Mapping cropping intensity trends in China during 1982–2013

Long range continuous monitoring information of cropping intensity is useful for sustainable agricultural management but still limited. This study filled this information gap through delivering spatiotemporal continuous datasets of cropping intensity in China during the past 30 years. Cropping intensity data were derived by a wavelet features-based method based on the long-term weekly global EVI2 (Enhance Vegetation Index with two bands) at 0.05° spatial resolution (5 km) from 1982 to 1999 and 8-day composite 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance products from 2001 to 2013. The remote-sensing estimated images in 2013 agreed well with field survey data (overall accuracy = 91.63%) and the national agricultural census data (r² = 0.89). Results revealed that the cropping intensity remarkably increased during 1982–1999 but slightly declined during 2001–2013. The overall cropping intensity increased from 1.34 in the 1980s to 1.41 in the 1990s, and then dropped to an average of 1.36 after 2000. From 1982 to 1999, approximately 93,225 km² single-cropped areas changed to double-cropping, primarily those located in the North China plain. However, 39,883 km² double-cropped areas were turned back into single-cropping areas from 2001 to 2013, principally located in the North China plain, the Middle-lower Yangtze River plain, and the hill regions of the southern Yangtze River. This reverse trend of cropping intensity was due to combined effects from the corresponding reverse variations in agricultural population, increasing agricultural mechanical power, positive agricultural policy. The agricultural duty free policy has only immediate effects on stabilizing cropping intensity in croplands with more favorable biophysical conditions.     

广州白云机场一次罕见的降水相态变化天气特征分析

利用常规气象观测资料和NCEP2再分析资料,对2016年1月24日广州白云机场出现的一次强冷空气过程的环流背景、主要影响系统以及期间降水多相态转换的天气特征进行了分析。结果表明:本次天气过程来源于一次横槽转竖型寒潮,冷锋、中低空切变线和低空急流给华南地区带来了较长时间的降水天气;白云机场上空存在多条0℃等温线和逆温层,探空廓线多次穿过0℃等温线,使得地面出现了雨和冰粒的相态转换;降水相态变化时暖层温度、0℃层高度、位势厚度、边界层高度、气柱云水量和地表抬升指数等指数均发生较大变化;当降水相态为雨时,暖层温度≥2℃,0℃层高度≤975 h Pa;当降水相态为冰粒时,暖层温度≤0℃,0℃层高度≌1000 h Pa。     

ERA5再分析数据适用性初步评估

利用山东省及周边地区10个站点的地面和高空观测资料对ERA5再分析资料的适用性进行了初步评估。结果表明：再分析的海平面气压和2 m温度与实况资料的相关性明显优于2 m相对湿度和10 m风场;高空温度和相对湿度在对流层中低层的适用性要好于高层,而位势高度和风场在中高层适用性较好;海平面气压再分析与实况的相关有着最明显的季节变化,2 m温度、2 m相对湿度和10 m风速则在部分站点有较明显的季节变化,而10 m风向的相关系数更多地表现出站点之间的差异,高空要素的适用性,季节和区域差异不明显。另外,对比发现,ERA5的适用性总体上要优于ERA-Interim再分析资料,地面和对流层低层的相对湿度、风场提高更为明显。     

基于再分析资料与观测资料的中国低温阈值变化特征研究

采用1979—2013年中国192站逐日最低温度观测资料和NCEP/NCAR、NCEP/DOE、JRA-55、ERA-Interim再分析资料及1979—2004年均一化资料,分别计算低温阈值并对比分析其气候态、年际和年代际变化、长期趋势等特征。结果表明:与观测结果相比,均一化资料阈值在东北、内蒙古西部和两广等地偏低,在青藏高原东侧、新疆北部和黄河中下游偏高,线性趋势则相反;再分析资料阈值在南方偏低、东北偏高,在东部的可信度高于西部;再分析资料能显示内蒙古中西部的降温趋势和青藏高原的增温趋势,但在数值和范围上有差异,且均低估了观测资料反映的华北地区的显著升温现象;再分析资料能体现观测资料阈值的全区一致性、东北与其他地区反相的空间分布及其年际变化特征,仅JRA-55和ERA-Interim可再现低温阈值的年代际变化特征。     

