scholarly journals High-Resolution Polarimetric Radar Observations of Snow-Generating Cells

2014 ◽  
Vol 53 (6) ◽  
pp. 1636-1658 ◽  
Author(s):  
Matthew R. Kumjian ◽  
Steven A. Rutledge ◽  
Roy M. Rasmussen ◽  
Patrick C. Kennedy ◽  
Mike Dixon
Keyword(s):  
High Resolution ◽  
Radar Data ◽  
Radiosonde Data ◽  
Polarimetric Radar ◽  
Front Range ◽  
Ice Crystal ◽  
Horizontal Polarization ◽  
X Band ◽  
Northern Colorado ◽  
Differential Reflectivity

AbstractHigh-resolution X-band polarimetric radar data were collected in 19 snowstorms over northern Colorado in early 2013 as part of the Front Range Orographic Storms (FROST) project. In each case, small, vertically erect convective turrets were observed near the echo top. These “generating cells” are similar to those reported in the literature and are characterized by ~1-km horizontal and vertical dimensions, vertical velocities of 1–2 m s−1, and lifetimes of at least 10 min. In some cases, these generating cells are enshrouded by enhanced differential reflectivity ZDR, indicating a “shroud” of pristine crystals enveloping the larger, more isotropic particles. The anticorrelation of radar reflectivity factor at horizontal polarization ZH and ZDR suggests ongoing aggregation or riming of particles in the core of generating cells. For cases in which radiosonde data were collected, potential instability was found within the layer in which generating cells were observed. The persistence of these layers suggests that radiative effects are important, perhaps by some combination of cloud-top cooling and release of latent enthalpy through depositional and riming growth of particles within the cloud. The implications for the ubiquity of generating cells and their role as a mechanism for ice crystal initiation and growth are discussed.

A Comparative Study of Rainfall Retrievals Based on Specific Differential Phase Shifts at X- and S-Band Radar Frequencies

2006 ◽  
Vol 23 (7) ◽  
pp. 952-963 ◽  
Author(s):  
Sergey Y. Matrosov ◽  
Robert Cifelli ◽  
Patrick C. Kennedy ◽  
Steven W. Nesbitt ◽  
Steven A. Rutledge ◽  
...  
Keyword(s):  
Comparative Study ◽  
Radar Data ◽  
Drop Diameter ◽  
Horizontal Polarization ◽  
Differential Phase Shift ◽  
Phase Measurements ◽  
Differential Phase ◽  
X Band ◽  
Raindrop Size ◽  
Differential Reflectivity

Abstract A comparative study of the use of X- and S-band polarimetric radars for rainfall parameter retrievals is presented. The main advantage of X-band polarimetric measurements is the availability of reliable specific differential phase shift estimates, KDP, for lighter rainfalls when phase measurements at the S band are too noisy to produce usable KDP. Theoretical modeling with experimental raindrop size distributions indicates that due to some non-Rayleigh resonant effects, KDP values at a 3.2-cm wavelength (X band) are on average a factor of 3.7 greater than at 11 cm (S band), which is a somewhat larger difference than simple frequency scaling predicts. The non-Rayleigh effects also cause X-band horizontal polarization reflectivity, Zeh, and differential reflectivity, ZDR, to be larger than those at the S band. The differences between X- and S-band reflectivities can exceed measurement uncertainties for Zeh starting approximately at Zeh > 40 dBZ, and for ZDR when the mass-weighted drop diameter, Dm, exceeds about 2 mm. Simultaneous X- and S-band radar measurements of rainfall showed that consistent KDP estimates exceeding about 0.1° km−1 began to be possible at reflectivities greater than ∼26–30 dBZ while at the S band such estimates can generally be made if Zeh > ∼35–39 dBZ. Experimental radar data taken in light-to-moderate stratiform rainfalls with rain rates R in an interval from 2.5 to 15 mm h−1 showed availability of the KDP-based estimates of R for most of the data points at the X band while at the S band such estimates were available only for R greater than about 8–10 mm h−1. After correcting X-band differential reflectivity measurements for differential attenuation, ZDR measurements at both radar frequency bands were in good agreement with each other for Dm < 2 mm, which approximately corresponds to ZDR ≈ 1.6 dB. The ZDR-based retrievals of characteristic raindrop sizes also agreed well with in situ disdrometer measurements.

