The Potential of Applying THz Radar to Detect Small Drizzle Droplets in a Large Cloud Chamber

Drizzle formation is a critical microphysical process in warm clouds. The mechanism of controlling drizzle formation, i.e., collision coalescence, is poorly represented in models, thus leadings to large discrepancies compared with observations in terms of cloud dynamical structure, lifetime, and precipitation characteristics. It has been proposed that drizzle formation can be achieved in a laboratory convection-cloud chamber, allowing aerosol-cloud-drizzle interactions to be studied at a fundamental level. However, detection of rare drizzle drops in a large cloud chamber is a challenging task due to the limited sampling volume of in situ droplet detection instruments. Instead, we envision using THz radar to sample the cloud chamber with high temporal-spatial resolution, enabling frequent and continuous hydrometer detections without interfering with the flow within the chamber.

We find that the THz radar with small observational volume is critical for drizzle detection in a cloud chamber to allow a drizzle drop in the radar sampling volume to dominate over the background cloud droplets signal. Theoretical estimates indicate that using a radar volume of several cubic centimeters (cm³) enables detection of a drizzle embryo with diameter larger than 40 . However, the occurrence of drizzle drops within the radar volume decreases as the sampling volume is reduced, thus the selection of the radar volume size should consider both of the signal power and occurrence of the detected drizzle particle. Observations from the Pi Convection-Cloud Chamber are used to validate the theoretical estimates and the drizzle detection concept using the high-resolution radar. Finally, an experiment using the THz radar with cm-scale range resolution applied in a test spray chamber is conducted to demonstrate the feasibility of proposed drizzle detection concept.