Changes in Spring Snow Cover over the Eastern and Western Tibetan Plateau and Their Associated Mechanism

The spring snow cover(SC)over the western Tibetan Plateau(TP)(TPSC)(W_TPSC)and eastern TPSC(E_TPSC)have displayed remarkable decreasing and increasing trends,respectively,during 1985–2020.The current work investigates the possible mechanisms accounting for these distinct TPSC changes.Our results indicate that the decrease in W_TPSC is primarily attributed to rising temperatures,while the increase in E_TPSC is closely linked to enhanced precipitation.Local circulation analysis shows that the essential system responsible for the TPSC changes is a significant anticyclonic system centered over the northwestern TP.The anomalous descending motion and adiabatic heating linked to this anticyclone leads to warmer temperatures and consequent snowmelt over the western TP.Conversely,anomalous easterly winds along the southern flank of this anticyclone serve to transport additional moisture from the North Pacific,leading to an increase in snowfall over the eastern TP.Further analysis reveals that the anomalous anticyclone is associated with an atmospheric wave pattern that originates from upstream regions.Springtime warming of the subtropical North Atlantic(NA)sea surface temperature(SST)induces an atmospheric pattern resembling a wave train that travels eastward across the Eurasian continent before reaching the TP.Furthermore,the decline in winter sea ice(SIC)over the Barents Sea exerts a persistent warming influence on the atmosphere,inducing an anomalous atmospheric circulation that propagates southeastward and strengthens the northwest TP anticyclone in spring.Additionally,an enhancement of subtropical stationary waves has resulted in significant increases in easterly moisture fluxes over the coastal areas of East Asia,which further promotes more snowfall over eastern TP.

The Summer Snow Cover Anomaly over the Tibetan Plateau and Its Association with Simultaneous Precipitation over the Mei-yu–Baiu region

The summer snow anomalies over the Tibetan Plateau （TP） and their effects on climate variability are often overlooked,possibly due to the fact that some datasets cannot properly capture summer snow cover over high terrain.The satellite-derived Equal-Area Scalable Earth grid （EASE-grid） dataset shows that snow still exists in summer in the western part and along the southem flank of the TP.Analysis demonstrates that the summer snow cover area proportion （SCAP） over the TP has a significant positive correlation with simultaneous precipitation over the mei-yu-baiu （MB） region on the interannual time scale.The close relationship between the summer SCAP and summer precipitation over the MB region could not be simply considered as a simultaneous response to the Silk Road pattern and the SST anomalies in the tropical Indian Ocean and tropical central-eastern Pacific.The SCAP anomaly has an independent effect and may directly modulate the land surface heating and,consequently,vertical motion over the western TP,and concurrently induce anomalous vertical motion over the North Indian Ocean via a meridional vertical circulation.Through a zonal vertical circulation over the tropics and a Kelvin wave-type response,anomalous vertical motion over the North Indian Ocean may result in an anomalous high over the western North Pacific and modulate the convective activity in the western Pacific warm pool,which stimulates the East Asia-Pacific （EAP） pattern and eventually affects summer precipitation over the MB region.

Persistence of Snow Cover Anomalies over the Tibetan Plateau and the Implications for Forecasting Summer Precipitation over the Meiyu-Baiu Region

The present reported study investigated the persistence of snow anomalies over the Tibetan Plateau(TP) from the preceding seasons to summer and the relationship between the previous snow cover anomaly and summer precipitation over East Asia. The results showed that, relative to other snow indices, such as the station observational snow depth(SOSD) index and the snow water equivalent(SWE) index, the snow cover area proportion(SCAP) index calculated from the SWE and the percentage of visible snow of the Equal-Area Scalable Earth Grids(EASE-grids) dataset has a higher persistence in interannual anomalies, particularly from May to summer. As such, the May SCAP index is significantly related to summer precipitation over the Meiyu-Baiu region. The persistence of the SCAP index can partly explain the season-delayed effect of snow cover over the TP on summer rainfall over the Meiyu-Baiu region besides the contribution of the soil moisture bridge. The preceding SST anomaly in the tropical Indian Ocean and ENSO can persist through the summer and affect the summer precipitation over the Meiyu-Baiu region. However, the May SCAP index is mostly independent of the simultaneous SSTs in the tropical Indian Ocean and the preceding ENSO and may affect the summer precipitation over the Meiyu-Baiu region independent of the effects of the SST anomalies. Therefore, the May SCAP over the TP could be regarded as an important supplementary factor in the forecasting of summer precipitation over the Meiyu-Baiu region.