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

2018 ◽  
Vol 33 (5) ◽  
pp. 1477-1495 ◽  
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.

scholarly journals Retrieval of lower-order moments of the drop size distribution using CSU-CHILL X-band polarimetric radar: a case study

2020 ◽  
Vol 13 (9) ◽  
pp. 4727-4750
Author(s):  
Viswanathan Bringi ◽  
Kumar Vijay Mishra ◽  
Merhala Thurai ◽  
Patrick C. Kennedy ◽  
Timothy H. Raupach
Keyword(s):  
Size Distribution ◽  
Lower Order ◽  
Drop Size ◽  
Radar Data ◽  
Drop Size Distribution ◽  
Polarimetric Radar ◽  
X Band ◽  
Double Moment ◽  
Differential Reflectivity

Abstract. The lower-order moments of the drop size distribution (DSD) have generally been considered difficult to retrieve accurately from polarimetric radar data because these data are related to higher-order moments. For example, the 4.6th moment is associated with a specific differential phase and the 6th moment with reflectivity and ratio of high-order moments with differential reflectivity. Thus, conventionally, the emphasis has been to estimate rain rate (3.67th moment) or parameters of the exponential or gamma distribution for the DSD. Many double-moment “bulk” microphysical schemes predict the total number concentration (the 0th moment of the DSD, or M0) and the mixing ratio (or equivalently, the 3rd moment M3). Thus, it is difficult to compare the model outputs directly with polarimetric radar observations or, given the model outputs, forward model the radar observables. This article describes the use of double-moment normalization of DSDs and the resulting stable intrinsic shape that can be fitted by the generalized gamma (G-G) distribution. The two reference moments are M3 and M6, which are shown to be retrievable using the X-band radar reflectivity, differential reflectivity, and specific attenuation (from the iterative correction of measured reflectivity Zh using the total Φdp constraint, i.e., the iterative ZPHI method). Along with the climatological shape parameters of the G-G fit to the scaled/normalized DSDs, the lower-order moments are then retrieved more accurately than possible hitherto. The importance of measuring the complete DSD from 0.1 mm onwards is emphasized using, in our case, an optical array probe with 50 µm resolution collocated with a two-dimensional video disdrometer with about 170 µm resolution. This avoids small drop truncation and hence the accurate calculation of lower-order moments. A case study of a complex multi-cell storm which traversed an instrumented site near the CSU-CHILL radar is described for which the moments were retrieved from radar and compared with directly computed moments from the complete spectrum measurements using the aforementioned two disdrometers. Our detailed validation analysis of the radar-retrieved moments showed relative bias of the moments M0 through M2 was <15 % in magnitude, with Pearson’s correlation coefficient >0.9. Both radar measurement and parameterization errors were estimated rigorously. We show that the temporal variation of the radar-retrieved mass-weighted mean diameter with M0 resulted in coherent “time tracks” that can potentially lead to studies of precipitation evolution that have not been possible so far.

scholarly journals Photogrammetric Analysis of the 2013 El Reno Tornado Combined with Mobile X-Band Polarimetric Radar Data

2015 ◽  
Vol 143 (7) ◽  
pp. 2657-2683 ◽  
Author(s):  
Roger M. Wakimoto ◽  
Nolan T. Atkins ◽  
Kelly M. Butler ◽  
Howard B. Bluestein ◽  
Kyle Thiem ◽  
...  
Keyword(s):  
Cross Sections ◽  
Radar Data ◽  
Polarimetric Radar ◽  
The North ◽  
Free Air ◽  
Debris Particles ◽  
X Band ◽  
Differential Reflectivity ◽  
First Time ◽  
The Relationship

Abstract This study presents rapid-scanning X-band polarimetric radar data combined with photogrammetry of the El Reno tornado of 31 May 2013. The relationship between the hook echo, weak-echo hole (WEH), weak-echo column (WEC), and the rotational couplet with the visual characteristics of the tornado are shown. For the first time, cross-correlation coefficient (ρhv) and differential reflectivity (ZDR) data are included in the photogrammetric analyses. The tornado was accompanied by a large tornadic debris signature (TDS) with a diameter ~2 km wide during the analysis time. The center of the TDS was not collocated with the WEH and the rotational couplet. Instead, the TDS was displaced ~1 km to the north and within the weak-echo notch of the hook echo. A “debris overhang” was identified in vertical cross sections of the ρhv fields. The overhang was located in a weak-echo trench and a notch of high ρhv, consistent with the position of the tornado updraft. The updraft was hypothesized to be carrying small debris particles to heights that produced the overhang signature. A U-shaped band of high ρhv and ZDR was resolved in a vertical cross section and positioned at the periphery of the WEC during one of the analysis times. It was proposed that the band formed as a result of hydrometeors encircling the WEC while being surrounded on all sides by relatively hydrometeor-free air. The characteristics of the scatterers within the WEC were resolved and believed to be composed of a low concentration of very small, randomly oriented, debris particles, even in the presence of strong centrifuging, and a general absence of hydrometeors.

