Though difficulties and uncertainties associated with measuring and parameterizing cirrus cloud properties and the complex processes involved exist, the fact that cirrus clouds are a key component in the Earth's radiative budget is well established. Numerous studies have demonstrated the intricate details involved in putting together a complete and accurate portrayal of the radiative properties of cirrus clouds. For example, analyses have reported on the sensitivity to ice crystal sizes, shapes, and concentrations, cloud top height, optical depth, etc. and how these factors change within and between regions of the globe, e.g.,. Furthermore, recent studies, such as that from , and references therein, highlight the intricacies of representing cirrus clouds accurately in simulations and reveal that this issue leads to questions in regard to the radiative role of cirrus clouds in the present and future climate.

In situ observations and subsequent analyses of cirrus microphysical properties such as ice water content (IWC), ice crystal concentration (Nice), and ice crystal size contribute to the construction of a more accurate characterization of cirrus clouds by providing values that are the basis for creating and validating parameterizations developed for general circulation models (GCMs). These three properties are found to vary naturally over several orders of magnitude ; therefore it is more reasonable and useful to explore them in the context of their relationship to other environmental variables (e.g., temperature). This in turn allows us to infer other information such as the mechanism of ice crystal formation and growth and go on to develop classifications of cirrus clouds based on these relationships.

An analysis of a large database of cirrus data from showed that there is a bimodal frequency distribution of IWC as a function of temperature. They hypothesized that the two modes are representative of the two formation pathways of cirrus ice crystals, homogeneous and heterogeneous ice nucleation. Both modes are observed over the complete cirrus temperature range, and the peak values of the modes increase with temperature. Furthermore, the low and high IWC modes correspond to respective Nice. While that study points to differences in nucleation pathways as being the key to understanding these bi-modalities, studies like suggest that differences in larger scale dynamics are the important factor. Their study reported that two populations of ice crystals were observed in particle size distributions (PSDs) from the Small Particles in Cirrus (SPARTICUS) campaign. They found a narrow small-particle mode and a broader large-particle mode (separated by a level area in the distribution, usually between about 40 and 100 µm). However, this bimodality was not consistently evident. Further, they found that subtropical and anvil cirrus types were more likely to display a bimodal PSD, while ridge-crest and frontal cirrus PSDs were more typically monomodal. An analysis of other microphysical properties also demonstrated strong ties to the large-scale dynamics of the environment in which they were observed.

A scan of the literature surrounding cirrus clouds shows that classification schemes based on large-scale dynamics or meteorology are commonly used, e.g.,. Typically, the cirrus clouds are classified as “synoptic” or “convective”, or they are classified based on more specific meteorology. However, has recently proposed new definitions for a cirrus classification scheme based on the origin of the ice crystals – in situ and liquid origin cirrus clouds. The details of this scheme are further discussed in Sect. .

Briefly, in , various cirrus production and development scenarios are discussed. These scenarios are explored through extensive and detailed modeling work from a microphysics box model, MAID (Model for Aerosol and Ice Dynamics), and compared to in situ observations from several airborne campaigns. However, the frequently observed high IWC values in combination with high Nice are not represented in the model simulations, thus indicating that “classic” cirrus microphysics does not lead to such conditions. One feature that is not included in the MAID model is the possibility for preexisting ice. Preexisting ice means that the ice crystals are formed in the mixed-phase regime at warmer temperatures (T>235 K), but are eventually incorporated into a cirrus cloud where they contribute to the overall microphysics. This pathway could lead to a cirrus cloud that contains many large ice crystals and thus the high IWC values, particularly if the crystals first developed in an environment that allows them to grow larger.

The analysis presented here follows from by using observational evidence to further explore and explain the two distinct types of cirrus proposed – in situ and liquid origin cirrus clouds. used model results and a more broad campaign-case method to introduce this concept. The following study seeks to demonstrate the existence of these two cirrus cloud types by delving more deeply into how the microphysical properties differ from one type to the other. Specifically, we focus on IWC, Nice, and ice crystal size. This is especially important for fully understanding cirrus clouds and how they should be properly represented in modeling scenarios as changes in microphysical properties will affect the radiative properties of cirrus clouds, both locally and globally.

