved cloud properties contributes to the albedo error. When albedos computed with cloud properties derived from nadir and oblique views are averaged, the albedo error can partially cancel. The absolute error in the narrowband 0.64 μm (3.7 μm) albedo averaged over a 1° × 1° domain is less than 1.5% (0.6%), 5.0% (4.1%), and 7.1% (11%) in order of increasing inhomogeneity, when albedos computed with cloud properties derived from viewing zenith angles between 0° and 60° are averaged and when the solar zenith angle is between 10° and 50°. When the solar zenith angle is 70°, the error increases to up to +24% (+37%) for all three scenes.
A Slingo - One of the best experts on this subject based on the ideXlab platform.
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Predicting cloud‐droplet Effective Radius and indirect sulphate aerosol forcing using a general circulation model
Quarterly Journal of the Royal Meteorological Society, 1996Co-Authors: Alun Jones, A SlingoAbstract:Various methods for predicting cloud-droplet Effective Radius in the Hadley Centre general circulation model are compared with aircraft and satellite retrievals, and are used to estimate the indirect radiative forcing by anthropogenic sulphate aerosols since the beginning of the industrial era. The effects both of different parametrization approaches and of different input sulphate data sets are examined; however, there is no clear evidence to prefer either of the two sulphate data sets used in the study. Two of the parametrizations generate distributions of present-day Effective Radius which are similar to each other and compare favourably with observations, yet provide very different estimates of the indirect effect, ranging from −0.5 to −1.5 W m−2 in the global annual mean. A sensitivity experiment in which it is assumed that droplet concentrations are not determined by sulphate concentrations in continental air reduces this global-mean forcing to −0.3 to −0.8 W m−2. This sensitivity demonstrates the need for a much better understanding of the link between sulphate aerosol mass concentrations, cloud condensation nuclei, and cloud-droplet number concentrations.
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predicting cloud droplet Effective Radius and indirect sulphate aerosol forcing using a general circulation model
Quarterly Journal of the Royal Meteorological Society, 1996Co-Authors: Alun Jones, A SlingoAbstract:Various methods for predicting cloud-droplet Effective Radius in the Hadley Centre general circulation model are compared with aircraft and satellite retrievals, and are used to estimate the indirect radiative forcing by anthropogenic sulphate aerosols since the beginning of the industrial era. The effects both of different parametrization approaches and of different input sulphate data sets are examined; however, there is no clear evidence to prefer either of the two sulphate data sets used in the study. Two of the parametrizations generate distributions of present-day Effective Radius which are similar to each other and compare favourably with observations, yet provide very different estimates of the indirect effect, ranging from −0.5 to −1.5 W m−2 in the global annual mean. A sensitivity experiment in which it is assumed that droplet concentrations are not determined by sulphate concentrations in continental air reduces this global-mean forcing to −0.3 to −0.8 W m−2. This sensitivity demonstrates the need for a much better understanding of the link between sulphate aerosol mass concentrations, cloud condensation nuclei, and cloud-droplet number concentrations.
Andrew J. Heymsfield - One of the best experts on this subject based on the ideXlab platform.
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extinction ice water content Effective Radius algorithms for calipso
Geophysical Research Letters, 2005Co-Authors: Andrew J. Heymsfield, D M Winker, Gerd-jan Van ZadelhoffAbstract:[1] Interrelationships between volume extinction coefficient (σ), ice water content (IWC), and Effective Radius (re), each dependent upon the particle size distribution (PSD) and temperature (T), are developed using in-situ microphysical measurements at low and mid-latitudes, remote sensing data, and model results. The ratio [], proportional to re, increases with T. Lower values of [] are observed near cloud top and base than within a cloud layer, although [] changes more with temperature than relative height within the cloud. For equivalent radar reflectivities (Ze) below about −28 dB, the minimum detectable with forthcoming spaceborne cloud radar, [] is a nearly constant value. IWC increases almost linearly with σ, with a temperature-dependence noted.
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Extinction‐ice water content‐Effective Radius algorithms for CALIPSO
Geophysical Research Letters, 2005Co-Authors: Andrew J. Heymsfield, Dave Winker, Gerd-jan Van ZadelhoffAbstract:[1] Interrelationships between volume extinction coefficient (σ), ice water content (IWC), and Effective Radius (re), each dependent upon the particle size distribution (PSD) and temperature (T), are developed using in-situ microphysical measurements at low and mid-latitudes, remote sensing data, and model results. The ratio [], proportional to re, increases with T. Lower values of [] are observed near cloud top and base than within a cloud layer, although [] changes more with temperature than relative height within the cloud. For equivalent radar reflectivities (Ze) below about −28 dB, the minimum detectable with forthcoming spaceborne cloud radar, [] is a nearly constant value. IWC increases almost linearly with σ, with a temperature-dependence noted.
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Preliminary Parameterization for Effective Radius for SCMs
2000Co-Authors: Greg M. Mcfarquhar, Andrew J. HeymsfieldAbstract:Using a single-column model (SCM), Somerville et al. (1999) showed that different parameterizations of Effective Radius, re, could result in variations in the outgoing longwave radiation (OLR) and downwelling shortwave flux (DSWF) on the order of 20 W m and 30 W m, respectively, and differences in solar and longwave radiative heating rates of 0.3!C day. Because many cloud radiative properties for both ice and water clouds (e.g., extinction coefficient, single-scattering albedo, asymmetry parameter) are parameterized in terms of water content and re, the representation of re is critical for an accurate estimate of cloud forcing.
