Raindrop shapes and fall velocities in “turbulent times”

Abstract

Abstract. Raindrop shapes and fall velocities measured by 2-dimensional video disdrometer are presented for 2 high-wind/turbulent events. The shapes were reconstructed using a relatively new technique. 10 m height wind sensor data are used to derive proxy-indicators for turbulent intensities. Our results show strong gusts, directional wind shifts (i.e. shear) and/or inferred high turbulence intensity are correlated with reduced fall speeds, reaching values ∼25 %–30 % less than the expected values, i.e. sub-terminal fall speeds. Significant percentage (20 %–35 %) of asymmetric drops (> 2 mm) deviating from the most probable axisymmetric shapes were also detected for some events with high turbulent intensities.

Analysis of Raindrop Shapes and Scattering Calculations: The Outer Rain Bands of Tropical Depression Nate

“…Regarding (i), even though it is assumed (e.g., [24]) that horizontal velocities of precipitation particles do not differ from prevalent wind velocities (e.g., [24,32]) during steady conditions, it is only the 2DVD instrument which is capable of providing direct observational evidence of this in natural rain. It is consistent with previous studies [11,33] from a line convection event and a hurricane event which showed excellent agreement of the 2DVD-based drop horizontal velocities at near-ground level with the wind measurements at 10 m height. Currently, a 3D sonic anemometer has been installed at the UAH site to measure the ambient winds and turbulent intensity…”

Analysis of Raindrop Shapes, Fall Velocities, and Scattering Calculations during Tropical Storm Nate

“…It is around this time, that the fall velocities also show a small but noticeable number of drops having lower than expected velocities. This may provide further evidence that turbulence may contribute toward enhanced amplitude of mixed mode oscillations, as was observed in two earlier studies, one during category-1 hurricane Irma, and the other during a highly organized line convection [23], [11]. (1)…”

“…[25], [26]) during steady conditions, it is only the 2DVD instrument which is capable of providing direct observational evidence on a drop-by-drop basis in natural rain. Examples have been demonstrated in previous studies ( [9], [23]) for a line convection event and a hurricane event which showed excellent agreement of the 2DVD-based drop horizontal velocities at near-ground level with the wind measurements at 10 m height. Such agreement has also been observed for several other events also (not published).…”

“…slowing down of the drops). This is in contrast to the hurricane (category-1) Irma event which had lower than expected velocities during turbulence , as well an embedded line convection period where low fall speeds were associated with a rapid change in wind direction (see [23] for both cases). Such changes were not observed with the anemometer readings during TS Nate Regarding point (iii), we have seen from Fig.…”

Raindrop fall velocity in turbulent flow: an observational study

“…On the other hand, Nemes et al (2017) calculated the response time of low density snow particles (mean size of 1 mm and density estimated as 0.05 g cc −1 ) as 50 ms which is much smaller than the Kolmogorov time scale (τ η = 0.1 s). This contrast reflects the low snow-to-fluid density ratio and Nemes et al (2017) experimentally found that the snow particles' mean settling velocity in turbulence was nearly twice the terminal velocity in still air (termed as a "preferential sweeping" mechanism; Wang and Maxey, 1993).…”