Characteristics of the Lifecycle of Open Cellular Convection During COMBLE

During cold-air outbreaks (CAO) when cold air blows off boreal continents or the Arctic ice over open water, a characteristic cloud pattern forms with linearly organized clouds streets and, further downwind, an open cellular structure. Despite the ubiquity of this CAO cloud regime over high-latitude oceans, its properties are rather poorly understood, including the lifecycle of open cellular convection. From December 2019 to May 2020, the Cold-air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) was conducted along the coast of northern Norway, with the goal to examine the processes in marine CAOs. Among the deployed instruments was a vertically scanning cloud radar known as the Ka-Band ARM zenith radar (KAZR). Open cellular convection observed by the KAZR during intense CAO conditions undergoes an apparent lifecycle. For an objective investigation of the open cellular convection and its lifecycle, individual open cells are identified using a watershed segmentation technique based on the KAZR equivalent reflectivity factor. The individual identified cells are then classified using a self-organizing maps (SOM) algorithm. The algorithm uses vertical distributions of equivalent reflectivity factor, Doppler velocity, and spectrum width as inputs. The resulting master SOM is interpreted as classifying clouds by their intensity and age, i.e. lifecycle stage. Thus, the algorithm allows analyzing the characteristics of the lifecycle. For instance, cells categorized as developing have substantially lower mean cloud top heights ~3.5 km on average, compared to cells classified as mature or dissipating with cloud tops ~4.5 km. Moreover, developing cells contain substantial liquid and dissipating cells are almost fully glaciated. The mean microwave radiometer observed liquid water path for intense developing cells is 300 g m-2, while for intense dissipating cells it is only 50 g m-2. Furthermore, the technique of identifying and classifying individual cells is applied to model output using a radar simulator.