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Sean Waugh and Terry J. Schuur

Abstract

Radiosonde observations are used the world over to provide critical upper-air observations of the lower atmosphere. These observations are susceptible to errors that must be mitigated or avoided when identified. One source of error not previously addressed is radiosonde icing in winter storms, which can affect forecasts, warning operations, and model initialization. Under certain conditions, ice can form on the radiosonde, leading to decreased response times and incorrect readings. Evidence of radiosonde icing is presented for a winter storm event in Norman, Oklahoma, on 24 November 2013. A special sounding that included a particle imager probe and a GoPro camera was flown into the system producing ice pellets. While the iced-over temperature sensor showed no evidence of an elevated melting layer (ML), complementary Particle Size, Image, and Velocity (PASIV) probe and polarimetric radar observations provide clear evidence that an ML was indeed present. Radiosonde icing can occur while passing through a layer of supercooled drops, such as frequently found in a subfreezing layer that often lies below the ML in winter storms. Events that have warmer/deeper MLs would likely melt any ice present off the radiosonde, minimizing radiosonde icing and allowing the ML to be detected. This paper discusses the hypothesis that the absence of an ML in the radiosonde data presented here is more likely to occur in winter storms that produce ice pellets, which tend to have cooler/shallower MLs. Where sounding data do appear to be compromised by icing, polarimetric radar data might be used to identify MLs for nowcasting purposes and numerical model initialization.

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Terry J. Schuur and Steven A. Rutledge

Abstract

Model simulations of a symmetric mesoscale convective system (MCS; observations discussed in Part I) were conducted using a 2D, time-dependent numerical model with bulk microphysics. A number of charging mechanisms were considered based on various laboratory studies. The simulations suggest that noninductive ice–ice charge transfer in the low liquid water content regime, characteristic of MCS stratiform regions, is sufficient to account for observed charge density magnitudes, and as much as 70% of the total charge in the stratiform region. The remaining 30% is contributed by charge advection from the convective region. The strong role of in situ charging is consistent with previous water budget studies, which indicate that roughly 70% of the stratiform precipitation results from condensation in the mesoscale updraft. Thus both in situ charging and charge advection (the two previously identified hypotheses) appear to be important contributors to the electrical budget of the stratiform region. The simulations also indicate that once these charge densities are achieved, the sink of charge resulting from particle fallout becomes approximately equal to the rate of charge generation. This result is consistent with the quasi-steady layered structure that is commonly observed in these systems. Two noninductive charging parameterizations are tested and both are found to reproduce some of the observed stratiform charge structures. The evaporation–condensation charging and melting charging mechanisms are also investigated, but found to be insignificant.

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Terry J. Schuur and Steven A. Rutledge

Abstract

The kinematic, microphysical, and electrical structures of two mesoscale convective systems (MCSs) observed during the 1991 Cooperative Oklahoma Profiler Studies (COPS91) experiment are analyzed. Profiles of the vertical electric field structure and charge density were obtained from a series of balloon-borne electric field meter (EFM) flights into each MCS. Contrasting electric field structures were found in the stratiform regions of these MCSs. In both systems, the EFM data indicate that the MCS charge structure was characterized by horizontally extensive regions of charge and charge density magnitudes on the order of what is typically observed in convective cores (⩽5 nC m−3). However, the vertical electric field profiles were each related to unique MCS precipitation and kinematic structures, with a five-layer charge profile (at T ⩽ 0°C) associated with the “symmetric” MCS and a simpler three-layer charge profile (at T ⩽ 0°C) associated with the “asymmetric” MCSs.

The observational analysis identified several kinematic, thermodynamic, and microphysical differences between the two systems that offer at least some explanation for the observed electrical structures. First, ice particles detrained from the convective line of the symmetric MCS had much shorter “residence times” in the unfavorable growth/charging region associated with the transition zone downdraft compared to the asymmetric case. Second, upon entering the trailing stratiform region, ice particles in the symmetric system were immersed in an environment that was more conducive to in situ charging via noninductive charging mechanisms. Strong mesoscale ascent in the stratiform region of the symmetric MCS led to the presence of supercooled cloud water, and hence significant electrification. There are also indications that fallspeed differences between particle types may be responsible for producing some of the charge transitions in the electric field profiles. In contrast, strong mesoscale ascent was not present in the asymmetric case, and hence conditions were less favorable for noninductive charging.

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Alexander V. Ryzhkov, Scott E. Giangrande, and Terry J. Schuur

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As part of the Joint Polarization Experiment (JPOLE), the National Severe Storms Laboratory conducted an operational demonstration of the polarimetric utility of the Norman, Oklahoma (KOUN), Weather Surveillance Radar-1988 Doppler (WSR-88D). The capability of the KOUN radar to estimate rainfall is tested on a large dataset representing different seasons and different types of rain. A dense gauge network—the Agricultural Research Service (ARS) Micronet—is used to validate different polarimetric algorithms for rainfall estimation. One-hour rain totals are estimated from the KOUN radar using conventional and polarimetric algorithms and are compared with hourly accumulations measured by the gauges. Both point and areal rain estimates are examined. A new “synthetic” rainfall algorithm has been developed for rainfall estimation. The use of the synthetic polarimetric algorithm results in significant reduction in the rms errors of hourly rain estimates when compared with the conventional nonpolarimetric relation: 1.7 times for point measurements and 3.7 times for areal rainfall measurements.

