Variability of Raindrop Size Distribution during a Regional Freezing Rain Event in the Jianghan Plain of Central China

The characteristics of the raindrop size distribution(DSD)during regional freezing rain(FR)events that occur throughout the phase change(from liquid to solid)are poorly understood due to limited observations.We investigate the evolution of microphysical parameters and the key formation mechanisms of regional FR using the DSDs from five disdrometer sites in January 2018 in the Jianghan Plain(JHP)of Central China.FR is identified via the size and velocity distribution measured from a disdrometer,the discrete Fréchet distancemethod,surface temperature,human observations,and sounding data.With the persistence of precipitation,the emergence of graupel or snowflakes significantly reduces the proportion of FR.The enhancement of this regional FR event is mainly dominated by the increase in the number concentration of raindrops but weakly affected by the diameters.To improve the accuracy of quantitative precipitation estimation for the FR event,a modified second-degree polynomial relation between the shapeμand slopeΛof gamma DSDs is derived,and a new Z-R(radar reflectivity to rain rate)relationship is developed.The mean values of mass-weighted mean diameters(D_(m))and generalized intercepts(lgN_(w))in FR are close to the stratiform results in the northern region of China.Both the melting of tiny-rimed graupels and large-dry snowflakes are a response to the formation of this regional FR process in the JHP,dominated by the joint influence of the physical mechanism of warm rain,vapor deposition,and aggregation/riming coupled with the effect of weak convective motion in some periods.

Analysis on A Synoptic Process of Freezing Rain in Anshan City

Based on MICAPS(Meteorological Information Combine Analysis and Process System) and observational data in Anshan Observatory,the freezing rain event occurred rarely in Anshan on February 24 in 2010 was analyzed.The results showed that there existed the inversion layer from the ground to 850 hpa level even to 700 hPa level when the local surface temperature was below 0 ℃.The temperature stratification structure of coldness-warmth-coldness was prevailing from the upper level to the ground with the existence of the ice melting layer of a certain thickness.Thus the freezing event occurred in Anshan.

Simulation of a Freezing Rain and Snow Storm Event over Southern China in January 2008 Using RIEMS 2.0

The Regional Integrated Environmental Model System （RIEMS 2.0） with NCEP Reanalysis II is utilized to simulate the severe freezing rain and snow storm event over southern China in January 2008, which caused severe damage in the region. The relationships between the freezing rain process and the large-scale cir- culation, in terms of the westerly and low-level jets, water vapor transportation, and northerly wind area/intensity indices, were analyzed to tmderstand the mechanisms of the freezing rain occurrence. The results indicate the fol- lowing： （1） RIEMS 2.0 reproduced the pattern of precipi- tation in January 2008 well, especially for the temporal evolution of daily precipitation averaged over the Yangtze River valley and southern China; （2） RIEMS 2.0 repro- duced the persistent trough in the South Branch of the westerlies, of which the southwesterly currents trans- ported abundant moisture into southern China; （3） RIEMS 2.0 reasonably reproduced the pattern of frequencies of light and moderate rain, although it overestimated the frequency of rain in southern China. This study shows that RIEMS 2.0 can be feasibly applied to study extreme weather and climate events in East Asia.

Application of Automatic Diagnostic Analysis on Atmospheric Horizontal Inversion Area and Freezing Rain Falling Area

The automatic diagnostic analysis on atmospheric horizontal inversion area is realized through changing the single atmospheric inversion into horizontal one based on the sounding data in Micaps system;the basic conceptual model INCLUDING atmospheric horizontal inversion area,ground 0 ℃ line,high altitude wet area,extreme point 0 ℃ line in inversion layer is buillt after the research on relationship between atmospheric horizontal inversion area and freezing rain falling area;automatic diagnostic analysis on freezing rain falling area is developed based on the conceptual model,the method is in accord well with the actual situation (freezing rain disaster weather occurred in most part of the south in beginning of 2008) and fitting rate reaches 94.4%,quantitative correlation coefficient between the area of freezing rain falling area and stations reporting freezing rain is 0.839.

