Understanding the Extratropical Liquid Water Path Feedback in Mixed-Phase Clouds with an Idealized Global Climate Model

Abstract A negative shortwave cloud feedback associated with higher extratropical liquid water content in mixed-phase clouds is a common feature of global warming simulations, and multiple mechanisms have been hypothesized. A set of process-level experiments performed with an idealized global climate model (a dynamical core with passive water and cloud tracers and full Rotstayn-Klein single-moment microphysics) show that the common picture of the liquid water path (LWP) feedback in mixed-phase clouds being controlled by the amount of ice susceptible to phase change is not robust. Dynamic condensate processes—rather than static phase partitioning—directly change with warming, with varied impacts on liquid and ice amounts. Here, three principal mechanisms are responsible for the LWP response, namely higher adiabatic cloud water content, weaker liquid-to-ice conversion through the Bergeron-Findeisen process, and faster melting of ice and snow to rain. Only melting is accompanied by a substantial loss of ice, while the adiabatic cloud water content increase gives rise to a net increase in ice water path (IWP) such that total cloud water also increases without an accompanying decrease in precipitation efficiency. Perturbed parameter experiments with a wide range of climatological LWP and IWP demonstrate a strong dependence of the LWP feedback on the climatological LWP and independence from the climatological IWP and supercooled liquid fraction. This idealized setup allows for a clean isolation of mechanisms and paints a more nuanced picture of the extratropical mixed-phase cloud water feedback than simple phase change.

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Sensitivity studies of different aerosol indirect effects in mixed-phase clouds

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-15045-2009 ◽

2009 ◽

Vol 9 (4) ◽

pp. 15045-15081

Author(s):

U. Lohmann ◽

C. Hoose

Keyword(s):

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Mixed Phase ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Activation Effect ◽

Aerosol Effect ◽

Sensitivity Studies ◽

Mixed Phase Clouds

Abstract. Aerosols affect the climate system by changing cloud characteristics. Using the global climate model ECHAM5-HAM, we investigate different aerosol effects on mixed-phase clouds: The glaciation effect, which refers to a more frequent glaciation due to anthropogenic aerosols, versus the de-activation effect, which suggests that ice nuclei become less effective because of an anthropogenic sulfate coating. The glaciation effect can partly offset the indirect aerosol effect on warm clouds and thus causes the total anthropogenic aerosol effect to be smaller. It is investigated by varying the parameterization for the Bergeron-Findeisen process and the threshold coating thickness of sulfate (SO4-crit), which is required to convert an externally mixed aerosol particle into an internally mixed particle. Differences in the net radiation at the top-of-the-atmosphere due to anthropogenic aerosols between the different sensitivity studies amount up to 0.5 W m−2. This suggests that the investigated mixed-phase processes have a major effect on the total anthropogenic aerosol effect.

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The importance of mixed-phase clouds for climate sensitivity in the global aerosol-climate model ECHAM6-HAM2

10.5194/acp-2018-97 ◽

2018 ◽

Cited By ~ 1

Author(s):

Ulrike Lohmann ◽

David Neubauer

Keyword(s):

Southern Ocean ◽

Climate Sensitivity ◽

Liquid Water ◽

Climate Model ◽

Global Climate Model ◽

Supercooled Liquid ◽

Mixed Phase ◽

Radiation Budget ◽

Shortwave Radiation ◽

Mixed Phase Clouds

Abstract. Clouds are important in the climate system because of their large influence on the radiation budget. On the one hand, they scatter solar radiation and with that cool the climate. On the other hand, they absorb and re-emit terrestrial radiation, which causes a warming. How clouds change in a warmer climate is one of the largest uncertainties for the equilibrium climate sensitivity (ECS). While a large spread in the cloud feedback arises from low-level clouds, it was recently shown that also mixed-phase clouds are important for ECS. If mixed-phase clouds in the current climate contain too few supercooled cloud droplets, too much ice will change to liquid water in a warmer climate. As shown by Tan et al. (2016), this overestimates the negative cloud phase feedback and underestimates ECS in the CAM global climate model (GCM). Here we are using the newest version of the ECHAM6-HAM2 GCM to investigate the importance of mixed-phase clouds for ECS. Although we also considerably underestimate the fraction of supercooled liquid water globally in the reference version of ECHAM6-HAM2 GCM, we do not obtain increases in ECS in simulations with more supercooled liquid water in the present-day climate, contrary to the findings by Tan et al. (2016). We hypothesize that it is not the global supercooled liquid water fraction that matters, but only how well low- and mid-level mixed-phase clouds with cloud top temperatures in the mixed-phase temperature range between 0 and −35 ºC are simulated. These occur most frequent in mid-latitudes, in particular over the Southern Ocean where they determine the amount of absorbed shortwave radiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over the Southern Ocean is only overestimated if all clouds below 0 ºC consist exclusively of ice and only in this simulation is ECS is significantly smaller than in all other simulations. Hence, the negative cloud phase feedback seems to be important only if the optically thin low- and mid-level mid-latitude clouds have the wrong phase (ice instead of liquid water) in the absence of overlying clouds. In all other simulations, changes in cloud feedbacks associated with cloud amount and cloud top pressure, dominate.

