楚科奇海浮冰区夏季短期颗粒有机物通量及其主要组分

中国首次北极科学考察期间 ,于 8月 2 0- 2 3日在楚科奇海浮冰区联合冰站实施了为期 4天的短期颗粒有机物通量研究。结果显示真光层的颗粒有机碳通量为 1 .582mgCm- 2 day- 1 ,2 2 0m深层为 1 .339mgCm- 2 day- 1 ,而相应的沉降颗粒物总通量分别高达 8.788和 1 0 .30 3mgm- 2 day- 1 。显示北极浮冰区的夏季融冰季节后期 ,颗粒有机碳通量的水平较低。与颗粒有机碳通量水平相似 ,生源硅和活性磷的通量水平也较低。对硅藻通量组份的分析表明 ,真光层沉降硅藻的优势种为Nitzschiacf.seriata、Naviculaglacialis和Melosirasp .,而 2 2 0m层则Lepto cylindrussp .占绝对优势 ,其丰度数量百分比均超过 70 %。硅藻碳通量的绝对值较低 ,为0 .1 0 7- 0 .1 1 3mgCm- 2 day- 1 。然而 ,真光层大型桡足类的碳估算值高达 1 0 8.67mgCm- 2day- 1 ,占浮游动物总碳量的 95 .3 % ,大型浮游动物的表观碳通量高于浮游植物碳通量 2- 3个数量级 ,显示楚科奇海夏季融冰期高浮游动物碳量、低浮游植物碳量的特点。但浮游动物表观碳量高的主要原因与浮游动物的昼夜垂直运动有关 ,却并非是实际向深层海洋传输的碳量     

土壤N2O释放通量季节变化的主要环境驱动因素研究

根据对贵州玉米－油菜轮作田、大豆－冬小麦轮作田和休耕地N2O释放通量的田间观测，研究了气候、农业活动和土壤性状对田间N2O释放通量季节变化的影响。结果表明，当温度适宜于反硝化作用进行时，降雨事件与N2O季节释放峰间存在明显的驱动－响应关系；形成N2O释放峰的最佳WFPS在48％－80％，气温在15－25℃；自然土壤冬春季N2O释放通量与温度间存在指数函数关系。施肥增加了土壤N2O释放峰的强度，而翻耕能形成弱的N2O释放峰，农业活动在一定程度上改变了释放通量的季节变化模式。土壤有机质含量的微弱差异对N2O的释放产生长远影响，三试验田N2O释放通量季节变化与土壤有效N含量间不存在直接的定量关系。     

Fluxes of CO2, CH4 and N2O from alpine grassland in the Tibetan Plateau

Using static chamber technique, fluxes of CO2, CH4 and N2O were measured in the alpine grassland area from July 2000 to July 2001, determinations of mean fluxes showed that co2 and N2O were generally released from the soil, while the alpine grassland accounted for a weak CH4 sink. Fluxes of CO2, CH4 and N2O ranged widely. The highest co2 emission occurred in August, whereas almost 90% of the whole year emission occurred in the growing season. But the variations of CH4 and N2O fluxes did not show any clear patterns over the one-year-experiment. During a daily variation, the maximum co2 emission occurred at 16:00, and then decreased to the minimum emission in the early morning. Daily pattern analyses indicated that the variation in co2 fluxes was positively related to air temperatures (R2=0.73) and soil temperatures at a depth of 5 cm (R2=0.86), whereas daily variations in CH4 and N2O fluxes were poorly explained by soil temperatures and climatic variables. co2 emissions in this area were much lower than other grasslands in plain areas.     

Net water vapour exchange over a mixed needle and broad-leaved forest in Changbai Mountain during autumn

Water vapour and CO2 fluxes were measured by the eddy-covariance technique above a mixed needle and broad-leaved forest with affiliated meteorological measurements in Changbai Mountain as part of China's FLUX projects since late August in 2002. Net water vapour exchange and environmental control over the forest were examined from September 1 to October 31 in 2002. To quantify the seasonal dynamics, the transition period was separated into leafed, leaf falling and leafless stages according to the development of leaf area. The results showed that (a) seasonal variation of water vapour exchange was mainly controlled by net radiation (Rn) which could account for 78.5%, 63.4% and 56.6% for leafed, leaf falling and leafless stages, respectively, while other environmental factors' effects varied evidently; (b) magnitude of water vapour flux decreased remarkably during autumn and daily mean of water vapour exchange was 24.2 mg m-2 s-1 (100%), 14.8 mg m-2 s-1 (61.2%) and 10.3 mg m-2 s-1 (42.6%) for leafed, leaf falling and leafless stage, respectively; and (c) the budget of water vapour exchange during autumn was estimated to be 87.1 kg H2O m-2, with a mean of 1427.2 g H2O d-1 varying markedly from 3104.0 to 227.5 g H2O m-2 d-1.     

Results from One-Year Continuous Operation of a Large Aperture Scintillometer over a Heterogeneous Land Surface

A large-aperture scintillometer (LAS) was operated continuouslyduring a period of more than one year over a heterogeneous land surface in Central Europeat the transition between marine and continental climates. The LAS measurements of the refractiveindex structure parameter, C_N ², were used to estimate the sensible heat flux. Thiswas possible for about 60to 80% of the time under daytime conditions during thesummer, with lower values obtained for the cold season (October to March). Using datafrom a three-week long field experiment, the LAS-based heat flux was compared with a weighedaverage of local heat flux measurements over the main land use classes (forest, agriculture,water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used forcomparison with the heat flux values of a numerical weather prediction model. An over-predictionof the model heat flux was found in summer but the modelled values were lower than the LASderived data during the cold season.     

