A Model Evaluation of Biological Effects on Seasonal Variation of Air–Sea CO2 Flux in the Yellow and East China Seas

Based on a coupled physical-biogeochemical model of the Yellow and East China Seas (YECS), the influence of biological activity on the seasonal variation of the air–sea CO₂ flux is evaluated. The solution of a sensitivity experiment that excludes biological activity is compared with that of a reference experiment that includes the full processes. The comparison reveals that biological activity results in a much stronger seasonal variation of surface dissolved inorganic carbon (DIC) and, hence, the ratio of total alkalinity to DIC in the northern parts of the YECS. The increased ratio resulting from biological DIC consumption contributes to the undersaturated partial pressure of CO₂ at the sea surface with respect to the atmosphere, causing the central Yellow Sea in summer and autumn to shift from being a CO₂ source to a sink; this same shift also occurs over the Changjiang Bank in summer. In the southern YECS, the biological effect is relatively weak. The comparison further reveals that low water temperature, instead of biological activity, is the dominant factor causing the YECS to become a carbon sink in spring. The biological effect on the variation of DIC (both at the surface and in the water column) differs greatly among the three representative regions of the YECS because of differences in primary production and hydrodynamic conditions. Particle-tracking simulations quantify the regional difference in horizontal advection. In the northern region, weaker horizontal advection causes the longer residence time of low DIC water induced by biological consumption. Over the entire YECS, biological activity contributes to about one-third of the total annual absorption of atmospheric CO₂.     

非均匀地表陆面过程参数化研究

地表固有的非均匀性影响近地层大气的垂直结构,甚至改变局地天气条件,亦使得大气数值模式大尺度网格面积水热通量的计算对其具有较强的敏感性。为了提高气候模式性能,非均匀陆面过程参数化已是当前大气边界层和陆面过程模式研究的热点和难点问题之一。本文在调研国内外大量文献的基础上,综述了近年来非均匀地表陆面过程参数化的研究现状,分析和比较了不同的参数化方法的优缺点以及数值模式模拟结果对它们的响应,提出了目前尚待继续探讨和解决的几个关键性问题。     

Modulation of El Nino-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific

The El Nin o-Southern Oscillation (ENSO) is modulated by many factors; most previous studies have emphasized the roles of wind stress and heat flux in the tropical Pacific. Freshwater flux (FWF) is another environmental forcing to the ocean; its effect and the related ocean salinity variability in the ENSO region have been of increased interest recently. Currently, accurate quantifications of the FWF roles in the climate remain challenging; the related observations and coupled ocean-atmosphere modeling involve large elements of uncertainty. In this study, we utilized satellite-based data to represent FWF-induced feedback in the tropical Pacific climate system; we then incorporated these data into a hybrid coupled ocean-atmosphere model (HCM) to quantify its effects on ENSO. A new mechanism was revealed by which interannual FWF forcing modulates ENSO in a significant way. As a direct forcing, FWF exerts a significant influence on the ocean through sea surface salinity (SSS) and buoyancy flux (Q B ) in the western-central tropical Pacific. The SSS perturbations directly induced by ENSO-related interannual FWF variability affect the stability and mixing in the upper ocean. At the same time, the ENSO-induced FWF has a compensating effect on heat flux, acting to reduce interannual Q B variability during ENSO cycles. These FWF-induced processes in the ocean tend to modulate the vertical mixing and entrainment in the upper ocean, enhancing cooling during La Nin a and enhancing warming during El Nin o, respectively. The interannual FWF forcing-induced positive feedback acts to enhance ENSO amplitude and lengthen its time scales in the tropical Pacific coupled climate system.     

Characteristics of Drag Coefficient in Different Coastal Regions of the South China Sea Under Tropical Cyclones–An Observational Study