How does tree age influence damage and recovery in forests impacted by freezing rain and snow?

The response and recovery mechanisms of forests to damage from freezing rain and snow events are a key topic in forest research and management. However, the relationship between the degree of damage and tree age, i.e., whether seedlings, young trees, or adult trees are most vulnerable, remains unclear and is rarely reported. We investigated the effect of tree age on the degrees of vegetation damage and subsequent recovery in three subtropical forest types-coniferous, mixed, and broad-leaved —in the Tianjing Mountains, South China, after a series of rare icy rain and freezing snow events in 2008. The results showed that damage and recovery rates were both dependent on tree age, with the proportion of damaged vegetation increasing with age(estimated by diameter at breast height, DBH) in all three forest types and gradually plateauing. Significant variation occurred among forest types. Young trees in the coniferous forest were more vulnerable than those in the broad-leaved forest. The type of damage also varied with tree age in different ways in the three forest types. The proportion of young seedlings that were uprooted(the most severe type of damage) was highest in the coniferous forest. In the mixed forest, young trees were significantly more likely to be uprooted than seedlings and adult trees, while in the broad-leaved forest, the proportion of uprooted adult trees was significantly higher than that of seedlings and young trees. There were also differences among forest types in how tree age affected damage recovery. In the coniferous forest, the recovery rate of trees with broken trunks or crowns(DBH > 2.5 cm) increased with tree age. However, in the mixed and broad-leaved forests, no obvious correlation between the recovery rate of trees with broken trunks or crowns and tree age was observed. Trees with severe root damage did not recover; they were uprooted and died. In these forests, vegetation damage and recovery showed tree age dependencies, which varied with tree shape, forest type, and damage type. Understanding this dependency will guide restoration after freezing rain and snow disturbances.

Indications of stratospheric anomalies in the freezing rain and snow disaster in South China,2008

NCEP/NCAR reanalysis data were used to characterize stratospheric temperature and water-vapor anomalies before and after the freezing rain and snow disaster of South China in 2008,and the influence of stratospheric circulation anomalies on the troposphere.Stratospheric temperature and water-vapor anomalies provided good leading indicators of this weather event.The period from December 1st 2007 to February 28th 2008 was divided into 18 pentads.During the 6th pentad,temperature decreased significantly at 10 hPa in the near-polar stratospheric region,and the decreasing trend strengthened and extended downward and southward to middle and lower latitudes.During the 14th-18th pentads,the temperature decrease reached its maximum and extended to 30°N.This coincided with the widespread freezing rain and snow event.By the end of January 2008,the temperature decrease ended in the near-polar stratospheric region,but continued in the mid-latitude area of the troposphere as the freezing rain and snow weather persisted.Similar to the temperature variations,positive anomalies of relative humidity in the stratospheric near-polar region also strengthened and extended downward and southward,coinciding with the freezing rain and snow event.Along with the significant relationship between the freezing rain and snow disaster and stratospheric circulation anomalies,the stratospheric polar vortex changed its shape in late December,intensifying and spreading downward from the top of the stratosphere and southward to the Asian continent,resulting in a deepening of the East Asian Trough and a strengthening of meridional circulation.Before the occurrence of the freezing rain and snow event,temperature and vapor increases in the stratosphere transferred downward to the troposphere,along with a stratospheric flow in the near-polar region southward to lower latitudes.

Analysis on the Long-Lasting Freezing Rain and Snowstorm Event at the Beginning of 2008

