Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth’s climate. An accepted ice nucleation pathway, known as ...deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below −40 °C and the absence of ice nucleation below water saturation at −35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.
We present a new cloud scheme for the ECHAM‐HAM global climate model (GCM) that includes prognostic cloud fraction and allows for subsaturation and supersaturation with respect to ice separately in ...the cloud‐free and cloudy air. Stratiform clouds form by convective detrainment, turbulent vertical diffusion, and large‐scale ascent. For each process, the corresponding cloud fraction is calculated, and the individual updraft velocities are used to determine cloud droplet/ice crystal number concentrations. Further, convective condensate is always detrained as supercooled cloud droplets at mixed‐phase temperatures (between 235 and 273 K), and convectively detrained ice crystal number concentrations are calculated based on the updraft velocity. Finally, the new scheme explicitly calculates condensation/evaporation and deposition/sublimation rates for phase‐change calculations. The new cloud scheme simulates a reasonable present‐day climate, reduces the previously overestimated cirrus cloud fraction, and in general improves the simulation of ice clouds. The model simulates the observed in‐cloud supersaturation for cirrus clouds, and it allows for a better representation of the tropical to extra‐tropical ratio of the longwave cloud radiative effect. Further, the ice water path, the ice crystal number concentrations, and the supercooled liquid fractions in mixed‐phase clouds agree better with observations in the new model than in the reference model. Ice crystal formation is dominated by the liquid‐origin processes of convective detrainment and homogeneous freezing of cloud droplets. The simulated ice clouds strongly depend on model tuning choices, in particular, the enhancement of the aggregation rate of ice crystals.
Plain Language Summary
This paper describes a new cloud scheme for the global climate model ECHAM‐HAM that better represents the ice cloud formation processes. It calculates the formation of clouds by convection, turbulent vertical diffusion, and large‐scale ascent. For each cloud formation process, the scheme calculates the cloud volume and the number concentration and size of cloud droplets and ice crystals. Further, it calculates how cloud droplets and ice crystals grow with time until they are large enough to form precipitation and are removed from the cloud. We show how the introduction of new formulations of the cloud processes affects the simulated clouds. The new ice cloud fraction compares better to satellite observations. In‐cloud properties including ice crystal number concentrations, the fraction of supercooled liquid clouds, and the radiative effects of clouds are also compared to observations. We conclude that the new cloud scheme better captures the observed properties of ice clouds and improves our capability to simulate and understand ice clouds.
Key Points
A new cloud scheme with prognostic, process‐based cloud fraction and in‐cloud water vapor was developed for ECHAM‐HAM
The new cloud scheme is stable within the GCM and improves the simulation of ice clouds
A prerequisite to applying 10Be in natural archives for solar and geomagnetic reconstructions is to know how 10Be deposition reflects atmospheric production changes. However, this relationship ...remains debated. To address this, we use two state‐of‐the‐art global models GEOS‐Chem and ECHAM6.3‐HAM2.3 with the latest beryllium production model. During solar modulation, both models suggest that 10Be deposition reacts proportionally to global production changes, with minor latitudinal deposition biases (<5%). During geomagnetic modulation, however, 10Be deposition changes are enhanced by ∼15% in the tropics and attenuated by 20%–35% in subtropical and polar regions compared to global production changes. Such changes are also hemispherically asymmetric, attributed to asymmetric production between hemispheres. For the solar proton event in 774/5 CE, 10Be shows a 15% higher deposition increase in polar regions than in tropics. This study highlights the importance of atmospheric mixing when comparing 10Be from different locations or to independent geomagnetic field records.
Plain Language Summary
The cosmogenic radionuclide beryllium‐10 (10Be) deposition in natural archives can be used to reconstruct solar and geomagnetic changes in the past. Understanding how 10Be deposition reflects atmospheric production rate changes is crucial for these applications. However, this relationship remains debated. To address this issue, we use two state‐of‐the‐art global models, GEOS‐Chem 14.1.1 and ECHAM6.3‐HAM2.3, along with the latest beryllium production model (CRAC: Be). When responding to solar modulation, both models indicate that 10Be deposition corresponds proportionally to global production rate changes, with a minor latitudinal bias. However, during geomagnetic modulation, 10Be deposition changes significantly compared to global production rate changes. 10Be deposition also shows varying hemispheric responses to geomagnetic modulation, attributed to the asymmetric production between hemispheres. For the extreme solar proton event in 774/5 CE, 10Be shows a higher deposition flux increase in the polar regions compared to the tropics. These findings underscore the need to account for atmospheric mixing on 10Be deposition from different locations, especially for the changes due to the geomagnetic field variations.
