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The coincidence of extreme cold, precipitation, and freezing weather is one of the most significant natural disasters in winter, with extensive impacts on public transportation, the energy industry, and the ecological landscape. Despite the importance of such events, comprehensive analyses of the performance of cutting-edge numerical weather prediction models at convection-permitting scales remain inadequate. This paper investigates the prolonged freezing event that occurred from January 24 to 28, 2018 in South China using the Weather Research and Forecasting (WRF) model with a horizontal grid spacing of 3 km. The focus is on evaluating the performance of the WRF model and the impact of microphysical parameterizations and nudging techniques. The results show that the WRF model successfully captures the main features of the prolonged freezing event, including the low-level collision of cold and warm air in the lower reaches of the Yangtze River valley and the spatial distribution of precipitation. However, the model overestimates the strength of warm air from the south, leading to a northward shift of the precipitation belt. The choice of physical parameterizations significantly influences model performance, with microphysical schemes of P3, Thompson, and MYDM showing better model skill in terms of root mean square error (RMSE). The analysis also highlights the importance of nudging techniques, particularly grid nudging, which significantly improves model performance. Furthermore, the inclusion of local, very dense station observations from the China Meteorological Administration (CMA) during the nudging process provides additional improvement. A further investigation reveals that the accuracy in reproducing low-level convergence over South China is the main factor influencing model performance across different sensitivity simulations. This study underscores the value of convection-permitting simulations in accurately reproducing extreme freezing events over China. • The WRF model successfully reproduces the main features of the 2018 prolonged freezing event in South China. • The P3, Thompson, and MYDM schemes show better model skill in terms of root mean square error (RMSE). • Grid nudging with local station observations from CMA significantly improves model performance. [ABSTRACT FROM AUTHOR]

The predictability of precipitation type in a January 2017 winter storm over the northeastern United States and southeastern Canada is examined using a convective-scale initial-condition ensemble with the Weather Research and Forecasting (WRF) Model. Real-time forecasts of the event by Environment and Climate Change Canada predicted 15–25 cm of snow accumulation in Montreal, Quebec, Canada. However, the initial 4 h of the event had 5–8 mmof freezing rain instead, followed by 7 cm of snow. While the total liquid-equivalent precipitation was consistent with the forecast, the unexpected freezing rain caused significant disruption in the Montreal region. The fraction of freezing precipitation (freezing rain and/or ice pellets) over the initial 4 h in Montreal varied greatly across the ensemble, with some members producing nearly all snow and others producing nearly all freezing precipitation. In members with larger fractions of freezing precipitation (as opposed to snow), the cyclone’s midlevel trough was displaced slightly to the northwest, and its downstream (eastern) edge was narrower, the latter of which was traced back to model initialization. These differences increased the midlevel southerly flow into southern Quebec, which both enhanced the horizontal warm advection and decreased the vertical cold advection leading up to the event. The consequent midlevel warming over Montreal in these members produced an above-zero layer that melted falling precipitation, leading to freezing upon contact with the ground. This case study highlights the value of convective-scale ensembles for identifying mechanisms by which initial synoptic-scale uncertainties lead to high-impact localized errors in precipitation type. [ABSTRACT FROM AUTHOR]

A freezing rain event, in which the Meteorological Centre of Canada's 2.5-km numerical weather prediction system significantly underpredicted the quantity of freezing rain, is examined. The prediction system models precipitation types explicitly, directly from the Milbrandt-Yau microphysics scheme. It was determined that the freezing rain underprediction for this case was due primarily to excessive refreezing of rain, originating from melting snow and graupel, in and under the temperature inversion of the advancing warm front ultimately depleting the supply of rain reaching the surface. The refreezing was caused from excessive collisional freezing between rain and graupel. Sensitivity experiments were conducted to examine the effects of a temperature threshold for collisional freezing and on varying the values of the collection efficiencies between rain and ice-phase hydrometeors. It was shown that by reducing the rain-graupel collection efficiency and by imposing a temperature threshold of 258C, above which collisional freezing is not permitted, excessive rain-graupel collection and graupel formation can be controlled in the microphysics scheme, leading to an improved simulation of freezing rain at the surface. [ABSTRACT FROM AUTHOR]

Recent studies document a polarimetric radar signature of refreezing. The signature is characterized by a low-level enhancement in differential reflectivity ZDR and a decrease in the copolar correlation coefficient ρhv within a region of decreasing radar reflectivity factor at horizontal polarization ZH 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.Amodified 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 Tw 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. [ABSTRACT FROM AUTHOR]

The article offers information related to the weather condition of various states of the U.S. as of July and August 2015. It mentions the temperature and inches of rain in U.S. states including Washington, Illinois, and North Carolina. It features several types of weathers and conditions occurred in the country including desert rain, wildfire, and freeze.

