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.

Ground-based Remote Sensing of Cirrus Clouds

Cirrus ◽  
2002 ◽  
Author(s):  
Kenneth Sassen ◽  
Gerald Mace
Keyword(s):  
Remote Sensing ◽  
Multispectral Imaging ◽  
Cirrus Clouds ◽  
Radiative Energy ◽  
Radiation Balance ◽  
Cirrus Cloud ◽  
Microphysical Properties ◽  
Research Aircraft ◽  
Cloud Modeling

Cirrus clouds have only recently been recognized as having a significant influence on weather and climate through their impact on the radiative energy budget of the atmosphere. In addition, the unique difficulties presented by the study of cirrus put them on the “back burner” of atmospheric research for much of the twentieth century. Foremost, because they inhabit the frigid upper troposphere, their inaccessibility has hampered intensive research. Other factors have included a lack of in situ instrumentation to effectively sample the clouds and environment, and basic uncertainties in the underlying physics of ice cloud formation, growth, and maintenance. Cloud systems that produced precipitation, severe weather, or hazards to aviation were deemed more worthy of research support until the mid- 1980s. Beginning at this time, however, major field research programs such as the First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE; Cox et al. 1987), International Cirrus Experiment (ICE; Raschke et al. 1990), Experimental Cloud Lidar Pilot Study (ECLIPS; Platt et al. 1994), and the Atmospheric Radiation Measurement (ARM) Program (Stokes and Schwartz 1994) have concentrated on cirrus cloud research, relying heavily on ground-based remote sensing observations combined with research aircraft. What has caused this change in research emphasis is an appreciation for the potentially significant role that cirrus play in maintaining the radiation balance of the earth-atmosphere system (Liou 1986). As climate change issues were treated more seriously, it was recognized that the effects, or feedbacks, of extensive high-level ice clouds in response to global warming could be pivotal. This fortunately came at a time when new generations of meteorological instrumentation were becoming available. Beginning in the early 1970s, major advancements were made in the fields of numerical cloud modeling and cloud measurements using aircraft probes, satellite multispectral imaging, and remote sensing with lidar, short-wavelength radar, and radiometers, all greatly facilitating cirrus research. Each of these experimental approaches have their advantages and drawbacks, and it should also be noted that a successful cloud modeling effort relies on field data for establishing boundary conditions and providing case studies for validation. Although the technologies created for in situ aircraft measurements can clearly provide unique knowledge of cirrus cloud thermodynamic and microphysical properties (Dowling and Radke 1990), available probes may suffer from limitations in their response to the wide range of cirrus particles and actually sample a rather small volume of cloud during any mission.

scholarly journals A microphysics guide to cirrus clouds – Part 1: Cirrus types

2015 ◽  
Vol 15 (21) ◽  
pp. 31537-31586 ◽  
Author(s):  
M. Krämer ◽  
C. Rolf ◽  
A. Luebke ◽  
A. Afchine ◽  
N. Spelten ◽  
...  
Keyword(s):  
Parameter Space ◽  
Field Measurements ◽  
Cirrus Clouds ◽  
Radiative Properties ◽  
Formation Mechanisms ◽  
Liquid Droplets ◽  
Atmospheric Conditions ◽  
Model Simulations ◽  
Microphysical Properties

Abstract. The microphysical and radiative properties of cirrus clouds continue to be beyond understanding and thus still represent one of the largest uncertainties in the prediction of the Earth's climate (IPCC, 2013). Our study aims to provide a guide to cirrus microphysics, which is compiled from an extensive set of model simulations, covering the broad range of atmospheric conditions for cirrus formation and evolution. The model results are portrayed in the same parameter space as field measurements, i.e. in the Ice Water Content-Temperature (IWC-T) parameter space. We validate this cirrus analysis approach by evaluating cirrus data sets from seventeen aircraft campaigns, conducted in the last fifteen years, spending about 94 h in cirrus over Europe, Australia, Brazil as well as Southern and Northern America. Altogether, the approach of this study is to track cirrus IWC development with temperature by means of model simulations, compare with observations and then assign, to a certain degree, cirrus microphysics to the observations. Indeed, the field observations show characteristics expected from the simulated cirrus guide. For example, high/low IWCs are found together with high/low ice crystal concentrations Nice. An important finding from our study is the classification of two types of cirrus with differing formation mechanisms and microphysical properties: the first cirrus type is rather thin with lower IWCs and forms directly as ice (in-situ origin cirrus). The second type consists predominantly of thick cirrus originating from mixed phase clouds (i.e. via freezing of liquid droplets – liquid origin cirrus), which are completely glaciated while lifting to the cirrus formation temperature region (< 235 K). In the European field campaigns, in-situ origin cirrus occur frequently at slow updrafts in low and high pressure systems, but also in conjunction with faster updrafts. Also, liquid origin cirrus mostly related to warm conveyor belts are found. In the US and tropical campaigns, thick liquid origin cirrus which are formed in large convective systems are detected more frequently.

