Dependence of Marine Boundary Layer Cloud Properties on Environmental Conditions during Cold Air Outbreaks over the North Atlantic: Results from CAESAR

The accurate representation of mixed-phase marine boundary layer clouds (MBCs) in models is challenging. There is a compelling need to determine how MBC properties and formation mechanisms vary with surface, environmental and aerosol conditions in high-latitudes during cold air outbreaks (CAOs). During the CAO Experiment in the Sub-Arctic Region (CAESAR) field campaign, the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) C-130 aircraft was deployed to Kiruna, Sweden with a variety of in situ and remote sensors to sample Arctic air masses as they transformed downstream from the CAO origin at the ice edge. This study uses CAESAR measurements to examine the vertical structure of MBCs and its transition downstream during CAOs, with a focus on the dependence of thermodynamic and cloud microphysical properties in the marine boundary layer (BL) on environmental conditions. The 16 March 2024 flight (RF07) is chosen as a case study because this was a full-fetch flight aligned well with cloud streets, where a very cold CAO was sampled with 36 dropsondes released allowing for a fine-scale characterization of the thermodynamic variation of the boundary layer. Wyoming W-band radar (WCR) measurements are utilized for detecting the vertical structure of MBCs and for classifying whether the MBCs were closed-cell or open-cell structured MBCs. RF07 observations show that after the transition from closed-cell to open-cell structure MBCs, the average planetary BL height (PBLH) and cloud top height (CLT) increased by 21% and 20%, surface humidity increased by 33%, horizontal wind speed increased by 16%, the CAO index M decreased by 10%, and the BL inversion strength decreased by 56%. Different methods are applied to calculate the cloud top entrainment rate and examine the cloud top instability. Irradiance measurements from the upward- and downward-looking visible and infrared radiometers are used to calculate radiative fluxes at cloud top. Depolarization and the ratio of total backscattering to molecular return (LSR) measurements from Multi-function Airborne Raman Lidar (MARLi) are used for cloud phase identification. Additionally, dropsonde data is used for detecting the low-level jet (LLJ). The correlation in properties before and after this transition with precipitation, wind shear, cloud phase, dry air entrainment, and cloud top radiative fluxes is described.