Sub-regional snow cover distribution across the southern Appalachian Mountains

Abstract

Snowfall in the Southern Appalachian Mountain region of the eastern US is characterized by much spatiotemporal variability. Annual snowfall totals vary by up to 75 cm, and variations in snowfall intensity can lead to large differences in the local snowfall distribution. Research has shown that the synoptic pattern associated with the snowfall strongly influences the regional-scale distribution of snow cover. However, topographic variability results in locally complex snow cover patterns that are not well understood or documented. In this study, we characterize the snow covered area (SCA) and fractional snow cover associated with different synoptic patterns in 14 individual sub-regions. We analyze 63 snow events using Moderate-resolution Imaging Spectroradiometer standard snow cover products to ascertain both qualitative and quantitative differences in snow cover across sub-regions. Among sub-regions, there is significant variation in the snow cover pattern from individual synoptic classes. Furthermore, the percent SCA follows the regional snowfall climatology, and sub-regions with the highest elevations and northerly latitudes exhibit the greatest variability. Results of the sub-regional analysis provide valuable guidance to forecasters by contributing a deeper understanding of local snow cover patterns and their relationship to synoptic-scale circulation features.     

MAJOR SNOWFALL EVENTS IN THE GREAT PLAINS: TEMPORAL ASPECTS AND RELATIVE IMPORTANCE

The purpose of this research is to develop a better understanding of the spatial and temporal aspects of central Great Plains snowstorms by: (1) identifying trends in the frequency of snow events, (2) determining the relative importance of significant snowfalls (those producing snowfall amounts of 10 cm or more), and (3) identifying temporal and/or latitudinal variations in the frequency of significant snowstorms. Using NCDC data from cooperative program weather stations along a north-south transect, summary statistics, correlation coefficients, and frequency histograms were generated. The results provided evidence of a highly variable spatial and temporal record. Significant snowstorms are more critical to total snowfall amounts in Nebraska, Kansas, and Texas. Although strong, regionally coherent temporal trends were not detected, a predominance of January through March snow events was observed. The greater frequency of January snowstorms coincides with the mean position of the polar front, while February and March snowfall frequency reflects the increased incidence of Colorado cyclones. Decadally, snowfall variability corresponds with a change in mean temperatures and predominant upper atmospheric flow that occurred around 1950. This change brought less snow to the Southern and Central Plains states, but more snow to the north. [Key words: snowfall, Great Plains, temporal/spatial variability.]     

The Box: How the Shipping Container Made the World Smaller and the World Economy Bigger

Contemporary research into extratropical cloud systems optimizes the increase in resolution of visible (VIS) and thermal infra-red (IR) sensors, and the ability to retrieve wind and atmospheric moisture variables at mesoscales using microwave radiometry. These passively-acquired remote sensing data are used to develop synoptic climatological (conceptual and simple statistical) ‘models’ of mesoscale cyclones in cold-air outbreaks (mesocyclones, ‘polar lows’) occurring over the otherwise data-void southern oceans. Mesocyclones present a limitation to successful weather forecasting for New Zealand and coastal Chile, southern Australia and South Africa, during the cold season. The synoptic climatological analyses show that: 1) the patterns of mesocyclone cloud vortex origins, movement and dissipation (‘mesocyclone regimes’), exhibit spatial dependence and have associations with upper-ocean conditions; 2) mesocyclone ‘outbreaks’ are embedded within characteristic larger-scale anomaly fields of tropospheric pressure, height, and layer thickness (mean temperature); and 3) composite (statistical average) models of cloud system structure based on the microwave retrievals of marine weather reveal mesocyclones to be relatively dry in comparison with synoptic cyclones, yet very windy. These analyses should permit the development of methods to better predict these important cold-season storms over southern middle latitudes, and a fuller assessment of their significance for the larger hydroclimatic system.     