scholarly journals Determining stages of cirrus evolution: a cloud classification scheme

2017 ◽  
Vol 10 (5) ◽  
pp. 1653-1664 ◽  
Author(s):  
Benedikt Urbanek ◽  
Silke Groß ◽  
Andreas Schäfler ◽  
Martin Wirth
Keyword(s):  
Classification Scheme ◽  
Cirrus Clouds ◽  
Radiative Properties ◽  
Temperature Fields ◽  
High Spectral Resolution ◽  
Cloud Properties ◽  
Lidar Measurements ◽  
Lee Wave

Abstract. Cirrus clouds impose high uncertainties on climate prediction, as knowledge on important processes is still incomplete. For instance it remains unclear how cloud microphysical and radiative properties change as the cirrus evolves. Recent studies classify cirrus clouds into categories including in situ, orographic, convective and liquid origin clouds and investigate their specific impact. Following this line, we present a novel scheme for the classification of cirrus clouds that addresses the need to determine specific stages of cirrus evolution. Our classification scheme is based on airborne Differential Absorption and High Spectral Resolution Lidar measurements of atmospheric water vapor, aerosol depolarization, and backscatter, together with model temperature fields and simplified parameterizations of freezing onset conditions. It identifies regions of supersaturation with respect to ice (ice-supersaturated regions, ISSRs), heterogeneous and homogeneous nucleation, depositional growth, and ice sublimation and sedimentation with high spatial resolution. Thus, all relevant stages of cirrus evolution can be classified and characterized. In a case study of a gravity lee-wave-influenced cirrus cloud, encountered during the ML-CIRRUS flight campaign, the applicability of our classification is demonstrated. Revealing the structure of cirrus clouds, this valuable tool might help to examine the influence of evolution stages on the cloud's net radiative effect and to investigate the specific variability of optical and microphysical cloud properties in upcoming research.

Combining cirrus cloud CALIPSO (IIR-CALIOP) and aircraft measurements with climate modeling to evaluate the spatial and seasonal radiative impact of homogeneous ice nucleation

2021 ◽  
Author(s):  
David L. Mitchell ◽  
John F. Mejia ◽  
Anne Garnier ◽  
Yuta Tomii ◽  
Martina Krämer ◽  
...  
Keyword(s):  
Experimental Design ◽  
Climate Modeling ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Cirrus Clouds ◽  
Radiative Properties ◽  
Cirrus Cloud ◽  
Modeling Studies ◽  
Northern And Southern Hemispheres

