scholarly journals Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM

2008 ◽  
Vol 8 (7) ◽  
pp. 2115-2131 ◽  
Author(s):  
U. Lohmann
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Hydrological Cycle ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Convective Precipitation ◽  
Anthropogenic Aerosol ◽  
Convective Clouds ◽  
Double Moment ◽  
Aerosol Effects

Abstract. Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei and may have an influence on the hydrological cycle. Here we investigate aerosol effects on convective clouds by extending the double-moment cloud microphysics scheme developed for stratiform clouds, which is coupled to the HAM double-moment aerosol scheme, to convective clouds in the ECHAM5 general circulation model. This enables us to investigate whether more, and smaller cloud droplets suppress the warm rain formation in the lower parts of convective clouds and thus release more latent heat upon freezing, which would then result in more vigorous convection and more precipitation. In ECHAM5, including aerosol effects in large-scale and convective clouds (simulation ECHAM5-conv) reduces the sensitivity of the liquid water path increase with increasing aerosol optical depth in better agreement with observations and large-eddy simulation studies. In simulation ECHAM5-conv with increases in greenhouse gas and aerosol emissions since pre-industrial times, the geographical distribution of the changes in precipitation better matches the observed increase in precipitation than neglecting microphysics in convective clouds. In this simulation the convective precipitation increases the most suggesting that the convection has indeed become more vigorous.

scholarly journals Global anthropogenic aerosol effects on convective clouds in ECHAM5-HAM

2007 ◽  
Vol 7 (5) ◽  
pp. 14639-14674 ◽  
Author(s):  
U. Lohmann
Keyword(s):  
Liquid Water ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Anthropogenic Aerosol ◽  
Water Path ◽  
Convective Clouds ◽  
Stratiform Clouds ◽  
Aerosol Effects

Abstract. Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei and may have an influence on the hydrological cycle. Here we investigate aerosol effects on convective clouds by extending the double moment cloud microphysics scheme developed for stratiform clouds to convective clouds in the ECHAM5 general circulation model. This increases the liquid water path in the tropics and reduces the sensitivity of the liquid water path with increasing aerosol optical depth in better agreement with observations and large-eddy simulation studies. In simulations in which greenhouse gases and aerosols emissions are increased since pre-industrial times, accounting for microphysics in convective clouds matches most closely the observed increase in precipitation. The total anthropogenic aerosol effect since pre-industrial time is slightly reduced from −1.6 to −1.9 W m−2 when microphysics are only included in stratiform clouds to −1.5 W m−2 when microphysics are included both in stratiform and convective clouds.

scholarly journals Influence of Giant CCN on warm rain processes in the ECHAM5 GCM

2008 ◽  
Vol 8 (14) ◽  
pp. 3769-3788 ◽  
Author(s):  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
Indirect Effect ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Precipitation Amount ◽  
Circulation Model ◽  
Anthropogenic Activity ◽  
Total Water ◽  
Anthropogenic Aerosol ◽  
Aerosol Indirect Effect ◽  
Cloud Properties

Abstract. Increased Cloud Condensation Nuclei (CCN) load due to anthropogenic activity might lead to non-precipitating clouds because the cloud drops become smaller (for a constant liquid water content) and, therefore, less efficient in rain formation (aerosol indirect effect). Adding giant CCN (GCCN) into such a cloud can initiate precipitation (namely, drizzle) and, therefore, might counteract the aerosol indirect effect. The effect of GCCN on global climate on warm clouds and precipitation within the ECHAM5 General Circulation Model (GCM) is investigated. Therefore, the newly introduced prognostic rain scheme (Posselt and Lohmann, 2007) is applied so that GCCN are directly activated into rain drops. The ECHAM5 simulations with incorporated GCCN show that precipitation is affected only locally. On the global scale, the precipitation amount does not change. Cloud properties like total water (liquid + rain water) and cloud drop number show a larger sensitivity to GCCN. Depending on the amount of added GCCN, the reduction of total water and cloud drops account for up to 20% compared to the control run without GCCN. Thus, the incorporation of the GCCN accelerate the hydrological cycle so that clouds precipitate faster (but not more) and less condensed water is accumulated in the atmosphere. An estimate of the anthropogenic aerosol indirect effect on the climate is obtained by comparing simulations for present-day and pre-industrial climate. The introduction of the prognostic rain scheme lowered the anthropogenic aerosol indirect effect significantly compared to the standard ECHAM5 with the diagnostic rain scheme. The incorporation of the GCCN changes the model state, especially the cloud properties like TWP and Nl. The precipitation changes only locally but globally the precipitation is unaffected because it has to equal the global mean evaporation rate. Changing the cloud properties leads to a local reduction of the aerosol indirect effect and, hence, partly compensating for the increased anthropogenic CCN concentrations in that regions. Globally, the aerosol indirect effect is nearly the same for all simulations.