In this paper,main characteristics of the long-lasting freezing rain and snowstorm event in southern China at the beginning of 2008,features of the related atmospheric circulation and the causes thereof are analyzed.During the event,patterns of the atmospheric circulation stayed stable;the polar vortex located in the northern part of the Eastern Hemisphere was strong with little movement;the cold front from the polar region and the active warm air mass from the tropical ocean confronted each other for a long time;the blocking high to the west of Baikal remained strong and steady;the trough over central and western Asia maintained its position for quite long with a group of little troughs splitting from it frequently;the dominant wind at 700 hPa was southwesterly while shears and vortexes at 850 hPa developed continually,providing the necessary low-level convergence for subsequent precipitation.Meanwhile,in the mid troposphere,eddies were generated over the Tibetan Plateau and positive vorticity disturbances in the Sichuan Basin propagated eastward to the coastal regions of eastern China.The western Pacific subtropical high was intensive with westward and northward migrations.The subtropical frontal zone was puissant and the north-south temperature gradient was large.Quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau remained stable.Warm air masses over the tropical ocean were active,so was the trough in the southern branch of the westerlies over the Bay of Bengal.There were four episodes associated with this event.The first one was featured with the interaction of strong cold and warm air,while the other three with the quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau as well as vigorous penetration of cold air from the north.The existence of the inversion layer and the thick melting layer were one of the main reasons for the long-lasting freezing rains.The main reason for the snowstorms was that the positive vorticity over the Sichuan Basin propagated eastward to the coastal regions of eastern China.Abundant water vapor and intense updraft also favored the heavy snows.

Effects of the Upstream Temperature Anomaly on Freezing Rain and Snowstorms over Southern China in Early 2008

By using ERA-Interim data, the temperature anomaly of the freezing rain and snowstorm event that occurred from Ii to 22 January 2008 in southern China was analyzed. During this period, diabatic heating and temperature advection caused the temperature to increase anomalously over the Tibetan Plateau. The anomalously high temperature moving from the Tibetan Plateau to southern Chin.a played several roles. First, the upper-level subtropical jet over China was split into two parts in the north-south direction, which affected the development of freezing rain in southern China; second, a ridge formed because of the warmer air moving to China, which hindered the transport of cold air from its upstream blocking high, forced the cold air to gather behind the ridge, and facilitated the severe cold air outbreak in the later period of the event; third, an inversion layer formed because of the lower-level cold air and upper-level warmer air over southern China, which was conducive to the development of the event over southern China; and finally, because of the temperature anomaly, opposite wind directions appeared at the lower levels （below 700 hPa）, which helped transport of warm-moist and cold-dry air to the event area.

A NUMERICAL DIAGNOSIS OF LEADING SIGNALS FACILITATING A “NORTH-RIDGE SOUTH-TROUGH” DIPOLE FOR THE EARLY-2008 SOUTH-CHINA FREEZING-RAIN EVENTS

Previous studies emphasize the important role of a "north-ridge versus south-trough" dipole (affecting the latitudes from 20° to 75° N around the Tibetan Plateau) of anomalous geopotential height ( Z ) in the early-2008 abnormal cryogenic freezing-rain-and-snow events in the southern part of China. The present study intends to extract the leading signal facilitating the dipole based on the numerical outputs of a full Z-linear model for diagnosing the global Z . Using this model built on full primitive equations in spherical-isobaric coordinates, we can further split the anomaly of Z-Zfζ-uβ (representing the Z component not explicitly associated with the Coriolis parameter f and its meridional derivative β ) into 15 components. With the model-output Zfζ-uβ (mainly accounting for the dipole under the geostrophic balance) and Z-Zfζ-uβ matrices as the left and right singular vectors respectively, a maximum covariance analysis (MCA) is performed to extract the significant 2-4-day leading signal carried by the MCA Z-Zfζ-uβ mode in the upstream area of the dipole. This leading signal is mainly attributed to 1) the abnormally strong westerlies centered around the exit region of the Atlantic jet-stream and 2) the corresponding anomalous 950-300 hPa anticyclone to the south of the abnormally strong center of westerlies. The energy of the positive wave center around this jet exit region favors the downstream north-ridge while the energy of the negative wave center associated with the anomalous anticyclone favors the downstream south-trough.

State of the art and practice of pavement anti-icing and de-icing techniques

Pavement snow and icing are worldwide problems, but effective countermeasures are just beginning to be developed in China. The two most common snow and ice removal methods are mechanical clearance and chemical melting, and the advantages and disadvantages of each approach are discussed here, including environmental and structural damage caused by corrosive snow melting agents. New developments in chemical melting agents and mechanical equipment are discussed, and an overview of alternative thermal melting systems is presented, including the use of geothermy and non-geothermal heating systems utilizing solar energy, electricity, conductive pavement materials, and infrared/microwave applications. Strategic recommendations are made for continued enhancement of public safety in snow and ice conditions.

