Determination of Angular Distribution Models for Indian desert scene: Comparison with other desert models

In this paper, the shortwave and longwave anisotropy for clear sky Indian desert scene has been estimated using long-term surface data, radiative transfer calculations and Helmholtz reciprocity for missing values. This study is important in the perspective of the low inclination satellites like Megha–Tropiques (MT) mission, carrying Scanner for Radiation Budget (ScaRaB) payload, which will provide broadband radiative fluxes at the top of the atmosphere (TOA). Due to low inclination angle, the angular models for clear sky land scenes for the MT-ScaRaB orbits will be dominated by desert points.The Angular Distribution Models (ADMs) determined in this study were compared with existing desert models. It is observed that for longwave radiation, the largest disagreement is observed for higher values of viewing zenith angle, especially for the summer season, where the difference in flux can reach up to 13 W/m². For the shortwave radiation, higher values of both solar zenith angle and viewing zenith angle cause largest incongruity in the computed albedo from the different models, suggesting the need of caution in interpretation of the flux computations from these bins. In fact at the higher solar zenith angle bin, the disparity in albedo can go up to 6.4%.     

The Geostationary Earth Radiation Budget Edition 1 data processing algorithms

The Geostationary Earth Radiation Budget (GERB) instrument is the first to measure the earth radiation budget from a geostationary orbit. This allows a full sampling of the diurnal cycle of radiation and clouds – which is important for climate studies, as well as detailed process studies, e.g. the lifecycle of clouds or particular aerosol events such as desert storms. GERB data is now for the first time released as Edition 1 data for public scientific use. In this paper we summarise the algorithms used for the Edition 1 GERB data processing and the main validation results. Based on the comparison with the independent CERES instrument, the Edition 1 GERB accuracy is 5% for the reflected solar radiances and 2% for the emitted thermal radiances.     

高光谱遥感地形影响建模与仿真

针对地形起伏对高光谱遥感图像几何变形和辐射变化的影响,建立高光谱遥感地形影响模型.该模型利用传感器位置、姿态和视场角建立模拟图像像元坐标和地面空间坐标之间的成像几何关系,利用地表反射率、数字高程模型等数据,考虑大气辐射传输过程,计算起伏地形下传感器入瞳辐亮度图像,并经过空间分辨率转换,生成最终遥感模拟图像,实现高光谱遥感地形影响精确建模.利用西藏驱龙地区Hyperion数据和其它相关数据进行仿真分析,将模拟图像和原始图像进行对比,结果比较吻合,表明该模型具有较好的模拟效果.     

Mixed surface reflection functions determined from a priori knowledge and satellite radiance

A method to derive mesoscale area means of surface solar flux densities from a priori knowledge and actual cloudfree satellite radiances is presented. It is based on the concept of the mixed reflection function which can be derived from existing data. Herewith and with actual atmospheric data derived from the operational meteorological network the cloudfree radiation field is computed. By comparison of computed and measured satellite radiance the surface albedo of the model is tuned. In a case study this method is applied to an agricultural region called La Mancha, Spain, and comprehensively checked against airborne radiance measurements. The surface albedo can be determined to about ± 0.01.     

Retrieval of aerosol optical characteristics from polarization measurements of reflected solar radiation above the oceans

The reflected near infrared solar radiation observed from space above the oceans is due mainly to the atmosphere scattering, as the ocean surface is nearly black. The molecular Rayleigh contribution is also minimized at infrared wavelengths and it can be evaluated. It is shown that the degree of polarization is much more sensitive to the aerosol properties than the radiance. Measurements of polarization at two wavelengths and with an angular scanning are simulated and an inversion algorithm is proposed. It aims at finding an “equivalent aerosol model,” which reproduces the optical thickness and the asymmetry factor of the actual aerosol at all wavelengths in the solar spectrum.     