&lt;p&gt;Many global climate modeling studies over the last decade have attempted to evaluate the relative contributions of homo- and heterogeneous ice nucleation (henceforth hom and het) in cirrus clouds, and the radiative contribution of hom relative to het.&amp;#160; There is likely a spatial and seasonal dependence here.&amp;#160; Since the microphysical and radiative properties of hom- and het-dominated cirrus clouds are likely very different, the outcome of such studies may be important to climate science.&amp;#160; But since the physics determining the competition between hom and het is very complex, involving poorly constrained variables, results from such modeling studies have often contradicted each other.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This study takes a different approach by using CALIPSO satellite effective diameter (D&lt;sub&gt;e&lt;/sub&gt;) retrievals from cirrus clouds, validated by recent in situ measurements (obtained from 24 field campaigns consisting of 150 flights), to constrain the cloud microphysics module (i.e., version 2 of the Morrison-Gettelman scheme or MG2) in the Whole Atmosphere Community Climate Model version 6 (WACCM6). [As a side-note, the ice particle number concentration N was calculated from the retrieved D&lt;sub&gt;e&lt;/sub&gt; and the in situ climatological ice water content and shown to be consistent with N retrievals based on a CloudSat-CALIPSO lidar-radar method]. The MG2 cirrus cloud ice particle size distribution was constrained to conform with these D&lt;sub&gt;e&lt;/sub&gt; retrievals that depend on temperature (T), latitude, season and land fraction (land vs. ocean).&amp;#160; The treatment of ice particle fall speeds was also revised.&amp;#160; Two 40-year WACCM6 simulations were differenced to obtain the radiative contribution of hom; one based on the retrieved D&lt;sub&gt;e&lt;/sub&gt; and one based on retrieved D&lt;sub&gt;e&lt;/sub&gt; corresponding to het conditions (where retrieved N was minimal).&amp;#160; The experimental design assumes hom-affected cirrus occur only outside the &amp;#177; 30 &amp;#176;latitude zone since cirrus within this zone exhibited the lowest N and were thus used to produce the D&lt;sub&gt;e&lt;/sub&gt; &amp;#8211; T look-up tables corresponding to het conditions.&amp;#160; These D&lt;sub&gt;e&lt;/sub&gt; &amp;#8211; T relationships for het conditions were applied to the entire planet in one simulation (labeled HET) while the other simulation (labeled CALCAL for CALIPSO-calibrated) is based on the actual D&lt;sub&gt;e&lt;/sub&gt; retrievals.&amp;#160; CALCAL &amp;#8211; HET differences in the cloud radiative effect (CRE) reveal the estimated CRE effect due to hom.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The results show CALCAL &amp;#8211; HET CRE differences of 2.4 and 2.5 W m&lt;sup&gt;-2&lt;/sup&gt; in the northern and southern hemispheres, respectively.&amp;#160; These CRE differences are largely due to cirrus-induced changes in mixed phase clouds.&amp;#160; However, top-of-model (TOM) CALCAL &amp;#8211; HET differences in total net forcing did not match these CRE differences due to mid-level increases in relative humidity in HET relative to CALCAL, so that these TOM differences were 1.8 and 2.0 W m&lt;sup&gt;-2&lt;/sup&gt; in the northern and southern hemispheres, respectively.&amp;#160; Radiative contributions from hom were minimal during the summer months (JJA) since shortwave and longwave cloud forcing tends to cancel then.&amp;#160; Other studies show this is true for the tropics (reinforcing the realism of our experimental design from a radiation purview).&amp;#160; During non-summer months, the TOM CALCAL &amp;#8211; HET difference in total net forcing was 2.4 W m&lt;sup&gt;-2&lt;/sup&gt; in both hemispheres.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

scholarly journals A microphysics guide to cirrus clouds – Part 1: Cirrus types

2016 ◽  
Vol 16 (5) ◽  
pp. 3463-3483 ◽  
Author(s):  
Martina Krämer ◽  
Christian Rolf ◽  
Anna Luebke ◽  
Armin Afchine ◽  
Nicole Spelten ◽  
...  
Keyword(s):  
Parameter Space ◽  
Field Measurements ◽  
Cirrus Clouds ◽  
Radiative Properties ◽  
Formation Mechanisms ◽  
Atmospheric Conditions ◽  
Jet Streams ◽  
Model Simulations ◽  
Microphysical Properties

Abstract. The microphysical and radiative properties of cirrus clouds continue to be beyond understanding and thus still represent one of the largest uncertainties in the prediction of the Earth's climate (IPCC, 2013). Our study aims to provide a guide to cirrus microphysics, which is compiled from an extensive set of model simulations, covering the broad range of atmospheric conditions for cirrus formation and evolution. The model results are portrayed in the same parameter space as field measurements, i.e., in the Ice Water Content-Temperature (IWC-T) parameter space. We validate this cirrus analysis approach by evaluating cirrus data sets from 17 aircraft campaigns, conducted in the last 15 years, spending about 94 h in cirrus over Europe, Australia, Brazil as well as South and North America. Altogether, the approach of this study is to track cirrus IWC development with temperature by means of model simulations, compare with observations and then assign, to a certain degree, cirrus microphysics to the observations. Indeed, the field observations show characteristics expected from the simulated Cirrus Guide. For example, high (low) IWCs are found together with high (low) ice crystal concentrations Nice. An important finding from our study is the classification of two types of cirrus with differing formation mechanisms and microphysical properties: the first cirrus type forms directly as ice (in situ origin cirrus) and splits in two subclasses, depending on the prevailing strength of the updraft: in slow updrafts these cirrus are rather thin with lower IWCs, while in fast updrafts thicker cirrus with higher IWCs can form. The second type consists predominantly of thick cirrus originating from mixed phase clouds (i.e., via freezing of liquid droplets – liquid origin cirrus), which are completely glaciated while lifting to the cirrus formation temperature region (< 235 K). In the European field campaigns, slow updraft in situ origin cirrus occur frequently in low- and high-pressure systems, while fast updraft in situ cirrus appear in conjunction with jet streams or gravity waves. Also, liquid origin cirrus mostly related to warm conveyor belts are found. In the US and tropical campaigns, thick liquid origin cirrus which are formed in large convective systems are detected more frequently.

scholarly journals The origin of midlatitude ice clouds and the resulting influence on their microphysical properties

