A Clearer View of Mixed-phase Conditions in Clouds

Riihimaki, L., Pacific Northwest National Laboratory

Cloud Processes

Convective Processes

Riihimaki L, J Comstock, E Luke, T Thorsen, and Q Fu. 2017. "A case study of microphysical structures and hydrometeor phase in convection using radar Doppler spectra at Darwin, Australia." Geophysical Research Letters, 44(14), 10.1002/2017GL074187.

Science

Observing mixed-phase (liquid and ice) conditions in clouds is a significant challenge. Cloud phase identification from lidar is one of the most trusted methods of identifying supercooled liquid. However, the lidar beam cannot see through optically thick clouds, so researchers get an incomplete picture of clouds. Scientists at the U.S. Department of Energy led a research team that used radar measurements to identify regions of a tropical deep convective cloud with both liquid droplets and ice crystals at temperatures colder than 0°C.

Impact

The coexistence of liquid and ice in the cloud changes how ice crystals grow, but a lack of observations of this mixed-phase state hinders progress in representing it well in models. Because falling ice crystals melt into rain when they reach warmer air, data that show where liquid and ice coexist in a cloud can help improve understanding of when and how much it will rain.

Summary

In this study, researchers identified cloud phase using data collected from a vertically pointing cloud radar at a DOE Atmospheric Radiation Measurement (ARM) Climate Research Facility observatory in Darwin, Australia. This instrument has previously helped scientists identify cloud phase in shallow, stable stratiform arctic clouds, but this is one of few studies to use the cloud radar to find phase in a convective cloud. Applying a new technique, researchers identified radar parameters (e.g., spectrum width, spectrum left slope, and spectrum right slope) that were sensitive to the presence of mixed-phase regions and used a clustering algorithm to determine those regions. The resulting detailed view provided two interesting insights on the phase conditions within the tropical convective cloud. First, the distribution of mixed-phase conditions existed in multiple regions, or microstructures, within the cloud and contained both liquid and ice rather than one large layer. Second, at the observed vertical velocities, researchers found conditions conducive to ice growth at the expense of liquid droplets—a process thought to be primarily relevant for stratiform clouds.