Aerosol-cloud-interactions account for the largest uncertainty in current estimates of changes in radiative forcing. This is particularly true for the role of ice nuclei (IN) in forming ice or mixed-phase clouds. The mixture of supercooled water and ice is microphysically unstable but yet mixed-phase clouds might persist over several days where they significantly influence energy budgets and radiation.<br/>The mechanisms of ice nucleation in such clouds are poorly understood. Reasons for this include uncertainties in ice nucleation parameterizations and the common assumption that laboratory-derived parameters can be directly applied to atmospheric conditions. We will show that assumptions on (parameterized) IN surface properties, such as contact angle distributions fitted to laboratory experiments, cannot be necessarily extrapolated to wide ranges of atmospheric conditions due to the ambiguity of assumed contact angle distributions.<br/>In addition to parameterized contact angles, uncertainty in ice nucleation is introduced due to assumptions on the time-dependence of ice nucleation. In general, models use either stochastic (time-dependent) or deterministic (time-independent) approaches in order to predict the number of ice particles. Based on sensitivity studies, we show that indeed time is the least sensitive parameter as compared to other parameters included in the classical nucleation theory (CNT), including temperature, and IN properties (size, contact angle). An analysis of parameter spaces for which CNT predicts (nearly) time-independent behavior reveals that deterministic approaches might be sufficiently accurate for a variety of ice nuclei.  <br/>While most of the experimental and model analyses have been performed for equilibrium conditions, in additional studies we will show the extent to which trends in sensitivities of various parameters behave under conditions as encountered in clouds. Under those conditions, thermodynamic equilibrium is not reached due to the continuous adjustment of saturation due to condensation, drop and ice growth. This analysis gives insight on to which parameters have to be included in comprehensive descriptions of ice nucleation to be used in models.

Ice nucleation in clouds: Sensitivities to physicochemical IN properties and cloud microphysics

249th National Meeting (2015)

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