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

Haag, Werner O.; Kärcher, B.; Ström, Johan; Minikin, Andreas; Lohmann, Ulrike; Ovarlez, J.; Stohl, Andreas

doi:10.5194/acp-3-1791-2003

Cited by 164 publications

(205 citation statements)

References 39 publications

(46 reference statements)

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“…This supports the assumption that the cloud is rather stable, maybe with a tendency to evaporate at that stage. High RHi values of approximately 140-150 % RHi are needed for homogeneous ice nucleation (DeMott et al, 1998;Koop et al, 2000;Haag et al, 2003;Kärcher, 2012). Thus the high RHi values of up to 150 % following the transient cirrus cloud on 3 November 2010 suggest that homogeneous ice nucleation may play a role in cirrus cloud formation in this situation.…”

Section: Summary and Discussionmentioning

confidence: 98%

“…Radiative properties of mid-latitude cirrus clouds strongly depend on their microphysical parameters, e.g. ice crystal shape, size distribution and number concentration (Stephens et al, 1990;Haag and Kärcher, 2004;Fusina et al, 2007), which are influenced by ambient conditions, especially dynamics , temperature and supersaturation (Heymsfield, 1977;Khvorostyanov and Sassen, 1998b) as well as the nucleation mode, i.e. the composition of the ambient aerosol (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Potential of airborne lidar measurements for cirrus cloud studies

et al. 2014

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Abstract. Aerosol and water vapour measurements were performed with the lidar system WALES of Deutsches Zentrum für Luft-und Raumfahrt (DLR) in October and November 2010 during the first mission with the new German research aircraft G55-HALO. Curtains composed of lidar profiles beneath the aircraft show the vertical and horizontal distribution and variability of water vapour mixing ratio and backscatter ratio above Germany. Two missions on 3 and 4 November 2010 were selected to derive the water vapour mixing ratio inside cirrus clouds from the lidar instrument. A good agreement was found with in situ observations performed on a second research aircraft flying below HALO. ECMWF analysis temperature data are used to derive relative humidity fields with respect to ice (RHi) inside and outside of cirrus clouds from the lidar water vapour observations. The RHi variability is dominated by small-scale fluctuations in the water vapour fields while the temperature variation has a minor impact. The most frequent in-cloud RHi value from lidar observations is 98 %. The RHi variance is smaller inside the cirrus than outside of the cloud. 2-D histograms of relative humidity and backscatter ratio show significant differences for in-cloud and out-of-cloud situations for two different cirrus cloud regimes. Combined with accurate temperature measurements, the lidar observations have a great potential for detailed statistical cirrus cloud and related humidity studies.

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Section: Summary and Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Potential of airborne lidar measurements for cirrus cloud studies

et al. 2014

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“…However, at lower RH ice conditions, the agreement is excellent. Observations suggest that in cirrus clouds, RH ice is mostly below 120 % (Haag et al, 2003), meaning the lower RH ice values where the DSF works better are most relevant to observed cirrus clouds.…”

Section: B Hande Et Al: Seasonal Variability Of Saharan Desert Dmentioning

confidence: 99%

Seasonal variability of Saharan desert dust and ice nucleating particles over Europe

et al. 2015

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Abstract. Dust aerosols are thought to be the main contributor to atmospheric ice nucleation. While there are case studies supporting this, a climatological sense of the importance of dust to atmospheric ice nucleating particle (INP) concentrations and its seasonal variability over Europe is lacking. Here, we use a mesoscale model to estimate Saharan dust concentrations over Europe in 2008. There are large differences in median dust concentrations between seasons, with the highest concentrations and highest variability in the lower to mid-troposphere. Laboratory-based ice nucleation parameterisations are applied to these simulated dust number concentrations to calculate the potential INP resulting from immersion freezing and deposition nucleation on these dust particles. The potential INP concentrations increase exponentially with height due to decreasing temperatures in the lower and mid-troposphere. When the ice-activated fraction increases sufficiently, INP concentrations follow the dust particle concentrations. The potential INP profiles exhibit similarly large differences between seasons, with the highest concentrations in spring (median potential immersion INP concentrations nearly 10 5 m −3 , median potential deposition INP concentrations at 120 % relative humidity with respect to ice over 10 5 m −3 ), about an order of magnitude larger than those in summer. Using these results, a best-fit function is provided to estimate the potential INPs for use in limited-area models, which is representative of the normal background INP concentrations over Europe. A statistical evaluation of the results against field and laboratory measurements indicates that the INP concentrations are in close agreement with observations.

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“…At temperatures between 200 and 235 K cirrus ice crystals form primarily by homogenous freezing of supercooled deliquesced aerosol (DeMott et al, 2003;Heymsfield and Sabin, 1989), which occurs if the saturation ratio with respect to ice, S, (i.e., the ratio of water vapor partial pressure to its equilibrium value over ice) reaches a characteristic threshold value, S hom (Koop et al, 2000). Heterogeneous freezing of water upon existing aerosol particles (termed "ice nuclei", IN) can also occur (at S lower than S hom ) and contribute to ice crystal concentrations (DeMott et al, 2003;Froyd et al, 2009), especially in polluted and dust-rich regions (Barahona et al, 2010a;Haag et al, 2003). The level of water vapor supersaturation (i.e., S − 1) is the thermodynamic driver for ice formation, and is generated by expansion of air parcels forced by large scale dynamics, gravity waves, and small scale turbulence (Kim et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

Dynamical states of low temperature cirrus

Barahona

Nenes

2011

Atmos. Chem. Phys.

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Abstract. Low ice crystal concentration and sustained incloud supersaturation, commonly found in cloud observations at low temperature, challenge our understanding of cirrus formation. Heterogeneous freezing from effloresced ammonium sulfate, glassy aerosol, dust and black carbon are proposed to cause these phenomena; this requires low updrafts for cirrus characteristics to agree with observations and is at odds with the gravity wave spectrum in the upper troposphere. Background temperature fluctuations however can establish a "dynamical equilibrium" between ice production and sedimentation loss (as opposed to ice crystal formation during the first stages of cloud evolution and subsequent slow cloud decay) that explains low temperature cirrus properties. This newly-discovered state is favored at low temperatures and does not require heterogeneous nucleation to occur (the presence of ice nuclei can however facilitate its onset). Our understanding of cirrus clouds and their role in anthropogenic climate change is reshaped, as the type of dynamical forcing will set these clouds in one of two "preferred" microphysical regimes with very different susceptibility to aerosol.

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

Cited by 164 publications

References 39 publications

Potential of airborne lidar measurements for cirrus cloud studies

Potential of airborne lidar measurements for cirrus cloud studies

Seasonal variability of Saharan desert dust and ice nucleating particles over Europe

Dynamical states of low temperature cirrus

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