A Comparison of Two Canopy Radiative Models in Land Surface Processes

This paper compares the predictions by two radiative transfer models-the two-stream approximation model and the generalized layered model （developed by the authors） in land surface processes -for different canopies under direct or diffuse radiation conditions. The comparison indicates that there are significant differences between the two models, especially in the near infrared （NIR） band. Results of canopy reflectance from the two-stream model are larger than those from the generalized model. However, results of canopy absorptance from the two-stream model are larger in some cases and smaller in others compared to those from the generalized model, depending on the cases involved. In the visible （VIS） band, canopy reflectance is smaller and canopy absorptance larger from the two-stream model compared to the generalized model when the Leaf Area Index （LAI） is low and soil reflectance is high. In cases of canopies with vertical leaf angles, the differences of reflectance and absorptance in the VIS and NIR bands between the two models are especially large. Two commonly occurring cases, with which the two-stream model cannot deal accurately, are also investigated. One is for a canopy with different adaxial and abaxial leaf optical properties; and the other is for incident sky diffuse radiation with a non-uniform distribution. Comparison of the generalized model within the same canopy for both uniform and non-uniform incident diffuse radiation inputs shows smaller differences in general. However, there is a measurable difference between these radiation inputs for a canopy with high leaf angle. This indicates that the application of the two-stream model to a canopy with different adaxial and abaxial leaf optical properties will introduce non-negligible errors.     

A Comparison of Two Canopy Radiative Models in Land Surface Processes

This paper compares the predictions by two radiative transfer models-the two-stream approximation model and the generalized layered model (developed by the authors) in land surface processes-for different canopies under direct or diffuse radiation conditions. The comparison indicates that there are significant differences between the two models, especially in the near infrared (NIR) band. Results of canopy reflectance from the two-stream model are larger than those from the generalized model. However, results of canopy absorptance from the two-stream model are larger in some cases and smaller in others compared to those from the generalized model, depending on the cases involved. In the visible (VIS) band, canopy reflectance is smaller and canopy absorptance larger from the two-stream model compared to the generalized model when the Leaf Area Index (LAI) is low and soil reflectance is high. In cases of canopies with vertical leaf angles, the differences of reflectance and absorptance in the VIS and NIR bands between the two models are especially large.Two commonly occurring cases, with which the two-stream model cannot deal accurately, are also investigated. One is for a canopy with different adaxial and abaxial leaf optical properties; and the other is for incident sky diffuse radiation with a non-uniform distribution. Comparison of the generalized model within the same canopy for both uniform and non-uniform incident diffuse radiation inputs shows smaller differences in general. However, there is a measurable difference between these radiation inputs for a canopy with high leaf angle. This indicates that the application of the two-stream model to a canopy with different adaxial and abaxial leaf optical properties will introduce non-negligible errors.     

从宽带太阳直接辐射小时或日曝辐量反演气溶胶光学厚度研究

从宽带的太阳直接辐射1天或1小时累计量(曝辐量)气象观测资料反演气溶胶光学厚度的一个有效方法是很有用的.作者把太阳曝辐量与"等效"的瞬时太阳直接辐射关联起来,建立了一个与曝辐量"等效"的瞬时太阳直接辐射的模式;应用该模式和一个"等效"波长模式,发展了一个从太阳直接曝辐量反演气溶胶光学厚度的方法.作者还从试验上比较分析了由某时刻的宽带太阳直接辐射、每小时或1天的太阳直接辐射曝辐量反演得到的气溶胶光学厚度以及由太阳光度计探测的气溶胶光学厚度.试验结果表明,由日太阳直接辐射曝辐量反演得到的气溶胶光学厚度可理解为辐射加权的日平均光学厚度.     