Despite improvements in forecasting extreme weather events, noncompliance with weather warnings among the public remains a problem. Although there are likely many reasons for noncompliance with weather warnings, one important factor might be people's past experiences with false alarms. The research presented here explores the role of false alarms in weather-related decision making. Over a series of trials, participants used an overnight low temperature forecast and advice from a decision aid to decide whether to apply salt treatment to a town's roads to prevent icy conditions or take the risk of withholding treatment, which resulted in a large penalty when freezing temperatures occurred. The decision aid gave treatment recommendations, some of which were false alarms, i.e., treatment was recommended but observed temperatures were above freezing. The rate at which the advice resulted in false alarms was manipulated between groups. Results suggest that very high and very low false alarm rates led to inferior decision making, but that lowering the false alarm rate slightly did not significantly affect compliance or decision quality. However, adding a probabilistic uncertainty estimate in the forecasts improved both compliance and decision quality. These findings carry implications about how weather warnings should be communicated to the public. [ABSTRACT FROM AUTHOR]

At subzero temperatures, cloud particles can contain both ice and liquid water fractions. Wet growth of precipitation particles occurs when supercooled cloud liquid is accreted faster than it can freeze on impact. With a flexible framework, the theory of wet growth of hail is extended to the case of the inhomogeneities of surface temperature and of liquid coverage over the surface of the particle. The theory treats the heat fluxes between its wet and dry parts and radial heat fluxes from the sponge layer through the liquid skin to the air. The theory parameterizes effects of nonsphericity of hail particles on their growth by accretion. Gradual internal freezing of any liquid soaking the hail or graupel particle's interior during dry growth ('riming') is treated as well. In this way, the microphysical recycling envisaged by Pflaum in a paper in 1980 is treated, with alternating episodes of wet and dry growth. The present paper, the first of a two-part paper, describes the scheme to treat wet growth, accounting for dependencies on condensate content, temperature, and particle size. Comparison with the laboratory experiments is presented. [ABSTRACT FROM AUTHOR]

A series of lake-effect freezing drizzle events occurred southeast of Lake Michigan during the 2009-2010 cool season. These events occurred under an anomalous flow pattern, were not anticipated by forecasters, and in more than one case, led to the issuance of advisories for slick travel conditions and ice accrual. Given the potential impacts of such events on the public and aviation communities--as well as limited previous research on lake-effect freezing precipitation--two case studies are performed. Despite their rarity, a common synoptic and mesoscale evolution is found in these freezing drizzle cases. Whereas the thermodynamic environment initially is supportive for lake-effect snow, a lowering capping inversion and a loss of moisture above this level diminishes the potential production of cloud ice. While this typically results in the end of lake-effect precipitation, these cases transitioned to freezing drizzle and are hypothesized to have resulted from a lack of cloud condensation nuclei in the air mass arriving from the lakes. Mesoscale model soundings projected the evolution of the thermodynamic environment. A conceptual summary of these events is presented that, given suggestive model guidance, includes tools to help forecasters better anticipate future similar events. [ABSTRACT FROM AUTHOR]

Large parts of western and central Europe face a 20-50 % future reduction in snowfall on Hellmann days (days with daily-mean temperatures below freezing). This strong reduction occurs in addition to the expected 75 % decrease of the number of Hellmann days near the end of the twenty first century. The result is insensitive to the exact freezing-level threshold, but is in sharp contrast with the winter daily precipitation, which increases under most global warming scenarios. Not only climate model simulations show this. Observational records also reveal that probabilities for precipitation on Hellmann days have been larger in the past. The future reduction is a consequence of the freezing-level threshold becoming a more extreme quantile of the temperature distribution in the future. Only certain circulation types permit these quantiles to be reached, and it is shown that these have intrinsically low precipitation probability. [ABSTRACT FROM AUTHOR]

The article offers information on the weather map used on May 13, 2009 in the U.S. It notes that the map depicted a strong weather system that moved through the Midwest, Great Lakes region, and much of Ontario along with subfreezing weather that approached Dakotas. The presentation of the map with very strong temperature contrast in northern Quebec which identified an east-west font is mentioned.