Characteristics of abrupt changes of snow cover and seasonal freeze-thaw layer in the Tibetan Plateau and their impacts on summer precipitation in China

In this paper, a variation series of snow cover and seasonal freeze-thaw layer from 1965 to 2004 on the Tibetan Plateau has been established by using the observation data from meteorological stations. The sliding T-test, M-K test and B-G algorithm are used to verify abrupt changes of snow cover and seasonal freeze-thaw layer in the Tibetan plateau. The results show that the snow cover has not undergone an abrupt change, but the seasonal freeze-thaw layer obviously witnessed a rapid degradation in 1987, with the frozen soil depth being reduced by about 15 cm. It is also found that when there ~s less snow in the plateau region, precipitation in South China and Southwest China increases. But when the frozen soil is deep, precipitation in most of China apparently decreases. Both snow cover and seasonal freeze-thaw layer on the plateau can be used to predict the summer precipitation in China. However, if the impacts of snow cover and seasonal freeze-thaw layer are used at the same time, the predictability of summer precipitation can be significantly improved. The significant correlation zone of snow is located in middle reaches of the Yangtze River covering the Hexi Corridor and northeastern Inner Mongolia, and the seasonal freeze-thaw layer exists in Mt. Nanling, northern Shannxi and northwestern part of North China. The significant correlation zone of simultaneous impacts of snow cover and seasonal freeze-thaw layer is larger than that of either snow cover or seasonal freeze-thaw layer. There are three significant correlation zones extending from north to south： the north zone spreads from Mr. Daxinganling to the Hexi Corridor, crossing northern Mt. Taihang and northern Shannxi; the central zone covers middle and lower reaches of the Yangtze River; and the south zone extends from Mt. Wuyi to Yunnan and Guizhou Plateau through Mt. Nanling.

A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon

The e?ect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SMMR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo e?ect, which is compensated by weaker upward sensible heat ?ux associated with colder surface temperature, whereas the e?ects of snow melting and evaporation are relatively smaller. The anomalies of surface heat ?uxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May. The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this in?uence is only obvious in early summer and almost disappears in later stages.

The Effects of Anomalous Snow Cover of the Tibetan Plateau on the Surface Heating

On the basis of snow data and AWS (Automatic Weather Station) data obtained from the Tibetan Plateau in recent years (1993 to 1999), the features of sensible heat, latent heat and net long-wave radiations are estimated, and their variations in more-snow year (1997/ 1998) and less-snow year (1996/ 1997) are analyzed comparatively. The relationships between snow cover of the Tibetan Plateau and plateau’s surface heating to the atmospheric heating are also discussed. The difference between more-snow and less-snow year in spring is remarkably larger than that in winter. Therefore, the effect of anomalous snow cover of the Tibetan Plateau in winter on the plateau heating appears more clearly in the following spring of anomalous snow cover. Key words Tibetan Plateau - Snow cover - Effects - Surface heat fluxes This research was supported by the National Key Programme for Developing Basic Sciences G1998040900 (I), National Natural Science Foundation of China (40075018) and Sichuan Youth Science and Technology Fund.

