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

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.

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

2021 ◽  
Author(s):  
Qiang Li ◽  
Silke Groß
Keyword(s):  
Air Traffic ◽  
Cirrus Clouds ◽  
Cirrus Cloud ◽  
Mean Values ◽  
Significance Level ◽  
Ice Cloud ◽  
Cloud Properties ◽  
Lidar Measurements ◽  
Traffic Reduction ◽  
Linear Depolarization Ratio

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 the 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 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, which showed similar meteorological conditions for March and April as they were found for 2020. Comparing these years shows that the cirrus cloud occurrence was reduced by about 30 % with smaller cloud thicknesses found in April 2020. The average thickness of cirrus clouds was reduced to 1.18 km in April 2020 compared to a value of 1.40 km under normal conditions. In addition, the cirrus clouds measured in April 2020 possess smaller mean values of the particle linear depolarization ratio (PLDR) than the previous years at high significance level, especially at colder temperatures (T 

scholarly journals The roles of dynamical variability and aerosols in cirrus cloud formation

2003 ◽  
Vol 3 (2) ◽  
pp. 1415-1451 ◽  
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.

Study of cirrus cloud properties and occurrence over Europe during the COVID-19 based on the lidar measurements of CALIPSO

2021 ◽  
Author(s):  
Qiang Li ◽  
Silke Groß
Keyword(s):  
United States ◽  
Radiative Forcing ◽  
The United States ◽  
Air Traffic ◽  
Cirrus Clouds ◽  
Cirrus Cloud ◽  
Good Opportunity ◽  
Mean Values ◽  
Significance Level ◽  
Cloud Properties

&lt;p&gt;Cirrus clouds have a wide global coverage providing considerable radiative forcing on the Earth&amp;#8217;s climate system. Due to their inadequate representation in the global models, cirrus clouds can lead to large uncertainties in the climate prediction. To date, experimental and theoretical efforts have been widely carried out to study the anthropogenic effects such as aviation that may change the formation and microphysic and optical properties of cirrus clouds. Unfortunately, however, solid observational studies are still rare for us to draw any robust conclusion on anthropogenic influence on cirrus. During the COVID-19 pandemic the civil air traffic over Europe was significantly reduced. This unique situation provides a good opportunity to study the effect of air traffic on cirrus. In this work, based on the analysis of the CALIPSO measurements we present the changes of cirrus cloud properties and occurrence over Europe in March and April 2020 compared with the reference results in the previous years under normal conditions. The comparison shows that the cirrus cloud occurrence was reduced by about 30% with smaller cloud thicknesses found in April 2020. The average thickness of cirrus clouds was reduced to 1.18 km in April 2020 compared to a value of 1.40 km under normal conditions. In addition, the cirrus clouds measured in April 2020 possess smaller mean values of the particle linear depolarization ratio (PLDR) than the previous years at a high significance level, especially at colder temperatures (T&lt;-50&lt;sup&gt;o&lt;/sup&gt;C). The same exercises are extended to the observations over China and the United States. Besides the regional discrimination of cirrus clouds, we reach the final summary that cirrus clouds show significant changes in both March and April over Europe, no changes in both months over China, and significant changes only in April over the United States.&lt;/p&gt;

scholarly journals An automated cirrus classification

2018 ◽  
Vol 18 (9) ◽  
pp. 6157-6169 ◽  
Author(s):  
Edward Gryspeerdt ◽  
Johannes Quaas ◽  
Tom Goren ◽  
Daniel Klocke ◽  
Matthias Brueck
Keyword(s):  
Formation Mechanism ◽  
Cirrus Clouds ◽  
Radiative Properties ◽  
Radiation Budget ◽  
Cloud Formation ◽  
Back Trajectory ◽  
Formation Mechanisms ◽  
Cirrus Cloud ◽  
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 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.

scholarly journals The roles of dynamical variability and aerosols in cirrus cloud formation

2003 ◽  
Vol 3 (3) ◽  
pp. 823-838 ◽  
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.