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The Definition and Significance of an Effective Radius for Ice Clouds
Journal of the Atmospheric Sciences, 1998Co-Authors: Greg M. Mcfarquhar, Andrew J. HeymsfieldAbstract:Abstract Single scattering shortwave properties of ice clouds are frequently derived in terms of the “Effective” Radius (re) of the ice crystal population. Substantial discrepancies between definitions exist, making interpretation and comparison of various radiative parameterization schemes and satellite retrieval techniques difficult. Each definition of Effective Radius can be related to more physically based parameters, such as the crystal dimension of median mass. For vertically inhomogeneous clouds, the relative importance of different cloud heights in determining the retrieved re is examined, and a definition of re for vertically inhomogeneous clouds composed of hexagonal columns is proposed. This definition shows reasonable agreement with the re values that would be retrieved using visible and near-infrared channels for some microphysical data acquired in tropical ice clouds near Kwajalein, Marshall Islands, in the mid-1970s. Because only the upper parts of a thick ice cloud are detected by satellit...
Qilong Min - One of the best experts on this subject based on the ideXlab platform.
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Simultaneously retrieving cloud optical depth and Effective Radius for optically thin clouds
Journal of Geophysical Research, 2005Co-Authors: Qilong Min, Minzheng DuanAbstract:[1] A new technique for simultaneously retrieving cloud optical depth and Effective Radius has been proposed. This approach is based on the angular distribution of scattered light in the forward scattering lobe of cloud drops. The angular distributions can be observed by multiple shadowband scans. Radiative transfer modeling simulations demonstrate that accuracies for cloud optical depth, Effective Radius, and liquid water path are 2%, 10%, and 2 gm−2, respectively, for given possible instrument noise and uncertainties. Further, we have tested different measurement strategies and achieved consistent accuracies. This technique will provide an approach to deal with the issue of “CLOWD (cloud with low optical (water) depth).”
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Effective Radius of cloud droplets by ground based remote sensing relationship to aerosol
Journal of Geophysical Research, 2003Co-Authors: Stephen E. Schwartz, Byung-gon Kim, Mark A. Miller, Qilong MinAbstract:[1] Enhancement of cloud droplet number concentration by anthropogenic aerosols has previously been demonstrated by in-situ measurements, but there remains large uncertainty in the resultant enhancement of cloud optical depth and reflectivity. Detection of this effect is made difficult by the large inherent variability in cloud liquid water path (LWP); the dominant influence of LWP on optical depth and albedo masks any aerosol influences. Here we use ground-based remote sensing of cloud optical depth (τc) by narrowband radiometry and LWP by microwave radiometry to determine the dependence of optical depth on LWP, thereby permitting examination of aerosol influence; the method is limited to complete overcast conditions with single layer clouds, as determined mainly by millimeter wave cloud radar. Measurements in north central Oklahoma on 13 different days in the year 2000 show wide variation in LWP and optical depth on any given day, but with near linear proportionality between the two quantities; variance in LWP accounts as much as 97% of the variance in optical depth on individual days and for about 63% of the variance in optical depth for the whole data set. The slope of optical depth vs. LWP is inversely proportional to the Effective Radius of cloud droplets (re); event-average cloud droplet Effective Radius ranged from 5.6 ± 0.1 to 12.3 ± 0.6 μm (average ± uncertainty in the mean). This Effective Radius is negatively correlated with aerosol light scattering coefficient at the surface as expected for the aerosol indirect effect; the weak correlation (R2 = 0.24) might be due in part to vertically decoupled structure of aerosol particle concentration and possible meteorological influence such as vertical wind shear. Cloud albedo and radiative forcing for a given LWP are highly sensitive to Effective Radius; for solar zenith angle 60° and typical LWP of 100 g m−2, as Effective Radius decreases from 10.2 to 5.8 μm determined on different days, the resultant decrease in calculated net shortwave irradiance at the top of the atmosphere (Twomey forcing) is about 50 W m−2.
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Effective Radius of cloud droplets by ground‐based remote sensing: Relationship to aerosol
Journal of Geophysical Research, 2003Co-Authors: Byung-gon Kim, Stephen E. Schwartz, Mark A. Miller, Qilong MinAbstract:[1] Enhancement of cloud droplet number concentration by anthropogenic aerosols has previously been demonstrated by in-situ measurements, but there remains large uncertainty in the resultant enhancement of cloud optical depth and reflectivity. Detection of this effect is made difficult by the large inherent variability in cloud liquid water path (LWP); the dominant influence of LWP on optical depth and albedo masks any aerosol influences. Here we use ground-based remote sensing of cloud optical depth (τc) by narrowband radiometry and LWP by microwave radiometry to determine the dependence of optical depth on LWP, thereby permitting examination of aerosol influence; the method is limited to complete overcast conditions with single layer clouds, as determined mainly by millimeter wave cloud radar. Measurements in north central Oklahoma on 13 different days in the year 2000 show wide variation in LWP and optical depth on any given day, but with near linear proportionality between the two quantities; variance in LWP accounts as much as 97% of the variance in optical depth on individual days and for about 63% of the variance in optical depth for the whole data set. The slope of optical depth vs. LWP is inversely proportional to the Effective Radius of cloud droplets (re); event-average cloud droplet Effective Radius ranged from 5.6 ± 0.1 to 12.3 ± 0.6 μm (average ± uncertainty in the mean). This Effective Radius is negatively correlated with aerosol light scattering coefficient at the surface as expected for the aerosol indirect effect; the weak correlation (R2 = 0.24) might be due in part to vertically decoupled structure of aerosol particle concentration and possible meteorological influence such as vertical wind shear. Cloud albedo and radiative forcing for a given LWP are highly sensitive to Effective Radius; for solar zenith angle 60° and typical LWP of 100 g m−2, as Effective Radius decreases from 10.2 to 5.8 μm determined on different days, the resultant decrease in calculated net shortwave irradiance at the top of the atmosphere (Twomey forcing) is about 50 W m−2.
