Exploration of remotely sensed forest structure and ultrasonic range sensor metrics to improve empirical snow models

Current methods to estimate snow accumulation and ablation at the plot and watershed levels can be improved as new technologies offer alternative approaches to more accurately monitor snow dynamics and their drivers. Here we conduct a meta‐analysis of snow and vegetation data collected in British Columbia to explore the relationships between a wide range of forest structure variables – obtained from Light Detection and Ranging (LiDAR), hemispherical photography (HP) and Landsat Thematic Mapper – and several indicators of snow accumulation and ablation estimated from manual snow surveys and ultrasonic range sensors. By merging and standardizing all the ground plot information available in the study area, we demonstrate how LiDAR‐derived forest cover above 0.5 m was the variable explaining the highest percentage of absolute peak snow water equivalent (SWE) (33%), while HP‐derived leaf area index and gap fraction (45° angle of view) were the best potential predictors of snow ablation rate (explaining 57% of variance). This study reveals how continuous SWE data from ultrasonic sensors are fundamental to obtain statistically significant relationships between snow indicators and structural metrics by increasing mean r² by 20% when compared to manual surveys. The relationships between vegetation and spectral indices from Landsat and snow indicators, not explored before, were almost as high as those shown by LiDAR or HP and thus point towards a new line of research with important practical implications. While the use of different data sources from two snow seasons prevented us from developing models with predictive capacity, a large sample size helped to identify outliers that weakened the relationships and suggest improvements for future research. A concise overview of the limitations of this and previous studies is provided along with propositions to consistently improve experimental designs to take advantage of remote sensing technologies, and better represent spatial and temporal variations of snow. Copyright © 2013 John Wiley & Sons, Ltd.     

半球形沉积谷场地对入射平面Rayleigh波的三维散射解析解

把半球形沉积谷和周围半空间场地分别用具有不同弹性模量和不同半径的球坐标描述。利用Fourier-Bessel级数展开法,在频域内给出了半空间中半球形沉积谷场地在平面Rayleigh波入射下三维散射问题的解析解。利用这一解析解计算分析了入射波频率、场地软硬对场地内地表位移幅值的影响。     

Effect of Macrophysical Parameters of Clouds on Broadband Solar Radiation (295-2800 nm) at a Subtropical Location

The present study describes the effect of clouds(macro-physical parameters) on global solar radiation(G).Data from four years of hourly measurements of G on a horizontal surface were used.These data were collected at the South Valley University(SVU) meteorological research station(26.2°N,32.7°E,96 m above mean see level.In addition,the cloud modification factor for G(CMFG) was estimated in three cases:high-level,mid-level,and low-level clouds.For every level,the variation of hourly CMFG as a function of cloud amount(CA) was illustrated.A third-order polynomial between hourly values of CMFG and CA was established.Furthermore,the effect of CA in the attenuation of G relative to its corresponding value in cloudless conditions is discussed.For cloud cover > 88%,G was reduced by 54%,34%,and 28% by low-,mid-,and high-level clouds,respectively.     

鱼眼影像技术反演植被冠层结构参数的研究进展

快速、可靠、精确的评估植被冠层结构参数在大气一植被相互作用的研究中起着举足轻重的作用。从冠层结构参数的反演原理、冠层间隙度的提取、冠层结构的反演模型和丛生指数3个方面论述了冠层结构参数即叶面积指数（LAI,leaf area index）和叶倾角分布（LAD,leaf angle distribution）的反演方法,并从鱼眼像片的采集、分析和模型的假设等方面分析影响冠层结构参数反演精度的原因,指出未来鱼眼影像技术虽然是LAI和LAD间接测量的理想手段,但是受观测环境、相机光学特性和冠层本身的影响,反演结果需要通过验证来消除不确定因素。     

Data‐driven modelling of shortwave radiation transfer to snow through boreal birch and conifer canopies

Hydrological and land surface models require simple but accurate methods to predict the solar radiation transmitted through vegetation to snow, backed up by direct comparisons to data. Twenty shortwave pyranometers were deployed in forest plots of varying canopy structures and densities in sparse birch forest near Abisko, Sweden, in spring 2011 and mixed conifer forest near Sodankylä, Finland, in spring 2012. Above‐canopy global and diffuse shortwave irradiances were also measured. These data were used to test a model that uses hemispherical photographs to explicitly estimate both diffuse radiation and direct beam transmission, as well as two models that apply bulk canopy parameters and versions of Beers Law. All three models predict canopy shortwave transmission similarly well for leafless birch forest, but for conifers, the bulk methods perform poorly. In addition, an existing model of multiple reflections between canopy and snow was found to be suitable for birch, but not conifers. A new bulk approach based on empirical relationships with hemisphere‐averaged sky view fraction showed improved performance for both sites; this suggests benefits of avoiding the use of plant area index calculated from optical methods, which can introduce errors. Furthermore, tests using common empirical diffuse radiation models were shown to underestimate shortwave transmission by up to 7% relative to using the data, suggesting that new diffuse models are required for high latitudes. Copyright © 2013 John Wiley & Sons, Ltd.     