遥感及再分析降水产品在缺资料干旱内陆盆地的适用性评估

山区降水较集中,但降水测站多位于山谷或人口密集区,代表性差。遥感和再分析降水产品能提供时空分布连续的数据,不受地形条件限制。柴达木盆地中心属干旱荒漠区,水是制约该区开发的首要条件,其四周属高寒山区,降水相对较多,但降水监测十分薄弱。为获取该区相对精确的降水时空分布信息,本文评估了4套高分辨率降水产品（CMADS、TRMM、GPM和MSWEP）的适用性。首先基于地面站点数据评估它们在不同时空尺度上的精度,并分析它们在柴达木盆地的空间分布和年内分配特征。然后,以盆地东南隅的无测站山区香日德河流域为研究区,利用降水产品驱动SWAT模型来评估它们的分布式水文模拟适用性。结果表明：① MSWEP在年、月尺度上与站点降水的吻合程度最高（R ≥ 0.79,PBIAS = 0.5%）,其次是GPM和TRMM,CMADS精度最低（R ≥ 0.64,PBIAS = 5.8%）;② 从降水精度与站点高程的关系来看,降水产品在相对低海拔区容易高估站点降水,而在相对高海拔区常低估实际降水;③ 在香日德河流域,MSWEP（NSE = 0.64）在基准期（2009—2012年）的径流模拟表现明显好于其它降水产品（NSE = 0.36~0.59）,变化期（2013—2016年）表现最好的是CMADS（NSE = 0.75,其余产品NSE = 0.53~0.68）。本研究可为缺资料干旱山区获取精确的降水时空信息和后续水资源的科学管理与规划提供重要支撑。     

夏季怒江流域水汽输送多支特征及对降水影响

基于高空间分辨率0.25°的ERA-Interim再分析资料、TRMM 3B43 Version7数据、气象站点实测数据等多源数据,本文采用一种新的流域边界水汽通量概化和提取方法,揭示了夏季怒江流域水汽输送多支特征,并分析了其对降水时空分异的影响。研究表明,在高黎贡山南部、北部,伯舒拉岭北部及念青唐古拉山中部,有4支区域性水汽输送高值区,多年平均输送通量分别达102.6 kg/(m·s)、66.3 kg/(m·s)、39.7 kg/(m·s)和41.3 kg/(m·s)。多支水汽输送不仅深刻影响流域水汽输送格局,而且对降水时空分异也有不同程度影响。年际变化上,中下游横断山区水汽输送对降水的影响较小,上游青藏高原区影响较大,尤其以那曲—比如—索县一带影响最为显著。空间分布上,流域降水与水汽输送通量呈显著正相关,受多支水汽输送影响形成多个区域性多雨带。     

A quantitative assessment of snow shielding effects on surface exposure dating from a western North American 10Be data compilation

To better assess the spatiotemporal variations of the snow shielding effect on surface exposure dating, we compiled a dataset of 1341 ¹⁰Be ages from alpine moraines and glacially eroded valleys across western North America, and conducted a sensitivity test with both modern and time-integrated snow data covering the same region. Our analyses reveal significant differences in snow shielding both across our geographic domain and through time. In our time-integrated experiments we find snow-based exposure age corrections as low as 3.5% in the Great Basin region and high as 28.4% in the Pacific Northwest for samples dating to the Last Glacial Maximum (LGM) when no wind-sweeping is assumed. As demonstrated with our time-varying snow conditions with a global climate model and a positive degree day model, modern snow conditions across western North America cannot account for the varying snow patterns under large scale climate shifts since the LGM. The snow-based exposure age corrections from the modern data differ from those calculated by our time-varying model by up to 17% across our model domain. In addition, we find that the ¹⁰Be ages calculated under two end-member scenarios regarding wind-sweeping effects, specifically whether boulders were shielded only when the total snow accumulation exceeded boulder heights or were always shielded when the snow was present, can differ by ∼7.6% on average for LGM aged samples. Our analyses provide a model-based estimates of the spatiotemporal variability and complexity of snow shielding effects on surface exposure dates across western North America and highlight the need to consider snow depth variations both spatially and temporally when conducting surface exposure dating in terrains where snowfall accumulation is significant.     