2015 ◽  
Vol 15 (23) ◽  
pp. 34243-34281 ◽  
Author(s):  
A. E. Luebke ◽  
A. Afchine ◽  
A. Costa ◽  
J. Meyer ◽  
C. Rolf ◽  
...  
Keyword(s):  
Bimodal Distribution ◽  
Cirrus Clouds ◽  
Ice Crystals ◽  
Radiative Properties ◽  
Ice Crystal ◽  
Ice Clouds ◽  
Microphysical Properties ◽  
Ice Crystal Size ◽  
Ice Water

Abstract. The radiative role of ice clouds in the atmosphere is known to be important, but uncertainties remain concerning the magnitude and net effects. However, through measurements of the microphysical properties of cirrus clouds, we can better characterize them, which can ultimately allow for their radiative properties to be more accurately ascertained. It has recently been proposed that there are two types of cirrus clouds – in situ and liquid origin. In this study, we present observational evidence to show that two distinct types of cirrus do exist. Airborne, in situ measurements of cloud ice water content (IWC), ice crystal concentration (Nice), and ice crystal size from the 2014 ML-CIRRUS campaign provide cloud samples that have been divided according to their origin type. The key features that set liquid origin cirrus apart from the in situ origin cirrus are a higher frequency of high IWC (> 100 ppmv), higher Nice values, and larger ice crystals. A vertical distribution of Nice shows that the in situ origin cirrus clouds exhibit a median value of around 0.1 cm−3, while the liquid origin concentrations are slightly, but notably higher. The median sizes of the crystals contributing the most mass are less than 200 μm for in situ origin cirrus, with some of the largest crystals reaching 550 μm in size. The liquid origin cirrus, on the other hand, were observed to have median diameters greater than 200 μm, and crystals that were up to 750 μm. An examination of these characteristics in relation to each other and their relationship to temperature provides strong evidence that these differences arise from the dynamics and conditions in which the ice crystals formed. Additionally, the existence of these two groups in cirrus cloud populations may explain why a bimodal distribution in the IWC-temperature relationship has been observed. We hypothesize that the low IWC mode is the result of in situ origin cirrus and the high IWC mode is the result of liquid origin cirrus.

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 origin of midlatitude ice clouds and the resulting influence on their microphysical properties

2016 ◽  
Vol 16 (9) ◽  
pp. 5793-5809 ◽  
Author(s):  
Anna E. Luebke ◽  
Armin Afchine ◽  
Anja Costa ◽  
Jens-Uwe Grooß ◽  
Jessica Meyer ◽  
...  
Keyword(s):  
Bimodal Distribution ◽  
Cirrus Clouds ◽  
Ice Crystals ◽  
Radiative Properties ◽  
Ice Crystal ◽  
Ice Clouds ◽  
Microphysical Properties ◽  
Ice Crystal Size ◽  
Ice Water

Abstract. The radiative role of ice clouds in the atmosphere is known to be important, but uncertainties remain concerning the magnitude and net effects. However, through measurements of the microphysical properties of cirrus clouds, we can better characterize them, which can ultimately allow for their radiative properties to be more accurately ascertained. Recently, two types of cirrus clouds differing by formation mechanism and microphysical properties have been classified – in situ and liquid origin cirrus. In this study, we present observational evidence to show that two distinct types of cirrus do exist. Airborne, in situ measurements of cloud ice water content (IWC), ice crystal concentration (Nice), and ice crystal size from the 2014 ML-CIRRUS campaign provide cloud samples that have been divided according to their origin type. The key features that set liquid origin cirrus apart from the in situ origin cirrus are higher frequencies of high IWC ( > 100 ppmv), higher Nice values, and larger ice crystals. A vertical distribution of Nice shows that the in situ origin cirrus clouds exhibit a median value of around 0.1 cm−3, while the liquid origin concentrations are slightly, but notably higher. The median sizes of the crystals contributing the most mass are less than 200 µm for in situ origin cirrus, with some of the largest crystals reaching 550 µm in size. The liquid origin cirrus, on the other hand, were observed to have median diameters greater than 200 µm, and crystals that were up to 750 µm. An examination of these characteristics in relation to each other and their relationship to temperature provides strong evidence that these differences arise from the dynamics and conditions in which the ice crystals formed. Additionally, the existence of these two groups in cirrus cloud populations may explain why a bimodal distribution in the IWC-temperature relationship has been observed. We hypothesize that the low IWC mode is the result of in situ origin cirrus and the high IWC mode is the result of liquid origin cirrus.

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.

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

2003 ◽  
Vol 3 (5) ◽  
pp. 1791-1806 ◽  
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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