Satellite Passive Sensing of the Marine Atmosphere Associated With Cold‐Air Mesoscale Cyclones

Contemporary research into extratropical cloud systems optimizes the increase in resolution of visible (VIS) and thermal infra‐red (IR) sensors, and the ability to retrieve wind and atmospheric moisture variables at mesoscales using microwave radiometry. These passively‐acquired remote sensing data are used to develop synoptic climatological (conceptual and simple statistical) ‘models’ of mesoscale cyclones in cold‐air outbreaks (mesocyclones, ‘polar lows’) occurring over the otherwise data‐void southern oceans. Mesocyclones present a limitation to successful weather forecasting for New Zealand and coastal Chile, southern Australia and South Africa, during the cold season. The synoptic climatological analyses show that: 1) the patterns of mesocyclone cloud vortex origins, movement and dissipation (‘mesocyclone regimes’), exhibit spatial dependence and have associations with upper‐ocean conditions; 2) mesocyclone ‘outbreaks’ are embedded within characteristic larger‐scale anomaly fields of tropospheric pressure, height, and layer thickness (mean temperature); and 3) composite (statistical average) models of cloud system structure based on the microwave retrievals of marine weather reveal mesocyclones to be relatively dry in comparison with synoptic cyclones, yet very windy. These analyses should permit the development of methods to better predict these important cold‐season storms over southern middle latitudes, and a fuller assessment of their significance for the larger hydroclimatic system.     

Annual floods in New England (USA) and Atlantic Canada: synoptic climatology and generating mechanisms

New England and Atlantic Canada are characterized by mixed flood regimes that reflect different storm types, antecedent land surface conditions, and flood seasonality. Mixed flood regimes are known to complicate flood risk analyses, yet the synoptic climatology and precipitation mechanisms that generate annual floods in this region have not been described in detail. We analyzed a set of long-term annual flood records at climate-sensitive stream gauges across the region and classified the synoptic climatology of each annual flood, quantitatively describing the precipitation mechanisms, and characterize flood seasonality. We find that annual floods here are dominantly generated by Great Lakes-sourced storms and Coastal lows, known locally as ‘nor’easters.’ Great Lakes storms tend to be associated with lower magnitude annual floods (<75th percentile) and Coastal lows are more clearly associated with higher magnitude events (>75th percentile). Tropical cyclones account for few of all annual floods, including extreme events, despite causing some of the region’s largest and most destructive floods. Late winter/early spring is when the greatest number of annual floods occur region wide, and rainfall is the dominant flood-producing mechanism. Rainfall in combination with snowmelt is also important. Both mechanisms are expected to be impacted by projected regional climate change. We find little evidence for associations between flood-producing synoptic storm types or precipitation mechanisms and large-scale atmospheric circulation indices or time periods, despite upward trends in New England annual flood magnitudes. To more completely investigate such associations, partial duration flood series that include more floods than just the largest of each year, and their associated synoptic climatologies and precipitation mechanisms, should be analyzed.     

A Climatology of Northwest Missouri Snowfall Events: Long-Term Trends and Interannual Variability

The goal of this study was to develop a 50-yr. statistical climatology of snowfall occurrences using data from a dense network of cooperative station observations covering northwest and central Missouri, and these records were provided by the Missouri Climate Center. This included a study of the long-term trends and interannual variability in snowfall occurrence as related to sea surface temperature variations in the Pacific Ocean basin associated with the El Niño and Southern Oscillation (ENSO) and the North Pacific Oscillation (NPO). These trends and variations were then related to four synoptic-scale flow regimes that produce these snowfalls in the Midwest. The results demonstrate that during the snowfall season (Oct-April) the northwest Missouri region can expect about eight snowfall events which produce ≥3 in. (>7.5 cm) of accumulation. While no significant long-term trend in overall snowfall occurrence was found, a decrease in the number of extreme events (≥10 in., >25 cm) was noted. Also, fewer snowfall events were found during El Niño years, while more heavy snowfall events occurred during "neutral" years, and these results could be related to synoptic-scale variability. A closer examination of the results demonstrated that El Niño/La Niña related variability in snowfall occurrence was superimposed on longer-term NPO-related variability.     

