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Dana M. Tobin
and
Matthew R. Kumjian

Abstract

Recent studies document a polarimetric radar signature of refreezing. The signature is characterized by a low-level enhancement in differential reflectivity Z DR and a decrease in the copolar correlation coefficient ρ hv within a region of decreasing radar reflectivity factor at horizontal polarization Z H toward the ground, called the refreezing layer (RFL). The evolution of the signature is examined during three winter storms in which the surface precipitation-type transitions from ice pellets to freezing rain. A modified quasi-vertical profile (QVP) technique is developed, which creates inverse-distance-weighted profiles using all available polarimetric data within a specified range from the radar location. Using this new technique reveals that the RFL descends in time prior to the transition from ice pellets to freezing rain and intersects the ground at the approximate transition time. Transition times are estimated using both crowdsourced and automated precipitation-type reports within a specified domain around the radar. These radar-estimated transition times are compared to a recently developed precipitation-classification algorithm based on Rapid Refresh (RAP) model wet-bulb temperature T w profiles to explore potential benefits of analyzing QVPs during transition events. The descent of the RFL in the cases analyzed herein is related to low-level warm-air advection (WAA). A simple method for forecasting the transition time using QVPs is presented for cases of constant WAA. The repeatability of the refreezing signature’s descent in ice pellet to freezing rain transition events suggests the potential for its use in operational settings to create or modify short-term forecasts.

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Dana M. Tobin
and
Matthew R. Kumjian

Abstract

A 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 (Z DR), specific differential phase (K DP), and linear depolarization ratio (LDR), and a reduction in copolar correlation coefficient (ρ hv) within a layer of decreasing radar reflectivity factor at horizontal polarization (Z H). 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 Z DR enhancements, and also closely replicate observed features in Z H, K DP, LDR, and ρ hv. Simulations that assume no increase in particle wobbling with freezing produce an even greater Z DR 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.

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Matthew S. Van Den Broeke
,
Dana M. Tobin
, and
Matthew R. Kumjian

Abstract

A powerful winter storm affected the south-central United States in early March 2014, accompanied by elevated convective cells with hail and high rates of sleet, freezing rain, and snow. During portions of the event the thermal profile exhibited a shallow surface cold layer and warm, unstable air aloft. Precipitation falling into the cold layer refroze into ice pellets and was accompanied by a polarimetric refreezing signature and numerous crowdsourced surface ice pellet reports. Quasi-vertical profiles of the polarimetric variables indicated an enhanced reflectivity factor Z HH below the melting layer bright band and enhanced low-level differential reflectivity Z DR values coincident with surface ice pellet reports. Freezing rain rate was highest in areas with high Z HH and specific differential phase K DP values at low levels. High snow rates were most closely associated with 1- and 1.5-km Z HH values, though K DP and Z DR also appeared to show some ability to distinguish high snow rate. Numerous elevated convective cells contained rotating updrafts that appeared to contribute to storm longevity and intensity. Most contained well-defined Z DR maxima or columns and relatively high base-scan Z DR values. Several contained polarimetric signatures consistent with heavy mixed-phase precipitation and hail; social media reports indicated that large hail was produced by some of the storms.

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Matthew R. Kumjian
,
Dana M. Tobin
,
Mariko Oue
, and
Pavlos Kollias

Abstract

Fully polarimetric scanning and vertically pointing Doppler spectral data from the state-of-the-art Stony Brook University Ka-band Scanning Polarimetric Radar (KASPR) are analyzed for a long-duration case of ice pellets over central Long Island in New York from 12 February 2019. Throughout the period of ice pellets, a classic refreezing signature was present, consisting of a secondary enhancement of differential reflectivity Z DR beneath the melting layer within a region of decreasing reflectivity factor at horizontal polarization Z H and reduced copolar correlation coefficient ρ hv. The KASPR radar data allow for evaluation of previously proposed hypotheses to explain the refreezing signature. It is found that, upon entering a layer of locally generated columnar ice crystals and undergoing contact nucleation, smaller raindrops preferentially refreeze into ice pellets prior to the complete freezing of larger drops. Refreezing particles exhibit deformations in shape during freezing, leading to reduced ρ hv, reduced co-to-cross-polar correlation coefficient ρ xh, and enhanced linear depolarization ratio, but these shape changes do not explain the Z DR signature. The presence of columnar ice crystals, though apparently crucial for instigating the refreezing process, does not contribute enough backscattered power to affect the Z DR signature, either.

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Dana M. Tobin
,
Matthew R. Kumjian
,
Mariko Oue
, and
Pavlos Kollias

Abstract

The discovery of a polarimetric radar signature indicative of hydrometeor refreezing has shown promise in its utility to identify periods of ice pellet production. Uniquely characterized well below the melting layer by locally enhanced values of differential reflectivity (Z DR) within a layer of decreasing radar reflectivity factor at horizontal polarization (ZH ), the signature has been documented in cases where hydrometeors were completely melted prior to refreezing. However, polarimetric radar features associated with the refreezing of partially melted hydrometeors have not been examined as rigorously in either an observational or microphysical modeling framework. Here, polarimetric radar data—including vertically pointing Doppler spectral data from the Ka-band Scanning Polarimetric Radar (KASPR)—are analyzed for an ice pellets and rain mixture event where the ice pellets formed via the refreezing of partially melted hydrometeors. Observations show that no such distinct localized Z DR enhancement is present, and that values instead decrease directly beneath enhanced values associated with melting. A simplified, explicit bin microphysical model is then developed to simulate the refreezing of partially melted hydrometeors, and coupled to a polarimetric radar forward operator to examine the impacts of such refreezing on simulated radar variables. Simulated vertical profiles of polarimetric radar variables and Doppler spectra have similar features to observations, and confirm that a Z DR enhancement is not produced. This suggests the possibility of two distinct polarimetric features of hydrometeor refreezing: ones associated with refreezing of completely melted hydrometeors, and those associated with refreezing of partially melted hydrometeors.

