Microscale characteristics of homogeneous freezing events in cirrus clouds

Kärcher, B.; Jensen, E.

doi:10.1002/2016gl072486

Cited by 12 publications

(16 citation statements)

References 30 publications

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“…The simulations capture the dependence of nucleated ICNC on the homogeneous freezing temperature, T ⋆ , which is, however, very weak. We note that across all the cases discussed here, T * ≈218 K with very little scatter (±0.25 K), corresponding to a narrow range of ice saturation ratios, S ⋆ ≈1.5 ± 0.02, characteristic for homogeneous freezing events (Kärcher & Jensen, ). Examining the full upper tropospheric temperature range would reveal significant variations of T ⋆ and nucleated ICNC.…”

Section: Stochastic Simulationsmentioning

confidence: 75%

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

Kärcher¹,

Jensen

Lohmann

2019

Geophysical Research Letters

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Effects of a spectrum of mesoscale gravity waves on homogeneous aerosol freezing in midlatitude cirrus are studied by means of parcel model simulations that are driven by random vertical wind speeds constrained by balloon measurements. Stochastic wave forcing with mean updraft speeds of 5–20 cm/s leads to substantial nucleated ice crystal number concentrations (ICNC) of 0.1–1 cm−3 in situations with slow large‐scale cooling, which by itself would generate fewer ice crystals. The stochastic nature of wave‐driven air parcel temperatures enhances ICNC even further, but the times required to reach freezing conditions unsupported by large‐scale cooling may vary widely. In the presence of wave forcing, ice crystals with low ICNC (<1–10 L−1) are also generated by homogeneous freezing, albeit only rarely. Comparisons with aircraft measurements suggest significant influences of heterogeneous ice‐nucleating particles and ice crystal sedimentation on ICNC, but quantifying their individual contributions remains elusive.

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Section: Stochastic Simulationsmentioning

confidence: 75%

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

Kärcher¹,

Jensen

Lohmann

2019

Geophysical Research Letters

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“…They interpreted these changes as being an effect of more frequent heterogeneous freezing on aviation exhaust particles. It has long been known that aircraft-emitted particles may act as efficient ice nuclei, leading to heterogeneous nucleation in regions with a favorable atmospheric state (including temperature and humidity) (e.g., Schumann, 1996;Jensen and Toon, 1997;Kärcher, 2007). Furthermore, aviation-induced aerosols and contrails can alter the properties of cirrus clouds (e.g., Tesche et al, 2016;Kärcher, 2017;Urbanek et al, 2018).…”

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confidence: 99%

Changes in cirrus cloud properties and occurrence over Europe during the COVID-19-caused air traffic reduction

Groß

2021

Atmos. Chem. Phys.

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Abstract. By inducing linear contrails and contrail cirrus, air traffic has a main impact on the ice cloud coverage and occurrence. During the COVID-19 pandemic, civil air traffic over Europe was significantly reduced, in March and April 2020, to about 80 % compared to the year before. This unique situation allows us to study the effect of air traffic on cirrus clouds. This work investigates, based on satellite lidar measurements, if and how cirrus cloud properties and occurrence changed over Europe in the course of COVID-19. Cirrus cloud properties are analyzed for different years between 2014 and 2019, which showed similar meteorological conditions for the month of April as in 2020. The meteorological conditions for March, however, were warmer and drier in 2020 than the previous years. The average thickness of cirrus clouds was reduced to 1.18 km in March 2020 compared to a value of 1.40 km under normal conditions, which is stronger than expected from the aviation reduction due to the less favorable meteorology for ice cloud formation. While the April results in 2020 were only slightly reduced, with an average thickness of 70 m thinner than the composite mean of the previous 6 years. Comparing the different years shows that the cirrus cloud occurrence was reduced by about 17 %–30 %, with smaller cloud thicknesses found in 2020 for both months. In addition, the cirrus clouds measured in 2020 possess smaller values of the particle linear depolarization ratio (PLDR) than the previous years at a high significance level for both months, especially at colder temperatures (T<-50 ∘C). The same analyses are extended to the observations over the USA and China. Besides the regional discrimination of cirrus clouds, we reach the final conclusion that cirrus clouds show significant changes in PLDR in both March and April over Europe, no changes in both months over China, and significant changes only in April over the USA.

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“…As illustrated in Figure , we make the assumption that the presence of cloud ice constrains q v locally to ice saturation, that is, q v ≃ q s . This does not hold during short‐term ice formation events (Kärcher & Jensen, ). For the purpose of this study, we identify the vapor PDF with a delta distribution:

PDF false(q_{v} false) = δ false(q_{v} - q_{s} false(T false) false) .

…”

Section: Resultsmentioning

confidence: 97%

“…As illustrated in Figure 1, we make the assumption that the presence of cloud ice constrains q v locally to ice saturation, that is, q v ≃ q s . This does not hold during short-term ice formation events (Kärcher & Jensen, 2017).…”

Section: Total Water Statisticsmentioning

confidence: 99%

On the Statistical Distribution of Total Water in Cirrus Clouds

Kärcher

Thornberry

Krämer

et al. 2018

Geophysical Research Letters

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Distributions of total water in cirrus provide important information on cloud-scale variability.Observations of water variables in cirrus are sparse, limiting our ability to constrain factors controlling their evolution and lifetimes. We present and analyze aircraft measurements of tropical and extratropical cirrus total water statistics. We show that observed distributions are replicated by a parametric model that only requires knowledge of in-cloud temperatures. We parameterize the temporal decay of cloud ice content and distribution skewness forced by ice crystal sedimentation and find that ice water content decays at a much faster rate than skewness in the absence of cloud ice and skewness sources. The sensitivity of skewness to changes in ice water content is small. Our methodology and findings may prove useful for studies addressing statistical cloud schemes and cirrus life cycles.Plain Language Summary Probability distributions are important modeling tools to describe the atmospheric effects of clouds. This study reports distributions of total (gas plus ice phase) water in high tropospheric ice clouds (cirrus) obtained from measurements with research aircraft in tropical, midlatitude, and Arctic locations. We make a first attempt to base the origin of the distributions on physical processes and to reconstruct them with a statistical approach based on knowledge of cloud temperatures. We thereby show that the observed, heavy tails of the total water statistics are linked to the presence of cloud ice crystals. While cloud ice mass diminishes by ice crystal settling, the tails decay at a much slower rate. We hope that our methodology proves to be useful in support of studies analyzing cirrus life cycles.

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Microscale characteristics of homogeneous freezing events in cirrus clouds

Cited by 12 publications

References 30 publications

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

Changes in cirrus cloud properties and occurrence over Europe during the COVID-19-caused air traffic reduction

On the Statistical Distribution of Total Water in Cirrus Clouds

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