An automated cirrus classification

Abstract. Cirrus clouds play an important role in determining the radiation budget of the earth, but many of their properties remain uncertain, particularly their response to aerosol variations and to warming. Part of the reason for this uncertainty is the dependence of cirrus cloud properties on the cloud formation mechanism, which itself is strongly dependent on the local meteorological conditions. In this work, a classification system (Identification and Classification of Cirrus or IC-CIR) is introduced to identify cirrus clouds by the cloud formation mechanism. Using reanalysis and satellite data, cirrus clouds are separated into four main types: orographic, frontal, convective and synoptic. Through a comparison to convection-permitting model simulations and back-trajectory-based analysis, it is shown that these observation-based regimes can provide extra information on the cloud-scale updraughts and the frequency of occurrence of liquid-origin ice, with the convective regime having higher updraughts and a greater occurrence of liquid-origin ice compared to the synoptic regimes. Despite having different cloud formation mechanisms, the radiative properties of the regimes are not distinct, indicating that retrieved cloud properties alone are insufficient to completely describe them. This classification is designed to be easily implemented in GCMs, helping improve future model–observation comparisons and leading to improved parametrisations of cirrus cloud processes.

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Technical note: An automated cirrus classification

10.5194/acp-2017-723 ◽

2017 ◽

Cited By ~ 2

Author(s):

Edward Gryspeerdt ◽

Johannes Quaas ◽

Tom Goren ◽

Daniel Klocke ◽

Matthias Brueck

Keyword(s):

Reanalysis Data ◽

Technical Note ◽

Cirrus Clouds ◽

Radiation Budget ◽

Back Trajectory ◽

Formation Mechanisms ◽

Mechanism Of Formation ◽

Cloud Properties

Abstract. Cirrus clouds play an important role in determining the radiation budget of the earth, but many of their properties remain uncertain, particularly their response to aerosol variations and to warming. Part of the reason for this uncertainty is the dependence of cirrus clouds on the mechanism of formation, which itself is strongly dependent on the local meteorological conditions. In this work, a classification system (Identification and Classification of Cirrus or IC-CIR) is introduced to identify cirrus clouds by their formation mechanisms. Using re-analysis and satellite data, cirrus clouds are separated in four main types: orographic, frontal, convective and in-situ. Through a comparison to convection-permitting model simulations and back-trajectory based analysis, it is shown that the regimes can provide extra information on the properties and origin of cirrus that could not be provided by the retrieved cloud properties or reanalysis data alone. This classification is designed to be easily implemented in GCMs, helping improve future model-observation comparisons and leading to improved parametrisations of cirrus cloud processes.

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The roles of dynamical variability and aerosols in cirrus cloud formation

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-3-1415-2003 ◽

2003 ◽

Vol 3 (2) ◽

pp. 1415-1451 ◽

Cited By ~ 4

Author(s):

B. Kärcher ◽

J. Ström

Keyword(s):

Probability Distributions ◽

Cirrus Clouds ◽

Cloud Formation ◽

Cirrus Cloud ◽

Ice Crystal ◽

Mesoscale Variability ◽

Aerosol Composition ◽

Airborne Measurements ◽

Cloud Properties

Abstract. The probability of occurrence of ice crystal number densities in young cirrus clouds is examined based on airborne measurements. The observations have been carried out at midlatitudes in both hemispheres at equivalent latitudes (~52–55° N/S) during the same season (local autumn in 2000). The in situ measurements considered in the present study include temperatures, vertical velocities, and ice crystal concentrations, the latter determined with high precision and accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities (1–10 cm−3) of small (diameter <20 μm) ice crystals. This mode dominates the probability distributions in both hemispheres and is shown to be caused by rapid cooling rates associated with updraft speeds in the range 10–100 cm s-1. A second mode containing larger crystals extends from ~1 cm−3 to low concentrations close to the detection threshold (~3×104cm−3) and is associated with lower updraft speeds. Results of a statistical analysis provide compelling evidence that the dynamical variability of vertical air motions on the mesoscale is the key factor determining the observed probability distributions of pristine ice crystal concentrations in cirrus. Other factors considered are variations of temperature as well as size, number, and ice nucleation thresholds of the freezing aerosol particles. The variability in vertical velocities is likely caused by atmospheric waves. Inasmuch as gravity waves are widespread, mesoscale variability in vertical velocities can be viewed as a universa feature of young cirrus clouds. Large-scale models that do not account for this subgrid-scale variability yield erroneous predictions of the variability of basic cirrus cloud properties. Climate change may bring about changes in the global distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work, these changes could significantly modify the probability distribution of cirrus ice crystal concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in cloud formation should be done before conclusions regarding the role of other anthropogenic factors, such as changes in aerosol composition, are made.