南极长城站能见度变化分析

利用南极长城站1986—2012年能见度人工观测资料,分析了能见度的变化特征和趋势。结果显示,高于10 km能见度出现频率为61.0%,低于1 km的频率为8.0%。11—3月能见度较好,6—10月能见度较差;海雾和降水是夏季能见度降低的主要原因,冬季则主要归因于频繁的降雪、吹雪或雪暴等灾害性天气。10 km能见度发生频数有显著减小倾向,10 km能见度呈上升趋势,且冬季的上升速度最快,大雾、吹雪和降雪减少是主导冬季能见度升高趋势的重要因素。长城站能见度显著的天气尺度周期为2.1—8.3 d,年际变化周期为2 a、4.1 a和6.9—8.2 a,又以4.1 a最为显著。2012年开展的能见度自动观测实验表明,20 km范围内的自动观测精度较高,适用于长城站的连续监测。     

CLIMATOLOGY OF SURFACE CYCLONE TRAJECTORY AND INTENSITY FOR HEAVY-SNOW EVENTS AT THREE MIDWESTERN STATIONS

In this paper, we assess interstorm cyclone variations in intensity and motion for heavy-snow events at three north-central U.S. stations: Minneapolis, Madison, and Indianapolis. Daily data for the period from 1966 to 1996 were used to select the 157 storm events. Heavy-snow cyclone track location (latitude and longitude) and intensity (central pressure) means and standard deviations are calculated for each station and related to snowfall magnitude. The most northwesterly of the three stations, Minneapolis, had the largest median heavy-snowfall total and also the largest standard deviation in heavy-snowfall amount. Statistical analysis of surface cyclone parameters reveals significant interstation differences in storm trajectory and intensity measures for all cyclones and cyclogenesis group subsets. Chi-square tests suggest that the distribution of storms by month and cyclogenesis type is significantly nonrandom for all stations. Summary findings indicate that Midwestern heavy-snow events are dominated by southern Great Plains cyclones and that storms from secondary cyclogenic regions contribute to significant differences at the three selected locations. [Key words: cyclones, snowfall, Midwest.]     

CLIMATOLOGICAL CLASSIFICATION OF SEVERE WIND-DRIVEN RAIN EVENTS AFFECTING THE SOUTHERN UNITED STATES

This paper follows recent studies of wind-driven rain (WDR), and statistically defines a “severe” WDR event. The research suggests a classification for severe WDR events by association with larger-scale weather systems. The study incorporates data for the period 1971 to 1995 and, from these data, the analysis reveals that severe WDR events are confined mainly to the southern United States and occur most frequently in the cold and transition seasons. Extratropical cyclones are the most common synoptic-scale weather systems associated with severe WDR events. Extratropical cyclones originating in three areas accounted for more than 70 percent of the severe events in the study. The cyclone types are referred to as Colorado Lows (CLs), Gulf Lows (GLs), and Texas Lows (TLs). The attendant synoptic-scale conditions of cyclones that produced severe WDR events differ significantly from extratropical cyclones originating in similar areas but not producing severe events. Key differences between the two cyclone populations are the cyclone track trajectories and the low and midlevel synoptic environments promoting cyclone intensification. [Key words: wind-driven rain (WDR), synoptic climatology, extratropical cyclones, United States.]     

中国区域降雪量和降雪频率对沙尘天气的影响作用分析

1970—2000年间气象台站降雪量和沙尘天气统计结果显示,在中国冬春季主要积雪覆盖区域,沙尘天气发生频次相对较低,各类沙尘天气基本发生在积雪覆盖率低、年降雪量少的区域。时间序列分析结果进一步显示,年降雪量和沙尘天气之间存在显著的负相关,降雪量的增多对沙尘天气的年发生次数具有明显的抑制作用。同时,年降雪频率也是影响沙尘天气爆发频次的重要因素之一。对于中国西北干旱少雪的地区,尤其体现在新疆北部地区,年降雪频率的增加能够显著地减少各类沙尘天气的发生次数。     

An Investigation of the Influence of Cyclonic Airstreams on Midwestern Snowfall

Although previous climatological studies have investigated the relationship between cyclone frequency and trajectory and regional weather, analyses of structural airflow features and precipitation have been confined to meteorological case studies. In the following paper, the influence of Colorado cyclone airstreams on Midwestern snowfall is investigated using isentropic streamline maps for five cold season months. Results indicate that Colorado cyclones account for a minor percentage of Midwestern snowfall events and amounts. When snowfall is induced by Colorado cyclogenesis, diffluence and instability in the cold sector portions of the cyclone are the dominant lifting mechanisms.     

