scholarly journals Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests

2010 ◽  
Vol 10 (3) ◽  
pp. 6375-6446 ◽  
Author(s):  
M. Salzmann ◽  
Y. Ming ◽  
J.-C. Golaz ◽  
P. A. Ginoux ◽  
H. Morrison ◽  
...  
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Wave Radiation ◽  
Anthropogenic Aerosols ◽  
Stratiform Cloud ◽  
Long Wave ◽  
Original Scheme ◽  
Cloud Ice

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.

scholarly journals Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests

2010 ◽  
Vol 10 (16) ◽  
pp. 8037-8064 ◽  
Author(s):  
M. Salzmann ◽  
Y. Ming ◽  
J.-C. Golaz ◽  
P. A. Ginoux ◽  
H. Morrison ◽  
...  
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Wave Radiation ◽  
Anthropogenic Aerosols ◽  
Stratiform Cloud ◽  
Long Wave ◽  
Original Scheme ◽  
Cloud Ice

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.

scholarly journals Prognostic parameterization of cloud ice with a single category in the aerosol-climate model ECHAM(v6.3.0)-HAM(v2.3)

2018 ◽  
Vol 11 (4) ◽  
pp. 1557-1576 ◽  
Author(s):  
Remo Dietlicher ◽  
David Neubauer ◽  
Ulrike Lohmann
Keyword(s):  
Vapor Deposition ◽  
General Circulation ◽  
Traditional Approach ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
High Temporal Resolution ◽  
Stratiform Cloud ◽  
Conversion Rates ◽  
Single Category ◽  
Cloud Ice

Abstract. A new scheme for stratiform cloud microphysics has been implemented in the ECHAM6-HAM2 general circulation model. It features a widely used description of cloud water with two categories for cloud droplets and raindrops. The unique aspect of the new scheme is the break with the traditional approach to describe cloud ice analogously. Here we parameterize cloud ice by a single category that predicts bulk particle properties (P3). This method has already been applied in a regional model and most recently also in the Community Atmosphere Model 5 (CAM5). A single cloud ice category does not rely on heuristic conversion rates from one category to another. Therefore, it is conceptually easier and closer to first principles. This work shows that a single category is a viable approach to describe cloud ice in climate models. Prognostic representation of sedimentation is achieved by a nested approach for sub-stepping the cloud microphysics scheme. This yields good results in terms of accuracy and performance as compared to simulations with high temporal resolution. Furthermore, the new scheme allows for a competition between various cloud processes and is thus able to unbiasedly represent the ice formation pathway from nucleation to growth by vapor deposition and collisions to sedimentation. Specific aspects of the P3 method are evaluated. We could not produce a purely stratiform cloud where rime growth dominates growth by vapor deposition and conclude that the lack of appropriate conditions renders the prognostic parameters associated with the rime properties unnecessary. Limitations inherent in a single category are examined.

scholarly journals Prognostic parameterization of cloud ice with a single category in the aerosol-climate model ECHAM(v6.3.0)-HAM(v2.3)

2017 ◽  
Author(s):  
Remo Dietlicher ◽  
David Neubauer ◽  
Ulrike Lohmann
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Traditional Approach ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
High Temporal Resolution ◽  
Conversion Rates ◽  
Single Category ◽  
Cloud Ice

Abstract. A new scheme for stratiform cloud microphysics has been implemented in the ECHAM6-HAM2 general circulation model. It features a widely used description of cloud water with two categories for cloud droplets and rain drops. The unique aspect of the scheme is the break with the traditional approach to describe cloud ice analogously. Here we parameterize cloud ice with a single, prognostic category as it has been done in regional models and most recently also in the global model CAM5. A single category does not rely on heuristic conversion rates from one category to another. At the same time it is conceptually easier and closer to first principles. This work shows that a single category is a viable approach to describe cloud ice in climate models. Prognostic representation of sedimentation is achieved by a nested approach for sub-stepping the microphysics scheme. This yields good results in terms of numerical stability and accuracy as compared to simulations with high temporal resolution. The improvement of the representation of cloud ice in ECHAM6-HAM2 is twofold. Not only are we getting rid of heuristic conversion rates but we also find that the prognostic treatment of sedimenting ice allows to unbiasedly represent the ice formation pathway from nucleation over growth by deposition and collisions to sedimentation.

scholarly journals Resolved Snowball Earth Clouds

2014 ◽  
Vol 27 (12) ◽  
pp. 4391-4402 ◽  
Author(s):  
Dorian S. Abbot
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Circulation Model ◽  
Cloud Amount ◽  
Cloud Microphysics ◽  
Snowball Earth ◽  
Geochemical Data ◽  
Microphysics Scheme ◽  
Cloud Resolving Model ◽  
Cloud Ice

