Radar Differential Reflectivity Research Articles

Inferring the Properties of Snow in Southern Ocean Shallow Convection and Frontal Systems Using Dual-Polarization C-Band Radar

Abstract We use dual-polarization C-band data collected in the Southern Ocean to examine the properties of snow observed during a voyage in the austral summer of 2018. Using existing forward modeling formalisms based on an assumption of Rayleigh scattering by soft spheroids, an optimal estimation algorithm is implemented to infer snow properties from horizontally polarized radar reflectivity, the differential radar reflectivity, and the specific differential phase. From the dual-polarization observables, we estimate ice water content qi, the mass-mean particle size Dm, and the exponent of the mass–dimensional relationship bm that, with several assumptions, allow for evaluation of snow bulk density, and snow number concentration. Upon evaluating the uncertainties associated with measurement and forward model errors, we determine that the algorithm can retrieve qi, Dm, and bm within single-pixel uncertainties conservatively estimated in the range 120%, 60%, and 40%, respectively. Applying the algorithm to open-cellular convection in the Southern Ocean, we find evidence for secondary ice formation processes within multicellular complexes. In stratiform precipitation systems we find snow properties and infer processes that are distinctly different from the shallow convective systems with evidence for riming and aggregation being common. We also find that embedded convection within the frontal system produces precipitation properties consistent with graupel. Examining 5 weeks of data, we show that snow in open-cellular cumulus has higher overall bulk density than snow in stratiform precipitation systems with implications for interpreting measurements from space-based active remote sensors.

Structure and evolution of organized precipitation bands: C-band doppler weather radar observations over Thumba (8.5o N, 77o E)

In the present communication, C-band polarimetric Doppler Weather Radar (C-DWR) observations are used to investigate the structure and evolution of organized precipitation bands during the Indian summer monsoon (ISM) of 2017, 2018 and 2019 over Thumba (8.50 N, 770 E), a coastal location in south India. The C-DWR observations show organized high radar reflectivity structures of ∼50 dBZ that organized into narrow bands of ∼200 km (North-South) in length and 10–20 km (East-West) in width with vertical extent of ∼6–8 km. These narrow bands of precipitation structures are observed to be formed over the Arabian Sea and subsequently propagating towards the radar site. The observed differential radar reflectivity (Zdr) and the correlation co-efficient values indicate the horizontally oriented droplets and non-uniform hydrometeors. The potential formation mechanisms of the precipitation bands are investigated using reanalysis winds. The results show that blocking effect induced by the Western Ghats around the radar site play a key role in the formation of organized precipitation bands. It is noted that the location of the blocking effect induced convergence (conducive for the formation of precipitating clouds) with respect to the orography is proportional to the intensity of the low-level winds. The significance of the present study lies in understating the structure and evolution of organized precipitation bands and discussing their potential formation mechanisms during the ISM using C-DWR observations.

Storm-Scale Polarimetric Radar Signatures Associated with Tornado Dissipation in Supercells

Abstract Polarimetric radar data from the WSR-88D network are used to examine the evolution of various polarimetric precursor signatures to tornado dissipation within a sample of 36 supercell storms. These signatures include an increase in bulk hook echo median raindrop size, a decrease in midlevel differential radar reflectivity factor (ZDR) column area, a decrease in the magnitude of the ZDR arc, an increase in the area of low-level large hail, and a decrease in the orientation angle of the vector separating low-level ZDR and specific differential phase (KDP) maxima. Only supercells that produced “long-duration” tornadoes (with at least four consecutive volumes of WSR-88D data) are investigated, so that signatures can be sufficiently tracked in time, and novel algorithms are used to isolate each storm-scale process. During the time leading up to tornado dissipation, we find that hook echo median drop size (D0) and median ZDR remain relatively constant, but hook echo median KDP and estimated number concentration (NT) increase. The ZDR arc maximum magnitude and ZDR–KDP separation orientation angles are observed to decrease in most dissipation cases. Neither the area of large hail nor the ZDR column area exhibit strong signals leading up to tornado dissipation. Finally, combinations of storm-scale behaviors and TVS behaviors occur most frequently just prior to tornado dissipation, but also are common 15–20 min prior to dissipation. The results from this study provide evidence that nowcasting tornado dissipation using dual-polarization radar may be possible when combined with TVS monitoring, subject to important caveats.