A simplified scheme of the generalized layered radiative transfer model

In this paper, firstly, a simplified version （SGRTM） of the generalized layered radiative transfer model （GRTM） within the canopy, developed by us, is presented. It reduces the information requirement of inputted sky diffuse radiation, as well as of canopy morphology, and in turn saves computer resources. Results from the SGRTM agree perfectly with those of the GRTM. Secondly, by applying the linear superposition principle of the optics and by using the basic solutions of the GRTM for radiative transfer within the canopy under the condition of assumed zero soil reflectance, two sets of explicit analytical solutions of radiative transfer within the canopy with any soil reflectance magnitude are derived： one for incident diffuse, and the other for direct beam radiation. The explicit analytical solutions need two sets of basic solutions of canopy reflectance and transmittance under zero soil reflectance, run by the model for both diffuse and direct beam radiation. One set of basic solutions is the canopy reflectance αf （written as α1 for direct beam radiation） and transmittance βf （written as β1 for direction beam radiation） with zero soil reflectance for the downward radiation from above the canopy （i.e. sky）, and the other set is the canopy reflectance （αb） and transmittance βb for the upward radiation from below the canopy （i.e., ground）. Under the condition of the same plant architecture in the vertical layers, and the same leaf adaxial and abaxial optical properties in the canopies for the uniform diffuse radiation, the explicit solutions need only one set of basic solutions, because under this condition the two basic solutions are equal, i.e., αf = αb and βf = βb. Using the explicit analytical solutions, the fractions of any kind of incident solar radiation reflected from （defined as surface albedo, or canopy reflectance）, transmitted through （defined as canopy transmittance）, and absorbed by （defined as canopy absorptance） the canopy and other properties per     

A Vegetated Urban Canopy Model for Meteorological and Environmental Modelling

An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ _l ) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model, and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling.     

Analysis of 40 year records of solar radiation data in Shanghai,Nanjing and Hangzhou in Eastern China

Summary In this paper, we analyze global, direct and diffuse solar radiation data on a horizontal surface observed at stations in Shanghai, Nanjing and Hangzhou for the period of 1961 to 2000. The data include monthly averages of the daily clearness index (G/G₀: the ratio of global to extraterrestrial solar radiation) and the diffuse fraction (D/G: the ratio of diffuse to global solar radiation. The present study has processed and analyzed the data, including variables or statistics of mean, and annual monthly and daily total, the diurnal variation and the frequency of daily totals of global solar radiation. A correlation between daily values of clearness index and diffuse fraction is obtained and recommended correlation equations were calculated. The annual variations and trend of yearly series are analyzed for daily global, direct and diffuse radiation on a horizontal surface, as well as for daily clearness index and diffuse fraction in Shanghai, Nanjing and Hangzhou. The results show: 1) the east China is characterized by a decrease in global and direct radiation and a little increase in diffuse radiation and a negative linear relationship was obtained between clearness index and diffuse fraction. 2) The annual variations of global, direct and diffuse radiation for Shanghai, Nanjing and Hangzhou are similar with relative low values of global and direct radiation in June due to the Meiyu period. 3) The acceleration of air pollution and decrease of relative sunshine are the possible causes for the decrease of global and direct radiation.     

A Double-Canyon Radiation Scheme for Multi-Layer Urban Canopy Models

We develop a double-canyon radiation scheme (DCEP) for urban canopy models embedded in mesoscale numerical models based on the Building Effect Parametrization (BEP). The new scheme calculates the incoming and outgoing longwave and shortwave radiation for roof, wall and ground surfaces for an urban street canyon characterized by its street and building width, canyon length, and the building height distribution. The scheme introduces the radiative interaction of two neighbouring urban canyons allowing the full inclusion of roofs into the radiation exchange both inside the canyon and with the sky. In contrast to BEP, we also treat direct and diffuse shortwave radiation from the sky independently, thus allowing calculation of the effective parameters representing the urban diffuse and direct shortwave radiation budget inside the mesoscale model. Furthermore, we close the energy balance of incoming longwave and diffuse shortwave radiation from the sky, so that the new scheme is physically more consistent than the BEP scheme. Sensitivity tests show that these modifications are important for urban regions with a large variety of building heights. The evaluation against data from the Basel Urban Boundary Layer Experiment indicates a good performance of the DCEP when coupled with the regional weather and climate model COSMO-CLM.     

