Reflectance variation in boreal landscape during the snow melting period using airborne imaging spectroscopy

We aim a better understanding of the effect of spring-time snow melt on the remotely sensed scene reflectance by using an extensive amount of optical spectral data obtained from an airborne hyperspectral campaign in Northern Finland. We investigate the behaviour of thin snow reflectance for different land cover types, such as open areas, boreal forests and treeless fells. Our results not only confirm the generally known fact that the reflectance of a melting thin snow layer is considerably lower than that of a thick snow layer, but we also present analyses of the reflectance variation over different land covers and in boreal forests as a function of canopy coverage. According to common knowledge, the highly variating reflectance spectra of partially transparent, most likely also contaminated thin snow pack weakens the performance of snow detection algorithms, in particular in the mapping of Fractional Snow Cover (FSC) during the end of the melting period. The obtained results directly support further development of the SCAmod algorithm for FSC retrieval, and can be likewise applied to develop other algorithms for optical satellite data (e.g. spectral unmixing methods), and to perform accuracy assessments for snow detection algorithms.A useful part of this work is the investigation of the competence of Normalized Difference Snow Index (NDSI) in snow detection in late spring, since it is widely used in snow mapping. We conclude, based on the spectral data analysis, that the NDSI -based snow mapping is more accurate in open areas than in forests. However, at the very end of the snow melting period the behavior of the NDSI becomes more unstable and unpredictable in non-forests with shallow snow, increasing the inaccuracy also in non-forested areas. For instance in peatbogs covered by melting snow layer (snow depth < 30 cm) the mean NDSI -0.6 was observed, having coefficient of variation as high as 70%, whereas for deeper snow packs the mean NDSI shows positive values.     

江汉盆地始新世中、晚期气候变化周期性的孢粉学证据

采用时间序列分析方法对江汉盆地明钾1孔的孢粉数据进行了处理，发现在中、晚始新世时期孢粉植物群的波动存在各种周期成分。长周期有1.2Ma、2.0Ma及5.0Ma，中周期有0.2Ma、0.4Ma、0.6Ma、0.8Ma，其中1.2Ma及0.2Ma、0.4Ma、0.6Ma的周期成分在第四纪孢粉植物群的演化中也存在，且与8.0Ma B.P.以来的构造气候旋回，亚旋回的准1.2Ma及0.4Ma周期相一致。这表明地球运动轨道变化对气候的制约作用由来已久。该孔孢粉时间序列的趋成分显示最少有4个水平，反映了孢粉植物群演化的4个阶段。曲线还揭示36Ma B.P.前后的波动周期及振幅都有明显的变异，可能预示了一次大的环境变革。这一变化在很大程度上与喜马拉雅一期运动和地球南半球一次大的海洋和气候变冷事件相吻合。     

NUMERICAL STUDY OF THE EFFECTS OF PERSISTENT WARMER SEASURFACE TEMPERATURE IN TROPICAL INDIAN OCEAN ONATMOSPHERIC CIRCULATION IN THE EARLY SUMMERIN EAST ASIA IN 1991

By employing the CCM1（R15L12）long-range spectral model, study is undertaken of the effects of sea surface temperature anomaly(SSTA) for tropical Indian ocean on circulation transformation in the early summer in East Asia in 1991. The results indicate that warmer SSTA contributes to the increasing of the temperature over the Plateau in early summer, resulting in the intensification of tropical easterly jet on 100 hPa and northward shift of Northern Hemisphere subtropical westerly jet in May. It is obviously favorable for the subtropical high enhancement over western Pacific Ocean in May and subtropical westerly jet maintaining at 35～40 °N in June, making the Mei-Yu come earlier and stay over the Changjiang basin in 1991. Furthermore, warmer SSTA is also advantageous to averaged temperature rise in East Asia land region and Nanhai monsoon development. These roles are helpful in accelerating the seasonal transition for East Asia in early summer.     

关于亚洲夏季风爆发及北半球季节突变的物理机理的诊断分析:Ⅱ青藏高原及邻近地区地表感热加热的作用

本文是系列文章的第二篇,首先分析了１９８９年亚洲夏季风爆发时期青藏高原及邻近地区地表感热通量和大气温度场季节变化的基本特征,着重讨论了春季高原地表感热加热和亚洲季风爆发的联系,然后分析了１９８０～１９８９年１０ａ南海季风爆发的气候学特征。上述工作表明,在春末初夏过渡季节,高原上空大气温度变化出现阶段性的跃升,并同亚洲夏季风阶段性的爆发有很好的对应关系。高原地表感热通量的持续增大导致了对流层高层局地反气旋式扰动环流的出现,使南亚反气旋北进的过程明显受到高原局地热力环流的调制,而热带东风急流入口区所产生的强烈的高层辐散,提供了有利于热带季风对流在南海地区首先爆发的动力学条件。此外,从５月份至６月中下旬,青藏高原、伊朗—阿富汗上空强大暖中心相继建立的结果,直接导致了热带地区上空大气南北温度梯度的反向依次在南海—孟加拉湾东部和阿拉伯海—印度次大陆由东向西相继建立,从而决定了亚洲季风建立的过程在不同地区爆发的时间不同。     