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Dynamic Processes in Cirrus Clouds: Concepts and Models

Cirrus ◽

10.1093/oso/9780195130720.003.0022 ◽

2002 ◽

Author(s):

David O’C. Starr ◽

Markus Quante

Keyword(s):

Cirrus Clouds ◽

Radiative Properties ◽

Cirrus Cloud ◽

Early Model ◽

Minimum Requirement ◽

Microphysical Properties ◽

Cloud Properties ◽

Cloud Models ◽

New Concepts

Advancement in the understanding of cirrus clouds and their life cycle comes through symbiotic use of models, observations, and related concepts (fig. 18.1). Models of cirrus clouds represent an integration of our knowledge of cirrus cloud properties and processes. They provide a capacity to extend knowledge and enhance understanding in ways that complement existing observational capabilities. Models can be used to develop new theories, such as parameterizations, and focus science issues and observational requirements and developments. For example, early model results of Starr and Cox (1985a) and Starr (1987b) predicted that fine cellular structure (~lkm or less) would be found in the upper part of extended stratiform cirrus clouds. This prediction was confirmed when high-frequency sensors were deployed both for active remote sensing (Sassen et al. 1990a, 1995) and later for in-situ measurements (Quante and Brown 1992; Gultepe et al. 1995; Quante et al. 1996). Sampling rates of 10Hz, or better, are now accepted as a minimum requirement for resolving cirrus cloud internal structure and circulation where 1-Hz or coarser measurements were previously used. Similarly, discrepancies between observed cloud radiative properties and calculations (theory) based on corresponding in-situ observations of cloud microphysical properties (Sassen et al. 1990b) led to the development of improved observing capabilities for small ice crystals (Arnott et al. 1994; Miloshevich and Heymsfield 1997; Lawson et al. 1998). Such sensors are now regarded as part of the standard complement when doing in-situ microphysical measurements in cirrus. At the same time, observations are absolutely essential in developing and evaluating cloud models. No cloud modeler wants to apply a model or theory too far beyond the limits of what can be observationally confirmed, at least in gross terms. The third aspect of this triad is concepts. Although models and observations can lead to predictions or diagnosis of unexpected relationships, they are each limited by the concepts that were used in their design and/or implementation. In the end, new concepts arising from analogy to other phenomena and/or from synergistic integration of existing knowledge can lead to new understanding, new models, new instruments, and new sampling strategies (fig. 18.1). Chapter 17 focuses on observations of internal cloud circulation and structure.

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

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-14573-2021 ◽

2021 ◽

Vol 21 (19) ◽

pp. 14573-14590

Author(s):

Qiang Li ◽

Silke Groß

Keyword(s):

Meteorological Conditions ◽

Air Traffic ◽

Cirrus Clouds ◽

Cloud Formation ◽

Final Conclusion ◽

Cirrus Cloud ◽

Average Thickness ◽

Ice Cloud ◽

Cloud Properties ◽

The Usa

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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Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-3-3267-2003 ◽

2003 ◽

Vol 3 (3) ◽

pp. 3267-3299 ◽

Cited By ~ 4

Author(s):

W. Haag ◽

B. Kärcher ◽

J. Ström ◽

A. Minikin ◽

U. Lohmann ◽

...

Keyword(s):

Relative Humidity ◽

Large Scale ◽

Climate Model ◽

Global Climate Model ◽

In Situ Measurements ◽

Cirrus Clouds ◽

Cloud Formation ◽

Formation Mechanisms ◽

Cirrus Cloud

Abstract. Factors controlling the distribution of relative humidity above ice saturation in the upper troposphere and lower stratosphere in the presence of cirrus clouds are examined with the help of microphysical trajectory simulations using a box model. Our findings are related to results from recent field campaigns and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with similar measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The comparison with field data reveals distinct interhemispheric differences in cirrus cloud freezing thresholds. Combining the present findings with recent results addressing the frequency distributions of updraft speeds and cirrus ice crystal number densities (Kärcher and Ström, 2993} provides evidence for the existence of complex heterogeneous freezing mechanisms in cirrus, at least in the polluted northern hemisphere, and further emphasizes the key role of gravity wave-induced dynamical variability in vertical air motion at the mesoscale. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale models of the atmosphere, in particular chemistry-transport and weather forecast models.