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Erica M. Griffin, Terry J. Schuur, and Alexander V. Ryzhkov

Abstract

This study implements a new quasi-vertical profile (QVP) methodology to investigate the microphysical evolution and significance of intriguing winter polarimetric signatures and their statistical correlations. QVPs of transitional stratiform and pure snow precipitation are analyzed using WSR-88D S-band data, alongside their corresponding environmental thermodynamic High-Resolution Rapid Refresh model analyses. QVPs of K DP and Z DR are implemented to demonstrate their value in interpreting elevated ice processes. Several fascinating and repetitive signatures are observed in the QVPs for differential reflectivity Z DR and specific differential phase K DP, in the dendritic growth layer (DGL), and at the tops of clouds. The most striking feature is maximum Z DR (up to 6 dB) in the DGL occurring near the −10-dBZ Z H contour within low K DP and during shallower and warmer cloud tops. Conversely, maximum K DP (up to 0.3° km−1) in the DGL occurs within low Z DR and during taller and colder cloud tops. Essentially, Z DR and K DP in the DGL are anticorrelated and strongly depend on cloud-top temperature. Analyses also show correlations indicating larger Z DR within lower Z H in the DGL and larger K DP within greater Z H in the DGL. The high-Z DR regions are likely dominated by growth of a mixture of highly oblate dendrites and/or hexagonal plates, or prolate needles. Regions of high K DP are expected to be overwhelmed with snow aggregates and crystals with irregular or nearly spherical shapes, seeded at cloud tops. Furthermore, QVP indications of hexagonal plate crystals within the DGL are verified using in situ microphysical measurements, demonstrating the reliability of QVPs in evaluating ice microphysics in upper regions of winter clouds.

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Qing Cao, Guifu Zhang, Edward A. Brandes, and Terry J. Schuur

Abstract

This study proposes a Bayesian approach to retrieve raindrop size distributions (DSDs) and to estimate rainfall rates from radar reflectivity in horizontal polarization ZH and differential reflectivity Z DR. With this approach, the authors apply a constrained-gamma model with an updated constraining relation to retrieve DSD parameters. Long-term DSD measurements made in central Oklahoma by the two-dimensional video disdrometer (2DVD) are first used to construct a prior probability density function (PDF) of DSD parameters, which are estimated using truncated gamma fits to the second, fourth, and sixth moments of the distributions. The forward models of ZH and Z DR are then developed based on a T-matrix calculation of raindrop backscattering amplitude with the assumption of drop shape. The conditional PDF of ZH and Z DR is assumed to be a bivariate normal function with appropriate standard deviations. The Bayesian algorithm has a good performance according to the evaluation with simulated ZH and Z DR. The algorithm is also tested on S-band radar data for a mesoscale convective system that passed over central Oklahoma on 13 May 2005. Retrievals of rainfall rates and 1-h rain accumulations are compared with in situ measurements from one 2DVD and six Oklahoma Mesonet rain gauges, located at distances of 28–54 km from Norman, Oklahoma. Results show that the rain estimates from the retrieval agree well with the in situ measurements, demonstrating the validity of the Bayesian retrieval algorithm.

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Charles M. Kuster, Pamela L. Heinselman, and Terry J. Schuur

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On 14 June 2011, an intense multicell thunderstorm produced one nonsevere and three severe downbursts within 35 km of the rapid-update, S-band phased array radar (PAR) at the National Weather Radar Testbed in Norman, Oklahoma, and the nearby polarimetric research Weather Surveillance Radar 1988-Doppler (KOUN). Data collected from these radars provided the opportunity to conduct a quantitative analysis of downburst precursor signature evolution depicted by 1-min PAR data and the associated evolution of differential reflectivity Z DR depicted by 5-min KOUN data. Precursors analyzed included descent of the reflectivity core, evolution of the magnitude and size of midlevel convergence (i.e., number of bins), and descending “troughs” of Z DR. The four downbursts exhibited midlevel convergence that rapidly increased to peak magnitude as the reflectivity core (65-dBZ isosurface) bottom and top descended. The Z DR troughs seen in the 5-min KOUN data appeared to descend along with the core bottom. Midlevel convergence size increased to a peak value and decreased as the reflectivity core descended in the three severe downbursts. In contrast, midlevel convergence size exhibited little change in the nonsevere downburst. The time scale of trends seen in the PAR data was 11 min or less and happened several minutes prior to each downburst’s maximum intensity. These results point to the importance of 1-min volumetric data in effectively resolving the evolution of downburst precursors, which could be beneficial to forecast operations.