欧亚500 hPa环流、雪盖和中国降水量3—4年耦合振荡

{{@ convertAbstractHtml(article.abstractinfoCn, "cn")}}   {{if article.graphicalAbstractinfoCn && article.graphicalAbstractinfoCn != ""}}{{@ article.graphicalAbstractinfoCn}}{{/if}}     

西太平洋副热带高压异常进退的对比分析

对1980年7月29日—8月16日西太平洋副高位置持续偏南和1983年7月16日—8月12日西太平洋副高位置持续偏北的两次季节内副高中期进退过程的环流特征和加热差异作了对比分析,讨论了东亚季风与对流活动、副高进退之间的关系.      

700年以来冲绳海槽南部黏土矿物来源及其对沉积环境的响应

本文对冲绳海槽南部HOBAB4-S2站700年以来的黏土矿物组成及结晶学特征进行了研究和分析,并对该区的物质来源以及黏土矿物所记录的东亚季风的演化历史进行了探讨。研究发现冲绳海槽南部HOBAB4-S2站黏土矿物组合总体是以伊利石（59%~77%,平均含量69%）和绿泥石（11%~17%,平均含量14%）为主,蒙脱石（5%~23%,平均含量12%）和高岭石含量（2%~6%,平均含量4%）则相对较低。通过黏土矿物研究对物源进行分析,发现冲绳海槽南部研究站位的黏土矿物中,伊利石、绿泥石以及高岭石主要来自于台湾岛上的河流,尤其是兰阳溪,蒙脱石则主要是来自于长江和东海大陆架的悬浮再沉积。蒙脱石/（伊利石+绿泥石）比值可以用来大致反映长江和东海大陆架与台湾岛对研究区物质输入的相对贡献。其相对贡献量的变化很好地记录了小冰期（1405 A.D.—1850 A.D.）东亚夏季风的减弱和台湾岛东北部-冲绳海槽南部地区相对湿润的气候特征。而该比值所指示的现代暖期（1960 A.D.）夏季风强度的减弱则可能主要是受到人类活动的影响。     

Hydrometeorological assessments and suspended sediment delivery from a central Himalayan glacier in the upper Ganga basin

Integrated hydrometeorological investigations are not frequently available at a regional scale over a longer time period, especially near the terminus of Indian Himalayan glaciers. An integrated approach to the collection of hydrological data has major advantages for understanding the runoff generation mechanisms at basin scale, particularly when coupled with meteorological observations. The current study involves time series analysis of hydrometeorological records collected near the terminus of the Chorabari Glacier, for four consecutive ablation seasons(June-Sept.) 2009-2012. The analysis shows that variation in rainfall was higher(c_v= 0.9) at the same elevation over proximal sites, while the intensity of extreme rainfall events was 121-160 mm/d. The diurnal temperature range(DTR) has a tendency to reduce over the ablation season because of the onset of the Indian Summer Monsoon(ISM) and then further increases during the ISM withdrawal indicating humid-temperate conditions. The peak discharge(Qpeak) was found to be higher during July and August. Snow and glacier melt contributed 76% of the total suspended sediment transport during peak ISM months(July and August) reflecting seasonal evolution of the hydrologic conduits. The results indicate that Karakoram and western Himalayan glaciers produce comparatively low sediment yield compared to central Himalayan glaciers. The hydrological variations are depicted through flow duration curves(FDC) for meltwater discharge and sediment load. The flow corresponding to Q_(50), Q_(75), and Q_(90)(where Qx is the discharge that is exceeded x percent of the time referred to as % dependability) are 4.2, 3.7, and 2.8 m~3/s; and the corresponding dependability for suspended sediment loads(SSLs) are 409.0, 266.0, and 157.2 t/d, respectively. The daily SSL and discharge(Q) from 2009 to 2012 were used to develop a sediment rating curve(SSL = 39.55 × Q~(1.588). R~2 = 0.8).Multiple regressions are used to determine the impacts of meteorological parameters on glacier melt.The meteorological conditions, hydrological characteristics, and suspended sediment delivery for the Chorabari Glacier provide insight on meltwater generation processes and sediment transport patterns during the ISM season.     

副高脊线的月内振荡对汉江上游秋汛的影响

利用1951—2014年中国台站逐日降水资料及NCEP/NCAR再分析资料,通过突变检验、周期分析等方法,研究了秋季9—10月副高脊线的年代际变化和月内振荡特征及其与汉江上游秋季降水存在的可能关联。结果表明:秋季副高脊线位置在1997年前后发生了突变,这与秋季副高强度的突变时间存在着较为明显的差异。秋季副高脊线位置发生突变后,与1997年之前相比明显偏北,且偏北日数增多,同时月内振荡也较脊线突变前偏强,与之对应的华西秋雨的降水量也随之增多。当其准双周周期较强时,汉江上游9—10月容易降水偏多;而相反地,脊线准双周周期弱时,则更易发生少雨。