Abstract
Abstract. Low ice crystal concentration and sustained in-cloud 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.
Highlights
Cirrus clouds are composed of ice crystals that form at high altitudes and temperatures typically below 235 K (Pruppacher and Klett, 1997)
Cold cirrus clouds will reside in the “dynamic equilibrium” regime if δT is below a characteristic threshold
High-amplitude, orographically-generated gravity waves are ubiquitous (Kim et al, 2003) but often lose intensity with altitude, weakening their contribution to the background spectrum of temperature fluctuations. δT can decrease enough at high altitude for cirrus to transition from a “pulse-decay” to a “dynamic equilibrium” state (Fig. 9)
Summary
Cirrus clouds are composed of ice crystals that form at high altitudes and temperatures typically below 235 K (Pruppacher and Klett, 1997). Large ice crystals tend to quickly fall out of freezing zones and have limited effect on new ice formation events; small crystals (typically those with terminal velocity, uterm, less or equal to the mean updraft u of the cirrus layer) fall slowly and can remain long enough in the upper part of the cloud to affect new freezing events This suggests that at low temperatures, preexisting (and typically small, Kramer et al, 2009) ice crystals may locally dehydrate the freezing zone sufficiently to inhibit the formation of new ice. The rate of crystal production is not uniform through the freezing zone, as the “local” saturation ratio, S, and updraft velocity, u (defined at the scale of individual cloud “parcels” ∼100–102 m, Pruppacher and Klett, 1997) may be affected by fluctuations in wind speed and temperature induced by gravity waves (Jensen et al, 2010; Karcher and Haag, 2004; Kim et al, 2003). In this work we analyze the range of conditions for which heterogeneous freezing may explain the features of cirrus clouds at low temperature, and propose an alternative view (based on a statistical description of cirrus formation and evolution) in which the interplay of temperature fluctuations, and ice crystal production and sedimentation leads to previously unidentified natural cirrus states of low ice crystal concentration and sustained high supersaturation
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