Toward Completing the Raindrop Size Spectrum: Case Studies Involving 2D-Video Disdrometer, Droplet Spectrometer, and Polarimetric Radar Measurements

AbstractAnalysis of drop size distributions (DSD) measured by collocated Meteorological Particle Spectrometer (MPS) and a third-generation, low-profile, 2D-video disdrometer (2DVD) are presented. Two events from two different regions (Greeley, Colorado, and Huntsville, Alabama) are analyzed. While the MPS, with its 50-μm resolution, enabled measurements of small drops, typically for drop diameters below about 1.1 mm, the 2DVD provided accurate measurements for drop diameters above 0.7 mm. Drop concentrations in the 0.7–1.1-mm overlap region were found to be in excellent agreement between the two instruments. Examination of the combined spectra clearly reveals a drizzle mode and a precipitation mode. The combined spectra were analyzed in terms of the DSD parameters, namely, the normalized intercept parameter NW, the mass-weighted mean diameter Dm, and the standard deviation of mass spectrum σM. The inclusion of small drops significantly affected the NW and the ratio σM/Dm toward higher values relative to using the 2DVD-based spectra alone. For each of the two events, polarimetric radar data were used to characterize the variation of radar-measured reflectivity Zh and differential reflectivity Zdr with Dm from the combined spectra. In the Greeley event, this variation at S band was well captured for small values of Dm (<0.5 mm) where measured Zdr tended to 0 dB but Zh showed a noticeable decrease with decreasing Dm. For the Huntsville event, an overpass of the Global Precipitation Measurement mission Core Observatory satellite enabled comparison of satellite-based dual-frequency radar retrievals of Dm with ground-based DSD measurements. Small differences were found between the satellite-based radar retrievals and disdrometers.

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Comparison of Raindrop Size Distribution between NASA’s S-Band Polarimetric Radar and Two-Dimensional Video Disdrometers

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-18-0339.1 ◽

2020 ◽

Vol 59 (3) ◽

pp. 517-533 ◽

Cited By ~ 1

Author(s):

Ali Tokay ◽

Leo Pio D’Adderio ◽

David A. Marks ◽

Jason L. Pippitt ◽

David B. Wolff ◽

...

Keyword(s):

Size Distribution ◽

Drop Diameter ◽

Two Dimensional ◽

Polarimetric Radar ◽

Raindrop Size Distribution ◽

Precipitation Measurement ◽

Ground Validation ◽

Raindrop Size ◽

Differential Reflectivity ◽

Global Precipitation

AbstractThe ground-based-radar-derived raindrop size distribution (DSD) parameters—mass-weighted drop diameter Dmass and normalized intercept parameter NW—are the sole resource for direct validation of the National Aeronautics and Space Administration (NASA) Global Precipitation Measurement (GPM) mission Core Observatory satellite-based retrieved DSD. Both Dmass and NW are obtained from radar-measured reflectivity ZH and differential reflectivity ZDR through empirical relationships. This study uses existing relationships that were determined for the GPM ground validation (GV) program and directly compares the NASA S-band polarimetric radar (NPOL) observables of ZH and ZDR and derived Dmass and NW with those calculated by two-dimensional video disdrometer (2DVD). The joint NPOL and 2DVD datasets were acquired during three GPM GV field campaigns conducted in eastern Iowa, southern Appalachia, and western Washington State. The comparative study quantifies the level of agreement for ZH, ZDR, Dmass, and log(NW) at an optimum distance (15–40 km) from the radar as well as at distances greater than 60 km from radar and over mountainous terrain. Interestingly, roughly 10%–15% of the NPOL ZH–ZDR pairs were well outside the envelope of 2DVD-estimated ZH–ZDR pairs. The exclusion of these pairs improved the comparisons noticeably.

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Validation of GPM Precipitation Products by Comparison with Ground-Based Parsivel Disdrometers over Jianghuai Region

Water ◽

10.3390/w11061260 ◽

2019 ◽

Vol 11 (6) ◽

pp. 1260 ◽

Cited By ~ 3

Author(s):

Zuhang Wu ◽

Yun Zhang ◽

Lifeng Zhang ◽

Xiaolong Hao ◽

Hengchi Lei ◽

...