A new MODIS daily cloud free snow cover mapping algorithm on the Tibetan Plateau

Because of similar reflective characteristics of snow and cloud, the weather status seriously affects snow monitoring using optical remote sensing data. Cloud amount analysis during 2010 to 2011 snow seasons shows that cloud cover is the major limitation for snow cover monitoring using MOD10A1 and MYD10A1. By use of MODIS daily snow cover products and AMSR-E snow wa- ter equivalent products （SWE）, several cloud elimination methods were integrated to produce a new daily cloud flee snow cover product, and information of snow depth from 85 climate stations in Tibetan Plateau area （TP） were used to validate the accuracy of the new composite snow cover product. The results indicate that snow classification accuracy of the new daily snow cover product reaches 91.7% when snow depth is over 3 cm. This suggests that the new daily snow cover mapping algorithm is suitable for monitoring snow cover dynamic changes in TP.

The Impact of Tibetan Plateau Snow Cover on the Summer Temperature in Central Asia

The current work examines the impact of the snow cover extent(SCE)of the Tibetan Plateau(TP)on the interannual variation in the summer(June−July−August)surface air temperature(SAT)over Central Asia(CA)(SAT_CA)during the 1979−2019 period.The leading mode of the summer SAT_CA features a same-sign temperature anomalies in CA and explains 62%of the total variance in SAT_CA.The atmospheric circulation associated with a warming SAT_CA is characterized by a pronounced high-pressure system dominating CA.The high-pressure system is accompanied by warm advection as well as descending motion over CA,favoring the warming of the SAT_CA.Analysis shows that the interannual variation in the summer SAT_CA is significantly positively correlated with the April SCE over the central-eastern TP.In April,higher than normal SCE over the central-eastern TP has a pronounced cooling effect on the column of the atmosphere above the TP and can persist until the following early summer.Negative and positive height anomalies appear above and to the west of the TP.In the following months,the perturbation forcing generated by the TP SCE anomalies lies near the western center of the Asian subtropical westerly jet(SWJ),which promotes atmospheric waves in the zonal direction guided by the Asian SWJ.Associated with this atmospheric wave,in the following summer,a significant high-pressure system dominates CA,which is a favorable condition for a warm summer SAT_CA.

Distribution of winter-spring snow over the Tibetan Plateau and its relationship with summer precipitationin Yangtze River

The distribution of winter-spring snow cover over the Tibetan Plateau （TP） and its relationship with summer precipitation in the middle and lower reaches of Yangtze River Valley （MLYRV） during 2003-2013 have been investigated with the moderate-resolution imaging spectrometer （MODIS） Terra data （MOD10A2） and precipitation observations. Results show that snow cover percentage （SCP） remains approximately 20% in winter and spring then tails off to below 5% with warmer temperature and snow melt in summer. The lower and highest percentages present a declining tendency while the middle SCP exhibits an opposite variation. The maximum value appears from the middle of October to March and the minimum emerges from July to August. The annual and winter-spring SCPs present a decreasing tendency. Snow cover is mainly situated in the periphery of the plateau and mountainous regions, and less snow in the interior of the plateau, basin and valley areas in view of snow cover frequency （SCF） over the TP. Whatever annual or winter-spring snow cover, they all have remarkable declining tendency during 2003-2013, and annual snow cover presents a decreasing trend in the interior of the TP and increasing trend in the periphery of the TP. Hie multi-year averaged eight-day SCP is negatively related to mean precipitation in the MLYRV. Spring SCP is negatively related to summer precipitation while winter SCP is positively related to summer precipitation in most parts of the MLYRV. Hence, the influence of winter snow cover on precipitation is much more significant than that in spring on the basis of correlation analysis. The oscillation of SCF from southeast to northwest over the TP corresponds well to the beginning,development and cessation of the rain belt in eastern China.