scholarly journals Effects of Cloud Microphysics on Tropical Atmospheric Hydrologic Processes and Intraseasonal Variability

2005 ◽  
Vol 18 (22) ◽  
pp. 4731-4751 ◽  
Author(s):  
K. M. Lau ◽  
H. T. Wu ◽  
Y. C. Sud ◽  
G. K. Walker
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Water Cycle ◽  
Deep Convection ◽  
Intraseasonal Variability ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Hydrologic Processes ◽  
Warm Rain

Abstract The sensitivity of tropical atmospheric hydrologic processes to cloud microphysics is investigated using the NASA Goddard Earth Observing System (GEOS) general circulation model (GCM). Results show that a faster autoconversion rate leads to (a) enhanced deep convection in the climatological convective zones anchored to tropical land regions; (b) more warm rain, but less cloud over oceanic regions; and (c) an increased convective-to-stratiform rain ratio over the entire Tropics. Fewer clouds enhance longwave cooling and reduce shortwave heating in the upper troposphere, while more warm rain produces more condensation heating in the lower troposphere. This vertical differential heating destabilizes the tropical atmosphere, producing a positive feedback resulting in more rain and an enhanced atmospheric water cycle over the Tropics. The feedback is maintained via secondary circulations between convective tower and anvil regions (cold rain), and adjacent middle-to-low cloud (warm rain) regions. The lower cell is capped by horizontal divergence and maximum cloud detrainment near the freezing–melting (0°C) level, with rising motion (relative to the vertical mean) in the warm rain region connected to sinking motion in the cold rain region. The upper cell is found above the 0°C level, with induced subsidence in the warm rain and dry regions, coupled to forced ascent in the deep convection region. It is that warm rain plays an important role in regulating the time scales of convective cycles, and in altering the tropical large-scale circulation through radiative–dynamic interactions. Reduced cloud–radiation feedback due to a faster autoconversion rate results in intermittent but more energetic eastward propagating Madden–Julian oscillations (MJOs). Conversely, a slower autoconversion rate, with increased cloud radiation produces MJOs with more realistic westward-propagating transients embedded in eastward-propagating supercloud clusters. The implications of the present results on climate change and water cycle dynamics research are discussed.

scholarly journals Prognostic precipitation with three liquid water classes in the ECHAM5-HAM GCM

2015 ◽  
Vol 15 (5) ◽  
pp. 7783-7836
Author(s):  
V. Sant ◽  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
Liquid Water ◽  
Radiative Forcing ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Liquid Water Path ◽  
Anthropogenic Aerosol ◽  
Global Circulation Models ◽  
Aerosol Forcing ◽  
Ice Phase

Abstract. In order to improve the global representation of rain formation in marine stratiform clouds a new parameterization with three prognostic liquid water classes was implemented into the general circulation model ECHAM5 with the aerosol module HAM. The additionally introduced drizzle class improves the physical representation of the droplet spectrum and more importantly, improves the microphysical processes relevant for precipitation formation compared to the standard parameterization. In order to avoid a mismatch of the liquid and ice phase, the prognostic treatment of snow has been introduced too. This has a significant effect on the amount and altitude of ice clouds, which in turn does not only affect in- and outgoing radiation, but also the parameterized collection rates. With the introduction of a prognostic precipitation scheme a more realistic representation of both liquid and ice phase large-scale precipitation is achieved compared to a diagnostic treatment. An encouraging finding is that the sensitivity of the liquid water path to the anthropogenic aerosol forcing with the prognostic treatment is reduced by about 25%. Although the total net radiative forcing is increased from 1.4±0.4 to 1.6±0.4 W m−2 from the control to the prognostic model version, the difference is within the interannual variability. Altogether the results suggest that the treatment of precipitation in global circulation models has a significant influence on the phase and lifetime of clouds, but also hints towards the uncertainties related to a prognostic precipitation scheme.

scholarly journals Introduction of prognostic rain in ECHAM5: design and single column model simulations

2008 ◽  
Vol 8 (11) ◽  
pp. 2949-2963 ◽  
Author(s):  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Rain Water ◽  
Time Step ◽  
Surface Precipitation ◽  
Column Model ◽  
Single Column ◽  
Single Column Model