Keyword(s):

Shape Parameter ◽

Rain Rate ◽

Dual Frequency ◽

Raindrop Size Distribution ◽

Precipitation Measurement ◽

Retrieval Algorithms ◽

Raindrop Size ◽

Global Precipitation Measurement ◽

Global Precipitation ◽

Rain Retrieval

In this study, we evaluated the performance of rain-retrieval algorithms for the Version 6 Global Precipitation Measurement Dual-frequency Precipitation Radar (GPM DPR) products, against disdrometer observations and improved their retrieval algorithms by using a revised shape parameter µ derived from long-term Particle Size Velocity (Parsivel) disdrometer observations in Jianghuai region from 2014 to 2018. To obtain the optimized shape parameter, raindrop size distribution (DSD) characteristics of summer and winter seasons over Jianghuai region are analyzed, in terms of six rain rate classes and two rain categories (convective and stratiform). The results suggest that the GPM DPR may have better performance for winter rain than summer rain over Jianghuai region with biases of 40% (80%) in winter (summer). The retrieval errors of rain category-based µ (3–5%) were proved to be the smallest in comparison with rain rate-based µ (11–13%) or a constant µ (20–22%) in rain-retrieval algorithms, with a possible application to rainfall estimations over Jianghuai region. Empirical Dm–Ze and Nw–Dm relationships were also derived preliminarily to improve the GPM rainfall estimates over Jianghuai region.

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A Variational Approach for Retrieving Raindrop Size Distribution from Polarimetric Radar Measurements in the Presence of Attenuation

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-12-0101.1 ◽

2013 ◽

Vol 52 (1) ◽

pp. 169-185 ◽

Cited By ~ 12

Author(s):

Qing Cao ◽

Guifu Zhang ◽

Ming Xue

Keyword(s):

Variational Approach ◽

Simulated Data ◽

Real Data ◽

Radar Data ◽

Polarimetric Radar ◽

University Of Oklahoma ◽

Raindrop Size Distribution ◽

Radar Measurements ◽

Raindrop Size ◽

Differential Reflectivity

AbstractThis study presents a two-dimensional variational approach to retrieving raindrop size distributions (DSDs) from polarimetric radar data in the presence of attenuation. A two-parameter DSD model, the constrained-gamma model, is used to represent rain DSDs. Three polarimetric radar measurements—reflectivity ZH, differential reflectivity ZDR, and specific differential phase KDP—are optimally used to correct for the attenuation and retrieve DSDs by taking into account measurement error effects. Retrieval results with simulated data demonstrate that the proposed algorithm performs well. Applications to real data collected by the X-band Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) radars and the C-band University of Oklahoma–Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME) also demonstrate the efficacy of this approach.

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Uncertainties of GPM DPR Rain Estimates Caused by DSD Parameterizations

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-14-0003.1 ◽

2014 ◽

Vol 53 (11) ◽

pp. 2524-2537 ◽

Cited By ~ 43

Author(s):

Liang Liao ◽

Robert Meneghini ◽

Ali Tokay

Keyword(s):

Dual Wavelength ◽

Evaluation Procedure ◽

Dual Frequency ◽

Raindrop Size Distribution ◽

Precipitation Measurement ◽

Raindrop Size ◽

Comparison Of The Results ◽

Rain Rates ◽

Global Precipitation ◽

The Impact

AbstractA framework based on measured raindrop size distribution (DSD) data has been developed to assess uncertainties in DSD models employed in Ku- and Ka-band dual-wavelength radar retrievals. In this study, the rain rates and attenuation coefficients from DSD parameters derived by dual-wavelength algorithms are compared with those directly obtained from measured DSD spectra. The impact of the DSD gamma parameterizations on rain estimation from the Global Precipitation Measurement mission (GPM) Dual-Frequency Precipitation Radar (DPR) is examined for the cases of a fixed shape factor μ as well as for a constrained μ—that is, a μ–Λ relation (a relationship between the shape parameter and slope parameter Λ of the gamma DSD)—by using 11 Particle Size and Velocity (Parsivel) disdrometer measurements with a total number of about 50 000 one-minute spectra that were collected during the Iowa Flood Studies (IFloodS) experiment. It is found that the DPR-like dual-wavelength techniques provide fairly accurate estimates of rain rate and attenuation if a fixed-μ gamma DSD model is used, with the value of μ ranging from 3 to 6. Comparison of the results reveals that the retrieval errors from the μ–Λ relations are generally small, with biases of less than ±10%, and are comparable to the results from a fixed-μ gamma model with μ equal to 3 and 6. The DSD evaluation procedure is also applied to retrievals in which a lognormal DSD model is used.

