Rainfall Microphysics Influenced by Strong Wind during a Tornadic Storm

Abstract

Abstract Rainfall microphysical characteristics including raindrop fall speed, axis ratio, and canting angle were measured through field observations by using a High-Speed Optical Disdrometer (HOD) during and after tornadic severe storm passage. High and low wind and turbulence characteristics were observed during and after passage, respectively, which provided an opportunity to compare the effects of the different wind and turbulence characteristics on raindrop characteristics. During passage, 9.4% of the raindrops larger than 1.0 mm in volume equivalent diameter (D) were identified as sub-terminal, whereas only 0.5% of the raindrops of the same size were detected as sub-terminal after passage. Contrary to findings in literature, we could not find any distinct super-terminal fall speed behavior for raindrops with D<1.0 mm during or after passage. For raindrops with D>2.0 mm, deviations of the axis ratio distribution from the predicted distribution for the equilibrium raindrops were observed, and the deviations during passage were larger than those after passage. The deviations of the axis ratio distributions from the predicted distributions for the equilibrium raindrops were also observed for mid-sized (1.0<D<2.0 mm) raindrops; however, these deviations during and after passage were of similar magnitude. The canting angle distribution for raindrops with D>2.0 mm was found to have the mean value of approximately 0° both during and after passage and the standard deviation values of 24.7° during passage and 13.6° after passage. This study shows the clear influence of wind on various rainfall microphysical characteristics and documents the observed value ranges of these characteristics under strong wind that are of importance for a number of rainfall applications, including radar rainfall retrievals and rainfall modeling.