Abstract Recent general circulation model (GCM) simulations have challenged the idea that a snowball Earth would be nearly entirely cloudless. This is important because clouds would provide a strong warming to a high-albedo snowball Earth. GCM results suggest that clouds could lower the threshold CO2 needed to deglaciate a snowball by a factor of 10–100, enough to allow consistency with geochemical data. Here a cloud-resolving model is used to investigate cloud and convection behavior in a snowball Earth climate. The model produces convection that extends vertically to a similar temperature as modern tropical convection. This convection produces clouds that resemble stratocumulus clouds under an inversion on modern Earth, which slowly dissipate by sedimentation of cloud ice. There is enough cloud ice for the clouds to be optically thick in the longwave, and the resulting cloud radiative forcing is similar to that produced in GCMs run in snowball conditions. This result is robust to large changes in the cloud microphysics scheme because the cloud longwave forcing, which dominates the total forcing, is relatively insensitive to cloud amount and particle size. The cloud-resolving model results are therefore consistent with the idea that clouds would provide a large warming to a snowball Earth, helping to allow snowball deglaciation.

scholarly journals Improving the Southern Ocean cloud albedo biases in a general circulation model

2020 ◽  
Vol 20 (13) ◽  
pp. 7741-7751
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Paul Field ◽  
Kalli Furtado ◽  
Jonny Williams ◽  
...  
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Short Wave ◽  
Cloud Microphysics ◽  
Global Climate Models ◽  
Ice Crystals ◽  
Wave Radiation ◽  
Short Wave Radiation

Abstract. The present generation of global climate models is characterised by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases and subsequent problems with atmospheric dynamics. In this study, we modify cloud microphysics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to ∼4 W m−2 and seasonally much larger reductions compared to the control model. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud.

Aerosol Influence on Mixed-Phase Clouds in CAM-Oslo

2008 ◽  
Vol 65 (10) ◽  
pp. 3214-3230 ◽  
Author(s):  
Trude Storelvmo ◽  
Jón Egill Kristjánsson ◽  
Ulrike Lohmann
Keyword(s):  
General Circulation ◽  
Cloud Microphysics ◽  
Number Concentration ◽  
Mixed Phase ◽  
Ice Crystal ◽  
Anthropogenic Aerosols ◽  
Ice Nuclei ◽  
New Treatment ◽  
Cloud Ice ◽  
Mixed Phase Clouds

A new treatment of mixed-phase cloud microphysics has been implemented in the general circulation model, Community Atmosphere Model (CAM)-Oslo, which combines the NCAR CAM2.0.1 and a detailed aerosol module. The new treatment takes into account the aerosol influence on ice phase initiation in stratiform clouds with temperatures between 0° and −40°C. Both supersaturation and cloud ice fraction, that is, the fraction of cloud ice compared to the total cloud water in a given grid box, are now determined based on a physical reasoning in which not only temperature but also the ambient aerosol concentration play a role. Included in the improved microphysics treatment is also a continuity equation for ice crystal number concentration. Ice crystal sources are heterogeneous and homogeneous freezing processes and ice multiplication. Sink terms are collection processes and precipitation formation, that is, melting and sublimation. Instead of using an idealized ice nuclei concentration for the heterogeneous freezing processes, a common approach in global models, the freezing processes are here dependent on the ability of the ambient aerosols to act as ice nuclei. Additionally, the processes are dependent on the cloud droplet number concentration and hence the aerosols’ ability to act as cloud condensation nuclei. Sensitivity simulations based on the new microphysical treatment of mixed-phase clouds are presented for both preindustrial and present-day aerosol emissions. Freezing efficiency is found to be highly sensitive to the amount of sulphuric acid available for ice nuclei coating. In the simulations, the interaction of anthropogenic aerosols and freezing mechanisms causes a warming of the earth–atmosphere system, counteracting the cooling effect of aerosols influencing warm clouds. The authors find that this reduction of the total aerosol indirect effect amounts to 50%–90% for the specific assumptions on aerosol properties used in this study. However, many microphysical processes in mixed-phase clouds are still poorly understood and the results must be interpreted with that in mind.

A New Two-Moment Bulk Stratiform Cloud Microphysics Scheme in the Community Atmosphere Model, Version 3 (CAM3). Part I: Description and Numerical Tests

2008 ◽  
Vol 21 (15) ◽  
pp. 3642-3659 ◽  
Author(s):  
Hugh Morrison ◽  
Andrew Gettelman
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
General Circulation Models ◽  
Cloud Microphysics ◽  
Small Time ◽  
Vertical Resolution ◽  
Time Step ◽  
Stratiform Cloud ◽  
Microphysics Scheme ◽  
Microphysical Processes

Abstract 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.

scholarly journals Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 1: Aerosol trends and radiative forcing

