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Ribu Cherian - One of the best experts on this subject based on the ideXlab platform.
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gcm simulations of anthropogenic aerosol induced changes in aerosol extinction atmospheric heating and precipitation over india
2013Co-Authors: Ribu Cherian, Chandra Venkataraman, Johannes Quaas, S RamachandranAbstract:[1] The influence of anthropogenic emissions on aerosol distributions and the hydrological cycle are examined with a focus on monsoon precipitation over the Indian subcontinent, during January 2001 to December 2005, using the European Centre for Medium-Range Weather Forecasts-Hamburg (ECHAM5.5) general circulation model extended by the Hamburg Aerosol Module (HAM). The seasonal variability of aerosol optical depth (AOD) retrieved from the MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellite is broadly well simulated (R � 0.6–0.85) by the model. The spatial distribution and seasonal cycle of the precipitation observed over the Indian region are reasonably well simulated (R � 0.5 to 0.8) by the model, while in terms of absolute magnitude, the model underestimates precipitation, in particular in the south-west (SW) monsoon season. The model simulates significant anthropogenic aerosol-induced changes in clear-sky net surface solar radiation (dimming greater than � 7Wm � 2 ), which agrees well with the observed trends over the Indian region. A statistically significant decreasing precipitation trend is simulated only for the SW monsoon season overthe central-north Indian region, which is consistent with the observed seasonal trend over the Indian region. In the model, this decrease results from a reduction in convective precipitation, where there is an increase in Stratiform Cloud droplet number concentration (CDNC) and solar dimming that resulted from increased stability and reduced evaporation. Similarities in spatial patterns suggest that surface cooling, mainly by the aerosol indirect effect, is responsible for this reduction in convective activity. When changes in large-scale dynamics are allowed by slightly disturbing the initial state of the atmosphere, aerosol absorption in addition leads to a further stabilization of the lower troposphere, further reducing convective precipitation.
Bjorn Stevens - One of the best experts on this subject based on the ideXlab platform.
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sugar gravel fish and flowers mesoscale Cloud patterns in the trade winds
2020Co-Authors: Bjorn Stevens, Sandrine Bony, Helene Brogniez, Laureline Hentgen, Cathy Hohenegger, Christoph Kiemle, Tristan Lecuyer, Ann Kristin Naumann, Hauke Schulz, Pier SiebesmaAbstract:An activity designed to characterise patterns of mesoscale (20 km to 2000 km) organisation of shallow Clouds in the downstream trades is described. Patterns of mesoscale organisation observed from space were subjectively defined and learned by twelve trained scientists. The ability of individuals to communicate, learn and replicate the classification was evaluated. Nine‐hundred satellite images spanning the area from 48°W to 58°W 10°N to 20°N for the Boreal winter months (Dec‐Feb) over ten years (2007/2008 to 2016/2017) were classified. Each scene was independently labeled by six scientists as being dominated by one of six patterns (one of which was “no‐pattern”). Four patterns of mesoscale organisation could be labelled in a reproducible manner, and were labelled Sugar, Gravel, Fish, and Flowers. Sugar consists of small, low Clouds of low reflectivity, Gravel Clouds form along apparent gust fronts, Fish are skeletal network (often fishbone‐like) of Clouds, while Flowers are circular clumped features defined more by their Stratiform Cloud elements. Both Fish and Flowers are surrounded by large areas of clear air. These four named‐patterns were identified 40% of the time, with the most common pattern being Gravel. Sugar was identified the least and suggests that unorganised and very shallow convection is unlikely to dominate large areas of the downstream Tradewinds. Some of the patterns show signs of seasonal and inter‐annual variability, and some degree of scale selectivity. Comparison of typical patterns with radar imagery suggests that even this subjective and qualitative visual inspection of imagery appears to capture several important physical differences between shallow Cloud regimes, such as precipitation and radiative effects.
