Hemispheric Asymmetry in Cloud Phase Partition

Science

New research reveals a fundamental imbalance in the composition of mixed-phase clouds between Earth's two hemispheres. Using U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) User Facility and National Science Foundation airborne observations from 75°S to 87°N, clouds in the Southern Ocean were found to contain significantly higher occurrence frequencies and mass fractions of supercooled liquid water than their northern counterparts. These findings imply an asymmetric response of clouds to a changing Earth's system in the two hemispheres.

Impact

A critical finding is that a leading earth system model—the DOE Energy Exascale Earth System Model (E3SM)—fails to capture this real-world pattern of cloud phase distributions. Not only does the model miss the hemispheric asymmetry, but it also underestimates the amount of ice clouds in the Arctic.

These discrepancies suggest that the model simulations may be missing important differences in how warming will unfold in the Northern and Southern Hemispheres.

Summary

The amount of liquid water versus ice in a cloud controls whether it predominantly cools the Earth by reflecting sunlight or warms it by trapping heat. This is especially complicated for mixed-phase clouds, which exist at temperatures below freezing and contain a mixture of both liquid water droplets and ice crystals. By analyzing a large data set from in situ, high-resolution aircraft observations of 14 flight campaigns, this study revealed a crucial fact: there are significant differences in cloud phase partition and microphysical properties between the Northern and Southern Hemispheres. A leading earth system model failed to reproduce this hemispheric asymmetry, especially missing the “icy” nature of clouds in the Arctic. The findings suggest that under a changing Earth’s system, clouds in the two hemispheres may respond in different ways, a complexity that current models may be missing.