Impacts of snow cover and frozen soil in the Tibetan Plateau on summer precipitation in China

This paper presents an analysis of the mechanisms and impacts of snow cover and frozen soil in the Tibetan Plateau on the sum- mer precipitation in China, using RegCM3 version 3.1 model simulations. Comparisons of simulations vs. observations show that RegCM3 well captures these impacts. Results indicate that in a more-snow year with deep frozen soil there will be more precipita- tion in the Yangtze River Basin and central Northwest China, western Inner Mongolia, and Xinjiang, but less precipitation in Northeast China, North China, South China, and most of Southwest China. In a less-snow year with deep frozen soil, however, there will be more precipitation in Northeast China, North China, and southern South China, but less precipitation in the Yangtze River Basin and in northern South China. Such differences may be attributed to different combination patterns of melting snow and thawing frozen soil on the Plateau, which may change soil moisture as well as cause differences in energy absorption in the phase change processes of snow cover and frozen soil. These factors may produce more surface sensible heat in more-snow years when the fi＇ozen soil is deep than when the frozen soil is shallow. The higher surface sensible heat may lead to a stronger updraft over the Plateau, eventually contributing to a stronger South Asia High and West Pacific Subtropical High. Due to different values of the wind fields at 850 hPa, a convergence zone will form over the Yangtze River Basin, which may produce more summer pre- cipitation in the basin area but less precipitation in North China and South China. However, because soil moisture depends on ice content, in less-snow years with deep frozen soil, the soil moisture will be higher. The combination of higher frozen soil moisture with latent heat absorption in the phase change process may generate less surface sensible heat and consequently a weaker updraft motion over the Plateau. As a result, both the South Asia High and the West Pacific Subtropical High will be weaker, hence caus- ing more summer precipitation in northern China but less in southem China.

RESPONSE OF TIBETAN SNOW COVER TO GLOBAL WARMING

Daily snow depth records at 60 primary dimatc statons over the Tibetan Plateau for the penod 1957-1992 are used as basis for point and areal hme series developmat. A statistical lnodel consistng of a trend plus serially correated noise is fitted to the data to test for trend in time series of snow cover. Restilts of three trend estimatos based on difference of average, least squares fitting and slope AN (1) process imply that the increase in annual snow depth is a systematic developlnent as evidenced by the presence of a dotenniruStic trend. The spatial pattem of trend estimates convinced that the increase trends are almost onAnpresent over the wtire plateau. Further exandnation of interannual variation of snow cover over the Tiban Plateau and suffoce air tenrperature in winter over the nodriem hemisphere reveals a posihve correlation of +0.21 betWeen the two time series for period of 1957-1992.

Spatiotemporal patterns of snow cover retrieved from NOAA-AVHRR LTDR: a case study in the Tibetan Plateau, China

Snow cover plays an important role in hydrological processes and seasonal water balance.Especially in the Tibetan Plateau(TP),snow cover is an important source of the Yangtze River,Yellow River and Lancang River(SRYYL),which greatly influences regional water balance.In this study,we quantified the temporal trend and spatial variation of snow cover across the TP by calibrating and developing the Advance Very High Resolution Radiometer(AVHRR)Long Term Data Record(LTDR)-derived snow cover products during 1982-2011.We also examined the relationship of snow cover with temperature and precipitation over the TP during 1982–2011.The results indicate that seasonal snow cover generally starts to accumulate from central plateau in October,while significant melting starts to occur from the southeastern plateau in May of following year.The long-term variability of snow cover is characterized by the tendency for a slight decrease in the mean snow coverage during the period of hydrological year(HY)1982–1993 and a slight increase from HY2001 to 2011,but the total snow cover area remains relatively stable over the past 30 years.The results also show that temperature plays a critical role in controlling the snow cover days.