To investigate the values of 10-m drag coefficient (CD) in different coastal areas under the influence of tropical cyclones, the present study used the observational data from four towers in different coastal areas of the South China Sea (SCS) during six tropical cyclone (TC) passages, and employed the eddy covariance method and the flux profile method. The analysis of footprint showed that the fluxes at Zhizai Island (ZZI), Sanjiao Island (SJI) and Donghai Island (DHI) were influenced basically by the ocean, and the flux at Shangyang Town (SYT) was influenced mainly by the land. The results showed that the dependence relationships of CD on 10-m wind speed (U10) in four different coastal areas under the influence of TCs were different. CD at ZZI and SJI initially increased and then decreased as U10 increased, similar to the pattern over the ocean. CD at ZZI and SJI represented the values over shallow water with seawater depths of ~7 m and ~2 m, respectively. Moreover, the critical wind speed at which CD peaked gradually decreased as the seawater depth became shallower in the coastal areas. CD at DHI and SYT decreased monotonously as U10 increased, similar to the pattern over the land. CD at DHI represented the value over the transition zone from shallow water to coastal land, and CD at SYT represented the value over the coastal land. Meanwhile, the eddy covariance method and the flux profile method were compared at ZZI and SYT during TC passages. It was found that their CD values obtained by the two methods were close. Finally, the parameterizations of observed u* and CD as a function of U10 over four different coastal areas were given under the influence of high winds. These parameterizations of observed C may be used in high-resolution numerical models for landfalling TC forecast.     

Determination of soil heat flux in a tibetan short-grass prairie

Soil heat flux is examined using a new method considering soil thermal conduction and convection processes. Using this method, we determine that soil heat fluxes owing to soil thermal conduction and convection were significant for the Naqu site in the summer of 1998. Experimental analyses of the surface energy balance are given.     

太平洋海气CO2通量对气候事件的响应

应用一个嵌套了海洋生物地球化学循环的太平洋环流碳循环模式,分析了1960～2000年太平洋不同海区海气碳通量随时间的变化。模拟结果显示,赤道太平洋为大气CO₂的排放区,南、北太平洋(南、北纬15°至模式计算区域南、北边界)为吸收区。3个海区海气碳通量随时间均存在显著的波动,其中赤道太平洋海气碳通量年际波动最显著。3个海区海气碳通量年际波动对气候事件的响应并不一致,在El Niño年赤道太平洋冷舌的强度和总溶解无机碳(DIC)的浓度以及输出生产力均会受到上升流减弱的影响而降低,La Niña年这些海气碳通量控制要素的分布情况则正好相反,但在南北太平洋副热带以及高纬度海区,El Niño和La Niña对这些要素带来的影响却并不一定相反,对输出生产力的影响甚至是一致的。以海表温度(SST)为例考察海气碳通量与物理场之间的关系表明,在赤道太平洋上升流对DIC的影响是控制海气碳通量变化的主要因素,而在其他海区,尤其是副热带海区,由于垂直运动的年际变化较小,且生物生产力水平较低,SST的波动对海气碳通量年际变化的影响更加重要。     

青藏高原东部拉萨河下游地区大气湍流交换特征研究

地气之间物质和能量随湍流运动进行输送,涡度相关技术是研究地气交换过程和评估大气资源的重要手段,它对湍流特征和精确的通量观测研究具有重要的作用.本研究利用拉萨蔡公堂通量站的闭路涡度相关系统,观测了 2020年8-11月青藏高原东部拉萨河下游地区典型高寒草甸下垫面的通量特征,并分析了该区域生长季和非生长季不同大气稳定条件下...     

高风速相干结构对通量输送影响的实验研究

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送.     

安徽省寿县农田能量平衡评价

为了评估安徽省寿县国家气候观象台新增的近地层通量观测系统业务运行状况,利用2007年9月—2008年8月寿县近地层通量系统观测资料,分析了农田下垫面能量平衡情况。结果表明:全年能量平衡比率平均为0.89,但不同条件下存在差异:白天明显大于夜间,春、夏、秋季明显大于冬季,裸地和麦、稻田明显大于雪地,晴、昙、阴、雨天气情况下差异不大。总体来说,能量不平衡是通量观测中较为普遍的现象,一般认为不平衡程度在10%~30%为合理范围,寿县国家气候观象台全年的能量不平衡程度 (11%) 恰好属于这个范围,并接近不平衡程度的下限,说明涡度相关法在淮河流域农田生态系统通量观测中可靠性较高。     

应用涡动相关法计算水热、CO2通量的国内外进展概况

目前涡动相关法被认为是国内外测定CO2、水热通量的最可靠方法。国外应用此方法解决了均匀下垫面假设下森林和农田等生态系统的CO2、水热通量的计算问题，目前致力于非理想下垫面（真实地形）的CO2、水热通量的计算。而国内应用涡动相关技术起步较晚，用此方法测定水热通量已有一定的知识积累，但测定CO2通量还处于资料收集阶段。我国自然生态系统、环境外交和社会经济可持续发展都要求精确查明我国CO2的时空变化特征和动力学机制，这应是今后研究的重点，并提出了应用该方法存在的主要问题及可能的解决方法，指出值得深入研究的领域。