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The roles of dynamical variability and aerosols in cirrus cloud formation

Atmospheric Chemistry and Physics ◽

10.5194/acp-3-823-2003 ◽

2003 ◽

Vol 3 (3) ◽

pp. 823-838 ◽

Cited By ~ 141

Author(s):

B. Kärcher ◽

J. Ström

Keyword(s):

Gravity Waves ◽

Probability Distributions ◽

Cirrus Clouds ◽

Cloud Formation ◽

Small Scale ◽

Cirrus Cloud ◽

Ice Crystal ◽

Mesoscale Variability ◽

Cloud Properties

Abstract. The probability of occurrence of ice crystal number densities in young cirrus clouds is examined based on airborne measurements. The observations have been carried out at midlatitudes in both hemispheres at equivalent latitudes (52-55°N/S) during the same season (local autumn in 2000). The in situ measurements considered in the present study include temperatures, vertical velocities, and total ice crystal concentrations, the latter determined with high precision and accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities (1-10 cm-3) of small (diameter <20 mm) ice crystals. This mode dominates the probability distributions and is shown to be caused by rapid cooling rates associated with updraft speeds in the range 10-100 cm s-1. A second mode containing larger crystals extends from ~1 cm-3 to low concentrations close to the detection threshold (~3 x 10-4 cm-3) and could be associated with lower updraft speeds. Results of a statistical analysis provide compelling evidence that the dynamical variability of vertical air motions on the mesoscale is the key factor determining the observed probability distributions of pristine ice crystal concentrations in cirrus. Other factors considered are changes of temperature as well as size, number, and ice nucleation thresholds of the freezing aerosol particles. The variability in vertical velocities is caused by atmospheric gravity waves leading to small-scale temperature fluctuations. Inasmuch as gravity waves are widespread, mesoscale variability in vertical velocities can be viewed as a universal feature of young cirrus clouds. Large-scale models that do not account for this subgrid-scale variability yield erroneous predictions of the variability of basic cirrus cloud properties. Climate change may bring about changes in the global distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work, these changes could significantly modify the probability distribution of cirrus ice crystal concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in cloud formation should be done before conclusions regarding the role of other anthropogenic factors, such as changes in aerosol composition, are made.

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Cirrus clouds in a global climate model with a statistical cirrus cloud scheme

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-5449-2010 ◽

2010 ◽

Vol 10 (12) ◽

pp. 5449-5474 ◽

Cited By ~ 50

Author(s):

M. Wang ◽

J. E. Penner

Keyword(s):

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Lower Troposphere ◽

Circulation Model ◽

Cirrus Clouds ◽

Cloud Formation ◽

Cirrus Cloud ◽

Ice Crystal ◽

Homogeneous Freezing

Abstract. A statistical cirrus cloud scheme that accounts for mesoscale temperature perturbations is implemented in a coupled aerosol and atmospheric circulation model to better represent both subgrid-scale supersaturation and cloud formation. This new scheme treats the effects of aerosol on cloud formation and ice freezing in an improved manner, and both homogeneous freezing and heterogeneous freezing are included. The scheme is able to better simulate the observed probability distribution of relative humidity compared to the scheme that was implemented in an older version of the model. Heterogeneous ice nuclei (IN) are shown to decrease the frequency of occurrence of supersaturation, and improve the comparison with observations at 192 hPa. Homogeneous freezing alone can not reproduce observed ice crystal number concentrations at low temperatures (<205 K), but the addition of heterogeneous IN improves the comparison somewhat. Increases in heterogeneous IN affect both high level cirrus clouds and low level liquid clouds. Increases in cirrus clouds lead to a more cloudy and moist lower troposphere with less precipitation, effects which we associate with the decreased convective activity. The change in the net cloud forcing is not very sensitive to the change in ice crystal concentrations, but the change in the net radiative flux at the top of the atmosphere is still large because of changes in water vapor. Changes in the magnitude of the assumed mesoscale temperature perturbations by 25% alter the ice crystal number concentrations and the net radiative fluxes by an amount that is comparable to that from a factor of 10 change in the heterogeneous IN number concentrations. Further improvements on the representation of mesoscale temperature perturbations, heterogeneous IN and the competition between homogeneous freezing and heterogeneous freezing are needed.

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Climatological and radiative properties of midlatitude cirrus clouds derived by automatic evaluation of lidar measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-7605-2016 ◽

2016 ◽

Vol 16 (12) ◽

pp. 7605-7621 ◽

Cited By ~ 23

Author(s):

Erika Kienast-Sjögren ◽

Christian Rolf ◽

Patric Seifert ◽

Ulrich K. Krieger ◽

Bei P. Luo ◽

...

