Prior heterogeneous ice nucleation events increase likelihood of homogeneous freezing during the evolution of synoptic cirrus
Abstract. In-situ observations are currently used to classify synoptic cirrus as formed by homogeneous or heterogeneous ice nucleation based on ice residual analysis. We use large-eddy model UCLALES-SALSA to show the limitations of this method by demonstrating that prior heterogeneous ice nucleation events can shape the thermodynamic conditions for homogeneous freezing to occur more likely in the subsequent nucleation events.
In a case study of synoptic cirrus from NASA’s Midlatitude Airborne Cirrus Properties Experiment (MACPEX), observations suggest homogeneous freezing as the dominant nucleation mechanism. Simulations done with UCLALES-SALSA show that homogeneous freezing occurred after earlier heterogeneous ice nucleation events, with mineral dust acting as the ice-nucleating particles (INPs). Heterogeneous ice nucleation depleted INPs from cirrus forming altitudes, creating favourable conditions for homogeneous freezing at the time of observations.
This study modelled cirrus cloud properties based on measured conditions and compared simulated results with observed cloud structures. It is shown that modelling the impact of prior nucleation events on the vertical distribution of mineral dust and humidity in the model is necessary to reproduce the observed cloud characteristics. Heterogeneous ice nucleation primarily had a role in removal of ice-nucleation active mineral dust from cloud-forming altitudes well before arriving at the measurement location, while having limited role in forming ice crystals shortly before the time of measurements.
Model results also show that small-scale wave activity strongly influenced ice nucleation efficiency and overall cloud properties. While large-scale atmospheric dynamics typically dominate synoptic cirrus formation, they alone were insufficient to replicate the observed cloud characteristics.