2003-2010年青藏高原积雪及雪水当量的时空变化

利用MODIS逐日无云积雪产品与AMSR-E雪水当量产品进行融合,获取了青藏高原500 m分辨率的高精度雪水当量产品,通过研究青藏高原积雪时空动态变化特征,分析了积雪覆盖日数、雪水当量以及总雪量的季节及年际变化.结果表明:青藏高原地区降雪主要集中在高海拔山区,而高原腹地降雪较少,降雪在空间上分布极为不均;2003-2010年期间,平均积雪日数呈显著减少趋势,稳定积雪区面积在逐渐扩大,常年积雪区面积在不断缩小.与积雪日数时空变化相比,雪水当量增加的区域与积雪日数增加的区域基本一致,但喜马拉雅山脉在积雪日数减少的情况下雪水当量却在逐年增加,表明该地区温度升高虽然导致部分常年积雪向季节性积雪过渡,但降雪量却在增加.总的积雪面积年际变化呈波动下降的趋势,但趋势不显著,且减少的比例很少.最大积雪面积呈现波动上升后下降的趋势,平均累积积雪总量呈明显的波动下降趋势,年递减率为1.0×103m3·a-1.

Decadal Relationship Between Atmospheric Heat Source and Winter-Spring Snow Cover over the Tibetan Plateau and Rainfall in East China

By using a reverse computation method and the NCEP/NCAR daily reanalysis data from 1960 to 2004, the atmospheric heat source （AHS） was calculated and analyzed. The results show that AHS over the Tibetan Plateau （TP） and its neighboring areas takes on a persistent downtrend in spring and summer during the foregone 50 years, especially the latest 20 years. Snow depth at 50 stations over the TP in winter and spring presents an increase, especially the spring snow depth exhibits a sharp increase in the late 1970s. A close negative correlation exists between snow cover and AHS over the TP and its neighboring areas, as revealed by an SVD analysis, namely if there is more snow over the TP in winter and spring, then the weaker AHS would appear over the TP in spring and summer. The SVD analysis between AHS over the TP in spring and summer and rainfall at 160 stations indicates that the former has a negative correlation with summer precipitation in the middle and lower reaches of the Yangtze River, and a positive correlation with that in South China and North China. The SVD analysis of both snow cover over the TP in winter and spring and rainfall at the same 160 stations indicates that the former has a marked positive correlation with precipitation in the middle and lower reaches of the Yangtze River, and a reversed correlation in South China and North China. On the decadal scale, the AHS and winter and spring snow cover over the TP have a close correlation with the decadal precipitation pattern shift （southern flood and northern drought） in East China. The mechanism on how the AHS over the TP influences rainfall in East China is discussed. The weakening of AHS over the TP in spring and summer reduces the thermodynamic difference between ocean and continent, leading to a weaker East Asian summer monsoon, which brings more water vapor to the Yangtze River Valley and less water vapor to North China. Meanwhile, the weakening of AHS over the TP renders the position of the subtropical high further westward and the rain belt lasting longer in the Yangtze River Valley, which causes more rain there and less rain in North China, thus showing the pattern of ＂southern flood and northern drought＂ in the latest 20 years. It is inferred that the increase of snow cover over the TP brings about the reduction of surface temperature and then surface heat source, leading eventually to the weakening of AHS there.

2000-2005年青藏高原积雪时空变化分析

利用MODIS卫星反演的积雪资料以及同期气象资料,分析了2000-2005年青藏高原积雪分布特征、年际变化及其与同期气温和降水的关系,结果表明:青藏高原积雪分布极不均匀,四周山区多雪,腹地少雪;高原积雪期主要集中在10月到翌年5月;2000-2005年高原积雪年际变化差异较大,积雪面积总体上呈现冬春季减少、夏秋季增加的趋势;气温和降水是影响高原积雪变化的基本因子.冬季,高原积雪面积变化对降水更为敏感;春季,气温是影响高原积雪面积变化更主要的因素.