Protecting and manoeuvring of spacecraft in space debris environment

The paper gives an overview on the fields of debris research performed at the TUBS. The orbital debris flux of all objects larger than 1cm has been established and simulated by a mathematical model in the past mainly on the basis of simulating explosion fragments. However the flux in the millimeter and submillimeter size range seems to be largely influenced by collisions and their ejecta on high circular or on eccentric orbits. The angular distribution of the impact flux on targets at various altitudes and on various inclinations are presented. This angular distribution has also influence on the surface impact flux on a space station, where also the self shielding has to be considered. Results for the ISS are presented. The risk of impacts of larger not shieldable objects on a space station may become too high, so that collision avoidance manoeuvres must be envisaged, the feasibility of which using onboard detectors is discussed.     

Passive remote sensing of aerosols from space now and in the future

Aerosols modify scattered solar radiation leaving the atmosphere and this fact will be exploited to determine the aerosol optical depth. The interaction processes between solar radiation and aerosol particles are outlined. A quasi-linear relationship (‘conversion curves’) between the radiance at the satellite, L_sat, and the aerosol optical depth, a_Dλ, is found from both numerical and empirical studies. Because L_sat is not only controlled by a_Dλ, but also by a series of other atmospheric parameters (perturbing quantities), the concept of ‘favourable viewing conditions’ is presented, where the effects of the perturbing quantities are minimal. The paper ends with some lines of thought on a concept for a turbidity satellite.     

Annual solar UV exposure and biological effective dose rates on the Martian surface.

The ultraviolet (UV) environment of Mars has been investigated to gain an understanding of the variation of exposure throughout a Martian year, and link this flux to biological effects and possible survival of organisms at the Martian surface. To gain an idea of how the solar UV radiation varies between different regions, including planned landing sites of two future Mars surface missions, we modelled the total solar UV surface flux throughout one Martian year for two different dust scenarios. To understand the degree of solar UV stress on micro-organisms and/or molecules essential for life on the surface of Mars, we also calculated the biologically effective dose (BED) for T7 and Uracil in relevant wavelength regions at the Martian surface as a function of season and latitude, and discuss the biological survival rates in the presence of Martian solar UV radiation. High T7/Uracil BED ratios indicate that even at high latitudes where the UV flux is significantly reduced, the radiation environment is still hostile for life due to the persisting UV-C component of the flux.     

Clouds and Earth Radiant Energy System (CERES), a review: Past,present and future

The Clouds and Earth Radiant Energy System (CERES) project’s objectives are to measure the reflected solar radiance (shortwave) and Earth-emitted (longwave) radiances and from these measurements to compute the shortwave and longwave radiation fluxes at the top of the atmosphere (TOA) and the surface and radiation divergence within the atmosphere. The fluxes at TOA are to be retrieved to an accuracy of 2%. Improved bidirectional reflectance distribution functions (BRDFs) have been developed to compute the fluxes at TOA from the measured radiances with errors reduced from ERBE by a factor of two or more. Instruments aboard the Terra and Aqua spacecraft provide sampling at four local times. In order to further reduce temporal sampling errors, data are used from the geostationary meteorological satellites to account for changes of scenes between observations by the CERES radiometers.     

Model of the all-sky He II 30.4 nm solar flux

Because of the orbit characteristics of the vast majority of spacecraft, the solar flux has been generally measured at Earth or in the plane of the ecliptic. So far, most published studies did not consider the fact that the extreme ultraviolet (EUV) solar flux is largely anisotropic. Indeed, in the EUV, the distribution of very contrasted bright and dark features at the surface of the Sun produces both the obvious rotational (longitudinal) modulation of the flux, but also a strong latitudinal anisotropy. Although largely ignored up to now, the latitudinal anisotropy affects the physical conditions in the corona and heliosphere. We describe an empirical model of the all-sky He II 30.4 nm flux based on EIT/SOHO data. The 30.4 nm flux was found to be strongly anisotropic. The anisotropy I_pol/I_eq between the fluxes computed for viewpoints located above the solar poles and within the solar equatorial plane ranges from 0.9 at solar minimum to 0.6 at solar maximum. A 20% asymmetry was also discovered between the north and south polar fluxes.     