Tornado Formation and Intensity Prediction Using Polarimetric Radar Estimates of Updraft Area

AbstractA sample of 198 supercells are investigated to determine if a radar proxy for the area of the storm midlevel updraft may be a skillful predictor of imminent tornado formation and/or peak tornado intensity. A novel algorithm, a modified version of the Thunderstorm Risk Estimation from Nowcasting Development via Size Sorting (TRENDSS) algorithm is used to estimate the area of the enhanced differential radar reflectivity factor (ZDR) column in Weather Surveillance Radar – 1988 Doppler data; the ZDR column area is used as a proxy for the area of the midlevel updraft. The areas of ZDR columns are compared for 154 tornadic supercells and 44 non-tornadic supercells, including 30+ supercells with tornadoes rated EF1, EF2, and EF3; nine supercells with EF4+ tornadoes also are analyzed. It is found that (i) at the time of their peak 0-1 km azimuthal shear, non-tornadic supercells have consistently small (< 20 km2) ZDR column areas while tornadic cases exhibit much greater variability in areas, and (ii) at the time of tornadogenesis, EF3+ tornadic cases have larger ZDR column areas than tornadic cases rated EF1/2. In addition, all nine violent tornadoes sampled have ZDR column areas > 30 km2 at the time of tornadogenesis. However, only weak positive correlation is found between ZDR column area and both radar-estimated peak tornado intensity and maximum tornado path width. Planned future work focused on mechanisms linking updraft size and tornado formation and intensity is summarized and the use of the modified TRENDSS algorithm, which is immune to ZDR bias and thus ideal for real-time operational use, is emphasized.

Observed Bulk Hook Echo Drop Size Distribution Evolution in Supercell Tornadogenesis and Tornadogenesis Failure

AbstractThe time preceding supercell tornadogenesis and tornadogenesis “failure” has been studied extensively to identify differing attributes related to tornado production or lack thereof. Studies from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) found that air in the rear-flank downdraft (RFD) regions of non- and weakly tornadic supercells had different near-surface thermodynamic characteristics than that in strongly tornadic supercells. Subsequently, it was proposed that microphysical processes are likely to have an impact on the resulting thermodynamics of the near-surface RFD region. One way to view proxies to microphysical features, namely drop size distributions (DSDs), is through use of polarimetric radar data. Studies from the second VORTEX used data from dual-polarization radars to provide evidence of different DSDs in the hook echoes of tornadic and non-tornadic supercells. However, radar-based studies during these projects were limited to a small number of cases preventing result generalizations. This study compiles 68 tornadic and 62 non-tornadic supercells using Weather Surveillance Radar–1988 Doppler (WSR-88D) data to analyze changes in polarimetric radar variables leading up to, and at, tornadogenesis and tornadogenesis failure. Case types generally did not show notable hook echo differences in variables between sets, but did show spatial hook echo quadrant DSD differences. Consistent with past studies, differential radar reflectivity factor (ZDR) generally decreased leading up to tornadogenesis and tornadogenesis failure; in both sets, estimated total number concentration increased during the same times. Relationships between DSDs and the near-storm environment, and implications of results for nowcasting tornadogenesis, also are discussed.

Distinguishing Characteristics of Tornadic and Nontornadic Supercell Storms from Composite Mean Analyses of Radar Observations

AbstractAn improved understanding of common differences between tornadic and nontornadic supercells is sought using a large set of observations from the operational NEXRAD WSR-88D polarimetric radar network in the contiguous United States. In particular, data from 478 nontornadic and 294 tornadic supercells during a 7-yr period (2011–17) are used to produce probability-matched composite means of microphysical and kinematic variables. Means, which are centered on echo-top maxima and in a horizontal coordinate system rotated such that storm motion points in the positive x dimension, are created in altitude relative to ground level at times of peak echo-top altitude and peak midlevel rotation for nontornadic supercells and times at and prior to the first tornado in tornadic supercells. Robust differences between supercell types are found, with consistent characteristics at and preceding tornadogenesis in tornadic storms. In particular, the mesocyclone is found to be vertically aligned in tornadic supercells and misaligned in nontornadic supercells. Microphysical differences found include a low-level radar reflectivity hook echo aligned with and ~10 km right of storm center in tornadic supercells and displaced 5–10 km down-motion in nontornadic supercells, a low-to-midlevel differential radar reflectivity dipole that is oriented more parallel to storm motion in tornadic supercells and more perpendicular in nontornadic supercells, and a separation between enhanced differential radar reflectivity and specific differential phase (with unique displacement-relative correlation coefficient reductions) at low levels that is more perpendicular to storm motion in tornadic supercells and more parallel in nontornadic supercells.