1981-2010年青藏高原积雪日数时空变化特征分析

全球气候变暖大背景下,作为冰冻圈最为活跃和敏感因子,青藏高原积雪变化备受国内外关注.本文利用青藏高原(以下简称高原)1981-2010年地面观测积雪日数资料,较系统地分析了近30年来高原积雪日数的时空变化特点.主要结论如下:(1)近30年内高原平均年积雪日数出现了非常显著的减少趋势,减少幅度达4.81 d·(10a)-1,其中冬季减幅最为明显,为2.36 d·(10a)-1,其次是春季(2.05 d·(10a)-1),而夏季最少(0.21 d·(10a)-1);(2)30年间,积雪日数较少的年份多数出现在本世纪初10年内,且2010年属于异常偏少年,高原积雪日数在1997年左右发生了由多到少的气候突变;(3)在空间上,北部柴达木盆地及其附件区域部分气象台站观测的年积雪日数出现了不显著的增加趋势之外,高原91.5%的气象站年积雪日数呈减少趋势,且高寒内陆中东部和西南喜马拉雅山脉南麓等高原历年积雪日数高值区域减少最为明显;(4)由于受到气象台站所在地理位置、地形地貌、地表类型、海拔高度、局地气候以及大气环流等综合影响,高原平均年积雪日数的空间差异很大,最多达146 d,最少的则不足1 d,平均仅为38 d,其中高寒内陆中东部是积雪日数最长的区域,而东南部海拔和纬度较低的干热河谷地区积雪日数最少.

青藏高原MODIS逐日无云积雪面积数据集 (2002-2015年)

青藏高原积雪对于能量和水循环起着重要的反馈和调节作用,其消融过程也直接影响着融雪性河流流量的变化,积雪的存在以及长时间变化等也是区域气候、生态和灾害的影响和响应敏感因素之一。中分辨率成像光谱仪(MODIS)具有高时空分辨率的特点,被广泛应用于积雪遥感动态监测。青藏高原地区积雪的赋存变化较快,高原周边高山区具有冰雪资源丰富,大气对流活跃等特点,而光学遥感往往受云的影响,在日时间尺度上积雪覆盖监测需要考虑去云问题。在充分考虑青藏高原的地形和山地积雪特征的情况下,本套数据集采用了多种去云过程和步骤相结合,逐步实现保持积雪分类精度的情况下,完成逐日积雪的云量消除,形成了“青藏高原MODIS逐日无云积雪面积”的逐步综合分类算法,完成了“青藏高原MODIS逐日无云积雪面积数据集(2002~2015年)”。论文选取2009年10月1日至2011年4月30日中的两个积雪季为算法研究和精度验证试验数据,采用研究区145个地面台站提供的雪深数据作为地面参考。结果表明,在高原地区,当积雪深度>3 cm时,无云积雪产品总分类精度达到96.6%,积雪分类精度达89.0%,整个算法流程对MODIS积雪产品去云的精度损失较低,数据可靠性较高。

Role of ENSO in the interannual relationship between Tibetan Plateau winter snow cover and Northwest Pacific tropical cyclone genesis frequency

Previous studies have revealed a significantly negative correlation between prior winter snow cover over the Tibetan Plateau （TPSC） and tropical cyclone genesis frequency （TCF） over the western North Pacific （WNP） in the following typhoon season. This study revisited this relationship based on long-term observational data. The results showed that the interannual correlation between TCF over the WNP and TPSC experienced a shift in the early 1990s. This correlation is significant during only 1993-2012 and is considerably weak during 1976-1992. The possible reasons causing the shift were examined further, and the results demonstrated that the central Pacific （CP） E1 Nifio-Southern Oscillation （ENSO） has played a vital role in intensifying the interannual relationship between TCF over the WNP and TPSC since the early 1990s. During 1993-2012, TPSC was negatively related to CP ENSO. When TPSC was higher than （lower than） normal, CP ENSO was often in its cold （warm） phase. Such a combination remarkably enhances the relationship of TPSC with the zonal land-sea thermal difference and thus with the summer monsoon over the WNE Additionally, it enhances the modulation of TPSC on the dynamical environments controlling TCF. As a result, the linkage between TPSC and TCF was significantly strengthened in this period. In sharp contrast, due to the weak relationship between TPSC and ENSO followed by the weak modulation of TPSC on the summer monsoon over the WNP and the dynamical environment during 1976-1992, the linkage between TPSC and TCF was weak during this time period. The results from additional dynamical diagnostic analyses further showed that during 1993-2012 CP ENSO modulated the barotropic energy conversion of zonal winds over the WNP, contributing to the intensified relationship between TPSC and TCF. These results will improve seasonal forecasting of tropical cyclone activity over the WNP.