Sensing snow height and surface temperature variations in Greenland from GPS reflected signals

The in situ measurements of snow surface temperature (SST) and snow height (SH) are very difficult with high costs, particularly in Greenland Ice Sheet (GrIS). Since the snow depth variations coupling with surface temperature are related to GPS multipath, it is possible to estimate the snow depth and surface air temperature variations by incorporating GPS-Reflectometry (GPS-R). In this paper, the reflected signals from ground GPS receivers are used to sense the SST and SH variations based on the thermophysical behavior and variations of snow layer from April to June 2010 at SMM1 site and from March to December 2010 at MARG site in Greenland. The results show that the mean daily changes in the ionospheric geometrical-free linear combination (GPS-L4) of dual-frequency GPS signals are related to daily SST and SH variations. The nonparametric bootstrapping model in direct (forward) and inverse models are developed and applied to estimate the SST and SH variations. The mean biases of SST and SH estimates are 0.18 °C and 0.23 m at SMM1 site, respectively, and 3.8 °C and 0.13 m at MARG site, respectively.     

A review of global satellite-derived snow products

Snow cover over the Northern Hemisphere plays a crucial role in the Earth’s hydrology and surface energy balance, and modulates feedbacks that control variations of global climate. While many of these variations are associated with exchanges of energy and mass between the land surface and the atmosphere, other expected changes are likely to propagate downstream and affect oceanic processes in coastal zones. For example, a large component of the freshwater flux into the Arctic Ocean comes from snow melt. The timing and magnitude of this flux affects biological and thermodynamic processes in the Arctic Ocean, and potentially across the globe through their impact on North Atlantic Deep Water formation.     

Effects of concurrent snow and cloud cover on planetary albedo

The effects of snow and cloud cover on planetary albedo are examined using observations from NOAA polar orbiting satellites. Reflected radiation was measured in the visible range (0.5 – 0.7 μm). Planetary albedos resulting from different cloud/snow cover conditions are compared using Northern Hemisphere snow cover maps, surface weather charts, satellite photos and data on land surface types. None of the cases studied show that concurrent cloud and snow cover produces significantly different planetary albedos than cloud cover alone. Cloud cover alone is found to yield higher planetary albedos than snow cover alone; the difference being greatest over forested areas. With and without snow cover present, clear-sky planetary albedos over farming and grazing lands (snow(0.45), no snow(0.15)) are found to be significantly higher than those over forested regions (snow(0.33), no snow(0.11)). Variations in satellite zenith angle are not found to produce significant effects in most cases studied.     

An observation of snow melting process from remotely sensed data

Observations of satellite images have shown that the snow melting in mountainous area proceeds more repidly in the east-facing slope of the valley than in the west-facing one. The energy for melting snow consists of the total from the atmosphere and from the solar rays. The diurnal variation of the solar energy into the snow in the east-facing slope differs from that in the west-facing slope. This causes the heighest value of the instantaneous energy for melting snow to occur in the west-facing surface. As one of the reasons for the above tendency, the difference of the heighest value to melt snow may be taken into account.     

武汉中层、低热层大气角谱中频雷达观测

武汉中频雷达是利用中层、低热层中电子密度不均匀体的散射来测量大气的水平风场和电子密度剖面。雷达在计算风场的过程中可得到一些该层大气中电子密度不均匀体的寿命和空间尺度等参量值。结合这些参量和大气风场值可计算得到大气的角谱。其计算方法包括全相关分析技术的谱宽法和空间相关法。前者计算的值被认为是大气角谱的上限值。应用这两种方法,利用2001年2月9日武汉中频雷达的观测数据,对武昌上空中层、低热层大气的角谱进行了计算。结果得到大气角谱随高度增加略微增加,如在68km为6°,90km达最大为11°,其平均值为9.2°。利用2001年2月4—10日86km高度上的数据,得到一个7天平均的日变化曲线,发现大气角谱值在本地夜晚最小。     

