我国东南象限冬、春季雨带的相关分析

通过对我国东南象限冬、春季雨带的相关分析,发现当冬、春季之间地气形势稳定时,约有68%的春雨带位于冬雨带南方50~300 km的范围内,此统计结果可作为预报春雨的指标;当冬季地气形势出现“双跳型”时,冬、春季之间的雨带可相距500 km以上,此时冬、春季的地震时空变化有可能作为春雨位置的预报指标。     

An observational study of wind-induced waving of plants

The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s^–1 measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s^–1 measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.     

An Observational Study of Wind-Induced Waving of Plants

The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s⁻¹ measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s⁻¹ measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.     

春秋季节侧柏-山毛桃群落演替初期的小气候特征

开展森林小气候研究,对于协调生物与环境的关系、保护生物多样性以及实现森林的可持续高效管理,都是十分重要的。选择5月和10月典型的晴朗无风天气条件,以侧柏纯林为对照,研究了郁闭度为0.25、0.54和0.87的侧柏-山毛桃群落演替初期小气候特征。结果表明,侧柏-山毛桃混交林对春季和秋季林内空气温度的早间增温效应和午间降温效应,随着郁闭度增加而增大。春季的高温峰值出现在15时,晚于秋季的高温峰值13时。春季和秋季林内10 cm处地温日间呈持续增加趋势,春季以郁闭度为0.25的B点为最高,秋季侧柏纯林在12时前为最低,14时后为最高。春季林内空气相对湿度从07时到11时呈快速降低的趋势,然后逐渐缓慢回升;而秋季林内的相对湿度,随着郁闭度的增加而增大,但日间变化幅度比较小。无论春季还是秋季,侧柏和山毛桃的叶片温度日间变化均呈先升高后缓慢降低的趋势。春季郁闭度对侧柏和山毛桃的叶片温度影响不显著,但秋季11-15时,随着郁闭度的增加,叶片温度有降低的趋势。在太行山区,以山毛桃为保护树种,快速提高侧柏-山毛桃混交林的郁闭度,对有效地改善林内小气候,促进森林更新和管理都是十分重要的。     

Turbulent transport of scalar quantities within and above a paddy field

An experiment was conducted to study turbulent transport processes of scalar quantities within and above a rice plant canopy. A sonic anemometer-thermometer and a Lyman- humidiometer were used to measure the turbulent fluxes of sensible and latent heat and related turbulence statistics within a paddy field. The sensible and latent heat fluxes measured at two heights within and above the plant canopy showed that the upper layer of this plant canopy was an active source region and that the source strength of sensible and latent heat depended on the solar radiation and physiology of rice plants. Analysis of joint probability distributions of w and T and of w and q within this plant canopy showed that downdrafts were remarkably efficient for upward transport of sensible and latent heat in the daytime. The vertical fluxes of temperature and humidity variance were also divergent from the upper layer of plant canopies. The power spectra of temperature and humidity within the plant canopy decreased rapidly in the high frequency range, compared with the - 2/3 law relationship of nS(n) vs log n observed above plant canopies.     

Coherent eddies and temperature structure functions for three contrasting surfaces. Part I: Ramp model with finite microfront time

Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field.     

Turbulence Structure Within and Above a Canopy of Bluff Elements

Measurements of turbulence structure in a wind-tunnel model canopy of bluff elements show many of the features associated with vegetation canopies and roughness sublayers but also display features more characteristic of the inertial sublayer (ISL). Points of similarity include the existence of an inflexion point in the space-time averaged streamwise velocity at the canopy top, the variation with height of turbulent second moments and the departure of the turbulent kinetic energy budget from local equilibrium in and just above the canopy. Quadrant analysis shows characteristic dominance of sweep over ejection events within the canopy although sweeps are more frequent than usually seen in vegetation canopies. Points of difference are a u′, w′ correlation coefficient that is closer to the ISL value than to most canopy data, and a turbulent Prandtl number midway between canopy and ISL values. Within the canopy there is distinct spatial partitioning into two flow regimes, the wake and non-wake regions. Both time-mean and conditional statistics take different values in these different regions of the canopy flow. We explain many of these features by appealing to a modified version of the mixing-layer hypothesis that links the dominant turbulent eddies to the instability of the inflexion point at canopy top. However, it is evident that these eddies are perturbed by the quasi-coherent wakes of the bluff canopy elements. Based upon an equation for the instantaneous velocity perturbation, we propose a criterion for deciding when the eddies linked to the inflexion point will dominate flow structure and when that structure will be replaced by an array of superimposed element wakes. In particular, we show that the resemblance of some features of the flow to the ISL does not mean that ISL dynamics operate within bluff-body canopies in any sense.     

