Abstract
Abstract. We report on fall speed measurements of raindrops in light-to-heavy rain events from two climatically different regimes (Greeley, Colorado, and Huntsville, Alabama) using the high-resolution (50 µm) Meteorological Particle Spectrometer (MPS) and a third-generation (170 µm resolution) 2-D video disdrometer (2DVD). To mitigate wind effects, especially for the small drops, both instruments were installed within a 2∕3-scale Double Fence Intercomparison Reference (DFIR) enclosure. Two cases involved light-to-moderate wind speeds/gusts while the third case was a tornadic supercell and several squall lines that passed over the site with high wind speeds/gusts. As a proxy for turbulent intensity, maximum wind speeds from 10 m height at the instrumented site recorded every 3 s were differenced with the 5 min average wind speeds and then squared. The fall speeds vs. size from 0.1 to 2 and >0.7 mm were derived from the MPS and the 2DVD, respectively. Consistency of fall speeds from the two instruments in the overlap region (0.7–2 mm) gave confidence in the data quality and processing methodologies. Our results indicate that under low turbulence, the mean fall speeds agree well with fits to the terminal velocity measured in the laboratory by Gunn and Kinzer from 100 µm up to precipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1 and 1.5 mm sizes were examined in detail under the same conditions. The histogram shapes for the 1 and 1.5 mm sizes were symmetric and in good agreement between the two instruments with no evidence of skewness or of sub- or super-terminal fall speeds. The histograms of the smaller 0.5 and 0.7 mm drops from MPS, while generally symmetric, showed that occasional occurrences of sub- and super-terminal fall speeds could not be ruled out. In the supercell case, the very strong gusts and inferred high turbulence intensity caused a significant broadening of the fall speed distributions with negative skewness (for drops of 1.3, 2 and 3 mm). The mean fall speeds were also found to decrease nearly linearly with increasing turbulent intensity attaining values about 25–30 % less than the terminal velocity of Gunn–Kinzer, i.e., sub-terminal fall speeds.
Highlights
Knowledge of the terminal fall speed of raindrops as a function of size is important in modeling collisional breakup and coalescence processes (e.g., List et al, 1987), in the radarbased estimation of rain rate, in retrieval of drop size distribution using Doppler spectra at vertical incidence (e.g., Sekhon and Srivastava, 1971) and in soil erosion studies (e.g., Rosewell, 1986)
The principal instruments used in this study are the Meteorological Particle Spectrometer (MPS) and third-generation 2-D video disdrometer (2DVD), both located within a 2/3-scale Double Fence Intercomparison Reference (DFIR; Rasmussen et al, 2012) wind shield
We have reported on raindrop fall speed distributions using a high-resolution (50 μm) droplet spectrometer (MPS) collocated with moderate-resolution (170 μm) 2DVD to cover the entire size range expected in natural rain
Summary
Knowledge of the terminal fall speed of raindrops as a function of size is important in modeling collisional breakup and coalescence processes (e.g., List et al, 1987), in the radarbased estimation of rain rate, in retrieval of drop size distribution using Doppler spectra at vertical incidence (e.g., Sekhon and Srivastava, 1971) and in soil erosion studies (e.g., Rosewell, 1986). A relatively new instrument, the Meteorological Particle Spectrometer (MPS), is a droplet imaging probe built by Droplet Measurements Technologies (DMT, Inc.) under contract from the US Weather Service and designed for drizzle as small as 50 μm and raindrops up to 3 mm This instrument in conjunction with a lower-resolution 2-D video disdrometer (2DVD; Schoenhuber et al, 2008) is used in this paper to measure fall speed distributions in natural rain. This paper briefly describes the instruments used, presents fall speed measurements from two sites under relatively low wind conditions and one case from an unusual tornadic supercell with high winds and gusts, and ends with a brief discussion and summary of the results
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