利用青海湖对FY—1C、FY—2B气象卫星热红外通道进行在轨辐射定标

介绍利用青海湖辐射校正场对FY－1C、FY2B气象卫星热红外通道进行在轨辐射定标，先用CE312野外热红外辐射计在水面测量水表辐亮度，再经大气订正传递到卫星入瞳处，大气订正包括大气吸收削弱和大气产生热发射影响，这两部分对卫星信号的贡献通过辐射传输模式MODTRAN37计算出来，同时进行CE312与卫星通道光谱响应匹配，最终得到卫星入瞳处的表观辐亮度，这个辐亮度与卫星通道的计数值得到该通道绝对定标系数。我们对两颗卫星进行了多次定标，结果表明利用青海湖进行的在轨定标与星上定标系数相差5％左右，相当于3K的亮温差。     

Centre-to-limb variation of photospheric facular radiance and image resolution

We study the effect of the angular resolution on the determination of the angular properties of the facular radiance. We analyze photospheric intensity in the continuum, around the Ni 676.8 nm line, and longitudinal magnetic field along the line of sight, measured by the MDI instrument aboard SOHO with two spatial resolutions, 4″ and 1.2″ (2″ and 0.6″ pixels, respectively). The effect of the limited photometric sensitivity of the instrument and the limited information on the angular structure of the magnetic field tubes are considered. Our study of the high-resolution data shows that intensity contrast of magnetic features between 80 and 600 Gauss increases from centre to limb up to a maximum that occurs at higher heliocentric angles (θ) when obtained with higher resolution data than for lower resolution data. There is a suggestion that at heliocentric angles below about 75° there is only a monotonic increase in the contrast as one goes from cos (θ) = 1 to cos (θ) = 0.2.     

嫦娥一号干涉成像光谱仪2C级图像数据质量评价

嫦娥一号干涉成像光谱仪(IIM)获取的多波段图像数据对于月表矿物成分反演具有重要意义. 从目视质量、客观参数、几何配准精度、图像质量随时间的变化情况等方面对IIM 2C级的多波段图像数据质量进行了评价. 结果显示, 其两端波段范围的图像质量较差, 不能有效识别月表单元, 中间波段图像质量较好; 自然色图像显示IIM数据存在明显的横向色调不均一性. 利用图像的客观评价指标(辐射精度、清晰度、信息量)将干涉成像光谱仪图像数据与美国Clementine UV-VIS多光谱图像数据进行对比分析, 结果表明, IIM数据清晰度好于Clementine UV-VIS, 而在方差和信息量上弱于Clementine UV-VIS. 将IIM图像数据与Clementine多光谱数据统一到相同的空间分辨率, 进行配准精度评价, 其最大控制点配准误差为0.63pixel. 此外, 还对时间跨度两个月覆盖月表同一区域的IIM图像数据从辐射精度、信息量和清晰度方面进行了评价.      

旋转蛇形通道内换热的非稳态实验

在旋转状态下,研究了涡轮内冷蛇形通道的非稳态换热特性.实验主要针对旋转状态下,通道内流量的变化和系统旋转速度的变化来进行的.结果表明:对于旋转通道的非稳态过程,换热系数的变化呈波动变化过程,且主要发生在实验参数变化的阶段.加速旋转时,角加速度力的作用会加强进气通道前缘面的换热,而降低后缘面的;减速旋转时,情况相反.而且,角加速度力的作用效果容易在旋转开始变化的时刻显现.当旋转速度在2个值之间往复时,换热系数变化呈现滞后环状,旋转速度越高滞后环越大.     

双通道辐射量计算太阳极紫外辐射E10.7指数方法初探

从太阳极紫外辐射研究的重要性出发, 介绍了太阳极紫外辐射E_10.7指 数及其作用, 详细阐述了利用两个能道的太阳辐射观测值计算极紫外辐射E_10.7指数的计算方法. 利用该方法对实测太阳辐射数据进行处理, 计算获得了2000-2005年的每日E_10.7指数, 并将计算结果 与Solar2000模式的输出结果进行对比分析, 验证了该计算方法的可行性, 对比结果表明, 最大相对误差在20%以内, 平均相对误差均在10%以内.