FACTORS INFLUENCING THE VARIABILITY OF MIDWINTER SNOW-DEPTH DECREASE IN THE NORTHERN GREAT PLAINS OF THE UNITED STATES

Meltwater from snow provides an important source of fresh water for agriculture, domestic use, and hydroelectric power in many regions of the world. In the winter and early spring, however, rapid ablation of the snowpack can lead to environmental hazards such as flooding.

Most snowmelt research has relied on measured data, which have limited the spatial and temporal resolution of the studies. Generally, these studies have been conducted at single locations over periods of a few days to a few weeks. This project investigates snowdepth changes on a more expansive scale by using a one-dimensional mass- and energy-balance model (SNTHERM) to compute the convective and radiative fluxes associated with large snow-depth change events. Midwinter snow-depth changes and the variability in snow-depth changes are evaluated for a 25-year time period over the northern Great Plains region of the United States during February.

Statistical analyses indicate that variations in sensible heat flux have the greatest influence on midwinter variability of snow-depth changes. This is shown by the consistently strong relationship between modeled sensible heat-flux values and changes in snow depth over the entire study period and at nearly every station. Mean daily temperature followed by mean daily dew-point temperature show the next strongest statistical relationships with snow-cover ablation. Radiative fluxes and net solar radiation in particular, have low correlations with changes in snow depth and are therefore found to have little impact on snow-depth change variability during the month of February. Although net solar radiation is not highly correlated with the variability of decreases in depth, its energy input is nevertheless an important source of energy for ablation processes. [Key words: snow melt, modeling, Great Plains.]     

SPATIAL AND TEMPORAL CLIMATOLOGY OF SNOWSTORMS IN THE DEEP SOUTH

Significant snowstorm events occurring during the winters from 1948/49 through 1989/90 are studied using a network of 100 weather stations across the southern states of Mississippi, Alabama, Georgia and South Carolina. To qualify for entry into the snowstorm inventory, at least 10 out of the 100 sites need a minimum of 25 mm (1 inch) of snowfall. A total of 69 events are identified and further categorized into four magnitudes based upon areal coverage or total snowfall amount. The storms are also classified into regional categories based upon the dominant part of the study area affected. The spatial characteristics of the snowstorms are discussed and illustrated with maps showing snowfall distributions for representative storms. Most events impact the northern portions of the study area although some very major storms fit into a Miscellaneous category with unique snowfall distributions in the southern or central parts of the Deep South. The temporal character of snowstorm frequency reveals that a very low number of events occurred during the 1950s with peak occurrence during the 1960s. Snowstorms generally remained more frequent during the 1970s and 1980s compared to the 1950s.     

近60 a新疆塔城地区不同相态降水时空变化特征

基于1961—2020年9个国家气象观测站逐日资料,采用气候统计学方法分析了塔城地区不同相态降水的时空分布及变化规律,探讨了降水相态的变化成因及其可能影响。结果表明：(1)近60 a塔城地区年平均降水日数88.1 d,其中降雨日数最多,降雪日数次之,雨夹雪日数最少;3种相态降水在空间上呈现地区西北部多、中东部少的分布格局。(2)从不同相态降水日数的月际分布来看,降雨主要出现在4—9月,降雪在11月—翌年3月较多,3—4月和10—11月期间3种相态降水共存。(3)近60 a塔城地区各站不同相态降水的变化趋势存在一定的差异,总体呈现降雨日数增加而降雪日数减少的变化趋势,且降雨量的增速高于降雪量增速,其结果导致雪雨比率以-0.33%·(10a)^-1的速率减小。(4)气温增暖是塔城地区降水相态向多雨化转变的主要原因,同时北极涛动指数(Arctic oscillation index,AO)、北大西洋涛动指数(North Atlantic oscillation index,NAO)以及北半球极涡指数对降水相态的变化也有一定的影响。     