Numerical Study of the Response of an Atmospheric Surface Layer to a Spatially Nonuniform Plant Canopy

High-accuracy large-eddy simulations of neutral atmospheric surface-layer flow over a gapped plant canopy strip have been performed. Subgrid-scale (SGS) motions are parameterized by the Sagaut mixed length SGS model, with a modification to compute the SGS characteristic length self-adaptively. Shaw’s plant canopy model, taking the vertical variation of leaf area density into account, is applied to study the response of the atmospheric surface layer to the gapped dense forest strip. Differences in the region far away from the gap and in the middle of the gap are investigated, according to the instantaneous velocity magnitude, the zero-plane displacement, the potential temperature and the streamlines. The large-scale vortex structure, in the form of a roll vortex, is revealed in the region far away from the gap. The nonuniform spatial distribution of plants appears to cause the formation of the coherent structure. The roll vortex starts in the wake of the canopy, and results in strong fluctuations throughout the entire canopy region. Wind sweeps and ejections in the plant canopy are also attributed to the large vortex structure.     

Wave-like wind fluctuations observed in the stable surface layer over a plant canopy(2)

Observations of wind velocity and air temperature fluctuations were made in the nocturnal surface inversion layer over a sorghum field. Wave-like fluctuations of temperature and wind velocity with a period of 15–20 min were observed for about 2 hours, 3 to 5 hours before sunrise. Wave-like fluctuations of temperature were observed in the air layer above and within a plant canopy and were most noticeable at the top of the plant canopy. Spectral analysis of temperature and wind velocity fluctuations reveals a separation of energy into wave-like and turbulent fluctuations. Cospectral analysis shows that for both momentum and heat, vertical transports are partitioned almost equally in the frequency ranges characteristic of wave-like and turbulent fluctuations. This suggests wave- turbulence interactions at low frequencies in the air layer near a plant canopy.     

Joint probability analysis of momentum and heat fluxes at a deciduous forest

Turbulent transport processes for momentum and scalar quantities are examined by a joint probability distribution analysis using data observed within and above a deciduous forest. Characteristics of transport processes in the frequency domain were also analyzed using Tukey's procedure. The results confirm that sweep phenomena prevail within and at the top of a tall plant canopy and that downdrafts are more effective for vertical transport of momentum and scalar quantities. On the other hand, updrafts become more efficient for vertical transport in the daytime at levels about twice treetop height. The results show that within the forest, the sweep phenomenon prevails over a wide frequency range, while above the forest, prevalence of the ejection phenomenon is limited to low frequencies. It is again noted that the plant canopy plays an important role in the sweep-ejection cycle as well as in turbulent transport processes.     

A spectral and lag-correlation analysis of turbulence in a deciduous forest canopy

The processes influencing turbulence in a deciduous forest and the relevant length and time scales are investigated with spectral and cross-correlation analysis. Wind velocity power spectra were computed from three-dimensional wind velocity measurements made at six levels inside the plant canopy and at one level above the canopy. Velocity spectra measured within the plant canopy differ from those measured in the surface boundary layer. Noted features associated with the within-canopy turbulence spectra are: (a) power spectra measured in the canopy crown peak at higher wavenumbers than do those measured in the subcanopy trunkspace and above the canopy; (b) peak spectral values collapse to a relatively universal value when scaled according to a non-dimensional frequency comprised of the product of the natural frequency and the Eulerian time scale for vertical velocity; (c) at wavenumbers exceeding the spectral peak, the slopes of the power spectra are more negative than those observed in the surface boundary layer; (d) Eulerian length scales decrease with depth into the canopy crown, then increase with further depth into the canopy; (e) turbulent events below crown closure are more correlated with turbulent events above the canopy than are those occurring in the canopy crown; and (f) Taylor's frozen eddy hypothesis is not valid in a plant canopy. Interactions between plant elements and the mean wind and turbulence alter the processes that produce, transport and remove turbulent kinetic energy and account for the noted observations.     

