Linear unmixing of MODIS albedo composites to infer subpixel land cover type albedos

We use a linear unmixing approach to test how land use and forestry maps, in combination with the MODIS BRDF/albedo product, can be used to estimate land cover type albedos in boreal regions. Operational land use maps from three test areas in Finland and Canada were used to test the method. The resulting endmember albedo estimates had low standard errors of the mean and were realistic for the main land cover types. The estimated albedos were fairly consistent with albedo measurements conducted with a telescope mast and pure pixel albedos. Problems with the method are the possible errors in the land cover maps, lack of good quality winter MODIS albedo composites and the mismatch between the MODIS pixels and the true observation area. The results emphasize the role of tree species as determinant of forest albedo. Comprehensive spatial and temporal measurements of land cover albedo are usually not possible with in situ mast measurements, and the spatial resolution of MODIS albedo product is often too low to allow direct comparison of pixel albedos and land cover types in areas with heterogeneous vegetation. Hence, and since local forestry maps exist for most temperate and boreal regions, we believe that the proposed method will be useful in estimating average regional land cover type albedos as well as in tracking changes in them.     

中国地表月平均反照率的遥感反演

地表特征和下垫面物理性质在时空分布上的差异 ,造成地表能量分布的不均 ,地球表面的半球反射在气候领域是一个非常重要的参数 ,它在地 气能量交换中决定着能量在地 气之间的分配比率。反照率随地表覆盖类型的变化具有很大的差异 ,而这往往是形成区域小气候差异的原因。文中通过统计和双向反射模型 ,应用NOAA14 AVHRR数据并结合地理信息系统 ,反演计算了 1997年中国月平均反照率的分布 ,并对结果做了分析检验。     

光伏电站对局地气候的影响研究进展

清洁能源的快速发展有效降低了人类对传统化石能源的依赖,并为缓解全球气候变暖做出显著贡献。随着各地光伏装机规模的不断扩大,光伏电站这一人工设施对气候环境的影响日益引起人们的关注。针对这一问题,国内外学者自2000年陆续开展了相关研究。综合国内外现有研究成果,对研究方法、影响机理及光伏电站对气候的影响等方面进行总结。大量研究表明：光伏电站布设会在荒漠地区产生“光伏热岛效应”,从而引起局部区域气温上升,在城市地区布设能够在降低能源消耗的同时减少“城市热岛效应” 。此外,光伏组件还会对反照率、地表辐射平衡等产生影响,进而对局地甚至全球气候产生作用,其影响范围和程度尚无准确结论,还需要进一步深入探索。     

What controls the magnitude of the daytime heat sink in a desert city?

The aim of this study was to increase knowledge of the causes of cooling in desert cities. We used a time-series of Landsat images to characterize the changes in daytime land surface temperature during the period of rapid urbanization in Dubai. Changes in land cover and albedo were also quantified from Landsat data and the development of different land use types and variations in urban geometry were characterized. The results demonstrate that urban growth has promoted a heat sink and that all urban land use types contributed to this effect. Vegetation generated the largest cooling effect per unit surface area but impervious surfaces dominated the urban environment and are responsible for the majority of the heat sink created by the city. Changes in albedo were not causally related to the urban heat sink, however, variations in urban geometry, particularly the amount of shading cast by buildings, had some influence on the magnitude of cooling. This study provides evidence that the expansion of the heat sink during urbanization in a desert environment is influenced by the forms of land cover transition, the type of urban land use that is developed, the thermal properties of construction materials used and the geometry of the city environment that is constructed. Future research should concentrate upon understanding these mechanisms in order to plan future developments which maximize cooling and reduce the environmental impacts of desert cities.     

大气对植被指数的影响

通过辐射传输的模拟计算，研究了大气状况对植被指数的影响，提出可以用能更好地反映植被固有辐射特性的地面植被指数来进行全球植被监测。     

卫星遥感数据的像元地面反射率反演计算

本文探讨了一种在卫星遥感数据地面辐射改正的基础上,利用少量离散分布的观测数据逐点的计算研究区域卫星像片像元地面反射率的方法。该项研究在遥感应用基础和山地辐射状况的研究中有重要意义。     

海冰反照率参数化方案的研究回顾

在全球变暖的背景下,北极海冰在发生快速变化,海冰覆盖范围明显减小,厚度显著变薄,积雪/海冰—反照率正反馈机制在此背景下变得愈发重要。气候系统和单一海冰模式采用了从简单到复杂的海冰反照率参数化方案。首先对模式中的海冰反照率参数化进行了回顾,并结合对现有卫星反照率产品问题的分析,概述了前人对参数化的评估研究工作。在此基础上进一步讨论了气候模式中海冰反照率参数化方案存在的问题,一方面目前的反照率参数化对海冰融池和冰间水道等物理过程的考虑还不够完善,另一方面反照率参数化方案的发展受到观测数据可用性的制约。最后对参数化的发展方向进行了初步探讨。     

Polar amplification in a coupled climate model with locked albedo

In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling of the atmospheric CO₂ content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature amplification.     

Rapid vegetation responses and feedbacks amplify climate model response to snow cover changes

We investigate the response of a climate system model to two different methods for estimating snow cover fraction. In the control case, snow cover fraction changes gradually with snow depth; in the alternative scenarios (one with prescribed vegetation and one with dynamic vegetation), snow cover fraction initially increases with snow depth almost twice as fast as the control method. In cases where the vegetation was fixed (prescribed), the choice of snow cover parameterization resulted in a limited model response. Increased albedo associated with the high snow caused some moderate localized cooling (3–5°C), mostly at very high latitudes (>70°N) and during the spring season. During the other seasons, however, the cooling was not very extensive. With dynamic vegetation the change is much more dramatic. The initial increases in snow cover fraction with the new parameterization lead to a large-scale southward retreat of boreal vegetation, widespread cooling, and persistent snow cover over much of the boreal region during the boreal summer. Large cold anomalies of up to 15°C cover much of northern Eurasia and North America and the cooling is geographically extensive in the northern hemisphere extratropics, especially during the spring and summer seasons. This study demonstrates the potential for dynamic vegetation within climate models to be quite sensitive to modest forcing. This highlights the importance of dynamic vegetation, both as an amplifier of feedbacks in the climate system and as an essential consideration when implementing adjustments to existing model parameters and algorithms.     

The Energy Balance Experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements

An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m⁻² over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m⁻²; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m⁻² across the field, rising to 50 W m⁻² or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m⁻², 5%) per site during the day and 10 W m⁻² at night. The National Center for Atmospheric Research is supported by the National Science Foundation.