An improved hemispherical photography model for stream surface shortwave radiation estimations in a central U.S. hardwood forest

Hemispherical photographs of forest canopies can be used to develop sophisticated models that predict incident below canopy shortwave radiation on the surface of interest (i.e. soil and water). Hemispherical photographs were collected on eight dates over the course of a growing season to estimate leaf area index and to quantify solar radiation incident on the surface of two stream reaches based on output from Gap Light Analyser and Hemisfer software. Stream reaches were shaded by a mixed‐deciduous Ozark border forested riparian canopy. Hemispherical photo model results were compared to observed solar radiation sensed at climate stations adjacent to each stream reach for the entire 2010 water year. Modeled stream‐incident shortwave radiation was validated with above‐stream pyranometers for the month of September. On average, the best hemispherical photo models underestimated daily averages of solar radiation by approximately 14% and 12% for E–W and N–S flowing stream reaches, respectively (44.7 W/m² measured vs 38.4 W/m² modeled E–W, 46.8 W/m² vs. 41.1 W/m²N–S). The best hemispherical photo models overestimated solar radiation relative to in–Stream pyranometers placed in the center of each stream reach by approximately 7% and 17% for E–W and N–S stream reaches respectively (31.3 W/m² measured vs 33.5 W/m² modeled E–W, 31.5 W/m² vs. 37.1 W/m²N–S). The model provides a geographically transferable means for quantifying changes in the solar radiation regime at a stream surface due to changes in canopy density through a growing season, thus providing a relatively simple method for estimating surface and water heating in canopy altered environments (e.g. forest harvest). Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of The Atmosphere on UVB Radiation Reaching The Earth’s Surface:Dependence on Solar Zenith Angle

The atmosphere protects humans,plants,animals,and microorganisms from damaging doses of ultraviolet-B(UVB) solar radiation(280-320 nm) because it modifies the UVB reaching the Earth’s surface.This modification is a function of the solar radiation’s path length through the atmosphere and the amount of each attenuator along the path length.The path length is determined by solar zenith angle(SZA).The present work explains the dependence of hemispherical transmittance of UVB on SZA.The database used consists of five years of hourly UVB and global solar radiation measurements.From 2001 to 2005,the South Valley University(SVU) meteorological research station(26.20°N,32.75°E) carried out these measurements on a horizontal surface.In addition,the corresponding extraterrestrial UVB(UVBext) and broadband solar radiation(Gext) were estimated.Consequently,the hemispherical transmittance of UVB(KtUVB) and the hemispherical transmittance of global solar radiation(Kt) were estimated.Furthermore,the UVB redaction due to the atmosphere was evaluated.An analysis of the dependence between KtUVB and SZA at different ranges of Kt was performed.A functional dependence between KtUVB and SZA(KtUVB=-a(SZA)+b) for very narrow Kt-ranges(width of ranges was 0.01) was developed.The results are discussed,and the sensitivity of △KtUVB to △SZA for very narrow Kt-ranges was studied.It was found that the sensitivity of △KtUVB to △SZA slightly increases with increased Kt,which means KtUVB is sensitive to SZA as Kt increases.The maximum correlation(R) between KtUVB and SZA was equal to-0.83 for Kt= 0.76.     

牛顿流体柱-半球面渗透注浆形式扩散参数的研究

文献检索表明：目前国内外还没有一个能较准确地计算牛顿流体柱-半球面渗透注浆形式扩散参数（如半球体部分扩散半径与柱体部分扩散高度等）的理论公式,导致理论远滞后于工程应用。以牛顿流体流变方程及渗流运动方程为基础,研究了牛顿流体柱-半球面渗透注浆形式半球体部分扩散半径与柱体部分扩散高度的理论计算公式;分析了牛顿流体的流变性、注浆压力、地下水压力及配置流体的水体温度对牛顿流体柱-半球面渗透注浆形式半球体部分扩散半径与柱体部分扩散高度的影响;并设计注浆试验对其进行验证。结果表明：由牛顿流体柱-半球面渗透注浆形式半球体部分扩散半径与柱体部分扩散高度的理论公式计算得到的半球体部分扩散半径和柱体部分扩散高度理论值与注浆试验实测值间虽有30%左右的差异,但它们都处于可接受误差范围内。因此,研究成果可为实际注浆施工提供理论支撑与指导作用。     

半球月平均位势高度场均匀和非均匀异常指数的时滞性分析

将半球月平均位势高度场异常分解为半球均匀和非均匀两部分，构造了均匀异常指数车和非均匀异常指数η。对ξ,η的时滞性分析表明：北(南)半球车的时滞性较强，1月(7月)可达1a(3a)以上。η的时滞性较弱，显著时滞通常不足1个月；较强时滞出现在北半球夏季平流层，最长可达4个月。