Abstract. Prognostic equations for the rain mass mixing ratio and the rain drop number concentration are introduced into the large-scale cloud microphysics parameterization of the ECHAM5 general circulation model (ECHAM5-PROG). To this end, a rain flux from one level to the next with the appropriate fall speed is introduced. This maintains rain water in the atmosphere to be available for the next time step. Rain formation in ECHAM5-PROG is, therefore, less dependent on the autoconversion rate than the standard ECHAM5 but shifts the emphasis towards the accretion rates in accordance with observations. ECHAM5-PROG is tested and evaluated with Single Column Model (SCM) simulations for two cases: the marine stratocumulus study EPIC (October 2001) and the continental mid-latitude ARM Cloud IOP (shallow frontal cloud case – March 2000). In case of heavy precipitation events, the prognostic equations for rain hardly affect the amount and timing of precipitation at the surface in different SCM simulations because heavy rain depends mainly on the large-scale forcing. In case of thin, drizzling clouds (i.e., stratocumulus), surface precipitation is sensitive to the number of sub-time steps used in the prognostic rain scheme. Cloud microphysical quantities, such as cloud liquid and rain water within the atmosphere, are sensitive to the number of sub-time steps in both considered cases. This results from the decreasing autoconversion rate and increasing accretion rate.

scholarly journals EPIC simulations of Neptune’s dark spots using an active cloud microphysical model

2020 ◽  
Vol 496 (4) ◽  
pp. 4760-4768
Author(s):  
Nathan Hadland ◽  
Ramanakumar Sankar ◽  
Raymond Paul LeBeau ◽  
Csaba Palotai
Keyword(s):  
Northern Hemisphere ◽  
General Circulation ◽  
Large Scale ◽  
Dynamic Properties ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Cloud Formation ◽  
Explicit Calculation ◽  
Dark Spot ◽  
Vapour Density

ABSTRACT The Great Dark Spot (GDS-89) observed by Voyager 2 was the first of several large-scale vortices observed on Neptune, the most recent of which was observed in 2018 in the Northern hemisphere (NDS-2018). Ongoing observations of these features are constraining cloud formation, drift, shape oscillations, and other dynamic properties. In order to effectively model these characteristics, an explicit calculation of methane cloud microphysics is needed. Using an updated version of the Explicit Planetary Isentropic Coordinate General Circulation Model (EPIC GCM) and its active cloud microphysics module to account for the condensation of methane, we investigate the evolution of large-scale vortices on Neptune. We model the effect of methane deep abundance and cloud formation on vortex stability and dynamics. In our simulations, the vortex shows a sharp contrast in methane vapour density inside compared to outside the vortex. Methane vapour column density is analogous to optical depth and provides a more consistent tracer to track the vortex, so we use that variable over potential vorticity. We match the meridional drift rate of the GDS and gain an initial insight into the evolution of vortices in the Northern hemisphere, such as the NDS-2018.

scholarly journals Global response of parameterised convective cloud fields to anthropogenic aerosol forcing

2019 ◽  
Author(s):  
Zak Kipling ◽  
Laurent Labbouz ◽  
Philip Stier
Keyword(s):  
Large Scale ◽  
Climate Model ◽  
Global Climate ◽  
Convective Cloud ◽  
Climate Impact ◽  
Limited Area ◽  
Climate Modelling ◽  
Anthropogenic Aerosol ◽  
Convective Clouds ◽  
Aerosol Effects

Abstract. The interactions between aerosols and convective clouds represent some of the greatest uncertainties in the climate impact of aerosols in the atmosphere. A wide variety of mechanisms have been proposed by which aerosols may invigorate, suppress, or change the properties of individual convective clouds, some of which can be reproduced in high-resolution limited-area models. However, there may also be mesoscale, regional or global adjustments which modulate or dampen such impacts which cannot be captured in the limited domain of such models. The Convective Cloud Field Model (CCFM) provides a mechanism to explicitly simulate a population of convective clouds within each grid column at resolutions used for global climate modelling, so that a representation of the microphysical aerosol response within each parameterised cloud type is possible. Using CCFM within the global aerosol–climate model ECHAM–HAM, we demonstrate how the parameterised cloud field responds to the present-day anthropogenic aerosol perturbation in different regions. In particular, we show that in regions with strongly-forced deep convection and/or significant aerosol effects via large-scale processes, the changes in the convective cloud field due to microphysical effects is rather small; however in a more weakly-forced regime such as the Caribbean, where large-scale aerosol effects are small, a signature of convective invigoration does become apparent.

scholarly journals Introduction of prognostic rain in ECHAM5: design and Single Column Model simulations

2007 ◽  
Vol 7 (5) ◽  
pp. 14675-14706 ◽  
Author(s):  
R. Posselt ◽  
U. Lohmann
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Rain Water ◽  
Time Step ◽  
Surface Precipitation ◽  
Column Model ◽  
Single Column ◽  
Single Column Model