Mobile Radar Observations of the Evolving Debris Field Compared with a Damage Survey of the Shawnee, Oklahoma, Tornado of 19 May 2013

AbstractA detailed damage survey is combined with high-resolution mobile, rapid-scanning X-band polarimetric radar data collected on the Shawnee, Oklahoma, tornado of 19 May 2013. The focus of this study is the radar data collected during a period when the tornado was producing damage rated EF3. Vertical profiles of mobile radar data, centered on the tornado, revealed that the radar reflectivity was approximately uniform with height and increased in magnitude as more debris was lofted. There was a large decrease in both the cross-correlation coefficient (ρhv) and differential radar reflectivity (ZDR) immediately after the tornado exited the damaged area rated EF3. Low ρhv and ZDR occurred near the surface where debris loading was the greatest. The 10th percentile of ρhv decreased markedly after large amounts of debris were lofted after the tornado leveled a number of structures. Subsequently, ρhv quickly recovered to higher values. This recovery suggests that the largest debris had been centrifuged or fallen out whereas light debris remained or continued to be lofted. Range–height profiles of the dual-Doppler analyses that were azimuthally averaged around the tornado revealed a zone of maximum radial convergence at a smaller radius relative to the leading edge of lofted debris. Low-level inflow into the tornado encountering a positive bias in the tornado-relative radial velocities could explain the existence of the zone. The vertical structure of the convergence zone was shown for the first time.

Use of polarimetric radar measurements to constrain simulated convective cell evolution: a pilot study with Lagrangian tracking

Abstract. To probe the potential value of a radar-driven field campaign to constrain simulation of isolated convection subject to a strong aerosol perturbation, convective cells observed by the operational KHGX weather radar in the vicinity of Houston, Texas, are examined individually and statistically. Cells observed in a single case study of onshore flow conditions during July 2013 are first examined and compared with cells in a regional model simulation. Observed and simulated cells are objectively identified and tracked from observed or calculated positive specific differential phase (KDP) above the melting level, which is related to the presence of supercooled liquid water. Several observed and simulated cells are subjectively selected for further examination. Below the melting level, we compare sequential cross sections of retrieved and simulated raindrop size distribution parameters. Above the melting level, we examine time series of KDP and radar differential reflectivity (ZDR) statistics from observations and calculated from simulated supercooled rain properties, alongside simulated vertical wind and supercooled rain mixing ratio statistics. Results indicate that the operational weather radar measurements offer multiple constraints on the properties of simulated convective cells, with substantial value added from derived KDP and retrieved rain properties. The value of collocated three-dimensional lightning mapping array measurements, which are relatively rare in the continental US, supports the choice of Houston as a suitable location for future field studies to improve the simulation and understanding of convective updraft physics. However, rapid evolution of cells between routine volume scans motivates consideration of adaptive scan strategies or radar imaging technologies to amend operational weather radar capabilities. A 3-year climatology of isolated cell tracks, prepared using a more efficient algorithm, yields additional relevant information. Isolated cells are found within the KHGX domain on roughly 40 % of days year-round, with greatest concentration in the northwest quadrant, but roughly 5-fold more cells occur during June through September. During this enhanced occurrence period, the cells initiate following a strong diurnal cycle that peaks in the early afternoon, typically follow a south-to-north flow, and dissipate within 1 h, consistent with the case study examples. Statistics indicate that ∼ 150 isolated cells initiate and dissipate within 70 km of the KHGX radar during the enhanced occurrence period annually, and roughly 10 times as many within 200 km, suitable for multi-instrument Lagrangian observation strategies. In addition to ancillary meteorological and aerosol measurements, robust vertical wind speed retrievals would add substantial value to a radar-driven field campaign.