Synoptic Classification of Snowfall Events in the Great Smoky Mountains,USA

Mean annual snowfall in the Great Smoky Mountains National Park (GSMNP) exhibits considerable spatial variability, ranging from 30 cm in the valleys to 254 cm at higher elevations. Snowfall can be tied to a variety of synoptic classes (e.g., Miller A or B cyclones, 500 hPa cutoff lows), but the frequency and significance of different synoptic classes have not been fully assessed, particularly at higher elevations. In this paper, we manually classify all snowfall events during the period 1991 to 2004 according to a synoptic classification scheme, calculate mean annual snowfall by 850 hPa wind direction and synoptic class, and develop composite plots of various synoptic fields. Hourly observations from nearby first-order stations and 24 hr snowfall totals from five sites within the GSMNP are used to define snowfall events. NCEP/NCAR reanalysis data are used to develop composite plots of various synoptic fields for cyclones passing south and then east of the area (e.g., Miller A cyclones). Results indicate that over 50% of the mean annual snowfall at higher elevations occurs in association with low-level northwest flow, and that Miller A cyclones contribute the greatest amount to mean annual snowfall at all elevations.     

On the frequency of lake-effect snowfall in the Catskill Mountains

Meltwater from snow that falls in the Catskill/Delaware watershed in the Catskill Mountains in south-central New York contributes to reservoirs that supply drinking water to approximately nine million people in and near New York City (NYC). Using the Interactive Multisensor Snow and Ice Mapping System (IMS) 4-km snow maps from the National Oceanic and Atmospheric Administration’s National Ice Center, we identified and tracked 28 lake-effect (LE) storms that deposited snow in the Catskills (2004–2017). These storms, which generally originated from Lake Ontario, but sometimes from Lake Erie, represent an underestimate of the number of LE storms that contribute snowfall to the Catskills snowpack because snowstorms are not visible on IMS maps when they travel over already-snow-covered terrain. Using satellite, meteorological (including NEXRAD and National Weather Service Cooperative Observer Program), and reanalysis data, we identified conditions that contributed to the LE snowstorms and mapped snow-cover extent (SCE) following the storms. We show that LE snow makes an important contribution to the Catskills snowpack because of the frequency of events, even though the total amount of each event may, on average, be small. The IMS 4-km maps tend to overestimate SCE compared to MODerate-resolution Imaging Spectroradiometer and Landsat snow maps. Climate change could impact LE storms and the distribution of rain versus snow in the Catskills, which may affect future reservoir operations in the NYC Water Supply System and winter recreation in the Catskills.     

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。     

TEMPORAL CHARACTERISTICS OF PENNSYLVANIA SNOWFALL, 1950–1951 THROUGH 1989–1990

While speculation exists on global and regional climate change for temperature and precipitation, relatively little research is available on snowfall and its changes. Twenty-six sites were selected to describe and analyze various characteristics of snowfall in Pennsylvania from 1950–1951 through 1989–1990. Overall, the state experienced a significant stepwise change in seasonal snowfall total during this period. Abnormally high seasonal totals prevailed from 1957–1958 through 1971–1972. This rise in snowfall was accompanied by colder-than-normal temperatures and a dramatic increase in large daily snow events. Principal components analysis (PCA) revealed that the seasonal temporal patterns were not uniform across the state. The PCA revealed four distinct seasonal regions. These regions exhibited everything from nearly linear increases and decreases over time to cyclical formations. PCA performed on the months of November through April each unveiled between three and five separate temporal regions. PCA analyses generally identified an eastern region, a north-central region, and a western region across Pennsylvania. [Key words: regionalization, principal components, variability, snowfall, climate change, Pennsylvania.]     

东北地区暴雪天气的统计学划分方法及其时空分布特征

应用近49 a东北地区200个气象站冬半年降水资料和相对应的NCEP/NCAR 2.5o×2.5o温度场再分析资料,提出了基于降水相态变化特征的东北地区暴雪天气划分的统计学方法,将东北地区暴雪分为纯雪型暴雪(Ⅰ型暴雪)和雨雪混合型暴雪(Ⅱ型暴雪)两大类,并根据影响范围将其划分为大范围暴雪、区域暴雪和局地暴雪3类。在此基础上分析了不同类型东北区暴雪的时空分布特征。结果发现:两型暴雪出现频次基本相当,Ⅱ型大范围暴雪明显多于Ⅰ型,局地暴雪则相反;东北地区暴雪的年际变化较大且具有明显的年代际特征,20世纪70年代后期至80年代中期Ⅰ型暴雪明显多于Ⅱ型,21世纪初期Ⅱ型暴雪多于Ⅰ型,表明全球变暖在中国东北的区域响应显著;东北区暴雪主要集中在秋末和初春两季,但两型暴雪起止时间和峰值都不相同;两型暴雪出现频次都呈东多西少的分布,其中Ⅱ型暴雪频次自东南向西向北减小的梯度更大。从结果来看所提出的划分办法很好的反映了不同降水相态的东北地区暴雪的发生和变化规律具有很大差别,这种差异与以往东北地区暴雪个例的分析工作中反映的情况是一致的;证明了本划分方法是合理的,具有实际工作意义。     