X-Band Polarimetric Observations of Cross Coupling in the Ice Phase of Convective Storms in Taiwan

2014 ◽  
Vol 53 (6) ◽  
pp. 1678-1695 ◽  
Author(s):  
J. C. Hubbert ◽  
S. M. Ellis ◽  
W.-Y. Chang ◽  
Y.-C. Liou
Keyword(s):  
Electric Fields ◽  
Cross Coupling ◽  
Radar Data ◽  
Ice Crystals ◽  
Polarimetric Radar ◽  
Differential Phase ◽  
X Band ◽  
Linear Depolarization Ratio ◽  
Differential Reflectivity ◽  
Ice Phase

AbstractIn this paper, experimental X-band polarimetric radar data from simultaneous transmission of horizontal (H) and vertical (V) polarizations (SHV) are shown, modeled, and microphysically interpreted. Both range–height indicator data and vertical-pointing X-band data from the Taiwan Experimental Atmospheric Mobile-Radar (TEAM-R) are presented. Some of the given X-band data are biased, which is very likely caused by cross coupling of the H and V transmitted waves as a result of aligned, canted ice crystals. Modeled SHV data are used to explain the observed polarimetric signatures. Coincident data from the National Center for Atmospheric Research S-band polarimetric radar (S-Pol) are presented to augment and support the X-band polarimetric observations and interpretations. The polarimetric S-Pol data are obtained via fast-alternating transmission of horizontal and vertical polarizations (FHV), and thus the S-band data are not contaminated by the cross coupling (except the linear depolarization ratio LDR) observed in the X-band data. The radar data reveal that there are regions in the ice phase where electric fields are apparently aligning ice crystals near vertically and thus causing negative specific differential phase Kdp. The vertical-pointing data also indicate the presence of preferentially aligned ice crystals that cause differential reflectivity Zdr and differential phase ϕdp to be strong functions of azimuth angle.

scholarly journals Retrieval of Three-Dimensional Raindrop Size Distribution Using X-Band Polarimetric Radar Data

2010 ◽  
Vol 27 (8) ◽  
pp. 1265-1285 ◽  
Author(s):  
D-S. Kim ◽  
M. Maki ◽  
D-I. Lee
Keyword(s):  
Size Distribution ◽  
Attenuation Correction ◽  
Earth Science ◽  
Three Dimensional ◽  
Radar Data ◽  
Rain Attenuation ◽  
Polarimetric Radar ◽  
Horizontal Polarization ◽  
Differential Phase ◽  
X Band

Abstract An improved algorithm based on the self-consistent principle for rain attenuation correction of reflectivity ZH and differential reflectivity ZDR are presented for X-band radar. The proposed algorithm calculates the optimum coefficients for the relation between the specific attenuation coefficient and the specific differential phase, every 1 km along a slant range. The attenuation-corrected ZDR is calculated from reflectivity at horizontal polarization and from reflectivity at vertical polarization after attenuation correction. The improved rain attenuation correction algorithm is applied to the range–height indicator (RHI) scans as well as the plan position indicator (PPI) volume scan data observed by X-band wavelength (MP-X) radar, as operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan. The corrected ZH and ZDR values are in good agreement with those calculated from the drop size distribution (DSD) measured by disdrometers. The two governing parameters of a normalized gamma DSD, normalized number concentration NW, and drop median diameter D0 are estimated from the corrected ZH and ZDR, and specific differential phase KDP values based on the “constrained-gamma” method. The method is applied to PPI and RHI data of a typhoon rainband to retrieve the three-dimensional distribution of DSD. The retrieved DSD parameters show reasonable agreement with disdrometer data. The present results demonstrate that high-quality correction and retrieval DSDs can be derived from X-band polarimetric radar data.

scholarly journals Retrieval of Lower-Order Moments of the Drop Size Distribution using CSU-CHILL X-band Polarimetric Radar: A Case Study

