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1. Wind and Turbulence Effects on Raindrop Fall Speed

   https://doi.org/10.1175/JAS-D-22-0137.1  

   Testik, Firat Y. ; Bolek, Abdullah ( April 2023 , Journal of the Atmospheric Sciences)

   Abstract Wind and turbulence effects on raindrop fall speeds were elucidated using field observations over a 2-yr time period. Motivations for this study include the recent observations of raindrop fall speed deviations from the terminal fall speed predictions ( V t ) based upon laboratory studies and the utilizations of these predictions in various important meteorological and hydrological applications. Fall speed ( V f ) and other characteristics of raindrops were observed using a high-speed optical disdrometer (HOD), and various rainfall and wind characteristics were observed using a 3D ultrasonic anemometer, a laser-type disdrometer, and rain gauges. A total of 26 951 raindrops were observed during 17 different rainfall events, and of these observed raindrops, 18.5% had a subterminal fall speed (i.e., 0.85 V t ≥ V f ) and 9.5% had a superterminal fall speed (i.e., 1.15 V t ≤ V f ). Our observations showed that distributions of sub- and superterminal raindrops in the raindrop size spectrum are distinct, and different physical processes are responsible for the occurrence of each. Vertical wind speed, wind shear, and turbulence were identified as the important factors, the latter two being the dominant ones, for the observed fall speed deviations. Turbulence and wind shear had competing effects on raindrop fall. Raindrops of different sizes showed different responses to turbulence, indicating multiscale interactions between raindrop fall and turbulence. With increasing turbulence levels, while the raindrops in the smaller end of the size spectrum showed fall speed enhancements, those in the larger end of the size spectrum showed fall speed reductions. The effect of wind shear was to enhance the raindrop fall speed toward a superterminal fall. 

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2. Shapes and Fall Speeds of Freezing and Frozen Raindrops

   https://doi.org/10.1175/JHM-D-19-0204.1  

   Rahman, Kalimur ; Testik, Firat Y. ( June 2020 , Journal of Hydrometeorology)

   null (Ed.)

   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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3. Hydrometeor Size Sorting in the Asymmetric Eyewall of Hurricane Matthew (2016)

   https://doi.org/10.1029/2020JD032671  

   Laurencin, Chelsey N. ; Didlake, Jr., Anthony C. ; Loeffler, Scott D. ; Kumjian, Matthew R. ; Heymsfield, Gerald M. ( September 2020 , Journal of Geophysical Research: Atmospheres)

   Polarimetric radar observations of Hurricane Matthew's asymmetric eyewall were captured by WSR‐88D radars from 1500 UTC on 7 October 2016 to 0000 UTC on 8 October 2016. Raindrop size sorting was observed within the eyewall, marked by a differential reflectivity (Z_DR) enhancement region situated upwind of a specific differential phase (K_DP) enhancement region, both overlapping the maximum reflectivity. This signature indicated that the largest raindrops fell out of the eyewall updrafts faster than the smaller, abundant drops that were advected further downstream by the primary circulation. Airborne Doppler radar observations revealed an updraft structure in an azimuthal location consistent with the size‐sorting signature and previous observational studies of eyewall kinematic asymmetries.

   Given that a tropical cyclone's environment or internal dynamics can modulate the eyewall's kinematic and microphysical structure, we used a simple size‐sorting model that only includes sedimentation and advection of raindrops by the axisymmetric tangential wind to examine how an eyewall size‐sorting signature responds to artificial changes in the tangential wind speed and initial raindrop size distributions (DSDs). The axisymmetric tangential wind was retrieved from WSR‐88D radar observations using the Ground‐Based Velocity Track Display technique. The simple model was capable of producing an eyewall size‐sorting signature with an azimuthal separation between the simulated Z_DRand K_DPenhancements in general agreement with the observed separation (~20°) at low levels. Sensitivity tests showed that the azimuthal separation between the Z_DRand K_DPenhancements responded to changes in the tangential wind speed, but not to changes in the initial DSDs aloft.

    

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4. Microphysical Characteristics of an Asymmetric Eyewall in Major Hurricane Harvey (2017)

   https://doi.org/10.1029/2018GL080770  

   Feng, Ya‐Chien ; Bell, Michael M. ( January 2019 , Geophysical Research Letters)

   Microphysical and kinematic structures of major Hurricane Harvey's (2017) asymmetric eyewall are analyzed from ground‐based polarimetric and airborne Doppler radars. New polarimetric observations of differential reflectivity (Z_DR) and specific differential phase (K_DP) show asymmetric wavenumber‐1 patterns associated with vertical wind shear (VWS) but were shifted azimuthally with respect to the reflectivity (Z_H) asymmetry. AZ_DRcolumn was found upwind of theZ_Hmaximum in a region with strong updrafts estimated from multi‐Doppler synthesis, with higher values ofK_DPfound cyclonically downwind. Retrieved raindrop size distributions show that azimuthal variations of size and number concentration were determined by both the VWS and the size sorting process. The diameter of raindrops decreases, while the number concentration increases cyclonically downwind of VWS‐induced updrafts due to the differential terminal fall speed of raindrops and strong rotational flow at major hurricane wind speeds.

    

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5. Sub-cloud rain evaporation in the North Atlantic winter trade winds derived by pairing isotopic data with a bin-resolved microphysical model

   https://doi.org/10.5194/acp-23-12671-2023  

   Sarkar, Mampi ; Bailey, Adriana ; Blossey, Peter ; de Szoeke, Simon P. ; Noone, David ; Quiñones Meléndez, Estefanía ; Leandro, Mason D. ; Chuang, Patrick Y. ( January 2023 , Atmospheric Chemistry and Physics)

   Sub-cloud rain evaporation in the trade wind region significantly influences the boundary layer mass and energy budgets. Parameterizing it is, however, difficult due to the sparsity of well-resolved rain observations and the challenges of sampling short-lived marine cumulus clouds. In this study, sub-cloud rain evaporation is analyzed using a steady-state, one-dimensional model that simulates changes in drop sizes, relative humidity, and rain isotopic composition. The model is initialized with relative humidity, raindrop size distributions, and water vapor isotope ratios (e.g., δDv, δ18Ov) sampled by the NOAA P3 aircraft during the Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC), which was part of the larger EUREC4A (ElUcidating the RolE of Clouds–Circulation Coupling in ClimAte) field program. The modeled surface precipitation isotope ratios closely match the observations from EUREC4A ground-based and ship-based platforms, lending credibility to our model. The model suggests that 63 % of the rain mass evaporates in the sub-cloud layer across 22 P3 cases. The vertical distribution of the evaporated rain flux is top heavy for a narrow (σ) raindrop size distribution (RSD) centered over a small geometric mean diameter (Dg) at the cloud base. A top-heavy profile has a higher rain-evaporated fraction (REF) and larger changes in the rain deuterium excess (d=δD-8×δ18O) between the cloud base and the surface than a bottom-heavy profile, which results from a wider RSD with larger Dg. The modeled REF and change in d are also more strongly influenced by cloud base Dg and σ rather than the concentration of raindrops. The model results are accurate as long as the variations in the relative humidity conditions are accounted for. Relative humidity alone, however, is a poor indicator of sub-cloud rain evaporation. Overall, our analysis indicates the intricate dependence of sub-cloud rain evaporation on both thermodynamic and microphysical processes in the trade wind region. 

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