南极半岛及附近地区大风天气及其预报

南极半岛及附近地区经常出现大风天气,它是影响该地区的主要天气现象.造成大风的主要天气系统是极地气旋.极地气旋的强弱、移速和路径的不同,对南极半岛及附近地区的影响也不同.而周围的环境流场也是形成大风的重要因素.所以,分析天气形势背景,对做好该地区大风天气现象的预报是十分重要的.本文利用1985年1月到1987年10月中国南极长城站接收的智利南极弗雷气象中心播发的南半球地面传真天气图,北京气象中心绘制的南半球地面和500hPa高空天气图资料,结合长城站的地面气象观测资料,对影响南极半岛及附近地区的大风天气过程进行了分析和研究.主要根据地面天气图上的天气形势归纳为四种类型:(1)强极地高压型;(2).强副热带高压型;(3)南美大陆高压型;(4)副热带高压脊线南伸型.同时,结合分析结果提出了预报南极半岛及附近地区大风天气的思路.     

乌鲁木齐典型暴雪天气机理及成因分析

利用常规地面、高空观测资料、NCEP/NCAR 1°×1°再分析和FY卫星资料,针对乌鲁木齐1990年以来的3场典型暴雪天气,从高低空环流和天气系统配置、不稳定条件、水汽、动力及黑体亮温（TBB）变化等方面综合对比分析暴雪成因。结果表明：（1） 3场暴雪均发生在欧洲高压脊东南衰退,推动西西伯利亚低槽东移南下,与中纬度短波槽结合的环流形势下,高低空系统呈“后倾槽”结构,乌鲁木齐处在925~600 hPa西北急流与600~200 hPa强西南急流叠置区,且天山山脉的地形强迫抬升有利于暴雪的维持和加强。（2） 暴雪前850~700 hPa均有东南风存在,微差平流作用有利于平流逆温生成和加强,使得能量不断积聚,后期冷空气进入,冷锋锋生,层结不稳定发展,为暴雪天气提供热力条件,东南风和平流逆温维持时间越长,储存能量越多,降雪越强。（3） 暴雪区存在西南和偏西2条水汽输送通道,中层水汽输送对乌鲁木齐暴雪至关重要,850~600 hPa存在较强的水汽辐合,且700 hPa最强。水汽输送、辐合强度及持续时间共同决定暴雪强度。（4） TBB与降雪强弱有一定的对应关系,TBB越低,云顶高度越高,中尺度云团发展越旺盛,降雪越强,降雪前TBB（云顶高度）的第一次迅速降低（升高）预示降雪开始,降雪过程中TBB降低对应降雪强度增强,且TBB降幅越大、低TBB值维持时间越长,降雪越强。     

TEMPORAL STATISTICS: AN APPLICATION IN SNOWFALL CLIMATOLOGY

A method is introduced for the analysis of temporal distributions of climatic data based on a comparison of a variable's frequency distribution in the time dimension to a standard bell-shaped curve through time. The following statistics are defined using mean monthly snowfall data: the temporal mean, which is the average time for accumulation of 50% of the annual snowfall total; the temporal standard deviation, which characterizes the mean annual spread of snowfall accumulation about the mean; temporal skewness, which characterizes the symmetry of the annual snowfall distribution; temporal kurtosis, which measures the peakedness of the annual distribution; and temporal correlation, which quantifies the degree of association between two temporal distributions. Analysis of snowfall data demonstrates the utility of temporal statistics for quantifying temporal and geographic variations in a climatic variable's seasonal distribution. [Key words: snowfall, climatology, temporal statistics.]     

An Investigation of the Influence of Cyclonic Airstreams on Midwestern Snowfall

Although previous climatological studies have investigated the relationship between cyclone frequency and trajectory and regional weather, analyses of structural airflow features and precipitation have been confined to meteorological case studies. In the following paper, the influence of Colorado cyclone airstreams on Midwestern snowfall is investigated using isentropic streamline maps for five cold season months. Results indicate that Colorado cyclones account for a minor percentage of Midwestern snowfall events and amounts. When snowfall is induced by Colorado cyclogenesis, diffluence and instability in the cold sector portions of the cyclone are the dominant lifting mechanisms.     

