Dissipation of Mixed-Phase Arctic Clouds and its Relationship to Aerosol Properties

Abstract

Surface aerosol concentrations in the Arctic can reach extremely low values, perhaps low enough that low-level mixed-phase stratocumulus clouds cannot be sustained. Nonetheless, these clouds are persistent features of the Arctic boundary layer and have a profound impact on the local surface energy budget, which in turn has implications for seasonal ice melt and climate change. When these low-level clouds do dissipate, it is unclear if the dissipation is linked to the aerosol concentrations, and if it is not, then how these clouds sustain themselves in the face of extremely low aerosol concentrations. One possibility is that surface aerosol concentrations are not representative of the aerosol concentrations impacting the cloud layer. Little is known about how aerosol properties vary with height in the region and how aerosol populations both below and above the clouds impact the cloud properties. A second, related issue is that current global models poorly simulate the properties of these clouds, in part because not enough is known about their relationship to aerosol properties and the microphysical processes that control their macrophysical properties. This project will closely combine Atmospheric Radiation Measurement (ARM) facility observations at Barrow, Alaska (NSA) with high resolution modeling to address these issues.
There are four primary objectives: 1) To understand the relationship between cloud dissipation events and Arctic aerosol properties, 2) To investigate the vertical profiles of aerosol properties with respect to the boundary layer top, 3) To examine the respective roles of above-cloud and sub-cloud aerosol properties on aerosol-cloud interactions and cloud dissipation, and 4) To understand the microphysical processes that lead to cloud dissipation.
To accomplish these objectives, we will make use of both ground-based observations from the ARM NSA site and large eddy simulations. The observational and modeling components of the project will be tightly linked. Using the observational data in conjunction with reanalysis data, we will identify cases of cloud dissipation that may have been influenced by changing aerosol concentrations. In situ lidar data will be used characterize aerosol properties in the boundary layer and above the boundary layer top on cloud free days and to understand how these properties differ just after cloud dissipation in our identified cases. A select number of cases will be modeled with large eddy simulations to further understand the relationship between cloud properties/dissipation and aerosol particles above and below the boundary layer top. These simulations, and a series of sensitivity experiments, will be closely guided by and compared to the results of the observational analysis. The simulations will also be used to understand the microphysical processes occurring in the clouds, and to understand how the microphysical processes relate to the evolution of the observable cloud properties. Finally, results of the simulation analysis will be used to help infer information about the microphysical processes occurring in observed clouds at the ARM NSA site.
This project will lead to unprecedented understanding of Arctic aerosol and cloud properties, processes, and their interactions. The results will be of use to model developers to improve the representation of Arctic low-level clouds in regional and global models.