Abstract. Prognostic equations for the rain mass mixing ratio and the rain drop number concentration are introduced into the large-scale cloud microphysics parameterization of the ECHAM5 general circulation model (ECHAM5-RAIN). For this a rain flux from one level to the next with the appropriate fall speed is introduced. This maintains rain water in the atmosphere to be available for the next time step. Rain formation in ECHAM5-RAIN is, therefore, less dependent on the autoconversion rate than the standard ECHAM5 but shifts the emphasis towards the accretion rates in accordance with observations. ECHAM5-RAIN is tested and evaluated with two cases: the continental mid-latitude ARM Cloud IOP (shallow frontal cloud case – March 2000) and EPIC (a marine stratocumulus study – October 2001). The prognostic equations for rain hardly affect the amount and timing of precipitation at the surface in different Single Column Model (SCM) simulations for heavy precipitating clouds because heavy rain depends mainly on the large-scale forcing. In case of thin, drizzling clouds (i.e., stratocumulus), an increase in surface precipitation is caused by more sub-time steps used in the prognostic rain scheme until convergence is reached. Cloud microphysical quantities, such as liquid and rain water, are more sensitive to the number of sub-time steps for light precipitation. This results from the decreasing autoconversion rate and increasing accretion rate.

Modeling Water Vapor and Clouds as Passive Tracers in an Idealized GCM

2018 ◽  
Vol 31 (2) ◽  
pp. 775-786 ◽  
Author(s):  
Yi Ming ◽  
Isaac M. Held
Keyword(s):  
Water Vapor ◽  
General Circulation ◽  
Large Scale ◽  
Probability Distributions ◽  
Water Vapor Pressure ◽  
Substantial Reduction ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Cloud Microphysics ◽  
Cloud Fraction

This paper introduces an idealized general circulation model (GCM) in which water vapor and clouds are tracked as tracers, but are not allowed to affect circulation through either latent heat release or cloud radiative effects. The cloud scheme includes an explicit treatment of cloud microphysics and diagnoses cloud fraction from a prescribed subgrid distribution of total water. The model is capable of qualitatively capturing many large-scale features of water vapor and cloud distributions outside of the boundary layer and deep tropics. The subtropical dry zones, midlatitude storm tracks, and upper-tropospheric cirrus are simulated reasonably well. The inclusion of cloud microphysics (namely rain re-evaporation) has a modest but significant effect of moistening the lower troposphere in this model. When being subjected to a uniform fractional increase of saturated water vapor pressure, the model produces little change in cloud fraction. A more realistic perturbation, which considers the nonlinearity of the Clausius–Clapeyron relation and spatial structure of CO2-induced warming, results in a substantial reduction in the free-tropospheric cloud fraction. This is reconciled with an increase of relative humidity by analyzing the probability distributions of both quantities, and may help explain partly similar decreases in cloud fraction in full GCMs. The model provides a means to isolate individual processes or model components for studying their influences on cloud simulation in the extratropical free troposphere.

scholarly journals Global response of parameterised convective cloud fields to anthropogenic aerosol forcing

2020 ◽  
Vol 20 (7) ◽  
pp. 4445-4460
Author(s):  
Zak Kipling ◽  
Laurent Labbouz ◽  
Philip Stier
Keyword(s):  
Large Scale ◽  
Climate Model ◽  
Global Climate ◽  
Convective Cloud ◽  
Climate Impact ◽  
Limited Area ◽  
Climate Modelling ◽  
Anthropogenic Aerosol ◽  
Convective Clouds ◽  
Aerosol Effects

Abstract. The interactions between aerosols and convective clouds represent some of the greatest uncertainties in the climate impact of aerosols in the atmosphere. A wide variety of mechanisms have been proposed by which aerosols may invigorate, suppress or change the properties of individual convective clouds, some of which can be reproduced in high-resolution limited-area models. However, there may also be mesoscale, regional or global adjustments which modulate or dampen such impacts which cannot be captured in the limited domain of such models. The Convective Cloud Field Model (CCFM) provides a mechanism to simulate a population of convective clouds, complete with microphysics and interactions between clouds, within each grid column at resolutions used for global climate modelling, so that a representation of the microphysical aerosol response within each parameterised cloud type is possible. Using CCFM within the global aerosol–climate model ECHAM–HAM, we demonstrate how the parameterised cloud field responds to the present-day anthropogenic aerosol perturbation in different regions. In particular, we show that in regions with strongly forced deep convection and/or significant aerosol effects via large-scale processes, the changes in the convective cloud field due to microphysical effects are rather small; however in a more weakly forced regime such as the Caribbean, where large-scale aerosol effects are small, a signature of convective invigoration does become apparent.

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