An Extra Large Aperture Scintillometer For Long Range Applications

An incoherent scintillometer with 0.31-m aperture was testedalong a 9.8-km path over grassland. Scintillometer derived heat fluxes were comparedwith in situ eddy covariance measurements. Albeit with considerable scatter, the fluxescompared well during daytime. During nighttime credible fluxes are also obtained. The scintillometerfunctioned satisfactory for 96% of the 7-week period.     

Integrated Sensible Heat Flux Measurements of a Two-Surface Composite Landscape using Scintillometry

The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux (H) directly integrated over a two-field composite surface is evaluated. We describe a field experiment performed within the Alpilles/ReSeDa project in the south-east of France over a composite surface made up of wheat and bare soil (451 and 216 m long respectively) using two 0.15-m diameter scintillometers mounted at heights of 2.05 and 4.54 m. When compared against reference values obtained by the eddy correlation technique, LAS-measured sensible heat flux reveals a systematic overestimation of about 10%. A simple model describing the integration of the scintillometer signal along the beam for a two-field composite surface is described. A simulation of the experiment confirms that the bias observed isrelated to non-linearities in the integration process in relation with thebell-shape sensitivity curve of the instrument to the structure parameter for the refractive index it measures. The model is used to test the sensitivity of the LAS-derived H values to the composition of the pathlength (ratio of both surfaces) and to the contrast in sensible heat flux and roughness length between the two fields. Sensitivity tests to the aggregation scheme for roughness length (two of them are tested) and to the measurement height are also presented. The composition of the surface in combination with the contrast in sensible heat flux (in direct relation with the contrast in latent heat flux) explains most of the bias, with possible deviations ranging from -50 up to 80 W m^-2. A tentative semi-empirical method for correcting the bias is suggested, which only requires a crude estimate of the contrast in component sensible heat fluxes along the pathlength.     

Determination of Area-Averaged Sensible Heat Fluxes with a Large Aperture Scintillometer over a Heterogeneous Surface – Flevoland Field Experiment

To test the applicability of the scintillation method over a heterogeneous area an experiment was carried out in the summer of 1998 in Flevoland (The Netherlands). In the patchy area only four crops were grown namely sugar beet, potatoes, wheat and onions. From eddy covariance measurements it was found that the heterogeneity was mainly caused by differences in thermal properties. No variations in the aerodynamics roughness length were observed. Two large aperture scintillometers were installed at a height of 11.6 and 20.4 m. A good resemblance was found between the sensible heat fluxes derived from both LAS instruments and the area-averaged fluxes obtained from the in-situ eddy covariance measurements. The slight underestimation of the lower LAS could be assessed using a blending height model and an analytical footprint model. The results also indicated that when scintillometer measurements are made below the blending height the violation to Monin–Obukhov Similarity Theory is small and that reasonable fluxes can be obtained from path-averaged structure parameters.     

Measurements Of Turbulence Transfer In The Near-Surface Layer Over The Southeastern Tibetan Plateau

Turbulent flux measurements at Qamdo site over the Tibetan Plateau during TIPEX from May 18 to June 30, 1998 are presented. Sensible heat dominated,accounting for about 66% of the available energy (the sum of net radiation and soil heat flux) prior to the monsoon(dry period), reducing to about 31%, with latent heat increased to about 56% of available energy,in the monsoon season (wet period). Surface energy budget closure on average was about 0.80 (0.85)prior to the monsoon and 0.89 (0.76) during the monsoon using eddy correlation (profile) methods. The sum of latent and sensible heat fluxes calculated from the flux-profilemethod was smaller by about 15% than that from eddy correlation. Martano's method is used toestimate the surface aerodynamic roughness length z₀ and zero plane displacement d from singlelevel sonic anemometer data, giving d = 0.12 m and z₀ = 0.08 m. The overall neutral dragcoefficient (C_DN) and scalar coefficient (C_HN) were found to be C_DN = 0.0055and C_HN = 0.0059 in the southeastern area of Tibet. Their variations with the mean wind speed at 10 m are discussed.     

An OGCM Simulation of Seasonal and Interannual Variabilities in the Surface—Layer Pacific of the Equatorial Band

The heat budget is analyzed in the surface-layer (0-50 m) Pacific of the equatorial band (10°S-10°N),using the simulation of an ocean general circulation model from 1945 to 1993. The analysis indicates that downward net surface heat flux from the atmosphere and ocean advective heat fluxes play distinct roles in seasonal and interannual variabilities of surface-layer ocean temperature. The surface heat flux dominantly determines the ocean temperature in the seasonal time-scale. But, it has a negative feedback to the ocean temperature in the interannual time-scale. The interannual variability of ocean temperature is largely associated with the cold advection from off-equatorial divergent flow in the central Pacific and from upwelling in the cold tongue. Both the surface heat flux and ocean advective heat fluxes are important to the ocean temperature during an El Nino event. The ocean advective heat fluxes are further associated with local westward trade wind in the central Pacific. These results are largely consistent with some regional observational analyses.