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 subterminal, whereas only 0.5% of the raindrops of the same size were detected as subterminal after passage. Contrary to findings in literature, we could not find any distinct superterminal 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 midsized (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.
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Shapes and Fall Speeds of Freezing and Frozen Raindrops
Abstract This study investigates the shapes and fall speeds of freezing and frozen raindrops through field observations using an instrument called the high-speed optical disdrometer (HOD) that we developed recently. Our field observations showed that while the shapes of all of the observed freezing raindrops and a portion of the frozen raindrops (39% of the frozen raindrops that are larger than 1.0 mm in volume equivalent diameter D ) resemble the shapes of warm raindrops, majority of frozen raindrops (61% of the frozen raindrops with D > 1.0 mm) exhibited a distinct feature such as a spicule, bulge, cavity, or aggregation. Field observations of axis ratios (i.e., ratio of the vertical to horizontal chord) and fall speeds were compared with the predictions of available models. Separate empirical axis ratio parameterizations were developed for the freezing and frozen raindrops using the HOD field observations and extensions to an available shape model were also incorporated. For the fall speeds of freezing and frozen raindrops, field observations demonstrated a good agreement with the predictions of the available parameterizations. Frozen raindrops showed a larger scatter of fall speeds around the mean fall speed of a given drop size than those of the freezing raindrops due to the shape variety among the frozen raindrops with the aforementioned distinct features. The drag coefficients for the observed hydrometeors were compared with the predictions of the available drag coefficient models. Separate “drag coefficient–Reynolds number” relationships for freezing and frozen raindrops were developed.
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- Award ID(s):
- 1741250
- PAR ID:
- 10218752
- Date Published:
- Journal Name:
- Journal of Hydrometeorology
- Volume:
- 21
- Issue:
- 6
- ISSN:
- 1525-755X
- Page Range / eLocation ID:
- 1311 to 1331
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JHM-D-19-0204.1