中国茶树春霜冻害研究进展

茶树作为我国主要经济作物,在早春萌发时易遭受霜冻害。我国茶树春霜冻多发于长江中下游,霜冻灾害指标可按照获取方法、数据类别、气象数据的时间尺度进一步细分。在气候变化背景下,茶树春霜冻发生次数虽呈下降趋势,但其危害不可忽视;其中,江南茶区茶树春霜冻的发生频率由南向北逐渐增加,随海拔升高逐渐增大。茶树春霜冻影响评估目前多集中于江、浙、皖、赣等茶区,且逐步由定性向定量发展;风险评估主要基于自然灾害风险形成机制划分不同的风险等级。今后,完善茶树春霜冻气象指标、构建基于茶园小气候的茶树春霜冻灾害指标、阐明全国茶树春霜冻时空分布特征、开展精细化茶树春霜冻风险评估将备受关注。     

Large-eddy simulation of plant canopy flows using plant-scale representation

Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model. A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy.     

Daytime variations of ozone eddy fluxes to maize

The vertical fluxes of ozone, momentum and heat in the atmospheric surface layer have been measured by eddy correlation above both mature and senescent maize canopies. Aerodynamic formulae are applied to find that the bulk canopy surface resistancer _c to ozone uptake and destruction varies between 4.0 and 0.5 s cm⁻¹ during the daytime. Apparently, surface properties tend to control the removal of ozone at the surface of the earth. For a lush canopy, the stomatal diffusion resistance is the most important property, while changes in surface temperature have little effect. Destruction at the soil and exterior plant surfaces appears to account for 20–50% of the total loss if leaf mesophyll resistances are assumed to be very small. Free water at leaf surfaces may at times inhibit ozone removal by both senescent and healthy plants.     

A simple mathematical model of airflow in waving plant canopies

A simple mathematical model is proposed, which combines the effects of mean wind speed, plant spacing and the drag coefficients of individual plants to calculate the fluctuating airflow within a waving crop canopy. The model is non-linear and is only amenable to analytical treatment when linearized; however, the full non-linear version can be solved on an analogue computer.The linear and non-linear results show unexpectedly good agreement, and the success of the linearization allows clear conclusions to be drawn about the relationship between the elasticity, geometry and mean wind speed through a stand of plants and the fluctuating components of airflow and stalk motion.Finally, the analytical results are in qualitative agreement with wind-tunnel results from two model canopies with different characteristics.     

On the Association between Spring Arctic Sea Ice Concentration and Chinese Summer Rainfall： A Further Study

In our previous study,a statistical linkage between the spring Arctic sea ice concentration(SIC)and the succeeding Chinese summer rainfall during the period 1968–2005 was identified.This linkage is demonstrated by the leading singular value decomposition(SVD)that accounts for 19%of the co-variance.Both spring SIC and Chinese summer rainfall exhibit a coherent interannual variability and two apparent interdecadal variations that occurred in the late 1970s and the early 1990s.The combined impacts of both spri...     

青藏高原冬春季地温异常对长江中下游夏季旱涝影响的研究

探讨了青藏高原地温异常对长江中下游地区降水的影响。通过资料分析揭示：长江中下游地区夏季旱涝前期冬春青藏高原各层次的地温距平具有反位相分布和高方差分布的特征，前期冬、春季青藏高原地温的三维结构对长江中下游地区夏季降水异常具有“强信号”指示特征。从地面0cm到地下3．2m的地温距平分布为：涝年高原偏南部为正，中部和北部为负，旱年时则与此相反，高原中部和东南部是反位相最明显的地区。地温距平在近地表变化较快，地温距平的大值区在40cm层到1．6m层之间，1．6m到3．2m层地温距平变化较慢。资料分析表明前期青藏高原不同层次地温异常是后期长江中下游地区降水异常的一个重要原因。资料分析和数值试验都揭示了北半球环流型对青藏高原不同层次地温异常可能产生遥相关波形并形成季尺度低频波，此相关波列的激发和传播可能影响长江中下游地区后期的降水。     