Key Points
We used two state‐of‐the‐art global models incorporating the latest beryllium production rates to study the sources of 10Be deposition
10Be deposition shows strong regional bias compared to the global signal in response to geomagnetic modulation but not to solar modulation
10Be deposition shows varying hemispheric responses to geomagnetic modulation, attributed to the asymmetric production between hemispheres
Flood or Drought: How Do Aerosols Affect Precipitation? Rosenfeld, Daniel; Lohmann, Ulrike; Raga, Graciela B ...
Science (American Association for the Advancement of Science),
09/2008, Letnik:
321, Številka:
5894
Journal Article
Recenzirano
Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been ...reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.
The idea of modifying cirrus clouds to directly counteract greenhouse gas warming has gained momentum in recent years, despite disputes over its physical feasibility. Previous studies that analyzed ...modifications of cirrus clouds by seeding of ice nucleating particles showed large uncertainties in both cloud and surface climate responses, ranging from no effect or even a small warming to a globally averaged cooling of about 2.5 °C. We use two general circulation models that showed very different responses in previous studies, ECHAM6-HAM and CESM-CAM5, to determine which radiative and climatic responses to cirrus cloud seeding in a 1.5 × CO2 world are common and which are not. Seeding reduces the net cirrus radiative effect for −1.8 W m−2 in CESM compared with only −0.8 W m−2 in ECHAM. Accordingly, the surface temperature decrease is larger in CESM, counteracting about 70% of the global mean temperature increase due to CO2 and only 30% in ECHAM. While seeding impacts on mean precipitation were addressed in past studies, we are the first to analyze extreme precipitation responses to cirrus seeding. Seeding decreases the frequency of the most extreme precipitation globally. However, the extreme precipitation events occur more frequently in the Sahel and Central America, following the mean precipitation increase due to a northward shift of the Intertropical Convergence Zone. In addition, we use a quadratic climate damage metric to evaluate the amount of CO2-induced damage cirrus seeding can counteract. Seeding decreases the damage by about 50% in ECHAM, and by 85% in CESM over the 21 selected land regions. Climate damage due to CO2 increase is significantly reduced as a result of seeding in all of the considered land regions.
A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing. Those processes are manifest in a number ...of relationships that can be expressed as factors dlnX/dlnY driving aerosol effects on cloud radiative forcing. These factors include the relationships between cloud condensation nuclei (CCN) concentration and emissions, droplet number and CCN concentration, cloud fraction and droplet number, cloud optical depth and droplet number, and cloud radiative forcing and cloud optical depth. The relationship between cloud optical depth and droplet number can be further decomposed into the sum of two terms involving the relationship of droplet effective radius and cloud liquid water path with droplet number. These relationships can be constrained using observations of recent spatial and temporal variability of these quantities. However, we are most interested in the radiative forcing since the preindustrial era. Because few relevant measurements are available from that era, relationships from recent variability have been assumed to be applicable to the preindustrial to present-day change. Our analysis of Aerosol Comparisons between Observations and Models (AeroCom) model simulations suggests that estimates of relationships from recent variability are poor constraints on relationships from anthropogenic change for some terms, with even the sign of some relationships differing in many regions. Proxies connecting recent spatial/temporal variability to anthropogenic change, or sustained measurements in regions where emissions have changed, are needed to constrain estimates of anthropogenic aerosol impacts on cloud radiative forcing.
Mixed‐phase clouds (clouds that consist of both cloud droplets and ice crystals) are frequently present in the Earth's atmosphere and influence the Earth's energy budget through their radiative ...properties, which are highly dependent on the cloud water phase. In this study, the phase partitioning of cloud water is compared among six global climate models (GCMs) and with Cloud and Aerosol Lidar with Orthogonal Polarization retrievals. It is found that the GCMs predict vastly different distributions of cloud phase for a given temperature, and none of them are capable of reproducing the spatial distribution or magnitude of the observed phase partitioning. While some GCMs produced liquid water paths comparable to satellite observations, they all failed to preserve sufficient liquid water at mixed‐phase cloud temperatures. Our results suggest that validating GCMs using only the vertically integrated water contents could lead to amplified differences in cloud radiative feedback. The sensitivity of the simulated cloud phase in GCMs to the choice of heterogeneous ice nucleation parameterization is also investigated. The response to a change in ice nucleation is quite different for each GCM, and the implementation of the same ice nucleation parameterization in all models does not reduce the spread in simulated phase among GCMs. The results suggest that processes subsequent to ice nucleation are at least as important in determining phase and should be the focus of future studies aimed at understanding and reducing differences among the models.
Key Points
Phase partitioning of cloud water in GCMs is investigated
Cloud water phase in GCMs is compared to satellite observations
Ice nucleation parameterization influence on cloud water phase is investigated
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