2020 ◽  
Author(s):  
Viswanathan Bringi ◽  
Kumar Vijay Mishra ◽  
Merhala Thurai ◽  
Patrick C. Kennedy ◽  
Timothy H. Raupach
Keyword(s):  
Size Distribution ◽  
Lower Order ◽  
Drop Size ◽  
Radar Data ◽  
Drop Size Distribution ◽  
Polarimetric Radar ◽  
X Band ◽  
Double Moment ◽  
Differential Reflectivity

Abstract. The lower order moments of the drop size distribution (DSD) have generally been considered as difficult to retrieve accurately from polarimetric radar data because these are related to higher order moments. For example, the 4.5th moment is associated with specific differential phase, 6th moment with reflectivity and ratio of high order moments with differential reflectivity. Thus, conventionally, the emphasis has been to estimate rain rate (3.67th moment) or parameters of the exponential or gamma distribution. Many double-moment bulk microphysical schemes predict the total number concentration (the 0th moment or M0) and the mixing ratio (or equivalently, the 3rd moment M3). Thus, it is difficult to compare the model outputs directly with polarimetric radar observations or, given the model outputs, to forward model the radar observables. This article describes the use of double-moment normalization of DSDs and the resulting stable intrinsic shape that can be fitted to the generalized gamma (G-G) distribution. The two reference moments are M3 and M6 which are shown to be retrievable using the X-band radar reflectivity, differential reflectivity and specific attenuation (from the iterative ZPHI method). Along with the climatological shape parameters of the G-G fit to the scaled/normalized DSDs, the lower order moments are then retrieved more accurately than possible hitherto. The importance of measuring the complete DSD from 0.1 mm onwards is emphasized using, in our case, an optical array probe with 50 µm resolution collocated with a two-dimensional video disdrometer with 170 µm resolution. This avoids small drop truncation and hence the accurate calculation of lower order moments. A case study of a complex multi-cell storm which traversed an instrumented site near the CSU-CHILL radar is described for which the moments were retrieved and compared with directly computed moments from the complete spectrum measurements using the aforementioned two disdrometers. Our detailed validation analysis of the radar-retrieved moments showed relative bias of the moments M0 through M2 was  0.9. Both radar measurement and parameterization errors were estimated rigorously. We show that the temporal variation of the radar-retrieved characteristic diameter with M0 resulted in coherent time tracks that can potentially lead to studies of precipitation evolution that have not been possible so far.

scholarly journals Polarimetric Radar Signatures of Dendritic Growth Zones within Colorado Winter Storms

2015 ◽  
Vol 54 (12) ◽  
pp. 2365-2388 ◽  
Author(s):  
Robert S. Schrom ◽  
Matthew R. Kumjian ◽  
Yinghui Lu
Keyword(s):  
Electromagnetic Scattering ◽  
Polarimetric Radar ◽  
Front Range ◽  
Horizontal Polarization ◽  
Winter Storms ◽  
Radar Observations ◽  
Colorado State University ◽  
Radar Signature ◽  
Radar Signatures ◽  
Differential Reflectivity

AbstractX-band polarimetric radar observations of winter storms in northeastern Colorado on 20–21 February, 9 March, and 9 April 2013 are examined. These observations were taken by the Colorado State University–University of Chicago–Illinois State Water Survey (CSU-CHILL) radar during the Front Range Orographic Storms (FROST) project. The polarimetric radar moments of reflectivity factor at horizontal polarization ZH, differential reflectivity ZDR, and specific differential phase KDP exhibited a range of signatures at different times near the −15°C temperature level favored for dendritic ice crystal growth. In general, KDP was enhanced in these regions with ZDR decreasing and ZH increasing toward the ground, suggestive of aggregation (or riming). The largest ZDR values (~3.5–5.5 dB) were observed during periods of significant low-level upslope flow. Convective features observed when the upslope flow was weaker had the highest KDP (>1.5° km−1) and ZH (>20 dBZ) values. Electromagnetic scattering calculations using the generalized multiparticle Mie method were used to determine whether these radar signatures were consistent with dendrites. Particle size distributions (PSDs) of dendrites were retrieved for a variety of cases using these scattering calculations and the radar observations. The PSDs derived using stratiform precipitation observations were found to be reasonably consistent with previous PSD observations. PSDs derived where riming may have occurred likely had errors and deviated significantly from these previous PSD observations. These results suggest that this polarimetric radar signature may therefore be useful in identifying regions of rapidly collecting dendrites, after considering the effects of riming on the radar variables.