Significance Statement

There exist two pathways for the formation of ice pellets: refreezing of fully melted hydrometeors, and refreezing of partially melted hydrometeors. A polarimetric radar signature indicative of fully melted hydrometeor refreezing has been extensively documented in the past, yet no study has documented the refreezing of partially melted hydrometeors. Here, observations and idealized modeling simulations are presented to show different polarimetric radar features associated with partially melted hydrometeor refreezing. The distinction in polarimetric features may be beneficial to identifying layers of supercooled liquid drops within transitional winter storms.

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Dana M. Tobin
,
Matthew R. Kumjian
, and
Alan W. Black

Abstract

Data from the National Highway Traffic Safety Administration’s (NHTSA) Fatality Analysis Reporting System (FARS) database were used to identify vehicle-related fatalities that occurred during active precipitation from 2013 to 2017. Changes to FARS for 2013–present allow the identification of freezing rain, in addition to rain, snow, sleet, and precipitation mixtures as prevailing precrash atmospheric conditions. The characteristics of vehicle-related fatalities for each precipitation type identified in FARS were assessed in terms of total numbers, roadway surface conditions, location, and annual and diurnal variability. Vehicle-related fatalities were also matched to nearby Automated Surface Observing System (ASOS) and Automated Weather Observing System (AWOS) precipitation-type reports to determine their agreement with precipitation types documented in FARS. Of the vehicle-related fatalities examined, 8.6% occurred during precipitation, with these fatalities further divided by precipitation type of approximately 81% rain, 14% snow, and 5% sleet, freezing rain, and mixtures of precipitation. Unexpected discrepancies between the numbers of sleet- versus freezing-rain-related fatalities reveal that caution should be taken when using FARS to identify these precipitation types. ASOS/AWOS precipitation-type reports have moderate agreement with FARS at 20 mi (32.2 km), and are capable of distinguishing precipitation and nonprecipitation indicated in FARS. Rain and snow have good agreement between the databases, whereas sleet, freezing rain, and precipitation mixtures have significantly reduced agreement due to a combination of ASOS/AWOS limitations and suspected FARS limitations. To provide a more accurate account of precipitation types for fatal crashes, the use of crashes where FARS-identified precipitation types are confirmed by nearby ASOS/AWOS reports is suggested.

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Dana M. Tobin
,
Heather D. Reeves
,
Maci N. Gibson
, and
Andrew A. Rosenow

Abstract

Winter-weather conditions pose an extreme hazard to motorists, resulting in approximately 1000 fatalities annually on U.S. roadways. Minimizing adverse impacts of winter weather requires (i) the identification of hazardous weather conditions leading up to and at the time of fatal crashes, and (ii) effective, targeted messaging of those hazards to motorists. The first objective is addressed by matching motor-vehicle-related fatalities from 2008 to 2019 to nearby weather reports to determine how precipitation types and other observable weather conditions (i.e., precipitation intensity, obscurations, and visibility) change leading up to crashes. One-half of fatalities occur in snow, with 75% occurring in ongoing snowfall. Of fatalities during freezing precipitation, 41% occur near the onset of freezing precipitation. In addition, 42% of fatalities have deteriorating weather conditions prior to the crash, primarily visibility reductions of ≥25%. The second objective is addressed by examining language currently used in National Weather Service Winter Weather Warning, Watch, or Advisory (WSW) issuances for fatal crashes. Only one-third of fatalities have a WSW. These WSWs both identify a road hazard (e.g., “roads will become slick”) and provide an action item for motorists (e.g., “slow down and use caution while driving”) but do not clearly convey tiered road-hazard ratings. Examination of non-weather-related attributes of fatal crashes suggest that variable-message signs along highways may be useful to communicate road hazards, and that future messaging should urge motorists to leave additional space around their vehicles, slow down, prepare for rapidly deteriorating conditions, and teach strategies to regain control of their vehicle.

Significance Statement

We find that approximately 1000 fatalities occur each year on U.S. roadways during winter weather. To inform how to reduce fatalities in the future, we identify weather conditions leading up to and at the time of fatal crashes and determine whether road hazards were publicly messaged alongside weather warnings and advisories. Ongoing snowfall, the onset of freezing precipitation, and visibility reductions were prominent factors found in many fatal crashes, suggesting that these may be important factors to address in future safety studies. Winter-weather warnings and advisories often contain language cautioning road hazards, yet only one-third of fatalities occur during conditions with such official statements. However, these statements do not clearly indicate how hazardous roads will be.

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