The objective of this paper is to assess the performances of the proposed ice water content (IWC)–radar reflectivity Z and IWC–Z–temperature T relationships for accurate retrievals of IWC from radar in space or at ground-based sites, in the framework of the forthcoming CloudSat spaceborne radar, and of the European CloudNET and U.S. Atmospheric Radiation Measurement Program projects. For this purpose, a large airborne in situ microphysical database is used to perform a detailed error analysis of the IWC–Z and IWC–Z–T methods. This error analysis does not include the error resulting from the mass–dimension relationship assumed in these methods, although the expected magnitude of this error is bounded in the paper. First, this study reveals that the use of a single IWC–Z relationship to estimate IWC at global scale would be feasible up to -15 dBZ, but for larger reflectivities (and therefore larger IWCs) different sets of relationships would have to be used for midlatitude and tropical ice clouds. New IWC–Z and IWC–Z–T relationships are then developed from the large aircraft database and by splitting this database into midlatitude and tropical subsets, and an error analysis is performed. For the IWC–Z relationships, errors decrease roughly linearly from +210%/-70% for IWC = 10-4 g m-3 to +75%/-45% for IWC = 10-2 g m-3, are nearly constant (+50%/-33%) for the intermediate IWCs (0.03–1 g m-3), and then linearly increase up to +210%/-70% for the largest IWCs. The error curves have the same shape for the IWC–Z–T relationships, with a general reduction of errors with respect to the IWC–Z relationships. Comparisons with radar–lidar retrievals confirm these findings. The main improvement brought by the use of temperature as an additional constraint to the IWC retrieval is to reduce both the systematic overestimation and rms differences of the small IWCs (IWC < 0.01 g m-3). For the large IWCs, the use of temperature also results in a slight reduction of the rms differences but in a substantial reduction (by a factor of 2) of the systematic underestimation of the large IWCs, probably owing to a better account of the Mie effect when IWC–Z relationships are stratified by temperature. [ABSTRACT FROM AUTHOR]

Frost on roadways and bridges can present hazardous conditions to motorists, particularly when it occurs in patches or on bridges when adjacent roadways are clear of frost. To minimize materials costs, vehicle corrosion, and negative environmental impacts, frost-suppression chemicals should be applied only when, where, and in the appropriate amounts needed to maintain roadways in a safe condition for motorists. Accurate forecasts of frost onset times, frost intensity, and frost disappearance (e.g., melting or sublimation) are needed to help roadway maintenance personnel decide when, where, and how much frost-suppression chemical to use. A finite-difference algorithm (BridgeT) has been developed that simulates vertical heat transfer in a bridge based on evolving meteorological conditions at its top and bottom as supplied by a weather forecast model. BridgeT simulates bridge temperatures at numerous points within the bridge (including its upper and lower surface) at each time step of the weather forecast model and calculates volume per unit area (i.e., depth) of deposited, melted, or sublimed frost. This model produces forecasts of bridge surface temperature, frost depth, and bridge condition (i.e., dry, wet, icy/snowy). Bridge frost predictions and bridge surface temperature are compared with observed and measured values to assess BridgeT's skill in forecasting bridge frost and associated conditions. [ABSTRACT FROM AUTHOR]

Freezing precipitation, in the form of freezing rain, freezing drizzle, and/or wet snow, can damage transportation networks, infrastructure, and vegetation. Ten events with freezing precipitation (including freezing rain and wet snow) over the province of Manitoba, Canada were examined using surface observational datasets, reanalysis products and 4-km resolution Weather Research and Forecasting (WRF) products that were both a retrospective control (CTRL) simulation as well as a pseudo-global warming (PGW) simulation. All events tracked to the south and/or east of Manitoba and most (8 of 10) events were associated with a consistent large scale pattern of extratropical cyclone with 500 hPa trough, low surface pressure center nearby, and an atmospheric river. Local factors, such as the 400 m elevated terrain of Riding Mountain, influenced 2 events mainly by altering surface temperature to be favorable for freezing precipitation. These events in the PGW simulation occurred 40–120 km farther north on average, with freezing rain generally being enhanced and wet snow generally being reduced, although wet snow was introduced into events which originally only had freezing rain. This study further showed that power lines aligned west to east, perpendicular to the strongest winds, are most susceptible to the consequences of icing and accretion within the current climate as well as the thermodynamically forced future one. • Freezing rain, wet snow and their mixtures lead to major impacts in Manitoba, Canada. • Ten historic freezing precipitation events over this area are examined in detail. • Freezing precipitation is dramatically altered by the area's low topographic features. • Pseudo-global warming is assumed to examine future freezing precipitation. • With this warming, freezing precipitation types and their locations are altered. [ABSTRACT FROM AUTHOR]