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Erica M. Griffin, Terry J. Schuur, and Alexander V. Ryzhkov

Abstract

Quasi-vertical profiles (QVPs) obtained from a database of U.S. WSR-88D data are used to document polarimetric characteristics of the melting layer (ML) in cold-season storms with high vertical resolution and accuracy. A polarimetric technique to define the top and bottom of the ML is first introduced. Using the QVPs, statistical relationships are developed to gain insight into the evolution of microphysical processes above, within, and below the ML, leading to a statistical polarimetric model of the ML that reveals characteristics that reflectivity data alone are not able to provide, particularly in regions of weak reflectivity factor at horizontal polarization Z H. QVP ML statistics are examined for two regimes in the ML data: Z H ≥ 20 dBZ and Z H < 20 dBZ. Regions of Z H ≥ 20 dBZ indicate locations of MLs collocated with enhanced differential reflectivity Z DR and reduced copolar correlation coefficient ρ hv, while for Z H < 20 dBZ a well-defined ML is difficult to discern using Z H alone. Evidence of large Z DR up to 4 dB, backscatter differential phase δ up to 8°, and low ρ hv down to 0.80 associated with lower Z H (from −10 to 20 dBZ) in the ML is observed when pristine, nonaggregated ice falls through it. Positive correlation is documented between maximum specific differential phase K DP and maximum Z H in the ML; these are the first QVP observations of K DP in MLs documented at S band. Negative correlation occurs between minimum ρ hv in the ML and ML depth and between minimum ρ hv in the ML and the corresponding enhancement of Z HZ H = Z HmaxZ Hrain).

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Alexander V. Ryzhkov, Scott E. Giangrande, Valery M. Melnikov, and Terry J. Schuur

Abstract

Techniques for the absolute calibration of radar reflectivity Z and differential reflectivity Z DR measured with dual-polarization weather radars are examined herein.

Calibration of Z is based on the idea of self-consistency among Z, Z DR, and the specific differential phase K DP in rain. Extensive spatial and temporal averaging is used to derive the average values of Z DR and K DP for each 1 dB step in Z. Such averaging substantially reduces the standard error of the K DP estimate so the technique can be used for a wide range of rain intensities, including light rain.

In this paper, the performance of different consistency relations is analyzed and a new self-consistency methodology is suggested. The proposed scheme substantially reduces the impact of variability in the drop size distribution and raindrop shape on the quality of the Z calibration. The new calibration technique was tested on a large polarimetric dataset obtained during the Joint Polarization Experiment in Oklahoma and yielded an accuracy of Z calibration within 1 dB.

Absolute calibration of Z DR is performed using solar measurements at orthogonal polarizations and polarimetric properties of natural targets like light rain and dry aggregated snow that are probed at high elevation angles. Because vertical sounding is prohibited for operational Weather Surveillance Radar-1988 Doppler (WSR-88D) radars because of mechanical constraints, the existing methodology for Z DR calibration is modified for nonzenith elevation angles. It is shown that the required 0.1–0.2-dB accuracy of the Z DR calibration is potentially achievable.

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Michihiro S. Teshiba, Phillip B. Chilson, Alexander V. Ryzhkov, Terry J. Schuur, and Robert D. Palmer

Abstract

A method is presented by which combined S-band polarimetric weather radar and UHF wind profiler observations of precipitation can be used to extract the properties of liquid phase hydrometeors and the vertical velocity of the air through which they are falling. Doppler spectra, which contain the air motion and/or fall speed of hydrometeors, are estimated using the vertically pointing wind profiler. Complementary to these observations, spectra of rain drop size distribution (DSD) are simulated by several parameters as related to the DSD, which are estimated through the two polarimetric parameters of radar reflectivity (ZH) and differential reflectivity (Z DR) from the scanning weather radar. These DSDs are then mapped into equivalent Doppler spectra (fall speeds) using an assumed relationship between the equivolume drop diameter and the drop’s terminal velocity. The method is applied to a set of observations collected on 11 March 2007 in central Oklahoma. In areas of stratiform precipitation, where the vertical wind motion is expected to be small, it was found that the fall speeds obtained from the spectra of the rain DSD agree well with those of the Doppler velocity estimated with the profiler. For those cases when the shapes of the Doppler spectra are found to be similar in shape but shifted in velocity, the velocity offset is attributed to vertical air motion. In convective rainfall, the Doppler spectra of the rain DSD and the Doppler velocity can exhibit significant differences owing to vertical air motions together with atmospheric turbulence. Overall, it was found that the height dependencies of Doppler spectra measured by the profiler combined with vertical profiles of Z, Z DR, and the cross correlation (ρHV) as well as the estimated spectra of raindrop physical terminal fall speeds from the polarimetric radar provide unique insight into the microphysics of precipitation. Vertical air motions (updrafts/downdrafts) can be estimated using such combined measurements.

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