1961-2014年青藏高原积雪时空特征及其影响因子

利用青藏高原(下称高原)1961-2014年地面110个气象站积雪深度、积雪日数、气温和降水逐日资料,系统地分析了高原积雪深度和积雪日数时空特征,并进一步探究了高原积雪深度和积雪日数与气候因子和地理因子之间的关系。研究发现:1961-2014年高原年平均积雪深度和积雪日数分别为0.26 cm和23.78 d,空间和季节尺度上分布不均匀,且积雪深度和积雪日数大值并不完全重合;在整体变化趋势上,积雪深度和积雪日数均呈缓慢下降趋势,分别为-0.0080±0.0086 cm·(10a)^-1(p=0.36)和-0.64±0.47 d·(10a)^-1(p=0.17),但在数理统计上不显著,且各站点差异性大;积雪深度和积雪日数在春季、冬季和年表现为“减-增-减”的年代际变化特征,而在秋季为“增-减”的变化特征;气温与积雪深度和积雪日数均有较好的相关性,冬季的降水与积雪深度和积雪日数高度相关;积雪深度和积雪日数随海拔呈增加趋势,积雪日数与纬度也高度相关,但积雪深度与纬度的相关性不明显。

Impacts of snow cover duration on vegetation spring phenology over the Tibetan Plateau

Aims Snow cover occupies large percentage of land surface in Tibetan Plateau.Snow cover duration(SCD)during non-growing seasons plays a critical role in regulating alpine vegetation’s phenology by affecting the energy budgets of land surface and soil moisture con-ditions.Different period’s snow cover during non-growing season may have distinct effect on the vegetation’s phenology.Start of season(SOS)has been observed advanced under the ongoing cli-mate change in the plateau,but it still remains unclear how the SCD alters the SOS.This study attempts to answer the following questions:(i)What is the pattern of spatial and temporal variations for SCD and grassland SOS?(ii)Which period’s SCD plays a critical role in grassland’s SOS?Methods The remote sensing datasets from the Moderate Resolution Imaging Spectroradiometer(MODIS)were utilized to compute the SOS and SCD on the Tibetan Plateau over 2003-2015.The Asymmetric Gaussian function was applied to extract SOS.We also explored the spatial pattern and temporal variation of SOS and SCD.Then,by using linear correlation coefficients,we investigated the driving effects of different period’s non-growing season SCD on SOS.Important Findings The non-growing season SCD slightly decreased during 2003-2015,while SOS exhibited an overall advancing trend.Advanced trends in SOS were observed in the eastern plateau,and the delayed trends were mainly founded in western plateau.Snow cover area exhibited two separate peaks during autumn and late winter over the plateau.Extended SCD regions mainly distributed in middle-east of the plateau,while shrunken SCD distributed in other regions of the plateau.SCD of different seasons caused distinct effects on vegetation SOS.Lengthened autumn SCD advanced SOS over the eastern plateau.The slightly lengthened SCD postponed SOS over the western plateau.In the wet meadow regions,advanced SOS was positively associated with SCD during the entire non-growing season,whereas for the dry steppe,SCD over the preseason played a more dominant role.The SCD of previous autumn and winter also showed lag effect on SOS over meadow regions to a certain extent.This study confirmed the importance of SCD to phenological pro-cesses at the beginning of growing season and further suggested that role of SCD should be discriminated for different periods and for dif-ferent heat-water conditions.With the lag effects and SCD’s distinct effect of different seasons considered,predictions on the Tibetan Plateau’s spring phenology could be improved.