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。     

近50年青藏高原东部冬季积雪的时空变化特征

选取青藏高原东部地区1961-2010 年64 个测站的积雪数据,分析了冬季积雪日数的空间分布和年代际变化特征,结果表明：高原东部冬季积雪空间分布差异较大,巴颜喀拉山、唐古拉山和念青唐古拉山多雪且变率大,藏南谷地、川西干暖河谷地带及柴达木盆地少雪且变率小,这样的空间分布是由周边大气环流系统及复杂局地地形共同造成的;高原东部冬季积雪表现出“少—多—少”的年代际变化特征,分别在80 年代末和20 世纪末发生由少到多和由多到少的两次突变,尤其是20 世纪末的突变更为显著;降雪和气温的变化是影响积雪日数的重要因素,其中降雪的影响更为显著;80 年代末高原冬季降雪由少到多的突变是造成积雪日数发生相应变化的主要原因;20 世纪末高原冬季气温和降雪分别发生由低到高和由多到少突变,其影响叠加导致积雪日数发生了更为显著的突变。     

1970—2018年秦岭南北冷季降雪量时空变化及其影响因素

基于72个气象站点逐日观测数据,对1970/1971—2018/2019年秦岭南北冷季（11月~次年5月）降水类型（降雪、降雨和雨夹雪）进行识别;重点关注降雪时空变化特征,探讨降雪与气温、湿球温度的响应关系;依据“夏季-秋季-冬季”Niño 3.4区海温异常状态,细化4种不同发展过程的厄尔尼诺-南方涛动（ENSO）事件,分析降雪异常与不同ENSO事件的对应关系。结果表明：① 相比气候平均态（1970—2000年）,1990—2018年,秦岭南坡（山地暖温带）降雪量下降了3.1 mm,基本与关中平原降雪量（17.1 mm）持平;② 空间趋势上,低海拔河谷地带降雪量以年代波动为主,山地高海拔地区为降雪下降区;③ 秦岭高山地区气温或湿球温度每升高1.0℃,降雪量分别下降23.1 mm和24.3 mm;从地带性角度分析,由北向南气温或湿球温度每升高1.0℃,秦岭南北降雪量分别下降3.0 mm和2.8 mm;④ 当厄尔尼诺/拉尼娜持续型发生时,关中平原降雪异常偏多;当拉尼娜发展型发生时,秦岭山地和大巴山区降雪异常偏少。当厄尔尼诺发展型发生时,秦岭南北降雪异常呈现“东西分异”,秦岭山地东部和关中平原为降雪异常偏少区。     

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.     

秦岭南北降雪异常空间模态识别及其影响因素分析

识别降雪异常空间模态,明晰降雪异常的影响因素,对理解中国南北过渡带气候变化规律具有重要的实践意义。论文基于1970—2020年逐日气象数据,辅以湿球温度动态阈值法、经验正交分解法等气候诊断方法,对秦岭南北冷季(11月—次年5月)降雪异常空间模态进行识别,探讨了不同主导模态与海气异常的相关关系。结果表明：(1)秦岭南北冷季降雪异常存在2个主导模态。第1模态为“全区一致型”,降雪异常偏强区分布于关中平原、秦岭山地、汉江谷地和大巴山区东段;第2模态为“山地主导下降型”,反映山地降雪异常对气候变化的敏感性;(2)在时间变化上,第1模态以年际波动为主,20世纪90年代中期后,空间模态多处于负相位,即全区一致降雪偏少;第2模态以年代转折为主,近期空间模态多处于正相位,即山地降雪异常偏少;(3)在影响因素上,第1模态降雪异常与1月中高纬度500 hPa欧亚遥相关波列相关,第2模态降雪异常与冬季赤道中东太平洋海温异常密切相关。研究将降雪异常格局与环流异常机制组合研究,可为理解中国南北过渡带降雪异常预警信号提供理论基础。     

