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Rodney J. Kubesh and Kenneth V. Beard


The natural oscillations of moderate-size raindrops were studied in a seven-story fall column using a computer-controlled generator to produce isolated water drops at terminal speed. Instantaneous shapes were photographed to obtain oscillation sequences of single drops by a multiple-strobe technique. The oscillation frequencies were determined from fall-streak modulations that were photographed in backscattered light of the primary rainbow. Measurements were made at three levels for 2.0- and 2.5-mm diameter drops to assess the role of aerodynamic feedback as the source of drop oscillations.

Variations as large as 15% in axis ratio were observed at the bottom of the fall column, even though the initial oscillations were predicted to die out by viscous decay theory. Practically all oscillations were at the fundamental and first harmonic frequencies. The oscillation modes deduced from the axis ratio scatter indicated that the axisymmetric modes died away slowly and that transverse modes persisted. The slow decay of the axisymmetric modes is postulated to be caused by positive feedback of shape-induced changes in pressure and drag from the initial oscillations. The transverse mode is believed to persist because of transverse pressure perturbations associated with eddy shedding. Various types of feedback are considered that could explain the broad coupling between eddy shedding and oscillations.

The mean experimental axis ratios were higher than equilibrium values—an apparent consequence of shape changes from transverse modes. The deviation from equilibrium shape was generally consistent with previous field measurements of raindrop axis ratios. Use of empirical mean axis ratios in differential reflectivity calculations would change equilibrium values of ZDR by 20%–30%.

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Kenneth V. Beard and Rodney J. Kubesh


The oscillation frequencies and modes of small raindrops (1.04–1.54-mm diameter) were determined from laboratory experiments using water drops generated at terminal velocity at a fall distance sufficient for initial oscillations to damp out. Frequency information was obtained from fall streaks photographed in backscatter light near the primary and secondary rainbows. Streak data was interpreted with the aid of ray tracing through drops with spherical harmonic perturbations. Axis ratio data was used in conjunction with analyses of spherical harmonic perturbations to help determine the oscillation modes.

Two frequencies were present in all drop sizes. The significant oscillation modes for smaller drops (1.04–1.30 mm) were the transverse modes of the fundamental and first harmonic, whereas the significant oscillation modes for larger drops (1.40–1.54 mm) were the axisymmetric mode of the fundamental and the transverse mode of the first harmonic. Primary resonance appears to be responsible for the transverse modes because of the match in frequencies between the forcing and response and because the spatial pattern of the eddy shedding would tend to force these modes. Secondary resonance would account for the axisymmetric mode in larger drops, since this mode is a subharmonic of the forcing frequency and there is no requirement for the forcing pattern to match the response.

Our study shows that small raindrops oscillate as a resonant response to eddy shedding. The postulated oscillation modes are consistent with scatter and means found in the laboratory data and would produce the trends in axis ratios inferred for small raindrops from field studies (Goddard and Cherry; Chandrasekar et al.). Since the discovered secondary resonance does not require a good frequency match, eddy shedding also may be the cause of raindrop oscillations detected in the field studies for much larger sizes.

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Kenneth V. Beard, Rodney J. Kubesh, and Harry T. Ochs III


The resonant interactions between eddy shedding and drop oscillations postulated by Gunn for millimeter diameter raindrops were investigated in a series of laboratory measurements of axis ratio and fall behavior for water drops of d = 0.70–1.54 mm. Drops were produced at terminal velocity using a orifice-jet drop generator and allowed to fall several meters so that the initial oscillations (produced during jet break up) decayed to a negligible amplitude before the drop shape was recorded using stroboscopic photographs. The measured axis ratios had equilibrium values for the smallest sizes (d = 0.70–0.96 mm), but scattered somewhat above equilibrium at intermediate sizes (d = 1.04–1.29 mm). An order of magnitude larger scatter in axis ratio, extending above and below equilibrium, was found for the largest sizes (d = 1.40–1.54 mm) with the average axis ratio being larger than equilibrium. All but the smallest drops displayed a sideways drift varying from 1% of the terminal velocity for d = 0.82–1.04 mm, increasing sharply to 6% for d = 1.11 mm and gradually lowering to 2% at d = 1.54 mm.

The observed axis-ratio scatter and fall behavior were generally consistent with simple oscillation and drift responses to various size-dependent wake configurations as determined from observations of freely falling bodies in liquid tank experiments. The small axis-ratio scatter above equilibrium and maximum in drift corresponds to the onset of an asymmetric pulsating wake found by Magarvey and Bishop, whereas the large axis-ratio scatter above and below equilibrium is an apparent axisymmetric oscillation.

Since the measured variance in axis ratio is largest for the largest size investigated, resonant oscillations extend far beyond the 1 mm size postulated by Gunn. Although such a broad coupling between eddy shedding and drop oscillations is not well understood, its existence would explain the shift in axis ratio found for small raindrop sizes using an aircraft optical-array probe by Chandrasekar, Cooper and Bringi, and postulated from distrometer-ZDR observations by Goddard and Cherry.

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Brian Billings, Stephen A. Cohn, Rodney J. Kubesh, and William O. J. Brown


The best way to train the next wave of observational talent is through direct experience. In 2012 and again in 2014, students at St. Cloud State University (SCSU) welcomed deployments of professional atmospheric research equipment, allowing them to support and execute field projects. The Boundary Structure Experiments with Central Minnesota Profiling (BaSE CaMP) projects brought the Mobile Integrated Sounding System (MISS) from the National Center for Atmospheric Research’s (NCAR) Earth Observing Laboratory (EOL) to SCSU for a National Science Foundation–funded educational deployment. Its diverse instrumentation and ability to travel to interesting weather events and locations makes MISS extremely valuable for teaching students about both weather experiments and measurement strategies. In addition to the university project, outreach activities with MISS took place at high schools, regional conferences, and public events. MISS carries four instruments: a boundary layer wind profiler, a radio acoustic sounding system (RASS), radiosondes, and an instrumented 10-m tower. The type and time of MISS deployments were quite varied so students could participate around their class schedule, jobs, and other commitments. Each year the project had periods of fixed operations and mobile activity, where MISS was relocated to best observe current weather conditions. BaSE CaMP operations and results were incorporated into many classes in the meteorology program at SCSU. The original course request was for Radar and Satellite Meteorology, but other activities contributed to Atmospheric Dynamics, Physical Meteorology, and Meteorological Analysis Software courses.

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