2011 ◽  
Vol 11 (8) ◽  
pp. 24085-24125 ◽  
Author(s):  
E. M. Leibensperger ◽  
L. J. Mickley ◽  
D. J. Jacob ◽  
W.-T. Chen ◽  
J. H. Seinfeld ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Transport Model ◽  
Circulation Model ◽  
Anthropogenic Sources ◽  
So2 Emissions ◽  
Anthropogenic Aerosols ◽  
Climatic Effects ◽  
Anthropogenic Aerosol ◽  
Direct Forcing

Abstract. We use the GEOS-Chem chemical transport model combined with the GISS general circulation model to calculate the aerosol direct and indirect (warm cloud) radiative forcings from US anthropogenic sources over the 1950–2050 period, based on historical emission inventories and future projections from the IPCC A1B scenario. The aerosol simulation is evaluated with observed spatial distributions and 1980–2010 trends of aerosol concentrations and wet deposition in the contiguous US. The radiative forcing from US anthropogenic aerosols is strongly localized over the eastern US. We find that it peaked in 1970–1990, with values over the eastern US (east of 100° W) of −2.0 W m−2 for direct forcing including contributions from sulfate (−2.0 W m−2), nitrate (−0.2 W m−2), organic carbon (−0.2 W m−2), and black carbon (+0.4 W m−2). The aerosol indirect effect is of comparable magnitude to the direct forcing. We find that the forcing declined sharply from 1990 to 2010 (by 0.8 W m−2 direct and 1.0 W m−2 indirect), mainly reflecting decreases in SO2 emissions, and project that it will continue declining post-2010 but at a much slower rate since US SO2 emissions have already declined by almost 60 % from their peak. This suggests that much of the warming effect of reducing US anthropogenic aerosol sources may have already been realized by 2010, however some additional warming is expected through 2020. The small positive radiative forcing from US BC emissions (+0.3 W m−2 over the eastern US in 2010) suggests that an emission control strategy focused on BC would have only limited climate benefit.

scholarly journals Improved simulation of clouds over the Southern Ocean in a General Circulation Model

2019 ◽  
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Paul Field ◽  
Kalli Furtado ◽  
Jonny Williams ◽  
...  
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Supercooled Liquid ◽  
Short Wave ◽  
Global Climate Models ◽  
Ice Crystals ◽  
Wave Radiation ◽  
Short Wave Radiation

Abstract. The present generation of global climate models is characterized by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases, and subsequent problems with atmospheric dynamics. In this study we modify cloud micro-physics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to 4 W/m2, and seasonally much larger reductions. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud. We hypothesize that such abundant supercooled liquid cloud is the result of a paucity of ice nucleating particles in this part of the atmosphere.

Anthropogenic Aerosols Dominate Forced Multidecadal Sahel Precipitation Change through Distinct Atmospheric and Oceanic Drivers

2020 ◽  
Vol 33 (23) ◽  
pp. 10187-10204
Author(s):  
Haruki Hirasawa ◽  
Paul J. Kushner ◽  
Michael Sigmond ◽  
John Fyfe ◽  
Clara Deser
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Forced Response ◽  
Sea Surface ◽  
Atmospheric Response ◽  
Anthropogenic Aerosols ◽  
Coupled General Circulation Model ◽  
Large Ensembles ◽  
Primary Driver ◽  
Sahel Precipitation

AbstractSahel precipitation has undergone substantial multidecadal time scale changes during the twentieth century that have had severe impacts on the region’s population. Using initial-condition large ensembles (LE) of coupled general circulation model (GCM) simulations from two institutions, forced multidecadal variability is found in which Sahel precipitation declines from the 1950s to 1970s and then recovers from the 1970s to 2000s. This forced variability has similar timing to, but considerably smaller magnitude than, observed Sahel precipitation variability. Isolating the response using single forcing simulations within the LEs reveals that anthropogenic aerosols (AA) are the primary driver of this forced variability. The roles of the direct-atmospheric and the ocean-mediated atmospheric responses to AA forcing are determined with the atmosphere–land GCM (AGCM) components of the LE coupled GCMs. The direct-atmospheric response arises from changes to aerosol and precursor emissions with unchanged oceanic boundary conditions while the ocean-mediated response arises from changes to AA-forced sea surface temperatures and sea ice concentrations diagnosed from the AA-forced LE. In the AGCMs studied here, the direct-atmospheric response dominates the AA-forced 1970s − 1950s Sahel drying. On the other hand, the 2000s − 1970s wetting is mainly driven by the ocean-mediated effect, with some direct atmospheric contribution. Although the responses show differences, there is qualitative agreement between the AGCMs regarding the roles of the direct-atmospheric and ocean-mediated responses. Since these effects often compete and show nonlinearity, the model dependence of these effects and their role in the net aerosol-forced response of Sahel precipitation need to be carefully accounted for in future model analysis.

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