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simulations of trade wind cumuli under a strong inversion
2001Co-Authors: Bjorn Stevens, Andreas Chlond, Bruce A. Albrecht, Andrew S Ackerman, Joan Cuxart, Malcolm K. Macvean, Andrew R. Brown, D C Lewellen, Peter G Duynkerke, Roel NeggersAbstract:The fifth intercomparison of the Global Water and Energy Experiment Cloud System Studies Working Group 1 is used as a vehicle for better understanding the dynamics of trade wind cumuli capped by a strong inversion. The basis of the intercomparison is 10 simulations by 7 groups. These simulations are supplemented by many further sensitivity studies, including some with very refined grid meshes. The simulations help illustrate the turbulent dynamics of trade cumuli in such a regime. In many respects the dynamics are similar to those found in many previous simulations of trade cumuli capped by weaker inversions. The principal differences are the extent to which the Cloud layer is quasi-steady in the current simulations, evidence of weak countergradient momentum transport within the Cloud layer, and the development and influence of an incipient Stratiform Cloud layer at the top of the Cloud layer. Although many elements of the turbulent structure (including the wind profiles, the evolution of Cloud-base height, the statistics of the subCloud layer, and the nature of mixing in the lower and middle parts of the Cloud layer) are robustly predicted, the representation of the Stratiform Cloud amount by the different simulations is remarkably sensitive to a number of factors. Chief among these are differences between numerical algorithms. These sensitivities persist even among simulations on relatively refined grid meshes. Part of this sensitivity is attributed to a physically realistic positive radiative feedback, whereby a propensity toward higher Cloud fractions in any given simulation is amplified by longwave radiative cooling. The simulations also provide new insight into the dynamics of the transition layer at Cloud base. In accord with observations, the simulations predict that this layer is most identifiable in terms of moisture variances and gradients. The simulations help illustrate the highly variable (in both height and thickness) nature of the transition layer, and we speculate that this variability helps regulate convection. Lastly the simulations are used to help evaluate simple models of trade wind boundary layers. In accord with previous studies, mass-flux models well represent the dynamics of the Cloud layer, while mixing-length models well represent the subCloud layer. The development of the Stratiform Cloud layer is not, however, captured by the mass-flux models. The simulations indicate that future theoretical research needs to focus on interface rules, whereby the Cloud layer is coupled to the subCloud layer below and the free atmosphere above. Future observational studies of this regime would be of most benefit if they could provide robust Cloud statistics as a function of mean environmental conditions.
Andrew Gettelman - One of the best experts on this subject based on the ideXlab platform.
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two moment bulk Stratiform Cloud microphysics in the gfdl am3 gcm description evaluation and sensitivity tests
2010Co-Authors: Hugh Morrison, Andrew Gettelman, Marc Salzmann, Yi Ming, Jeanchristophe Golaz, Paul Ginoux, Martina Kramer, Leo J DonnerAbstract:Abstract. A new Stratiform Cloud scheme including a two-moment bulk microphysics module, a Cloud cover parameterization allowing ice supersaturation, and an ice nucleation parameterization has been implemented into the recently developed GFDL AM3 general circulation model (GCM) as part of an effort to treat aerosol-Cloud-radiation interactions more realistically. Unlike the original scheme, the new scheme facilitates the study of Cloud-ice-aerosol interactions via influences of dust and sulfate on ice nucleation. While liquid and Cloud ice water path associated with Stratiform Clouds are similar for the new and the original scheme, column integrated droplet numbers and global frequency distributions (PDFs) of droplet effective radii differ significantly. This difference is in part due to a difference in the implementation of the Wegener-Bergeron-Findeisen (WBF) mechanism, which leads to a larger contribution from super-cooled droplets in the original scheme. Clouds are more likely to be either completely glaciated or liquid due to the WBF mechanism in the new scheme. Super-saturations over ice simulated with the new scheme are in qualitative agreement with observations, and PDFs of ice numbers and effective radii appear reasonable in the light of observations. Especially, the temperature dependence of ice numbers qualitatively agrees with in-situ observations. The global average long-wave Cloud forcing decreases in comparison to the original scheme as expected when super-saturation over ice is allowed. Anthropogenic aerosols lead to a larger decrease in short-wave absorption (SWABS) in the new model setup, but outgoing long-wave radiation (OLR) decreases as well, so that the net effect of including anthropogenic aerosols on the net radiation at the top of the atmosphere (netradTOA = SWABS-OLR) is of similar magnitude for the new and the original scheme.