Parameterization of snow-free land surface albedo as a function of vegetation phenology based on MODIS data and applied in climate modelling

The aim of this study was to develop an advanced parameterization of the snow-free land surface albedo for climate modelling describing the temporal variation of surface albedo as a function of vegetation phenology on a monthly time scale. To estimate the effect of vegetation phenology on snow-free land surface albedo, remotely sensed data products from the Moderate-Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra platform measured during 2001 to 2004 are used. The snow-free surface albedo variability is determined by the optical contrast between the vegetation canopy and the underlying soil surface. The MODIS products of the white-sky albedo for total shortwave broad bands and the fraction of absorbed photosynthetically active radiation (FPAR) are analysed to separate the vegetation canopy albedo from the underlying soil albedo. Global maps of pure soil albedo and pure vegetation albedo are derived on a 0.5° regular latitude/longitude grid, re-sampling the high-resolution information from remote sensing-measured pixel level to the model grid scale and filling up gaps from the satellite data. These global maps show that in the northern and mid-latitudes soils are mostly darker than vegetation, whereas in the lower latitudes, especially in semi-deserts, soil albedo is mostly higher than vegetation albedo. The separated soil and vegetation albedo can be applied to compute the annual surface albedo cycle from monthly varying leaf area index. This parameterization is especially designed for the land surface scheme of the regional climate model REMO and the global climate model ECHAM5, but can easily be integrated into the land surface schemes of other regional and global climate models.     

A generalized layered radiative transfer model in the vegetation canopy

In this paper, a generalized layered model for radiation transfer in canopy with high vertical resolution is developed. Differing from the two-stream approximate radiation transfer model commonly used in the land surface models, the generalized model takes into account the effect of complicated canopy morphology and inhomogeneous optical properties of leaves on radiation transfer within the canopy. In the model, the total leaf area index （LAI） of the canopy is divided into many layers. At a given layer, the influences of diffuse radiation angle distributions and leaf angle distributions on radiation transfer within the canopy are considered. The derivation of equations serving the model are described in detail, and these can deal with various diffuse radiation transfers in quite broad categories of canopy with quite inhomogeneons vertical structures and uneven leaves with substantially different optical properties of adaxial and abaxial faces of the leaves. The model is used to simulate the radiation transfer for canopies with horizontal leaves to validate the generalized model. Results from the model are compared with those from the two-stream scheme, and differences between these two models are discussed.     

A Multi-layer Radiation Model for Urban Neighbourhoods with Trees

A neighbourhood-scale multi-layer urban canopy model of shortwave and longwave radiation exchange that explicitly includes the radiative effects of tall vegetation (trees) is presented. Tree foliage is permitted both between and above buildings, and mutual shading, emission and reflection between buildings and trees are included. The basic geometry is a two-dimensional canyon with leaf area density profiles and probabilistic variation of building height. Furthermore, the model accounts for three-dimensional path lengths through the foliage. Ray tracing determines the receipt of direct shortwave irradiance by building and foliage elements. View factors for longwave and shortwave diffuse radiation exchange are computed once at the start of the simulation using a Monte Carlo ray tracing approach; for subsequent model timesteps, matrix inversion rapidly solves infinite reflections and interception of emitted longwave between all elements. The model is designed to simulate any combination of shortwave and longwave radiation frequency bands, and to be portable to any neighbourhood-scale urban canopy geometry based on the urban canyon. Additionally, the model is sufficiently flexible to represent forest and forest-clearing scenarios. Model sensitivity tests demonstrate the model is robust and computationally feasible, and highlight the importance of vertical resolution to the performance of urban canopy radiation models. Full model evaluation is limited by the paucity of within-canyon radiation measurements in urban neighbourhoods with trees. Where appropriate model components are tested against analytic relations and results from an independent urban radiation transfer model. Furthermore, system response tests demonstrate the ability of the model to realistically distribute shortwave radiation among urban elements as a function of built form, solar angle and tree foliage height, density and clumping. Separate modelling of photosynthetically-active and near-infrared shortwave bands is shown to be important in some cases. Increased canyon height-to-width ratio and/or tree cover diminishes the net longwave radiation loss of individual canyon elements (e.g., floor, walls), but, notably, has little effect on the net longwave loss of the whole urban canopy. When combined with parametrizations for the impacts of trees on airflow and hydrological processes in the urban surface layer, the new radiation model extends the applicability of urban canopy models and permits more robust assessment of trees as tools to manage urban climate, air quality, human comfort and building energy loads.     

