Introduction

We conducted an in-depth study on Arctic low-level boundary layer clouds, focusing on the microphysical properties of cloud droplets and aerosols within the cloud's top layer. Our research aimed to better understand the processes behind monomodal and bimodal droplet size distributions, which are critical in understanding cloud behavior in polar regions. This work was part of the VERtical Distribution of Ice in Arctic Clouds (VERDI) campaign, where we utilized airborne instrumentation to gather data over the Mackenzie River delta and the Beaufort Sea.

Methodology

During the VERDI campaign, we used a variety of instruments aboard the Basler BT-67 research aircraft, such as the Cloud Droplet Probe (CDP), Cloud Imaging Probe (CIP), and other aerosol and radiation measurement tools. These instruments allowed us to capture detailed size distributions of cloud droplets in both liquid and mixed-phase clouds. Our focus was on the upper cloud layer, where processes such as turbulent mixing and droplet evaporation contribute to the formation of bimodal droplet size distributions.

Key Findings

We observed that monomodal droplet size distributions were common inside the cloud, with droplet sizes increasing with altitude. However, in the transition zone at the cloud top, we detected a shift to bimodal distributions, where two distinct droplet modes coexisted. Our analysis suggests that these bimodal distributions are likely caused by turbulent mixing and evaporation at the cloud top, rather than new droplet formation from entrained aerosols. Simulations using Direct Numerical Simulations (DNS) supported our findings, indicating that turbulent mixing at the cloud top is a significant factor in creating these bimodal patterns.

Conclusion

Our study provides new insights into the microphysical processes that govern cloud droplet size distributions in Arctic boundary layer clouds. The occurrence of bimodal size distributions at the cloud top underscores the importance of turbulent mixing in shaping cloud properties. These findings contribute to improving the representation of Arctic clouds in climate models, which is essential for understanding the broader impacts of Arctic amplification and global climate change.

This summary is based on the publication: Klingebiel, M., de Lozar, A., Molleker, S., Weigel, R., Roth, A., Schmidt, L., Meyer, J., Ehrlich, A., Neuber, R., Wendisch, M., and Borrmann, S. (2015). Arctic low-level boundary layer clouds: In situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds. Atmos. Chem. Phys., 15, 617–631. DOI: 10.5194/acp-15-617-2015.