Microphysical and Polarimetric Radar Modeling of Hydrometeor Refreezing

2021 ◽  
Author(s):  
Dana M. Tobin ◽  
Matthew R. Kumjian
Keyword(s):  
Polarimetric Radar ◽  
Horizontal Polarization ◽  
Radar Signature ◽  
Liquid Portion ◽  
Linear Depolarization Ratio ◽  
Bin Microphysics ◽  
Ice Shell ◽  
Differential Reflectivity ◽  
The Impact ◽  
Polar Correlation

AbstractA unique polarimetric radar signature indicative of hydrometeor refreezing during ice pellet events has been documented in several recent studies, yet the underlying microphysical causes remain unknown. The signature is characterized by enhancements in differential reflectivity (ZDR), specific differential phase (KDP), and linear depolarization ratio (LDR), and a reduction in co-polar correlation coefficient (ρhv) within a layer of decreasing radar reflectivity factor at horizontal polarization (ZH). In previous studies, the leading hypothesis for the observed radar signature is the preferential refreezing of small drops. Here, a simplified, one-dimensional, explicit bin microphysics model is developed to simulate the refreezing of fully melted hydrometeors, and coupled with a polarimetric radar forward operator to quantify the impact of preferential refreezing on simulated radar signatures. The modeling results demonstrate that preferential refreezing is insufficient by itself to produce the observed signatures. In contrast, simulations considering an ice shell growing asymmetrically around a freezing particle (i.e., emulating a thicker ice shell on the bottom of a falling particle) produce realistic ZDR enhancements, and also closely replicate observed features in ZH, KDP, LDR, and ρhv. Simulations that assume no increase in particle wobbling with freezing produce an even greater ZDR enhancement, but this comes at the expense of reducing the LDR enhancement. It is suggested that the polarimetric refreezing signature is instead strongly related to both the distribution of the unfrozen liquid portion within a freezing particle, and the orientation of this liquid with respect to the horizontal.

scholarly journals Validation of a Multilag Estimator on NJU-CPOL and a Hybrid Approach for Improving Polarimetric Radar Data Quality

2020 ◽  
Vol 12 (1) ◽  
pp. 180
Author(s):  
Shiqing Shao ◽  
Kun Zhao ◽  
Haonan Chen ◽  
Jianjun Chen ◽  
Hao Huang
Keyword(s):  
Theoretical Analysis ◽  
Correlation Coefficient ◽  
Hybrid Method ◽  
Radar Data ◽  
Noise Power ◽  
Signal Power ◽  
Polarimetric Radar ◽  
Moment Estimation ◽  
Spectrum Width ◽  
Differential Reflectivity

For the estimation of weak echo with low signal-to-noise ratio (SNR), a multilag estimator is developed, which has better performance than the conventional method. The performance of the multilag estimator is examined by theoretical analysis, simulated radar data and some specific observed data collected by a C-band polarimetric radar in previous research. In this paper, the multilag estimator is implemented and verified for Nanjing University C-band polarimetric Doppler weather radar (NJU-CPOL) during the Observation, Prediction and Analysis of Severe Convection of China (OPACC) field campaign in 2014. The implementation results are also compared with theoretical analysis, including the estimation of signal power, spectrum width, differential reflectivity, and copolar correlation coefficient. The results show that the improvement of the multilag estimator is little for signal power and differential reflectivity, but significant for spectrum width and copolar correlation coefficient when spectrum width is less than 2 ms−1, which implies a large correlation time scale. However, there are obvious biases from the multilag estimator in the regions with large spectrum width. Based on the performance analysis, a hybrid method is thus introduced and examined through NJU-CPOL observations. All lags including lag 0 of autocorrelation function (ACF) are used for moment estimation in this algorithm according to the maximum usable lag number. A case study shows that this hybrid method can improve moment estimation compared to both conventional estimator and multilag estimator, especially for weak weather echoes. The improvement will be significant if SNR decreases or the biases of noise power in the conventional estimator increase. In addition, this hybrid method is easy to implement on both operational and non-operational radars. It is also expected that the proposed hybrid method will have a better performance if applied to S-band polarimetric radars which have twice the maximum useable lags in the same conditions with C-band radars.

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