Точностные характеристики оценки модифицированной дифференциальной радиолокационной отражаемости при дистанционном зондировании неоднородного метеообразования

Точностные характеристики оценки модифицированной дифференциальной радиолокационной отражаемости при дистанционном зондировании неоднородного метеообразования

Observations of Convective Thermals with Weather Radar

AbstractIt is shown that the NEXRAD weather radar with enhanced detectability is capable of observing the evolution of convective thermals. The fields of radar differential reflectivity show that the upper parts of the thermals are observable due to Bragg scatter, whereas scattering from insects dominates in the lower parts. The thermal-top rise rate is between 1.5 and 3.7 m s−1 in the analyzed case. Radar observations of thermals also enable estimations of their maximum heights, horizontal sizes, and the turbulent dissipation rate within each thermal. These attributes characterize the intensity of convection.

Parameters of Cloud Ice Particles Retrieved from Radar Data

AbstractThe mean axis ratio (length/width) and the degree of orientation of cloud ice particles are retrieved from radar differential reflectivity (ZDR) and the copolar correlation coefficient (ρhv) measured with the S-band WSR-88D radar. Hardware differential phases and amplifications in the polarimetric channels affect measured ZDR and ρhv and are taken into consideration in the retrieval procedure. The retrieval is performed for particles in shapes of hexagonal prisms, which are closer to shapes of real cloud particles than frequently used spheroids. The median retrieved axis ratio for prisms is larger than that for spheroids. The statistical 1σ retrieval errors caused by fluctuations of radar returns are about 40% in areas of signal-to-noise ratios stronger than 10 dB. The values of the degree of orientation lie in an interval from 2° to 23°, which points to significant perturbations in the orientations of ice particles most likely caused by the wind field.

Storm Labeling in Three Dimensions (SL3D): A Volumetric Radar Echo and Dual-Polarization Updraft Classification Algorithm

This study presents a new storm classification method for objectively stratifying three-dimensional radar echo into five categories: convection, convective updraft, precipitating stratiform, nonprecipitating stratiform, and ice-only anvil. The Storm Labeling in Three Dimensions (SL3D) algorithm utilizes volumetric radar data to classify radar echo based on storm height, depth, and intensity in order to provide a new method for updraft classification and improve upon the limitations of traditional storm classification algorithms. Convective updrafts are identified by searching for three known polarimetric radar signatures: weak-echo regions (bounded and unbounded) in the radar reflectivity factor at horizontal polarization ([Formula: see text]), differential radar reflectivity ([Formula: see text]) columns, and specific differential phase ([Formula: see text]) columns. Additionally, leveraging the three-dimensional information allows SL3D to improve upon missed identifications of weak convection and intense stratiform rain in traditional two-dimensional classification schemes. This study presents the results of applying the SL3D algorithm to several cases of high-resolution three-dimensional composites of NEXRAD WSR-88D data in the contiguous United States. Comparisons with a traditional algorithm that uses two-dimensional maps of [Formula: see text] are also shown to illustrate the differences of the SL3D algorithm.

Climatological characteristics of raindrop size distributions in Busan, Republic of Korea

Abstract. Raindrop size distribution (DSD) characteristics within the complex area of Busan, Republic of Korea (35.12° N, 129.10° E), were studied using a Precipitation Occurrence Sensor System (POSS) disdrometer over a 4-year period from 24 February 2001 to 24 December 2004. Also, to find the dominant characteristics of polarized radar parameters, which are differential radar reflectivity (Zdr), specific differential phase (Kdp) and specific attenuation (Ah), T-matrix scattering simulation was applied in the present study. To analyze the climatological DSD characteristics in more detail, the entire period of recorded rainfall was divided into 10 categories not only covering different temporal and spatial scales, but also different rainfall types. When only convective rainfall was considered, mean values of mass-weighted mean diameter (Dm) and normalized number concentration (Nw) values for all these categories converged around a maritime cluster, except for rainfall associated with typhoons. The convective rainfall of a typhoon showed much smaller Dm and larger Nw compared with the other rainfall categories. In terms of diurnal DSD variability, we analyzed maritime (continental) precipitation during the daytime (DT) (nighttime, NT), which likely results from sea (land) wind identified through wind direction analysis. These features also appeared in the seasonal diurnal distribution. The DT and NT probability density function (PDF) during the summer was similar to the PDF of the entire study period. However, the DT and NT PDF during the winter season displayed an inverse distribution due to seasonal differences in wind direction.