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Bacteria in the ECHAM5-HAM global climate model

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-8645-2012 ◽

2012 ◽

Vol 12 (18) ◽

pp. 8645-8661 ◽

Cited By ~ 48

Author(s):

A. Sesartic ◽

U. Lohmann ◽

T. Storelvmo

Keyword(s):

Active Sites ◽

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Ice Nucleation ◽

Cloud Formation ◽

Liquid Water Path ◽

Water Path ◽

Ice Nuclei ◽

Ice Water

Abstract. Some bacteria are among the most active ice nuclei found in nature due to the ice nucleation active proteins on their surface, which serve as active sites for ice nucleation. Their potential impact on clouds and precipitation is not well known and needs to be investigated. Bacteria as a new aerosol species were introduced into the global climate model (GCM) ECHAM5-HAM. The inclusion of bacteria acting as ice nuclei in a GCM leads to only minor changes in cloud formation and precipitation on a global level, however, changes in the liquid water path and ice water path are simulated, specifically in the boreal regions where tundra and forests act as sources of bacteria. Although bacteria contribute to heterogeneous freezing, their impact is reduced by their low numbers compared to other heterogeneous IN. This result confirms the outcome of several previous studies.

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Sensitivity studies of different aerosol indirect effects in mixed-phase clouds

Atmospheric Chemistry and Physics ◽

10.5194/acp-9-8917-2009 ◽

2009 ◽

Vol 9 (22) ◽

pp. 8917-8934 ◽

Cited By ~ 119

Author(s):

U. Lohmann ◽

C. Hoose

Keyword(s):

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Mixed Phase ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Activation Effect ◽

Aerosol Effect ◽

Sensitivity Studies ◽

Mixed Phase Clouds

Abstract. Aerosols affect the climate system by changing cloud characteristics. Using the global climate model ECHAM5-HAM, we investigate different aerosol effects on mixed-phase clouds: The glaciation effect, which refers to a more frequent glaciation due to anthropogenic aerosols, versus the de-activation effect, which suggests that ice nuclei become less effective because of an anthropogenic sulfate coating. The glaciation effect can partly offset the indirect aerosol effect on warm clouds and thus causes the total anthropogenic aerosol effect to be smaller. It is investigated by varying the parameterization for the Bergeron-Findeisen process and the threshold coating thickness of sulfate (SO4-crit), which is required to convert an externally mixed aerosol particle into an internally mixed particle. Differences in the net radiation at the top-of-the-atmosphere due to anthropogenic aerosols between the different sensitivity studies amount up to 0.5 W m−2. This suggests that the investigated mixed-phase processes have a major effect on the total anthropogenic aerosol effect.

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Formation and Development of Orographic Mixed-Phase Clouds

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0348.1 ◽

2017 ◽

Vol 74 (11) ◽

pp. 3703-3724 ◽

Cited By ~ 3

Author(s):

Olga Henneberg ◽

Jan Henneberger ◽

Ulrike Lohmann

Keyword(s):

Liquid Water ◽

Sensitivity Study ◽

Horizontal Resolution ◽

Mixed Phase ◽

Liquid Water Path ◽

Water Path ◽

Orographic Forcing ◽

Mixed Phase Clouds ◽

Ice Water ◽

Saturation Vapor

Abstract Orographic forcing can stabilize mixed-phase clouds (MPCs), which are thermodynamically unstable owing to the different saturation vapor pressure over liquid water and ice. This study presents simulations of MPCs in orographically complex terrain over the Alpine ridge with the regional model COSMO using a horizontal resolution of 1 km. Two case studies provide insights into the formation of Alpine MPCs. Trajectory studies show that the majority of the air parcels lifted by more than 600 m are predominantly in the liquid phase even if they originate from glaciated clouds. The interplay between lifted and advected air parcels is crucial for the occurrence of MPCs. Within a sensitivity study, the orography is reduced to 80%, which changed both the total barrier height and steepness. The changes in total water path (TWP), liquid water path (LWP), and ice water path (IWP) vary in sign and strength as the affected precipitation does. LWP can experience changes up to 500% resulting in a transformation from an ice-dominated MPC to a liquid-dominated MPC. In further simulations with increased steepness and maintained surface height at Jungfraujoch, TWP experiences a reduction between 25% and 40% during different time periods, which results in reduced precipitation by around 30%. An accurate representation of the steepness and the height of mountains in models is crucial for the formation and development of MPCs.

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Bacteria in the ECHAM5-HAM global climate model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-1457-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 1457-1488 ◽

Cited By ~ 11

Author(s):

A. Sesartic ◽

U. Lohmann ◽

T. Storelvmo

Keyword(s):

Active Sites ◽

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Ice Nucleation ◽

Cloud Formation ◽

Liquid Water Path ◽

Water Path ◽

Ice Nuclei ◽

Ice Water

Abstract. Bacteria are the most active naturally occuring ice nuclei (IN) due to the ice nucleation active proteins on their surface, which serve as active sites for ice nucleation. Their potential impact on clouds and precipitation is not well known and needs to be investigated. Bacteria as a new aerosol species were introduced into the global climate model (GCM) ECHAM5-HAM. The inclusion of bacteria acting as IN in a GCM leads to only minor changes in cloud formation and precipitation on a global level, however, changes in the liquid water path and ice water path can be observed, specifically in the boreal regions where tundra and forests act as sources of bacteria.

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