Keyword(s):

Radiative Forcing ◽

Cirrus Clouds ◽

Radiative Properties ◽

Radiation Budget ◽

Shortwave Radiation ◽

Radiative Transfer Model ◽

Transfer Model ◽

Data Set ◽

Detection Scheme ◽

Lidar Measurements

Abstract. Cirrus, i.e., high, thin clouds that are fully glaciated, play an important role in the Earth's radiation budget as they interact with both long- and shortwave radiation and affect the water vapor budget of the upper troposphere and stratosphere. Here, we present a climatology of midlatitude cirrus clouds measured with the same type of ground-based lidar at three midlatitude research stations: at the Swiss high alpine Jungfraujoch station (3580 m a.s.l.), in Zürich (Switzerland, 510 m a.s.l.), and in Jülich (Germany, 100 m a.s.l.). The analysis is based on 13 000 h of measurements from 2010 to 2014. To automatically evaluate this extensive data set, we have developed the Fast LIdar Cirrus Algorithm (FLICA), which combines a pixel-based cloud-detection scheme with the classic lidar evaluation techniques. We find mean cirrus optical depths of 0.12 on Jungfraujoch and of 0.14 and 0.17 in Zürich and Jülich, respectively. Above Jungfraujoch, subvisible cirrus clouds (τ < 0.03) have been observed during 6 % of the observation time, whereas above Zürich and Jülich fewer clouds of that type were observed. Cirrus have been observed up to altitudes of 14.4 km a.s.l. above Jungfraujoch, whereas they have only been observed to about 1 km lower at the other stations. These features highlight the advantage of the high-altitude station Jungfraujoch, which is often in the free troposphere above the polluted boundary layer, thus enabling lidar measurements of thinner and higher clouds. In addition, the measurements suggest a change in cloud morphology at Jungfraujoch above ∼ 13 km, possibly because high particle number densities form in the observed cirrus clouds, when many ice crystals nucleate in the high supersaturations following rapid uplifts in lee waves above mountainous terrain. The retrieved optical properties are used as input for a radiative transfer model to estimate the net cloud radiative forcing, CRFNET, for the analyzed cirrus clouds. All cirrus detected here have a positive CRFNET. This confirms that these thin, high cirrus have a warming effect on the Earth's climate, whereas cooling clouds typically have cloud edges too low in altitude to satisfy the FLICA criterion of temperatures below −38 °C. We find CRFNET = 0.9 W m−2 for Jungfraujoch and 1.0 W m−2 (1.7 W m−2) for Zürich (Jülich). Further, we calculate that subvisible cirrus (τ < 0.03) contribute about 5 %, thin cirrus (0.03 < τ < 0.3) about 45 %, and opaque cirrus (0.3 < τ) about 50 % of the total cirrus radiative forcing.

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Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity

Atmospheric Chemistry and Physics ◽

10.5194/acp-3-1791-2003 ◽

2003 ◽

Vol 3 (5) ◽

pp. 1791-1806 ◽

Cited By ~ 108

Author(s):

W. Haag ◽

B. Kärcher ◽

J. Ström ◽

A. Minikin ◽

U. Lohmann ◽

...

Keyword(s):

Relative Humidity ◽

Large Scale ◽

Climate Model ◽

Global Climate Model ◽

In Situ Measurements ◽

Cirrus Clouds ◽

Formation Mechanisms ◽

Cirrus Cloud ◽

Homogeneous Freezing

Abstract. Factors controlling the microphysical link between distributions of relative humidity above ice saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the help of microphysical trajectory simulations. Our findings are related to results from aircraft measurements and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and weather forecast models.

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A decadal cirrus clouds climatology from ground-based and spaceborne lidars above the south of France (43.9° N–5.7° E)

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-6951-2013 ◽

2013 ◽

Vol 13 (14) ◽

pp. 6951-6963 ◽

Cited By ~ 19

Author(s):

C. Hoareau ◽

P. Keckhut ◽

V. Noel ◽

H. Chepfer ◽

J.-L. Baray

Keyword(s):

Observation Time ◽

Cirrus Clouds ◽

Cirrus Cloud ◽

Upper Troposphere ◽

Cloud Parameters ◽

Cloud Properties ◽

Total Observation Time ◽

The Mean ◽

Ground Based Lidar ◽

Total Observation

Abstract. This study provides an analysis of cirrus cloud properties at midlatitude in the southern part of France from ground-based and spaceborne lidars. A climatology of cirrus cloud properties and their evolution over more than 12 yr is presented and compared to other mid-latitude climatological studies. Cirrus clouds occur ~37% of the total observation time and remain quasi-constant across seasons with a variation within ~5% around the mean occurrence. Similar results are obtained from CALIOP and the ground-based lidar, with a mean difference in occurrence of ~5% between both instruments. From the ground-based lidar data, a slight decrease in occurrence of ~3% per decade is observed but found statistically insignificant. Based on a clustering analysis of cirrus cloud parameters, three distinct classes have been identified and investigations concerning their origin are discussed. Properties of these different classes are analysed, showing that thin cirrus in the upper troposphere represent ~50% of cloud cover detected in summer and fall, decreasing by 15–20% for other seasons.

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