我国高空风速的气候学特征

利用全国119个探空站1980~2006年13个等压面和地面附近的月平均风速资料,分析了不同高度年、季节平均风速的气候学特征。结果表明,全国平均200hPa以下风速随高度增加而增加,200hPa~30hPa之间风速随高度增加而降低,30hPa以上风速随高度增加而再呈增加趋势;春、秋季平均风速随高度变化与年平均相似,但冬、夏季的垂直分布差异明显;700hPa及其以上最大风速出现在1月,最小风速在7月或8月;850hPa和地面最大风速发生在4月;对流层风速年较差从下向上增加,在200hPa附近风速年较差最大。我国地面风速在东、西部地区较大,中部地区较小;500hPa年平均风速分布呈西低东高态势,最大中心出现在辽东半岛东部;200hPa年平均风速在江淮地区出现高值中心;500hPa冬季最大风速区在大陆南部,夏季北移并向西扩展;200hPa各季强风速区基本呈东西走向的带状分布,其中春季在江淮地区,夏季移至西北,秋季位于黄淮地区,冬季位置最南。     

Winter Cyclones and Circulation Patterns on the Western Great Lakes

The climatology of winter cyclones crossing the western Great Lakes was investigated. Data concerning storm strength, place of origin, and surface and 500 mb level synoptic characteristics were obtained from the months of October through February, 1955–1976, for 469 cyclones. November cyclones had the lowest mean pressure and strongest pressure gradient of the 5 months examined, but cyclonic frequencies were greatest in December and January. In all months, cyclones originating in the southwestern United States were significantly deeper than storms of northern origins, and these cyclones crossed the western Lakes most frequently in November. In November, cyclogenesis in the southwest was related to departures of the observed flow from the mean monthly mid-tropospheric circulation with trough development over the west. Yearly variations in the number of severe November cyclones were related to differences of mean monthly 500 mb flow.     

孟湾槽在百色市异常降雪天气中的作用

通过对1983年12月28日、2002年12月26日百色市两次异常降雪天气过程的对比分析,发现冬季孟加拉湾槽的发展东移在其中起到了关键性的作用。由于孟加拉湾槽前暖湿气流强盛,这种强盛的西南暖湿气流在低空冷空气堆(冷锋)上发生强烈的系统性上升运动,从而导致百色市及其邻近地区出现异常的雨雪天气。     

向极跨越70°N气旋的主要特征分析

北极地区以南生成并向北移动进入极区的气旋,在移动发展过程中常伴随大风、降水和升温等过程,对中低纬度地区物质和热量向极地输送起着重要作用,并对极区大气、海洋和海冰的变化产生一定影响。基于欧洲中期天气预报中心(ECMWF)发布的1979—2015年的海平面气压再分析数据产品,利用气旋自动识别和追踪算法,开展气旋的识别和追踪,获得向极跨越70°N气旋的数量、强度、活动轨迹及北向运动纬距等主要特征如下:该类气旋在数量上,春、冬季多于夏、秋季,年总数量和春、秋、冬季均呈减少趋势;强气旋易发于冬季,弱气旋多发于夏季;该类气旋活动轨迹,冬季集中分布在海上,夏季在陆地上;该类气旋北向运动纬距整体平均为9.2°,冬季平均最大,为10.2°,夏季平均最小,为7.3°;在年际变化上,年平均和春、冬季平均呈增长趋势,夏、秋季平均呈减少趋势;在年代际变化上,年平均和夏、冬季平均从1979—1988年到1989—1998年阶段都是减小的,到1999—2008年阶段是增大的,其后再减小,春、秋季则无明显趋势变化。     

毛乌素沙漠边缘地区春末夏初沙尘暴天气的物理量场结构

沙尘暴天气是毛乌素沙漠边缘内蒙古、甘肃、宁夏、陕西等地区春末夏初特有的一种灾害性天气。应用欧洲中心再分析资料的气压场和各种物理量场,选取了5个沙尘暴个例,分析了沙尘暴时期各层气压场的特征和物理量的空间结构。结果表明：蒙古高压是影响沙尘暴的主要天气系统。在沙尘暴发生时,蒙古高压前部整层都为宽广而强劲的西北气流,从地面可伸展到200 hPa附近,其物理属性是干、冷,其前部近乎于东北—西南向的区域,是锋区的密集带,也是各种物理量表现最为敏感的区域,垂直速度场为上升运动区,涡度场为正涡度区,高—低层散度差值场为正值,这些物理量都为沙尘暴的发生起到了推动和加强作用。