Measurements of Differential Reflectivity in Snowstorms and Warm Season Stratiform Systems

AbstractThe organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.

Bulk Hook Echo Raindrop Sizes Retrieved Using Mobile, Polarimetric Doppler Radar Observations

AbstractPolarimetric radar observations obtained by the NOAA/National Severe Storms Laboratory mobile, X-band, dual-polarization radar (NOXP) are used to investigate “hook echo” precipitation properties in several tornadic and nontornadic supercells. Hook echo drop size distributions (DSDs) were estimated using NOXP data obtained from 2009 to 2012, including during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Differences between tornadic and nontornadic hook echo DSDs are explored, and comparisons are made with previous observations of estimated hook echo DSDs made from stationary S- and C-band Doppler radars. Tornadic hook echoes consistently contain radar gates that are characterized by small raindrops; nontornadic hook echoes are mixed between those that have some small-drop gates and those that have almost no small-drop gates. In addition, the spatial distribution of DSDs was estimated using the high-spatial-resolution data afforded by NOXP. A unique polarimetric signature, an area of relatively low values of differential radar reflectivity factor ZDR south and east of the tornado, is observed in many of the tornadic cases. Also, because most data were obtained using 2-min volumetric updates, the evolution of approximated hook echo precipitation properties was studied during parts of the life cycles of three tornadoes. In one case, there is a large decrease in the percentage of large-raindrop gates and an increase in the percentage of small-raindrop gates in the minutes leading up to tornado formation. The percentage of large-drop gates generally increases prior to and during tornado dissipation. Near-storm environmental data are used to put forth possible relationships between bulk hook echo DSDs and tornado production and life cycle.

Polarimetric Radar Analysis of Raindrop Size Variability in Maritime and Continental Clouds

AbstractDuring the Queensland Cloud Seeding Research Program, the “CP2” polarimetric radar parameter differential radar reflectivity Zdr was used to examine the raindrop size evolution in both maritime and continental clouds. The focus of this paper is to examine the natural variability of the drop size distribution. The primary finding is that there are two basic raindrop size evolutions, one associated with continental air masses characterized by relatively high aerosol concentrations and long air trajectories over land and the other associated with maritime air masses with lower aerosol concentrations. The size evolution difference is during the growth stage of the radar echoes. The differential radar reflectivity in the growing continental clouds is dominated by large raindrops, whereas in the maritime clouds differential reflectivity is dominated by small raindrops and drizzle. The drop size evolution in many of the maritime air masses was very similar to those observed in the maritime air of the Caribbean Sea observed with the NCAR S-band polarimetric radar (S-Pol) during the Rain in Cumulus over the Ocean (RICO) experiment. Because the tops of the Queensland continental clouds ascended almost 2 times as fast as the maritime ones in their growth stage, both dynamical and aerosol factors may be important for the systematic difference in drop size evolution. Recommendations are advanced for future field programs to understand better the causes for the observed variability in drop size evolution. Also, considering the natural variability in drop size evolution, comments are provided on conducting and evaluating cloud seeding experiments.

Experimental and theoretical analyses on the microwave backscattering ability and differential radar reflectivity of non-spherical hydrometeors

This paper experimentally and theoretically examines the scattering properties of simulated non-spherical hydrometeors including water oblates, ice oblates and ice sphere–cone-oblates, in terms of the backscattering cross-section and the differential reflectivity. The experimental measurements of the backscattering cross-sections of non-spherical hydrometeor samples were performed in the Electromagnetic Scattering Laboratory of China National Space Industrial Corporation. Meanwhile, the backscattering cross-sections have been computed with the transition ( T) matrix method. The theoretical results are compared with the experimental data, showing that the calculations are consistent with the observations in general. Experimental and theoretical analyses indicate that the backscattering cross-section of non-spherical particles increases as the particle size parameter increases, and fluctuates when the sizes are larger under the effect of resonance scattering. Differential reflectivity Z DR of water oblates in natural rainfall is always greater than 0 dB whereas Z DR of hailstones may be negative. There is a good linear relationship between differential reflectivity and aspect ratio of a particle. These derivations agree with the literature and can be used to identify the presence of hail particles and distinguish between plate-type and columnar-type hydrometeors. In this study, the measuring experiment and the T-matrix method calculations for the scattering of simulated raindrop and ice particles are also briefly described.