Cirrus analyses often categorize naturally occurring, non-aviation-induced cirrus clouds into two groups based on the meteorology associated with their development. However, the recent study from introduced an updated classification of these two types, which instead refers to their origin – in situ and liquid. This classification is based on (i) the formation mechanism of the cloud particles (directly as ice or frozen liquid droplets), and is therefore tied to a temperature threshold of -38 ∘C, below which liquid water drops do not exist, and, (ii) the vertical velocity, which determines the thickness of the cirrus. By default, a meteorological classification is also embedded within this scheme, but with some modification. This is discussed at the end of this section.

Cirrus clouds whose ice crystals have formed and grown within an ice cloud only environment are referred to as in situ origin cirrus clouds. These clouds form via heterogeneous and homogeneous ice nucleation whereby an air parcel rises and cools to a point at which a freezing threshold (i.e., the supersaturation with respect to ice needed to initiate nucleation) is crossed, and ice crystals can form and continue to grow as conditions allow. The freezing threshold is determined with respect to ice nuclei in the case of heterogeneous ice nucleation or with respect to supercooled solution particles in the case of homogeneous ice nucleation. Homogeneous ice nucleation refers to the process by which supercooled particles in solution freeze. This development process is illustrated simply in the schematic shown in Fig. , left. In situ origin cirrus clouds may also be observed in the form of fall streaks, i.e., where large ice crystals have sedimented to lower altitudes/higher temperatures. However, this phenomena was not observed in the data set used for this analysis.

Cirrus clouds whose ice crystals originally formed as liquid drops lower in the atmosphere (T>235 K), which subsequently froze while being lifted into the cirrus temperature region of the atmosphere, are referred to as liquid origin cirrus clouds (Fig. , right). This difference is important because liquid and mixed-phase clouds develop and are controlled by different microphysical processes, such as the mechanism described by the Köhler equation, than those found in ice-only atmospheric environments. These warmer clouds exist in a regime that supplies a greater amount of water vapor for cloud particle formation and growth. Furthermore, the population of effective cloud condensation nuclei (CCN) can result in clouds with many liquid cloud particles. Heterogeneous drop freezing will be triggered in those particles containing an insoluble ice nucleus. Homogeneous drop freezing, which is something different from the homogeneous ice nucleation of aerosol particles in solution discussed in the previous paragraph, is also possible but will only occur at -38 ∘C if supercooled liquid water droplets still remain. These conditions also allow for other growth mechanisms, such as aggregation and riming, that are not always seen in the cirrus environment. Aggregation and riming can be important processes in liquid origin cirrus clouds, but mainly at higher vertical velocities (i.e., in strong convection). As shown in Fig. , we suggest that if the vertical motion is strong enough, any existing ice crystals or liquid droplets can also be lifted into the cirrus environment. Any ice crystals or frozen liquid drops observed within this space would then be identified as a cirrus cloud, regardless of their origin. Additionally, liquid origin cirrus clouds can be connected to in situ origin cirrus clouds. If the conditions allow for it (i.e., if the supersaturation reaches the homogeneous ice nucleation threshold), further ice nucleation events producing small ice crystals may take place in addition to the existing, large liquid origin cirrus crystals. The liquid origin cirrus type is where convective cirrus is classified. Warm conveyor belt cirrus and, in some cases, lee-wave-induced cirrus are also good candidates for inclusion in this category because they can involve a lifting of clouds to T<-38 ∘C.

Though this classification is based on the ice crystal origin, it can also be compared to categorization based on meteorology. For example, as noted in the introduction, classify observations from the SPARTICUS campaign into groups such as ridge-crest, frontal, subtropical jet stream, and anvil cirrus. explain that ridge-crest cirrus is comparable to in situ origin cirrus in a fast updraft case, while frontal, subtropical jet stream, and anvil cirrus fit into the liquid origin category and represent both slow and fast updraft cases. As discussed more thoroughly therein, this is further supported by observational similarities and differences between SPARTICUS and the campaigns (including ML-CIRRUS) used by for their analysis.

The primary data set used for this study comes from the recent ML-CIRRUS campaign, which took place in the spring of 2014. The campaign was based out of Oberpfaffenhofen, Germany using the HALO aircraft and comprises 16 flights in total covering various locations over the European continent . Only 13 flights are used in the analysis presented here. The remaining three flights have been excluded from our analysis because they were aimed at sampling pure contrail/aviation-induced cirrus or liquid clouds.