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a new two moment bulk Stratiform Cloud microphysics scheme in the community atmosphere model version 3 cam3 part ii single column and global results
2008Co-Authors: Andrew Gettelman, Hugh Morrison, Steven J. GhanAbstract:The global performance of a new two-moment Cloud microphysics scheme for a general circulation model (GCM) is presented and evaluated relative to observations. The scheme produces reasonable representations of Cloud particle size and number concentration when compared to observations, and it represents expected and observed spatial variations in Cloud microphysical quantities. The scheme has smaller particles and higher number concentrations over land than the standard bulk microphysics in the GCM and is able to balance the top-of-atmosphere radiation budget with 60% the liquid water of the standard scheme, in better agreement with retrieved values. The new scheme diagnostically treats both the mixing ratio and number concentration of rain and snow, and it is therefore able to differentiate the two key regimes, consisting of drizzle in shallow, warm Clouds and larger rain drops in deeper Cloud systems. The modeled rain and snow size distributions are consistent with observations.
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a new two moment bulk Stratiform Cloud microphysics scheme in the community atmosphere model version 3 cam3 part i description and numerical tests
2008Co-Authors: Hugh Morrison, Andrew GettelmanAbstract:A new two-moment Stratiform Cloud microphysics scheme in a general circulation model is described. Prognostic variables include Cloud droplet and Cloud ice mass mixing ratios and number concentrations. The scheme treats several microphysical processes, including hydrometeor collection, condensation/ evaporation, freezing, melting, and sedimentation. The activation of droplets on aerosol is physically based and coupled to a subgrid vertical velocity. Unique aspects of the scheme, relative to existing two-moment schemes developed for general circulation models, are the diagnostic treatment of rain and snow number concentration and mixing ratio and the explicit treatment of subgrid Cloud water variability for calculation of the microphysical process rates. Numerical aspects of the scheme are described in detail using idealized one-dimensional offline tests of the microphysics. Sensitivity of the scheme to time step, vertical resolution, and numerical method for diagnostic precipitation is investigated over a range of conditions. It is found that, in general, two substeps are required for numerical stability and reasonably small time truncation errors using a time step of 20 min; however, substepping is only required for the precipitation microphysical processes rather than the entire scheme. A new numerical approach for the diagnostic rain and snow produces reasonable results compared to a benchmark simulation, especially at low vertical resolution. Part II of this study details results of the scheme in single-column and global simulations, including comparison with observations.
Steven J. Ghan - One of the best experts on this subject based on the ideXlab platform.
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a new two moment bulk Stratiform Cloud microphysics scheme in the community atmosphere model version 3 cam3 part ii single column and global results
2008Co-Authors: Andrew Gettelman, Hugh Morrison, Steven J. GhanAbstract:The global performance of a new two-moment Cloud microphysics scheme for a general circulation model (GCM) is presented and evaluated relative to observations. The scheme produces reasonable representations of Cloud particle size and number concentration when compared to observations, and it represents expected and observed spatial variations in Cloud microphysical quantities. The scheme has smaller particles and higher number concentrations over land than the standard bulk microphysics in the GCM and is able to balance the top-of-atmosphere radiation budget with 60% the liquid water of the standard scheme, in better agreement with retrieved values. The new scheme diagnostically treats both the mixing ratio and number concentration of rain and snow, and it is therefore able to differentiate the two key regimes, consisting of drizzle in shallow, warm Clouds and larger rain drops in deeper Cloud systems. The modeled rain and snow size distributions are consistent with observations.