Streamflow trends and hydrological response to climatic change in Tarim headwater basin

This paper has studied the change of streamflow and the impact of climatic vari-ability conditions on regional hydrological cycle in the headwater of the Tarim River Basin. This study investigates possible causes of observed trends in streamflow in an environment which is highly variable in terms of atmospheric conditions, and where snow and ice melt play an important role in the natural hydrological regime. The discharge trends of three head streams have a significant increase trend from 1957 to 2002 with the Mann–Kendall test. Complex time-frequency distributions in the streamflow regime are demonstrated especially by Morlet wavelet analysis over 40 years. The purpose is to ascertain the nature of climatic factors spatial and temporal distribution, involved the use of EOF (Empirical Orthogonal Function) to compare the dominant temperature, precipitation and evaporation patterns from normally climatic records over the Tarim’s headwater basin. It shows that the first principal component was dominated since the 1990s for temperature and precipitation, which identifies the significant ascending trend of spatial and temporal pattern characteristics under the con-dition of the global warming. An exponential correlation is highlighted between surface air temperature and mean river discharge monthly, so the regional runoff increases by 10%–16% when surface air temperature rises by 1℃. Results suggest that headwater basins are the most vulnerable environments from the point of view of climate change, because their wa-tershed properties promote runoff feeding by glacier and snow melt water and their funda-mental vulnerability to temperature changes affects rainfall, snowfall, and glacier and ice melt.     

Assessment of ski condition reliability in the Spanish and Andorran Pyrenees for the second half of the 20th century

In this study temporal trends of 14 climate and snow parameters related to ski conditions were analyzed for 11 ski stations located in the central Pyrenees (Spain and Andorra). We also investigated whether there was a temporal association for the analyzed parameters, such that the occurrence in a particular year of good (or bad) climate or snow conditions as represented by one parameter was similarly reflected by the other parameters. The lack of reliable climate and snow measurements was overcome by the use of simulated climate data retrieved from a high resolution hindcast simulation available for the period 1960–2006. These data were also used as inputs for an energy and mass snow energy model to obtain snow series. The results showed trends in ski reliability parameters for the 1960–2006 period. The number of days having a snowpack deeper than 30 cm and 100 cm showed declines at low and mid altitudes. The start of the ski season appears progressively delayed for all stations, and the ski season shortened. The frequency of rainy days increased at 3 stations and decreased at 8, while the frequency of days having heavy snowfall increased at 8 stations and declined at 3. Days having potential for snowmaking declined at all stations. The number of days having a wind-chill < −20 °C also decreased markedly, as overall did the number of days having a wind speed greater than 80th percentile. The main findings from the assessment of temporal associations between climate and snow parameters were positive correlations between snow depth and windy conditions. Seasons having a higher frequency of very cold days had a lower frequency of heavy snowfall and rainy days. Thus, the adverse effects on the ski industry of lesser snow availability may have been partially negated by the occurrence of fewer days of closure because of high winds, or other adverse meteorological factors.     

塔里木河流域径流变化趋势及其对气候变化的响应

This paper has studied the change of streamflow and the impact of climatic variability conditions on regional hydrological cycle in the headwater of the Tarim River Basin. This study investigates possible causes of observed trends in streamflow in an environment which is highly variable in terms of atmospheric conditions, and where snow and ice melt play an important role in the natural hydrological regime. The discharge trends of three head streams have a significant increase trend from 1957 to 2002 with the Mann-Kendall test. Complex time-frequency distributions in the streamflow regime are demonstrated especially by Morlet wavelet analysis over 40 years. The purpose is to ascertain the nature of climatic factors spatial and temporal distribution, involved the use of EOF （Empirical Orthogonal Function） to compare the dominant temperature, precipitation and evaporation patterns from normally climatic records over the Tarim＇s headwater basin. It shows that the first principal component was dominated since the 1990s for temperature and precipitation, which identifies the significant ascending trend of spatial and temporal pattern characteristics under the condition of the global warming. An exponential correlation is highlighted between surface air temperature and mean river discharge monthly, so the regional runoff increases by 10%-16% when surface air temperature rises by 1 ℃. Results suggest that headwater basins are the most vulnerable environments from the point of view of climate change, because their watershed properties promote runoff feeding by glacier and snow melt water and their fundamental vulnerability to temperature changes affects rainfall, snowfall, and glacier and ice melt.     