An Analytical Solution for Raindrop Evaporation and Its Application to Radar Rainfall Measurements

Abstract An analytical solution for the evaporation of a single raindrop is derived in this paper. Based on this solution, a parameter D* is defined as the diameter of the raindrop that just evaporates completely after falling through a certain distance in a prescribed environment. The parameter D* is then used for studying the modification of raindrop size distribution by evaporation in a steady, still atmosphere. The results for the Marshall–Palmer distribution are used to discuss errors caused by rain evaporation in radar rainfall measurements. Quantitative estimation of these errors, or as an equivalent, estimation of the rain evaporation along the falling path, using both radar reflectivity Z and radar differential reflectivity ZDR techniques, is studied. The results show that, for the detection of rain evaporation, reflectivity is more sensitive than differential reflectivity, whereas for the estimation of rainfall rate R, an empirical ZDR–Z–R formula is more robust and accurate than a Z–R formula.

Laboratory Measurements of Axis Ratios for Large Raindrops

The oscillations of moderate to large raindrops are investigated using a seven-story fall column with shape data obtained from multiple-strobe photographs. Measurements are made at a fall distance of 25 m for drops of D = 2.5-, 2.9-, 3.6-, and 4.0-mm diameter, with additional measurements at intermediate distances to assess the role of aerodynamic feedback as the source of drop oscillations. Oscillations, initiated by the drop generator, are found to decay during the first few meters of fall and then increase to where the drops attained terminal speed near 10 m. Throughout the lower half of the fall column, the oscillation amplitudes are essentially constant. These apparently steady-state oscillations are attributed to resonance with vortex shedding. For D = 2.5 and 3.6 mm, the mean axis ratio is near the theoretical equilibrium value, a result consistent with axisymmetric (oblate/prolate mode) oscillations at the fundamental frequency. For D = 2.9 and 4.0 mm, however, the mean axis ratio is larger than the theoretical equilibrium value by 0.01 to 0.03, a characteristic of transverse mode oscillations. Comparison with previous axis ratio and vortex-shedding measurements suggests that the oscillation modes of raindrops are sensitive to initial conditions, but because of the prevalence of off-center drop collisions, the predominant steady-state response in rain is expected to be transverse mode oscillations. A simple formula is obtained from laboratory and field measurements to account for the generally higher average axis ratio of raindrops having transverse mode oscillations. In the application to light to heavy rainfall, the ensemble mean axis ratios for raindrop sizes of D = 1.5–4.0 mm are shifted above equilibrium values by 0.01–0.04, as a result of steady-state transverse mode oscillations maintained intrinsically by vortex shedding. Compared to the previous axis ratio formula based on wind tunnel measurements, the increased axis ratios for oscillating raindrops amount to a reduction of 0.1–0.4 dB in radar differential reflectivity ZDR, and an increase of about 0.5 mm for a reflectivity-weighted mean drop size of less than about 3 mm.

Differential Doppler Velocity: A Radar Parameter for Characterizing Hydrometeor Size Distributions

Abstract Observations of Doppler-resolved spectra of differential radar reflectivity provide estimates of particle shapes as a function of their terminal velocity, and they can be derived by having the antenna at a significant elevation angle. Turbulence tends to smear out the details of the actual spectra observed, but the difference in the mean values of velocity using horizontal and vertical polarizations, which the authors call the “differential Doppler velocity” (DDV), is unaffected. Larger raindrops fall faster and are oblate, so values of DDV are positive. If a gamma function is used for the raindrop size spectrum, then the observed DDV and ZDR correspond to particular values of median drop diameter D0 and the dispersion index m. The scaling parameter N0 is derived from Z. Estimates of m have a mean value of 5 but vary substantially. An error in rainfall rate of up to ±15% results if the rainfall rate is computed from Z and ZDR alone, and m is assumed constant at 5. An overestimation of more than 30% occurs if m is assumed to be 0. DDV values in stratiform ice are slightly negative. The values in ice are explicable in terms of a mixture of slowly falling oblate crystals and faster-falling spherical aggregates. In the bright band, DDV is consistent with the coexistence of oblate snowflakes and faster-falling raindrops.