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

2010 ◽  
Vol 10 (12) ◽  
pp. 5449-5474 ◽  
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.

Dynamic Processes in Cirrus Clouds: Concepts and Models

Cirrus ◽  
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.

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

2013 ◽  
Vol 13 (14) ◽  
pp. 6951-6963 ◽  
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.

scholarly journals Variability in cirrus cloud properties using a Polly<sup>XT</sup> Raman lidar over high and tropical latitudes

2020 ◽  
Vol 20 (7) ◽  
pp. 4427-4444 ◽  
Author(s):  
Kalliopi Artemis Voudouri ◽  
Elina Giannakaki ◽  
Mika Komppula ◽  
Dimitris Balis
Keyword(s):  
Optical Properties ◽  
Cirrus Clouds ◽  
Cirrus Cloud ◽  
Raman Lidar ◽  
Mean Values ◽  
Cloud Optical Depth ◽  
Geometrical Properties ◽  
Depolarisation Ratio ◽  
Cloud Properties ◽  
Lidar Ratio

Abstract. Measurements of geometrical and optical properties of cirrus clouds, performed with a multi-wavelength PollyXT Raman lidar during the period 2008 to 2016, are analysed. The measurements were performed with the same instrument, during sequential periods, in three places at different latitudes, Gwal Pahari (28.43∘ N, 77.15∘ E; 243 m a.s.l.) in India, Elandsfontein (26.25∘ S, 29.43∘ E; 1745 m a.s.l.) in South Africa and Kuopio (62.74∘ N, 27.54∘ E; 190 m a.s.l.) in Finland. The lidar dataset was processed by an automatic cirrus cloud masking algorithm, developed in the frame of this work. In the following, we present a statistical analysis of the lidar-retrieved geometrical characteristics (cloud boundaries, geometrical thickness) and optical properties of cirrus clouds (cloud optical depth, lidar ratio, ice crystal depolarisation ratio) measured over the three areas that correspond to subtropical and subarctic regions as well as their seasonal variability. The effect of multiple scattering from ice particles to the derived optical products is also considered and corrected in this study. Our results show that cirrus layers, which have a noticeable monthly variability, were observed between 6.5 and 13 km, with temperatures ranging from −72 to −27 ∘C. The observed differences on cirrus clouds' geometrical and optical properties over the three regions are discussed in terms of latitudinal and temperature dependence. The latitudinal dependence of the geometrical properties is consistent with satellite observations, following the pattern observed with CloudSat, with decreasing values towards the poles. The geometrical boundaries have their highest values in the subtropical regions, and overall, our results seem to demonstrate that subarctic cirrus clouds are colder, lower and optically thinner than subtropical cirrus clouds. The dependence of cirrus cloud geometrical thickness and optical properties on mid-cirrus temperatures shows a quite similar tendency for the three sites but less variability for the subarctic dataset. Cirrus clouds are geometrically and optically thicker at temperatures between −45 and −35 ∘C, and a second peak is observed at lower temperatures ∼-70 ∘C for the subarctic site. Lidar ratio values also exhibit a pattern, showing higher values moving toward the poles, with higher mean values observed over the subarctic site. The dependency of the mid-cirrus temperatures on the lidar ratio values and the particle depolarisation values is further examined. Our study shows that the highest values of the cirrus lidar ratio correspond to higher values of cirrus depolarisation and warmer cirrus. The kind of information presented here can be rather useful in the cirrus parameterisations required as input to radiative transfer models and can be a complementary tool for satellite products that cannot provide cloud vertical structure. In addition, ground-based statistics of the cirrus properties could be useful in the validation and improvement of the corresponding derived products from satellite retrievals.

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

2003 ◽  
Vol 3 (3) ◽  
pp. 3267-3299 ◽  
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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