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Development of an Operational Convective Nowcasting Algorithm Using Raindrop Size Sorting Information from Polarimetric Radar Data

Weather and Forecasting ◽

10.1175/waf-d-18-0025.1 ◽

2018 ◽

Vol 33 (5) ◽

pp. 1477-1495 ◽

Cited By ~ 2

Author(s):

Darrel M. Kingfield ◽

Joseph C. Picca

Keyword(s):

Model Simulation ◽

Algorithm Design ◽

Radar Data ◽

Polarimetric Radar ◽

Horizontal Polarization ◽

Spatiotemporal Resolution ◽

Raindrop Size Distribution ◽

Size Sorting ◽

Raindrop Size ◽

Differential Reflectivity

Abstract Raindrop size sorting is a ubiquitous microphysical occurrence in precipitating systems. Owing to the greater terminal fall speed of larger particles, a raindrop’s fall trajectory can be sensitive to its size, and strong air currents (e.g., a convective updraft) can enhance this sensitivity. Indeed, observational and numerical model simulation studies have confirmed these effects on raindrop size distributions near convective updrafts. One striking example is the lofting of liquid drops and partially frozen hydrometeors above the environmental 0°C level, resulting in a small columnar region of positive differential reflectivity ZDR in polarimetric radar data, known as the ZDR column. This signature can serve as a proxy for updraft location and strength, offering operational forecasters a tool for monitoring convective trends. Beneath the 0°C level, where WSR-88D spatiotemporal resolution is highest, anomalously high ZDR collocated with lower reflectivity factor at horizontal polarization ZH is often observed within and beneath convective updrafts. Here, size sorting creates a deficit in small drops, while relatively large drops and melting hydrometeors are present in low concentrations. As such, this unique raindrop size distribution and its related polarimetric signature can indicate updraft location sooner and more frequently than the detection of a ZDR column. This paper introduces a novel algorithm that capitalizes on the improved radar coverage at lower levels and automates the detection of this size sorting signature. At the algorithm core, unique ZH–ZDR relationships are created for each radar elevation scan, and positive ZDR outliers (often indicative of size sorting) are identified. Algorithm design, examples, performance, strengths and limitations, and future development are discussed.

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Physical Evaluation of GPM DPR Single- and Dual-Wavelength Algorithms

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-18-0210.1 ◽

2019 ◽

Vol 36 (5) ◽

pp. 883-902 ◽

Cited By ~ 9

Author(s):

Liang Liao ◽

Robert Meneghini

Keyword(s):

Path Integral ◽

Dual Wavelength ◽

Dual Frequency ◽

Error Sources ◽

Algorithm Performance ◽

Physical Evaluation ◽

Precipitation Measurement ◽

Raindrop Size ◽

Global Precipitation Measurement ◽

Global Precipitation

AbstractA physical evaluation of the rain profiling retrieval algorithms for the Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement (GPM) Core Observatory satellite is carried out by applying them to the hydrometeor profiles generated from measured raindrop size distributions (DSD). The DSD-simulated radar profiles are used as input to the algorithms, and their estimates of hydrometeors’ parameters are compared with the same quantities derived directly from the DSD data (or truth). The retrieval accuracy is assessed by the degree to which the estimates agree with the truth. To check the validity and robustness of the retrievals, the profiles are constructed for cases ranging from fully correlated (or uniform) to totally uncorrelated DSDs along the columns. Investigation into the sensitivity of the retrieval results to the model assumptions is made to characterize retrieval uncertainties and identify error sources. Comparisons between the single- and dual-wavelength algorithm performance are carried out with either a single- or dual-wavelength constraint of the path integral or differential path integral attenuation. The results suggest that the DPR dual-wavelength algorithm generally provides accurate range-profiled estimates of rainfall rate and mass-weighted diameter with the dual-wavelength estimates superior in accuracy to those from the single-wavelength retrievals.