Improvement of a soil-atmosphere-transfer model for the simulation of bare soil surface energy balances in semiarid areas

A soil-atmosphere-transfer model (SATM) was evaluated using observational data from the Tongyu Cropland Station and Audubon Research Ranch in semiarid areas, where the land cover was nearly bare soil during the simulation period. Simulations by the SATM at both sites were conducted using the new and original surface thermal roughness length parameterization schemes, respectively. Comparisons of simulations and observations have demonstrated that using the new surface thermal roughness length scheme in this model made sound improvements in the simulation of soil surface temperatures, sensible heat fluxes and net radiation fluxes in the daytime at both sites, compared to the original scheme, because the new scheme produced a larger aerodynamic resistance for turbulent heat transfer in the daytime. With respect to latent heat fluxes, the improvement was not as obvious as that attained for soil surface temperature since the soil water content in the surface layer in a semiarid area is a more important factor than surface soil temperature in controlling evaporation rate. Accordingly, it can be concluded that the new surface thermal roughness length parameterization scheme could improve the ability of the SATM to simulate bare soil surface energy budget with latent heat flux component being innegligible in semiarid areas.     

Computing solar heating in a fog layer: a new parameterization

Computation of solar radiation flux within a fog layer is essential during the dissipation stage. A new parameterization of the asymmetry factor g and of the single scattering albedo ω allows the use of a monospectral model (FB), already designed for cloudy layer by Fouquart and Bonnel (1980). This new parameterization is necessary because of the different droplet size distributions existing in clouds and fogs. A calculation method of new g and ω values, in function of the optical thickness δ and of the effective radius r_e of droplet size distribution is proposed.     

一个简单的雪晶辐射参数化方案及雪晶辐射对中尺度降水的影响

文章提出一个简单的雪晶辐射性质参数化方案, 并利用1998年6月8日华南暴雨资料研究了雪晶的辐射性质对于中尺度降水的影响。结果表明:雪晶的辐射性质对中尺度降水的影响是不可忽略的, 白天尤为显著; 它能够明显地改变中尺度降水的局部特征, 特别是降水中心的强度和位置, 而对降水的分布影响不大。因此, 建立独立的雪晶辐射参数化对提高中尺度模式对中尺度降水的预报能力是必要的。     

A cloud layer‐sunshine model for estimating direct,diffuse and total solar radiation

A previous study (Suckling and Hay, 1976a) described a method for calculating hourly values of the direct and diffuse solar radiation for cloudless sky conditions. This paper presents an extension which incorporates the effects of clouds through the use of hourly values of cloud amount and type for up to four layers and hourly bright sunshine totals. The latter data provide a more accurate measure of the length of time the direct radiation of the sun is not attenuated by cloud. On an average, the cloud layer‐sunshine (CLS) model estimated daily total solar radiation at five Canadian locations to within ±15 per cent of the measured values. This was an improvement over an earlier model (Davies et al., 1975) based on cloud data alone, but the relative advantage, as well as the overall errors themselves, were diminished as the averaging period was increased to five and ten days. The CLS model has the additional advantage of calculating the separate direct and diffuse components of the total solar radiation.     

考虑气压的晴空大气下行长波辐射参数化方案及其适用性分析

基于Prata晴空大气下行长波辐射参数化方案,针对其在高原地区及可降水量较小地区理论精度较差的缺点,考虑不同高度的大气柱发射的长波辐射量不同,提出了三种考虑气压的大气下行长波辐射参数化方案,通过全球ERA-5再分析数据进行最小二乘拟合确定经验常数并在全球不同区域对其适用性进行了分析.模拟结果表明,考虑气压的参数化方案有...     

北京地区雷电定位系统场地误差及其结构分析

本文利用1988,1989两年北京地区雷电定位系统(LLS)探测到的雷电资料,采用作者在文献[10]中提出的参数化方法,分别计算了该系统中各定向仪(DF)的场地误差曲线。结果表明,场地误差曲线不仅是测量方位角的偶周期函数,而且包含着奇周期的成份。同时还表明,同一场地具有相对稳定的误差曲线。 文中还讨论了场地误差产生的原因,并首次提出偶极辐射是产生场地误差的主要原因的观点,推导出了它可能产生的场地误差的函数形式。用此理论可很好地解释实际计算出的各站场地误差曲线的结构特征。     

Energy Balance Of A Sparse Coniferous High-Latitude Forest Under Winter Conditions

