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.
Raindrop fall velocity in turbulent flow: an observational study