Modelling the Effect of Tree Foliage on Sprayer Airflow in Orchards

The effect of tree foliage on sprayer airflow through pear trees in a fruit orchard was studied and modelled in detail. A new three-dimensional (3-D) computational fluid dynamics model that integrates the 3-D canopy architecture with a local closure model to simulate the effect of the stem and branches and leaves of trees separately on airflow was developed. The model was validated with field observations made in an experimental orchard (pcfruit, Sint-Truiden, Belgium) in spring and summer 2008 and was used to investigate the airflow from three air-assisted orchard sprayers (Condor V, Duoprop and AirJet quatt). Velocity magnitudes were measured before and behind leafless and fully-leafed pear canopies across the row while the operating sprayers are passing along the row, and were compared with the simulations. The simulation results predicted the measured values well with all the local relative errors within 20%. The effect of foliar density on airflow from the three air assisted sprayers was manifested by changing the magnitude and direction of the sprayers’ air velocity behind the canopy, especially at the denser regions of the canopy and by changing the pattern of velocity decay horizontally along the jet. The developed methodology will also allow a thorough investigation of atmospheric airflow in canopy structures.     

Possible Effects of Climate Change on Plant/Herbivore Interactions in Moist Tropical Forests

The interactions between plants and herbivores are key determinants of community structure world wide. Their role is particularly important in lowland tropical rain forests where rates of herbivory are higher, plants are better defended chemically and physically, and herbivores have specialized diets. In contrast to the temperate zone, most of the herbivory in the tropics occurs on ephemeral young leaves (>70%), which requires herbivores to have finely tuned host-finding abilities. As a consequence of these tight ecological and evolutionary linkages, the interplay between plants and herbivores in the tropics may be more susceptible to perturbations of climate change.Increases in global temperature, atmospheric CO2, and the length of the dry season are all likely to have ramifications for plant/herbivore interactions in the tropics. Here I extrapolate from our current and incomplete understanding of the mechanisms regulating plant/herbivore interactions and present a scenario for possible trends under a changing climate. Although elevated CO2 tends to enhance plant growth rates, the larger effects of increased drought stress will probably result in lower growth. In atmospheres experimentally enriched in CO2, the nutritional quality of leaves declines substantially due to a dilution of nitrogen by 10-30%. This response is buffered in plant species associated with nitrogen fixers. Elevated CO2 should also cause a slight decrease in nitrogen-based defenses (e.g., alkaloids) and a slight increase in carbon-based defenses (e.g., tannins). The most dramatic and robust predicted effect of climate change is on rates of herbivory. Lower foliar nitrogen due to CO2 fertilization of plants causes an increase in consumption per herbivore by as much as 40%, and unusually severe drought appears to cause herbivore populations to explode. In areas where elevated CO2 is combined with drying, rates of herbivory may rise 2-4 fold. The frequency of insect outbreaks is also expected to increase. Higher herbivory should further reduce plant growth rates, perhaps favoring plant species that are well-defended or fix nitrogen. The predicted increase in the number of herbivores is primarily due to relaxed pressure from predators and parasitoids. Elevated temperatures may increase herbivore developmental times, affording them partial escape from discovery by natural enemies, and drought appears to decimate parasitoid populations. The expected decline in parasitoid numbers may be due to direct effects of dry season drought or to the relative scarcity of herbivores during that period. As a consequence, the relative abundance of species will change, and overall biodiversity should decline.     

基于碳水通量耦合原理改进Penman-Monteith蒸散发模型

陆地蒸散(ET)涵括地表和潮湿叶片的蒸发和植物的蒸散发,是陆地水循环的重要组成部分。Penman-Monteith方程是估算陆地蒸散的重要方法,方程中的叶片或冠层气孔导度是提高估算精度的关键因子。根据碳水循环的耦合原理,植物光合作用模型可用于估算叶片或冠层气孔导度。植物光合作用模型可分为三类:1)使用总冠层导度的大叶模型(BL),2)区别阴、阳叶冠层导度的双大叶模型(TBL),3)区别阴、阳叶叶片导度的双叶模型(TL)。与这三类光合作用模型相对应,衍生出基于不同导度计算方法的三种蒸散估算模型。三种蒸散模型之间的主要区别在于是否进行从叶片尺度到冠层尺度的气孔导度集成。这三种模型中,双叶模型使用叶片尺度的气孔导度,集成度最低。反之,大叶模型使用冠层尺度的气孔导度,集成度最高。由于在Penman-Monteith中,蒸腾和气孔导度之间的关系是非线性的,气孔导度的集合会导致负偏差。因此,与通量测量相比,大叶蒸散模型的估算偏差最大,而双叶蒸散模型的估算偏差最小。