Measurements carried out in Northern Finland on radiation and turbulent fluxes over a sparse, sub-arctic boreal forest with snow covered ground were analysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12–24 March 1997) day and night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. The sensible heat flux above the forest was fairly large, reaching more than 100 W m^-2. The measurements of sensible heat flux within and above the forest revealed that the sensible heat flux from the snow surface is negligible and the sensible heat flux above the forest stems from warming of the trees. A simple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separately. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial transparency of the forest.Input to the model are the direct and diffuse incoming shortwave radiation.Measurements of the global radiation (direct plus diffuse incoming shortwaveradiation) above the forest revealed a considerable attenuation of the globalradiation at low solar elevation. A relation for the atmospheric turbidity asfunction of the solar elevation angle is suggested. The global radiation wassimulated for a three month period. For conditions with a cloud cover of lessthan 7 oktas good agreement between model predictions and measurementswere found. For cloud cover 7 and 8 oktas a considerable spread can beobserved. To apply the proposed energy balance model, the global radiationmust be separated into its diffuse and direct components. We propose a simpleempirical relationship between diffuse shortwave and global radiation asfunction of cloud cover.     

Calculations of river-runoff in the GISS GGM: impact of a new land-surface parameterization and runoff routing model on the hydrology of the Amazon River

This study examines the impact of a new land-surface parameterization and a river routing scheme on the hydrology of the Amazon basin, as depicted by the NASA/Goddard Institute of Space Studies (GISS) global climate model (GCM). The more physically realistic land surface scheme introduces a vegetation canopy resistance and a six-layer soil system. The new routing scheme allows runoff to travel from a river's headwater to its mouth according to topography and other channel characteristics and improves the timing of the peak flow. River runoff is examined near the mouth of the Amazon and for all of its sub-basins. With the new land-surface parameterization, river run-off increases significantly and is consistent with that observed in most basins and at the mouth. The representation of the river hydrology in small basins is not as satisfactory as in larger basins. One positive impact of the new land-surface parameterization is that it produces more realistic evaporation over the Amazon basin, which was too high in the previous version of the GCM. The realistic depiction of evaporation also affects the thermal regime in the lower atmosphere in the Amazon. In fact, the lower evaporation in some portions of the basin reduces the cloudiness, increases the solar radiation reaching the ground, increases the net radiation at the surface, and warms the surface as compared to observations. Further GCM improvement is needed to obtain a better representation of rainfall processes.     

A revised method for determining the direct and diffuse components of the total short‐wave radiation

A relationship, derived by Liu and Jordan (1960), under which the total short‐wave radiation may readily be subdivided into its direct and diffuse components is shown to vary both spatially and seasonally. This variability is attributed to changes in the importance of the multiple reflection of short‐wave radiation between the earth's surface and atmosphere. A revised relationship, which incorporates the influence of this process, is shown to have applicability at a large number of Canadian locations.     

Influence of the total atmospheric optical depth and cloud cover on solar irradiance components

Broadband solar irradiance data obtained in the spectral range 400–940 nm at Kwangju, South Korea from 1999–2000 have been analyzed to investigate the effects of cloud cover and atmospheric optical depth on solar radiation components. Results from measurements indicate that the percentage of direct and diffuse horizontal components of solar irradiance depend largely on total optical depth (TOD) and cloud cover. During summer and spring, the percentages of diffuse solar irradiance relative to the global irradiance were 5.0% and 4.9% as compared to 2.2% and 3.0% during winter and autumn. The diffuse solar irradiance is higher than the direct in spring and summer by 24.2%, and 40.6%, respectively, which may largely be attributed to the attenuation (scattering) of radiation by heavy dust pollution and large cloud amount. In cloud-free conditions with cloud cover ≤2/10, the fraction of the direct and diffuse components were 66.0% and 34.0%, respectively, with a mean daily global irradiance value of 7.92±2.91 MJ m⁻² day⁻¹. However, under cloudy conditions (with cloud cover ≥8/10), the diffuse and direct fractions were 97.9% and 2.2% of the global component, respectively. The annual mean TOD under cloudless conditions (cloud cover≤2/10) yields 0.74±0.33 and increased to as much as 3.15±0.67 under cloudy conditions with cloud amount ≥8/10. An empirical formula is derived for estimating the diffuse and direct components of horizontal solar irradiance by considering the total atmospheric optical depth (TOD). Results from statistical models are shown for the estimation of solar irradiance components as a function of TOD with sufficient accuracy as indicated by low standard error for each solar zenith angle (SZA).