Articles | Volume 9, issue 4
Research article
22 Apr 2016
Research article |  | 22 Apr 2016

Measuring droplet fall speed with a high-speed camera: indoor accuracy and potential outdoor applications

Cheng-Ku Yu, Pei-Rong Hsieh, Sandra E. Yuter, Lin-Wen Cheng, Chia-Lun Tsai, Che-Yu Lin, and Ying Chen

Abstract. Acquisition of accurate raindrop fall speed measurements outdoors in natural rain by means of moderate-cost and easy-to-use devices represents a long-standing and challenging issue in the meteorological community. Feasibility experiments were conducted to evaluate the indoor accuracy of fall speed measurements made with a high-speed camera and to evaluate its capability for outdoor applications. An indoor experiment operating in calm conditions showed that the high-speed imaging technique can provide fall speed measurements with a mean error of 4.1–9.7 % compared to Gunn and Kinzer's empirical fall-speed–size relationship for typical sizes of rain and drizzle drops. Results obtained using the same apparatus outside in summer afternoon showers indicated larger positive and negative velocity deviations compared to the indoor measurements. These observed deviations suggest that ambient flow and turbulence play a role in modifying drop fall speeds which can be quantified with future outdoor high-speed camera measurements. Because the fall speed measurements, as presented in this article, are analyzed on the basis of tracking individual, specific raindrops, sampling uncertainties commonly found in the widely adopted optical disdrometers can be significantly mitigated.

Short summary
How to accurately measure droplet fall speed in natural outdoor conditions has been a long-standing and highly challenging issue in the meteorological community. Results from this article are not only to demonstrate the great potential for high-speed imaging to provide a reliable measurement of droplet fall speed without suffering from sampling uncertainties but also to share a new approach and different thoughts about the retrieval of the droplet fall speed information.