An official website of the United States government
Abstract Stratocumulus clouds cover about a fifth of Earth’s surface, and due to their albedo and low-latitude location, they have a strong effect on Earth’s radiation budget. Previous studies using large-eddy simulations have shown that multiple equilibria (both stratocumulus-covered and cloud-free/scattered cumulus states) exist as a function of fixed SST, with relevance to equatorward advected air masses. Multiple equilibria have also been found as a function of atmospheric CO 2 , with a subtropical SST nearly 10 K higher in the cloud-free state and with suggested relevance to warm climate dynamics. In this study, we use a mixed-layer model with an added surface energy balance and the ability to simulate both the stratocumulus (coupled) and cloud-free/scattered cumulus (decoupled) states using a “stacked” mixed-layer approach to study both types of multiple equilibria and the corresponding hysteresis. The model’s simplicity and computational efficiency allow us to qualitatively explore the mechanisms critical to the stratocumulus cloud instability and hysteresis as well as isolate key processes that allow for multiple equilibria via mechanism-denial experiments not possible with a full-complexity model. For the hysteresis in fixed SST, we find that decoupling can occur due to either enhanced entrainment warming or a reduction in cloud-top longwave cooling. The critical SST at which decoupling occurs is highly sensitive to precipitation and entrainment parameterizations. In the CO 2 hysteresis, decoupling occurs in the simple model used even without the inclusion of SST–cloud cover feedbacks, and the width of the hysteresis displays the same sensitivities as the fixed SST case. Overall, the simple model analysis and results motivate further studies using higher complexity models.
more »
« less
Investigating the Effects of a Subtropical Stratocumulus Cloud Breakup in Warm Climates Using Cloud-Locking Experiments
Abstract In the warm, equable climate of the Eocene, constraints on CO2levels and low-latitude temperatures have generally precluded climate models from recreating key features of the climate, especially the above-freezing winter temperatures in the continental interiors suggested by fossil evidence of frost-intolerant species at high latitudes. Several cloud feedbacks have been suggested as mechanisms for enhanced wintertime warming, including a breakup of subtropical stratocumulus cloud decks at high CO2concentrations. It has been suggested that this breakup could lead to 8 K of global average warming, but how this low-latitude cloud feedback translates to high-latitude warming is not obvious and warrants quantification with a global climate model. In this study, we use cloud-locking experiments in the Community Earth System Model, version 2 (CESM2), in which homogeneous subtropical stratocumulus clouds are prescribed or completely removed to investigate the maximum warming achieved by a breakup of subtropical stratocumulus clouds in both a preindustrial and a high CO2climate. We use the present-day continental configuration and vegetation to make these results applicable to a future warm climate. We find that in the most dramatic case, the stratocumulus breakup leads to a significant globally averaged warming of about 4.5 K and contributes to some reduction in below-freezing days in the continental interiors at high latitudes. The resulting warming is limited by stabilizing low-cloud feedbacks induced elsewhere following the breakup. We conclude that a stratocumulus breakup may have played a nonnegligible role in past warm climates, even if it cannot, on its own, explain the key features of the Eocene.
more »
« less
- Award ID(s):
- 2303486
- PAR ID:
- 10697450
- Publisher / Repository:
- AMETSOC
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 38
- Issue:
- 17
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 4315 to 4330
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO 2 can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.more » « less
-
This study investigates cloud formation and transitions in cloud types at Summit, Greenland, during 16–22 September 2010, when a warm, moist air mass was advected to Greenland from lower latitudes. During this period there was a sharp transition between high ice clouds and the formation of a lower stratocumulus deck at Summit. A regional mesoscale model is used to investigate the air masses that form these cloud systems. It is found that the high ice clouds form in originally warm, moist air masses that radiatively cool while being transported to Summit. A sensitivity study removing high ice clouds demonstrates that the primary impact of these clouds at Summit is to reduce cloud liquid water embedded within the ice cloud and water vapor in the boundary layer due to vapor deposition on snow. The mixed-phase stratocumulus clouds form at the base of cold, dry air masses advected from the northwest above 4 km. The net surface radiative fluxes during the stratocumulus period are at least 20 W m −2 larger than during the ice cloud period, indicating that, in seasons other than summer, cold, dry air masses advected to Summit above the boundary layer may radiatively warm the top of the Greenland Ice Sheet more effectively than warm, moist air masses advected from lower latitudes.more » « less
-
Abstract High‐Resolution Multi‐scale Modeling Frameworks (HR)—global climate models that embed separate, convection‐resolving models with high enough resolution to resolve boundary layer eddies—have exciting potential for investigating low cloud feedback dynamics due to reduced parameterization and ability for multidecadal throughput on modern computing hardware. However low clouds in past HR have suffered a stubborn problem of over‐entrainment due to an uncontrolled source of mixing across the marine subtropical inversion manifesting as stratocumulus dim biases in present‐day climate, limiting their scientific utility. We report new results showing that this over‐entrainment can be partly offset by using hyperviscosity and cloud droplet sedimentation. Hyperviscosity damps small‐scale momentum fluctuations associated with the formulation of the momentum solver of the embedded large eddy simulation. By considering the sedimentation process adjacent to default one‐moment microphysics in HR, condensed phase particles can be removed from the entrainment zone, which further reduces entrainment efficiency. The result is an HR that can produce more low clouds with a higher liquid water path and a reduced stratocumulus dim bias. Associated improvements in the explicitly simulated sub‐cloud eddy spectrum are observed. We report these sensitivities in multi‐week tests and then explore their operational potential alongside microphysical retuning in decadal simulations at operational 1.5° exterior resolution. The result is a new HR having desired improvements in the baseline present‐day low cloud climatology, and a reduced global mean bias and root mean squared error of absorbed shortwave radiation. We suggest it should be promising for examining low cloud feedbacks with minimal approximation.more » « less
-
null (Ed.)Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One likely reason is that this region has few continental and anthropogenic sources of cloud-nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur-based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea-spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region’s expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean-cloud-climate feedbacks in the region.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JCLI-D-24-0371.1
