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Cirrus Clouds

Cirrus clouds, which are made up entirely of ice crystals, are an important subject of research at IEK-7. They influence the climate and limit the input of water into the stratosphere due to the sedimentation of large ice crystals. The ice water content of cirrus clouds was determined in ten aircraft measurement campaigns in the Arctic, in the midlatitudes and in the tropics (TROCCINOX, SCOUT-O3 and others). Measurement data are available for three versions of IEK-7's FISH Lyman-α hygrometer (high-altitude research aircraft GFD Learjet, DLR Falcon, M-55 Geophysica). The comprehensive FISH database was used to derive a climatology of the ice water content and to determine the HNO3 uptake in cirrus clouds. This climatology is used successfully today to calculate cirrus clouds in the CLaMS model. It has also been included in the global radiation models of other atmospheric research groups.

Cirrus guide (H): Pure ice clouds (cirrus) currently constitute one of the major uncertainties in the prediction of the Earth's climate. This can be attributed to the poor knowledge of the ice formation processes. A step forward in the representation and understanding of cirrus is provided by a guide to cirrus microphysics [3], 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. This cirrus analysis approach is validated by evaluating cirrus data sets from 17 aircraft campaigns, conducted in the last 15 years, spending in total about 94 h in cirrus over Europe, Australia, Brazil as well as South and North America. Altogether, the approach of this study was 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.
In conjunction with another study [4], it was possible to classify two types of cirrus with different formation mechanisms and microphysical properties (see Fig.). The first cirrus type forms directly as ice (in situ origin cirrus). The second type originates from mixed-phase clouds (i.e., via freezing of liquid droplets – liquid origin cirrus), which are completely glaciated during uplift to the cirrus formation temperature region (< 235 K). In slow updrafts, in-situ cirrus are optically thin with lower IWCs, so they do not block the incoming solar light and thus have a warming effect. In fast updrafts, in-situ cirrus are optically thicker with higher IWCs. They may shield the sun and thus have the potential for a cooling effect. Liquid origin cirrus show the highest IWCs: they are predominantly optically thick and thus might have a cooling effect.

Sketch of in-situ and liquid origin cirrus


Sketch of in-situ and liquid origin cirrus. In situ origin cirrus (green) are ice crystals forming directly from the gas phase. In-situ cirrus are preferably thin with lower IWC. They divide in two classes: (1) thin cirrus forming in slow updrafts consisting of few, large ice crystals with a large geographic coverage and a long lifetime, (2) thicker cirrus forming in fast updrafts consisting of many small ice crystals. Liquid origin cirrus (blue) are ice crystals originating from frozen liquid drops which are uplifted from further below in the atmosphere into the cirrus temperature range. Liquid origin cirrus are mostly thick with higher IWC and have larger ice crystals than in-situ cirrus.


Further, fifteen months of simultaneous observations of ice crystal number concentration in cirrus (Nice) and relative humidity with respect to ice (RHice) were analyzed as part of IAGOS (In-service Aircraft for a Global Observing System) [5]. Comparison of the cirrus properties measured by the high-precision, but smaller-scale observations from research aircraft and simpler, but larger-scale measurements from commercial aircraft show very good agreement (P).
In addition to the airborne observations, simulations of cirrus are performed with the newly developed large-scale Lagrangian microphysical process model ClaMS-Ice (M), extending the `Cirrus Guide’ by the view on the up to know not observed Arctic cirrus. Most importantly, we found that Arctic in-situ cirrus have higher values of maximum Nice which are caused by strong gravity wave activity over Greenland. On the other hand fewer liquid origin cirrus are found in the Arctic compared to observations at mid-latitudes. The aforementioned studies are complemented by experimental [6, 7, 8] and theoretical work including observations of sub-visible cirrus clouds by infrared limb-sounding in the lowermost stratosphere (LMS) [10] and simulations of TTL cirrus [9].

References:

  1. Schiller et al. 2008, Journal of Geophysical Research, 113, D24208.
  2. Krämer et al. 2009, Atmospheric Chemistry and Physics, 9, 3505-3522.
  3. Krämer, M. et al. 2016, Atmos. Chem. Phys. 16: (5), 3463-3483, doi:10.5194/acp-16-3463-2016.
  4. Luebke, A.E. et al. 2016, Atmos. Chem. Phys. 16: (9), 5793-5809,doi:10.5194/acp-16-5793-2016.
  5. Petzold et al., 2017, Faraday Discussions., doi:10.1039/C7FD0006E.
  6. Lübke, A.E. et al. 2013, Atmos. Chem. Phys. 13: 6447-6459, doi:10.5194/acp-13-6447-2013.
  7. Järvinen, E. et al. 2016, J. Atmos. Sci. 73: (10), 3885-3910, doi:10.1175/JAS-D-15-0365.1.
  8. Kienast-Sjögren, E. et al. 2016, Atmos. Chem. Phys. 16: (12), 7605-7621, doi:10.5194/acp-16-7605-2016.
  9. Spichtinger, P. et al. 2013, Atmos. Chem. Phys. 13: 9810-9818, doi:10.5194/acp-13-9801-2013.
  10. Spang, R. et al. 2015, Atmos. Chem. Phys. 15: 927-950, doi:10.5194/acp-15-927-2015.


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