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A Field Study of Footprint-Scale Variability of Raindrop Size Distribution

Journal of Hydrometeorology ◽

10.1175/jhm-d-17-0003.1 ◽

2017 ◽

Vol 18 (12) ◽

pp. 3165-3179 ◽

Cited By ~ 15

Author(s):

Ali Tokay ◽

Leo Pio D’Adderio ◽

Federico Porcù ◽

David B. Wolff ◽

Walter A. Petersen

Keyword(s):

Spatial Variability ◽

Size Distribution ◽

Dual Frequency ◽

Weighting Method ◽

Convective Clouds ◽

Raindrop Size Distribution ◽

Precipitation Measurement ◽

Raindrop Size ◽

Global Precipitation Measurement ◽

Global Precipitation

Abstract A network of seven two-dimensional video disdrometers (2DVD), which were operated during the Midlatitude Continental Convective Clouds Experiment (MC3E) in northern Oklahoma, are employed to investigate the spatial variability of raindrop size distribution (DSD) within the footprint of the dual-frequency precipitation radar (DPR) on board the National Aeronautics and Space Administration’s Global Precipitation Measurement (GPM) mission core satellite. One-minute 2DVD DSD observations were interpolated uniformly to 13 points distributed within a nearly circular DPR footprint through an inverse distance weighting method. The presence of deep continental showers was a unique feature of the dataset resulting in a higher mean rain rate R with respect to previous studies. As a measure of spatial variability for the interpolated data, a three-parameter exponential function was applied to paired correlations of three parameters of normalized gamma DSD, R, reflectivity, and attenuation at Ka- and Ku-band frequencies of DPR (Z_Ka, Z_Ku, k_Ka, and k_Ku, respectively). The symmetry of the interpolated sites allowed quantifying the directional differences in correlations at the same distance. The correlation distances d0 of R, k_Ka, and k_Ku were approximately 10 km and were not sensitive to the choice of four rain thresholds used in this study. The d0 of Z_Ku, on the other hand, ranged from 29 to 20 km between different rain thresholds. The coefficient of variation (CV) remained less than 0.5 for most of the samples for a given physical parameter, but a CV of greater than 1.0 was also observed in noticeable samples, especially for the shape parameter and Z_Ku.

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Analysis of Video Disdrometer and Polarimetric Radar Data to Characterize Rain Microphysics in Oklahoma

Journal of Applied Meteorology and Climatology ◽

10.1175/2008jamc1732.1 ◽

2008 ◽

Vol 47 (8) ◽

pp. 2238-2255 ◽

Cited By ~ 132

Author(s):

Qing Cao ◽

Guifu Zhang ◽

Edward Brandes ◽

Terry Schuur ◽

Alexander Ryzhkov ◽

...

Keyword(s):

Leading Edge ◽

Radar Data ◽

Size Distributions ◽

Polarimetric Radar ◽

Sampling Errors ◽

Radar Measurements ◽

Drop Size Distributions ◽

Two Parameters ◽

Differential Reflectivity ◽

Good Agreement

Abstract In this paper, data from three 2-dimensional video disdrometers (2DVDs) and an S-band polarimetric radar are used to characterize rain microphysics in Oklahoma. Sampling errors from the 2DVD measurements are quantified through side-by-side comparisons. In an attempt to minimize the sampling errors, a method of sorting and averaging based on two parameters (SATP) is proposed. The shape–slope (μ–Λ) relation of a constrained gamma (C-G) model is then refined for the retrieval of drop size distributions (DSDs) from polarimetric radar measurements. An adjustable term that is based on observed radar reflectivity and differential reflectivity is introduced to make the C-G DSD model more applicable. Radar retrievals using this improved DSD model are shown to provide good agreement with disdrometer observations and to give reasonable results, including in locations near the leading edge of convection where poorly sampled large drops are often observed.

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