Cloud Top Thermodynamic Phase Transition in Cold Air Outbreak Clouds Observed by Satellite

Arctic cold air outbreaks are common mesoscale events at high latitudes which can form extensive and well-ordered clouds and play a major role in radiative forcing. A better understanding of the microphysical properties of these clouds could greatly aid in the understanding of the arctic amplification phenomenon and cloud-phase climate feedback. We have used a combination of MODIS, CALIPSO and CloudSat publicly available satellite data to evaluate cold air outbreaks over the eastern north Atlantic in the 2019-2020 northern hemisphere winter. First, specific strong events were identified using the cold air outbreak index, defined as the difference between the surface skin temperature and the potential temperature at 850 hPa and calculated using MERRA-2 reanalysis data. These events generally exhibit three distinct regions. Closest to the sea-ice or sea-land border, at the beginning of the event, are the cloud streets, near two-dimensional convective patterns. In this region, there is a linear relationship between cloud top temperature and liquid water fraction, as expected due to the activation of ice nucleating particles. At the other end of the event is the convective closed cell region, and between the two is the transition regime. Past the cloud street regime, decoupling of the temperature and phase has been observed in multiple events. We hypothesize that this is due to secondary ice production, which could cause the cloud phase to be more dependent on cloud age than temperature. We will further investigate this phenomenon using CALIPSO-Cloudsat and ground-based measurements, with a goal of identifying the type and driver of the secondary ice production and creating a subgrid-scale cold air outbreak cloud parameterization which has the potential to improve the accuracy of global climate models in high latitude regions.