Ingo Sölch

Supersaturation Variability and Cirrus Ice Crystal Size Distributions

Small-scale dynamical variability affects atmospheric supersaturation and therefore the development of ice cloudsvia uptakeof watervapor on ice crystals.This variabilityand its implications forice growthare difficult to capture experimentally and theoretically. By interpreting supersaturation as a stochastic variable, the authors examinethe average temporal behaviorof, and the link between, supersaturationfluctuations and ice crystal size distributions in upper-tropospheric cirrus clouds. The authors classify cirrus types according to their ability to dampen supersaturation fluctuations owing to depositional growth of cloud ice and study how sizedistributionsinthemrespondtosupersaturationvariability,investigatingthepossibilityoftheoccurrence of ice-supersaturated states within cirrus. Typical time scales for growth and damping impacts on supersaturation are minutes and minutes to hours, respectively, and are highly variable among cirrus types and within single clouds. Transient deviations from saturated equilibrium states can occur depending on the ice crystal number concentration and size and on the strength of the small-scale dynamical forcing. Supersaturation preferentially occurs in cloud regions with few small ice crystals. The authors demonstrate that supersaturation fluctuations in very thin tropical tropopause cirrus create long-lived supersaturated states. Furthermore, they potentially generate few large ice crystals, broadening size distributions, and significantly enhance water mass fluxes due to sedimentation. Although not studied here, they may also allow new ice crystals to nucleate. Implications of these findings for those clouds to dehydrate air entering the lower stratosphere are discussed and future research needs are outlined.

Cloud resolving modeling of contrail evolution

Contrails are ice clouds that form behind aircraft. As a result of burning kerosene in the engines, water vapor is emitted that rapidly freezes and forms ice crystals. If the atmosphere is sufficiently moist and cold, these contrails expand and persist for many hours. This chapter describes the numerical modeling of contrails on a local scale with cloud resolving simulations. Emphasis is put on the description of microphysical modeling. With this methodological approach valuable information on contrail evolution for a multitude of atmospheric and aircraft parameters can be obtained.