The Unprecedented Freezing Disaster in January 2008 in Southern China and Its Possible Association with the Global Warming

The unprecedented disaster of low temperature and persistent rain, snow, and ice storms, causing widespread freezing in the Yangtze River Basin and southern China in January 2008, is not a local or regional event, but a part of the chain events of large-scale low temperature and snow storms in the same period in Asia. The severity and impacts of the southern China 2008 freezing disaster were the most significant among others. This disastrous event was characterized by three major features： （1） snowfall, freezing rain, and rainfall, the three forms of precipitation, coexisted with freezing rain being the dominant producer responsible for the disaster; （2） low temperature, rain and snow, and freezing rain exhibited extremely great intensity, with record-breaking measurements observed for eight meteorological variables based on the statistics made by China National Climate Center and the provincial meteorological services in the Yangtze River Basin and southern China; （3） the disastrous weathers persisted for an exceptionally long time period, unrecorded before in the meteorological observation history of China. The southern China 2008 freezing disaster may be resulted from multiple different factors that superimpose on and interlink with one another at the right time and place. Among them, the La Nina situation is a climate background that provided conducive conditions for the intrusions of cold air into southern China; the persistent anomaly of the atmospheric circulation in Eurasia is the direct cause for a succession of cold air incursions into southern China; and the northward transport of warm and moist airflows from the Bay of Bengal and South China Sea finally warranted the formation of the freezing rain and snow storms and their prolonged dominance in the southern areas of China. A preliminary discussion of a possible association of this disastrous event with the global warming is presented. This event may be viewed as a short-term regional perturbation to the global warming. There is not any possibility for this event to divert the long-term trend and the overall pattern of the global warming.

A Preliminary Analysis of Features and Causes of the Snow Storm Event over the Southern Areas of China in January 2008

Four successive freezing rain/heavy snowfall processes occurred in the southern part of China from 11 January to 2 February 2008 （named ＂0801 Southern Snow Disaster＂ hereafter）, during which a large-scale blocking circulation lasted for a long time over the mid-high latitudes of the Euro-Asian continent. This severe event is featured with a broad spatial scale, strong intensity, long duration, and serious damage. During the event, the blocking situation in the mid-high latitudes maintained quasi-sationary, but weather systems in the lower latitudes were active. Abundant water vapor was supplied, and favorable weather conditions for ice storms were formed over the large areas across the southern part of China. The results in this paper demonstrate that the significant factors responsible for the abnormal atmospheric circulation and this severe event include： 1） the very active Arctic Oscillation （AO）, which helped the permanent maintenance of the planetary-scale waves; 2） the continuous transfer of negative vorticity from the upstream region around 50°E into the blocking area, which caused the blocking situation reinforced repeatedly and sustained for a long time; and 3） the active air currents south of the Tibetan Plateau, which ensured abundant moisture supply to the southern areas of China. The 0801 Southern Snow Disaster was accompanied by extremely severe icing. In this paper, the data from Cloud-Profile Radar onboard the satellite CloudSat are used to study the dynamic and microphysical features of this event. The results show that there existed a melting layer between 2 and 4 km, and ice particles could be found above this layer and in the layer near the ground surface. Surface temperature kept between -4℃ and 0℃ with relative humidity over 90%, which provided the descending supercooled waterdrops with favorable synoptic and physical conditions to form glaze and ice at the surface via freezing, deposition and/or accretion. Causes of the event might be, as a whole, traced back to the planetary-scale systems. The study on the polar vortex anomaly in this paper reveals that changes in the polar vortex in the stratosphere preceded those in the troposphere, especially in early December 2007, while the intensification of the polar vortex in the troposphere delayed dramatically until middle January and early February of 2008. This implies that changes in the polar vortex in the stratosphere may be a precursor of the ensuing severe event and a meaningful clue for extended forecasts of such a disaster.