Despite having a clear picture of the properties associated with the two cirrus origin types, there are questions concerning whether they are also found in other locations and regions, i.e., how cirrus produced by other meteorological situations (e.g., anvil outflow cirrus) fit in to this classification scheme, and if the frequency with which they occur is similar. In an effort to begin exploring this idea, we have compared the results from ML-CIRRUS to the data from the Midlatitude Cirrus eXperiment (MidCiX), which took place in the spring of 2004 and was based out of Houston, Texas. Figure  shows the relationships between IWC, Nice, and Dice, mode in the same format as Fig.  for each campaign. The top panel shows the observations from the ML-CIRRUS campaign without any division between in situ and liquid origin cirrus. The bottom panel shows data from the MidCiX campaign. For this campaign, the IWC values were measured by the Closed-path Laser Hygrometer (CLH) from the University of Colorado , while the Nice and Dice, mode values come from a different CAPS instrument, but they also cover a similar size range as NIXE-CAPS. It is interesting to compare these two campaigns because they are representative of different dynamics. The MidCiX campaign took place in the springtime when the large scale dynamics in the US are shifting from the winter frontal systems to the summer convective systems. As a result, this data set is representative of cirrus stemming from jet streams, convection, and closed low pressure systems.

It can be seen in Fig.  that there is a difference in IWC, Nice, and Dice, mode values. The MidCiX IWC content values are much larger overall and appear at larger Dice, mode than in ML-CIRRUS. Also, these large IWC values are observed at both low and high Nice. From this comparison, we hypothesize, that conditions with more prevalent convection will lead to more liquid origin cirrus with higher IWC values. However, the very high Nice values reported by the CAPS could be an overestimation caused by ice crystal shattering. This data set has not been corrected by an interarrival-time-based algorithm for such features. Instead, the concentrations of the particles in the overlapping ranges of the CAS and CIP probes incorporated into the CAPS have been adjusted to each other seefor more details. However, an overestimation of Nice does not change the important message conveyed by this comparison in regard to the high IWC and large Dice, mode values, the appearance of which should be mostly unaffected by shattering.

Unfortunately, due to the important small scale features in these dynamic systems, the CLaMs-Ice model was unable to accurately portray each MidCiX flight, and therefore, we do not currently have the same information with respect to where the appropriate divisions between in situ origin and liquid origin cirrus cases should be. Although we cannot demonstrate it in the current analysis, we suspect that in MidCiX, and other campaigns sampling from similar dynamics, the liquid origin cirrus clouds are more prevalent relative to the in situ origin cirrus clouds than what is observed in the ML-CIRRUS data set. Further analysis and additional data, which can be found in an upcoming analysis, are necessary to answer this critical question.

The differences between the cirrus cloud origins that have been described here offer new insights into how cirrus can be classified. To demonstrate that two groups do in fact exist within one campaign data set, Fig.  shows the IWC-temperature relationship from ML-CIRRUS. Similar to Fig. , the data are presented in 5 K temperature bins and provide information on the frequency with which each variable occurs within a given temperature bin. Furthermore, the percentage by which each point is more representative of in situ origin cirrus (greens) or liquid origin cirrus (blues) is also shown here. The most frequent observations at low IWC are at low temperatures and are predominantly in situ origin cirrus while the most frequent observations at warmer temperatures are predominantly liquid origin cirrus and exhibit high IWC values. There is an overlap region in the mid-range temperatures where in situ origin cirrus becomes less prevalent and liquid origin cirrus becomes increasingly dominant, but there is still a distribution around the median fit line of the IWC-T relationship. It can be argued that at T>235 K the data will show 100 % liquid origin because we have selected for it in the data processing, but this is not true for T<235 K.

The emergence of two distinct groups of cirrus clouds is reminiscent of the bimodal IWC distribution from mentioned in the introduction, particularly since one group is more representative of low IWC, while the other is more representative high IWC. Thus, after completing this analysis, we now hypothesize that the two modes are the result of the presence of the two origin types. However, the heterogeneous and homogeneous ice nucleation mechanisms are still highly influential in driving the microphysical development of a cirrus cloud and will be discussed further in future work.

