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1. Raindrop fall velocities from an optical array probe and 2-D video disdrometer

   https://doi.org/10.5194/amt-11-1377-2018  

   Bringi, Viswanathan; Thurai, Merhala; Baumgardner, Darrel (January 2018, Atmospheric Measurement Techniques)

   Abstract. We report on fall speed measurements of raindrops in light-to-heavyrain events from two climatically different regimes (Greeley,Colorado, and Huntsville, Alabama) using the high-resolution(50 µm) Meteorological Particle Spectrometer (MPS) anda third-generation (170 µm resolution) 2-D videodisdrometer (2DVD). To mitigate wind effects, especially for thesmall drops, both instruments were installed within a 2∕3-scaleDouble Fence Intercomparison Reference (DFIR) enclosure. Two casesinvolved light-to-moderate wind speeds/gusts while the third casewas a tornadic supercell and several squall lines that passed overthe site with high wind speeds/gusts. As a proxy for turbulentintensity, maximum wind speeds from 10 m height at theinstrumented site recorded every 3 s were differenced withthe 5 min average wind speeds and then squared. The fall speedsvs. size from 0.1 to 2 and >0.7 mm were derived from theMPS and the 2DVD, respectively. Consistency of fall speeds from thetwo instruments in the overlap region (0.7–2 mm) gaveconfidence in the data quality and processing methodologies. Ourresults indicate that under low turbulence, the mean fall speedsagree well with fits to the terminal velocity measured in thelaboratory by Gunn and Kinzer from 100 µm up toprecipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1and 1.5 mm sizes were examined in detail under the sameconditions. The histogram shapes for the 1 and 1.5 mm sizeswere symmetric and in good agreement between the two instrumentswith no evidence of skewness or of sub- or super-terminal fallspeeds. The histograms of the smaller 0.5 and 0.7 mm dropsfrom MPS, while generally symmetric, showed that occasionaloccurrences of sub- and super-terminal fall speeds could not beruled out. In the supercell case, the very strong gusts andinferred high turbulence intensity caused a significant broadeningof the fall speed distributions with negative skewness (for drops of1.3, 2 and 3 mm). The mean fall speeds were also found todecrease nearly linearly with increasing turbulent intensityattaining values about 25–30 % less than the terminalvelocity of Gunn–Kinzer, i.e., sub-terminal fall speeds. 

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2. The evolution of fast turbulent deflagrations to detonations

   https://doi.org/10.1063/5.0144663  

   Hytovick, Rachel; Chambers, Jessica; Chin, Hardeo; Gamezo, Vadim_N; Poludnenko, Alexei; Ahmed, Kareem (April 2023, Physics of Fluids)

   We use advanced experimental techniques to explore turbulence-induced deflagration-to-detonation transition (tDDT) in hydrogen–air mixtures. We analyze the full sequence of turbulent flame evolution from fast deflagration-to-detonation using simultaneous direct measurements of pressure, turbulence, and flame, shock, and flow velocities. We show that fast turbulent flames that accelerate and develop shocks are characterized by turbulent flame speeds that exceed the Chapman–Jouguet deflagration speed in agreement with the tDDT theory and direct numerical simulation (DNS) results. Velocity and pressure evolutions are provided to detail the governing mechanisms that drive turbulent flame acceleration. Turbulent flame speeds and fluctuations are examined to reveal flow field characteristics of the tDDT process. This work contributes to the understanding of fundamental mechanisms responsible for spontaneous initiation of detonations by fast turbulent flames. 

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3. The role of thermal stratification on the co‐spectral properties of momentum transport above an Amazonian forest

   https://doi.org/10.1002/qj.5024  

   Mortarini, Luca; Katul, Gabriel; Mauricio_Cely‐Toro, Ivan; Quaresma_Dias‐Júnior, Cléo; Mammarella, Ivan (October 2025, Quarterly Journal of the Royal Meteorological Society)

   Abstract The influence of thermal stratification on the turbulent kinetic energy balance has been widely studied; however, its influence on the turbulent stress remains less explored in the presence of tall vegetated canopies and less ideal micrometeorological conditions. Here, the impact of thermal stratification on turbulent momentum flux is considered in the roughness sublayer (RSL) and the atmospheric surface layer (ASL) using the Amazon Tall Tower Observatory (ATTO) in Brazil. A scalewise co‐spectral budget (CSB) model is developed using standard closure schemes for the pressure–velocity decorrelation. The CSB revealed that the co‐spectrum between longitudinal () and vertical () velocity fluctuations is impacted by the energy spectrum of the vertical velocity and the much less studied longitudinal heat‐flux co‐spectrum , where are temperature fluctuations and is the longitudinal wavenumber. Under stable, very stable, and dynamic–convective conditions, the scaling exponent in for the inertial subrange (ISR) scales is dominated by instead of . A near scaling in robust to large variations in thermal stratification is found, whereas the Kolmogorov ISR scaling for is not found. The scale‐dependent decorrelation time between and is dominated by in the ISR, but is nearly constant for eddies larger than the vertical velocity integral scale, regardless of stability. Implications of these findings for generalized stability correction functions that are based on the turbulent stress budget instead of the turbulent kinetic energy budget are discussed. 

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4. Raindrop shapes and fall velocities in “turbulent times”

   Thurai, M.; Schönhuber, M.; Lammer, G.; Bringi, V. (June 2019, Advances in science and research)

   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. 10m 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. 

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5. Arctic observations and numerical simulations of surface wind effects on Multi-Angle Snowflake Camera measurements

   https://doi.org/10.5194/amt-14-1127-2021  

   Fitch, Kyle E.; Hang, Chaoxun; Talaei, Ahmad; Garrett, Timothy J. (January 2021, Atmospheric Measurement Techniques)

   null (Ed.)

   Abstract. Ground-based measurements of frozen precipitation are heavily influenced by interactions of surface winds with gauge-shield geometry. The Multi-Angle Snowflake Camera (MASC), which photographs hydrometeors in free-fall from three different angles while simultaneously measuring their fall speed, has been used in the field at multiple midlatitude and polar locations both with and without wind shielding. Here, we present an analysis of Arctic field observations – with and without a Belfort double Alter shield – and compare the results to computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC. MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (≤5ms-1) and the MASC is shielded. MASC-measured fall speeds that do not match KAZR-measured velocities tend to fall below a threshold value that increases approximately linearly with wind speed but is generally <0.5ms-1. For those events with wind speeds ≤1.5ms-1, hydrometeors fall with an orientation angle mode of 12∘ from the horizontal plane, and large, low-density aggregates are as much as 5 times more likely to be observed. Simulations in the absence of a wind shield show a separation of flow at the upstream side of the instrument, with an upward velocity component just above the aperture, which decreases the mean particle fall speed by 55 % (74 %) for a wind speed of 5 m s−1 (10 m s−1). We conclude that accurate MASC observations of the microphysical, orientation, and fall speed characteristics of snow particles require shielding by a double wind fence and restriction of analysis to events where winds are light (≤5ms-1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined. 

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