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A Stratiform Cloud parameterization for general circulation models
1994Co-Authors: Steven J. Ghan, L.r. Leung, J.e. Penner, C.c. Chuang, J. MccaaAbstract:The crude treatment of Clouds in General Circulation Models (GCMs) is widely recognized as a major limitation in the application of these models to predictions of global climate change. The purpose of this project is to develop a paxameterization for Stratiform Clouds in GCMs that expresses Stratiform Clouds in terms of bulk microphysical properties and their subgrid variability. In this parameterization, precipitating Cloud species are distinguished from non-precipitating species, and the liquid phase is distinguished from the ice phase. The size of the non-precipitating Cloud particles (which influences both the Cloud radiative properties and the conversion of non-precipitating Cloud species to precipitating species) is determined by predicting both the mass and number concentrations of each species.
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computationally efficient approximations to Stratiform Cloud microphysics parameterization
1992Co-Authors: Steven J. Ghan, Richard C EasterAbstract:Abstract Bulk Cloud microphysics parameterizations typically employ time steps of a few tens of seconds. Although the computational burden of these parameterizations is acceptable for the 1-day mesoscale Cloud simulations for which they were designed, the time steps are unacceptably short for direct application of them parameterizations to global-climate simulation. To increase the computational efficiency of bulk Cloud microphysics parameterizations, we introduce two approximations that are appropriate for Stratiform Clouds. By diagnosing rather than predicting rain and snow concentrations and by assuming instantaneous melting of snow, we have found that the permissible time step is increased tenfold (to 2–6 min) with little loss in accuracy for vertical motions and time scales characteristic of those resolved by general circulation models (GCMs). Such time steps are sufficiently long to permit application of bulk Cloud microphysical parameterizations to GCMs for multiyear global simulations. However, we...
Thomas C. Marshall - One of the best experts on this subject based on the ideXlab platform.
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Positive charge in the Stratiform Cloud of a mesoscale convective system
2001Co-Authors: Thomas C. Marshall, Maribeth Stolzenburg, W. David Rust, Earle Williams, R. BoldiAbstract:A balloon sounding of electric field in the trailing Stratiform Cloud of a bow echo mesoscale convective system reveals only two substantial in-Cloud positive charge regions. These charge regions are located at altitudes of 5.1–5.6 km and 6.4–6.8 km, above the level of 0°C at 4.2 km. The two positive charge regions are the likely sources of six positive Cloud-to-ground flashes with large peak currents (>32 kA) that occurred within 60 km of the balloon during its flight. The amount of charge transferred by three of these positive flashes that made Q bursts is calculated in the range of 97–196 C. Flashes of this sort are known to produce sprites and elves in the mesosphere. The positive charge regions in this Stratiform Cloud are substantially lower than the 10-km altitude commonly assumed for the positive charge in many sprite modeling studies.
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electrical and kinematic structure of the Stratiform precipitation region trailing an oklahoma squall line
1991Co-Authors: David W Rust, Terry J Schuur, Bradley F Smull, Thomas C. MarshallAbstract:Abstract An electric field sounding through the transition zone precipitation minimum that trailed an Oklahoma squall line on 18 June 1987 provides information about the electrical structure within a midlatitude trailing Stratiform Cloud. A single-Doppler radar analysis concurrent with the flight depicts a kinematic structure dominated by two mesoscale flow regimes previously identified in squall-line systems: a strong midlevel, front-to-rear flow coinciding with the Stratiform Cloud layer and a descending rear inflow that sloped from 6.5 km AGL at the Stratiform Cloud's trailing edge to 1.5 km AGL at the convective line. Electric field magnitudes as high as 113 kV m−1 were observed by the electric field sounding, which reveals an electric field structure comparable in magnitude and complexity to structures reported for convective cells of thunderstorms. The charge regions inferred with an approximation to Gauss' law have charge density magnitudes of 0.2–4.1 nC m−3 and vertical thicknesses of 130–1160 m; ...