Finally, classifying the data in this way could be more accurate for representing cirrus clouds in the climate system because it includes the potential for also classifying the clouds according to their radiative role. The distribution shown in Fig.  appears very similar to what is shown in Fig. 11 in . Further analysis is planned to evaluate this as well.

The analysis presented here has expanded upon and by showing that cirrus clouds can be divided into two groups according to the origin of their ice particles. Here, we have used airborne, in situ observations of IWC, Nice, and ice crystal size from the 2014 ML-CIRRUS campaign to demonstrate clear differences between the microphysical properties of each origin type. Notably, we demonstrate that observations of high IWC and Nice values in combination with large crystals are found in the liquid origin cirrus type. The highest frequency IWC values for in situ origin cirrus were observed to be between 1 and 10 ppmv, while they were 10–100 ppmv in the liquid origin cirrus. The Nice values appear to be similar between the origin types, but median values demonstrate that there is a difference. Using the modal Nice value for midlatitude cirrus (0.1 cm-3) as a guideline, it was found that median values of Nice for in situ cirrus are distributed around this value, but liquid origin cirrus clouds are above it. Similar to IWC, ice crystal size (both Rice and Dice, mode) proved to also show distinct differences dependent on origin. Dice, mode in the in situ origin clouds had median values that were mostly less than 200 µm with the largest particles reaching sizes of 550 µm. Ice crystals in the liquid origin cirrus had median Dice, mode values that were larger than the 200 µm guideline and even larger crystals of nearly 750 µm.

PSDs in 5 K temperature bins allowed a more in-depth look at the details of the cloud structures based on the different populations of ice crystals and how they change with temperature. Once again, it was clear that differences exist between the concentrations and sizes of the particles. In particular, as noted throughout this analysis, the liquid origin cirrus could be characterized by higher concentrations of particles and a size range that is noticeably broader and containing larger crystals. From this information combined with the existing knowledge concerning the details of cloud development in the cirrus environment versus lower in the atmosphere (mixed-phase regime), we could infer the mechanisms and conditions that contributed to create the PSD for each origin type. This indicates that the origin of the ice crystal matters and the influence of that origin can be observed. Moreover, an example was given demonstrating how the PSD for a liquid origin cirrus cloud can continue to be built upon through subsequent homogeneous ice nucleation after arriving in the cirrus regime.

The concept that the two different formation-based cirrus types have different microphysical properties has been demonstrated based on the observations from the midlatitude field campaign ML-CIRRUS. However, this campaign may not be representative of the midlatitudes as a whole. The cloud observations mostly took place in moderate updrafts, typical for the region over Europe that was probed during the campaign. A comparison between the results from ML-CIRRUS and MidCiX provides evidence to suggest that different dynamics will influence the relative frequency of occurrence of in situ versus liquid origin cirrus. Faster updrafts (e.g., convection) will result in higher IWCs and a larger influence from liquid origin cirrus, as demonstrated by the MidCiX data set. One of the uncertainties still existing within the work that is presented here is what the ratio of in situ to liquid origin clouds is on a local or even global scale. Thus, it would be informative to also analyze additional data from locations such as North America and Asia, where the dynamics are known to be more convective than what is typically observed over Europe.

The existence of these two cirrus groups also leads us to examine how we define a cirrus cloud. The major identifier of a cirrus cloud is that it is composed solely of ice. Other measurable properties may be assigned to different cloud samples to tell us more about the position, thickness, etc. of the cloud. However, as suggest, sub-classifications of cirrus based on their ice content would be useful. Information concerning the origin of an ice crystal and how that influences the microphysical properties of a cirrus cloud is something that moves our understanding of cirrus in a direction that begins to provide a more clear representation of the radiative role of cirrus clouds. As stated by the 2013 IPCC report , there remains a very large uncertainty in the role of ice clouds in the atmosphere. Simply put, it is unclear whether ice clouds have a warming or cooling effect on the atmosphere. suggest that in situ origin cirrus clouds may have the tendency toward a cooling effect, while the thicker liquid origin clouds may tend toward warming. Future work is planned to address this topic. While these clouds will be called “cirrus” in any case, the study presented here demonstrates that a categorization scheme based on the two origins is more appropriate for describing the variety of cirrus clouds.