New Instrument Provides Breakthrough in Cloud Microphysics

Summary

In the atmosphere as clouds form, grow, and dissipate, they mix with the air around them. This mixing of the water-saturated air inside the clouds with the drier air outside the clouds impacts the size and number of cloud droplets. The details of this mixing process, however, have been a source of controversy in the cloud microphysics community for decades. Two theories have been developed to describe how clouds mix with the environment: (1) homogeneous mixing, in which all droplets evaporate a little bit until the air becomes saturated; and (2) inhomogeneous mixing, in which some water droplets evaporate completely while others are unchanged.

In a recent study, researchers present in-cloud observations from a new instrument that finally settle the matter. The new instrument, Holographic Detector for Clouds (HOLODEC), was developed in part with support from the Atmospheric Radiation Measurement (ARM) Climate Research Facility. The HOLODEC takes detailed three-dimensional images of cloud droplets during aircraft flights, enabling the measurement of both the sizes and spatial distribution of cloud droplets within clouds at unprecedented scales. The holographic measurements show that in cumulus clouds, the data are in strong agreement with the inhomogeneous mixing hypothesis.  The droplet size distributions show large changes in number density as drier air mixes with the cloudy air, but a nearly unchanging mean droplet diameter. Essentially, clouds have distinct edges down to the centimeter scale. This result is important for correctly representing cloud microphysical processes within numerical weather and climate models, because the same amount of water divided into many small drops or a few large drops has very different optical properties.  The differences affect how much sunlight is reflected by clouds, as well as other aspects of the cloud development and lifecycle, such as precipitation development.

References

Beals, M. J., J. P. Fugal, R. A. Shaw, J. Lu, S. M. Spuler, and J. L. Stith. 2015. “Holographic Measurements of Inhomogeneous Cloud Mixing at the Centimeter Scale,” Science 350(6256), 87–90. DOI: 10.1126/science.aab0751.