青藏高原强降水日数的时空分布特征

 根据青海和西藏48个气象台站近48 a（1961-2008年）的逐日降水和气温资料,分别以日降水量超过5 mm和25 mm作为冬半年（11月～翌年3月）和夏半年（5～9月）强降水的临界值,分析了青藏高原冬、夏半年强降水日数的时空分布特征。结果表明：（1）高原强降水日数与总降水量的空间分布型非常相似,夏半年均表现为由东南向西北递减,而冬半年则为由高原腹地向四周递减。（2）夏（冬）半年强降水主要集中在7月上旬～8月中旬（11月上旬和3月中下旬）。（3）夏（冬）半年强降水存在准6 a（5～6 a）的年际振荡以及准10～11 a（15 a）的年代际振荡。（4）强降水日数变化趋势的空间差异较大,夏半年高原北（南）部强降水日数普遍以增加（减少）趋势为主,而冬半年除雅鲁藏布江流域呈减少趋势外,高原大多数地区均表现出显著增加趋势。     

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.     

锡林郭勒积雪日数时空分布规律研究

利用1971-2015年锡林郭勒地区15个气象观测站近45 a的逐日积雪日数资料,采用滑动T检验、Mann-Kendall检验、小波分析和EOF方法对研究区的积雪日数时空变化特征进行分析。结果表明：研究期内积雪日数在1996年发生了一次由多到少的突变,且日数变化存在7 a的主周期和11 a、22 a的副周期。积雪月际变化呈单峰型的分布特征,多雪期主要集中在12~2月,少雪期分布在10月份和4月份;研究区空间分布差异性显著,总体呈东多西少、南多北少的分布格局,区内大部地区属于稳定积雪区。对积雪日数及其影响因子进行聚类分析,将研究区划分为4种类型,分别为降雪量偏少-积雪日数偏高区、降雪量-积雪日数一致偏高区、降雪量-积雪日数中值区、降雪量-积雪日数一致偏少区。该区有3种异常分布型：第一模态为全区一致偏多（少）型;第二模态为北多（少）南少（多）型;第三模态为中西部多（少）,东南部少（多）型。     

博州不同级别降水及极端降水事件的时空变化

根据1961—2005年新疆博州（博尔塔拉蒙古自治州的简称,下同）4站逐日降水资料,用阈值检测方法计算出博州地区极端降雨（雪）的阈值,并用气候趋势系数、Kendall-τ秩次相关以及滑动t检验等分析了博州地区不同量级降水日数以及极端降水日数的变化特征。研究表明,博州地区极端降水阈值与年平均降水量的空间分布基本一致：山区大,盆地小,地区间差异极大。3—10月一日降水量≤0.2 mm的微量降雨日数大范围减少;年降水量增加的方式在不同子区域是不同的：①对于年平均降水量仅有100 mm左右的艾比湖一带而言,主要体现在中雨、小雪次数的增加上,其他量级的雨雪日数及强度增加趋势不显著,这种增量对干旱区而言很小,无法改变干旱区的本质。②博河上游地区夏半年主要体现在小雨、大雨次数的增加以及中雨强度的增加上,冬半年主要体现在小雪、大雪或极端降雪日数的增加以及大雪、暴雪强度的增加上。虽然博河上游地区大雨次数显著增加,但强度显著降低。这种增加方式导致博河上游地区冬季牧区易出现雪灾,夏季易出现洪灾。③博河中游地区主要体现在小雪、中雪、大雪（或极端降雪）、中雨频次以及小雨强度的增加上,而且一日降水量≤0.2 mm的微量降雨日数的减少趋势大于其他量级降雨总次数的增加趋势。降水日的这种变化方式在该区域气候显著偏暖的气候背景中,极易造成春夏阶段性极端干旱事件频发。