基于长时间序列ERA-Interim再分析资料的广东沿海波浪能资源分布特征分析

波浪能的开发利用作为海洋可再生能源发展的重要组成部分,持续受到社会和学界的广泛关注。广东省波浪能资源的开发利用具有得天独厚的优势,但其发展水平还处于探索阶段,亟须进一步加强。本文利用长达40年(1979—2018年)的ERA-Interim再分析波浪资料,从总体分布特征上分析了广东沿海波浪能资源及其长期变化趋势。粤东-珠江口-粤西沿岸一线,波浪能资源呈现"低-高-低-高-低"的分布特征,具有显著的季节变化特征;长期变化趋势上呈现显著的逐年线性递增特征,递增速率为0.054 3 kW/(m·a),其中1979—1994年变化较为快速,2001—2018年变化较为平缓。     

4种再分析气温资料在羌塘国家级自然保护区的适用性研究

利用1979 - 2018年羌塘国家级自然保护区边缘的3个气象站点狮泉河（西部）、 申扎（中部）和安多（东部）的观测气温（OT）, 与ERA-Interim（ET）、 NCEP/NCAR（NT1）、 NCEP/DOE（NT2）和JRA-55（JT）4种再分析气温资料, 从年际变化和年变化两方面采用多年气温变化趋势、 均方差、 相关性等参数方法在羌塘自然保护区进行了适用性研究。结果表明： 从多年平均气温年际变化来看, 4种再分析资料在狮泉河（西部）的适用性较差, JRA-55资料在安多（东部）和申扎（中部）的适用性较好, 且冬季再分析气温资料与观测气温的相关性好于夏季; 从多年平均气温年变化来看, 均表现为冬季（1月或12月）最低, 夏季（7月或8月）最高的“单峰型”变化。综合来看, JRA-55资料在羌塘自然保护区的适用性较好, ERA-Interim对多年气温趋势变化表现不准确, NCEP/NCAR和NCEP/DOE与观测气温相比显著偏低。     

近30年全球干旱半干旱区的蒸散变化特征

全球变暖加剧了气候系统能量和水分循环相互作用的变化,水分平衡变化导致极端旱涝事件频发。地表蒸散是能量水分循环的重要过程,是理解气候变化的关键环节。本文基于1982~2011年FLUXNET-MTE观测资料和ERA-Interim再分析资料,分析了全球干旱半干旱区蒸散的时空变化特征及典型区域的变幅、趋势和季节变化。结果表明:（1）干旱半干旱区多年平均蒸散量小于300 mm。冬季蒸散量最小,夏季最大且变率也最强。1990年代前后,干旱半干旱区蒸散发生了明显的年代际转变,暖季的年代际差异尤为明显。（2）近30年来,东半球干旱半干旱区蒸散量呈增加趋势,西半球呈减小趋势。典型区域来看,南非呈显著增加趋势[25.14 mm（10 a）-1],美国西南部呈显著减小趋势[-19.86 mm（10 a）-1];萨赫勒、中国北部和澳大利亚呈增加趋势,阿根廷及智利南部呈减小趋势。（3）蒸散变化与温度、降水的变化联系密切,三者具有相似的年循环变化,但三者间相关性在干旱半干旱区具有显著的差异性。