Improved Surface Reflectance from Remote Sensing Data with Sub-Pixel Topographic Information
Roupioz, L.F.S. ; Nerry, F. ; Jia, L. ; Menenti, M. - \ 2014
Remote Sensing 6 (2014)11. - ISSN 2072-4292 - p. 10356 - 10374.
landsat-tm data - satellite imagery - rugged terrain - atmospheric correction - spatial-resolution - illumination - products - albedo - china - model
Several methods currently exist to efficiently correct topographic effects on the radiance measured by satellites. Most of those methods use topographic information and satellite data at the same spatial resolution. In this study, the 30 m spatial resolution data of the Digital Elevation Model (DEM) from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) are used to account for those topographic effects when retrieving land surface reflectance from satellite data at lower spatial resolution (e.g., 1 km). The methodology integrates the effects of sub-pixel topography on the estimation of the total irradiance received at the surface considering direct, diffuse and terrain irradiance. The corrected total irradiance is then used to compute the topographically corrected surface reflectance. The proposed method has been developed to be applied on various kilometric pixel size satellite data. In this study, it was tested and validated with synthetic Landsat data aggregated at 1 km. The results obtained after a sub-pixel topographic correction are compared with the ones obtained after a pixel level topographic correction and show that in rough terrain, the sub-pixel topography correction method provides better results even if it tends to slightly overestimate the retrieved land surface reflectance in some cases.
Attributing the impacts of land-cover changes in temperate regions on surface temperature and heat fluxes to specific causes: Results from the first LUCID set of simulations
Boisier, J.P. ; Noblet-Ducoudré, N. de; Pitman, A.J. ; Cruz, F.T. ; Delire, C. ; Hurk, B.J.J.M. van den; Molen, M.K. van der; Müller, C. ; Voldoire, A. - \ 2012
Journal of Geophysical Research: Atmospheres 117 (2012)D12. - ISSN 2169-897X
climate system model - soil-moisture - sensitivity - feedbacks - forcings - exchange - forest - energy - biomes - albedo
Surface cooling in temperate regions is a common biogeophysical response to historical Land-Use induced Land Cover Change (LULCC). The climate models involved in LUCID show, however, significant differences in the magnitude and the seasonal partitioning of the temperature change. The LULCC-induced cooling is directed by decreases in absorbed solar radiation, but its amplitude is 30 to 50% smaller than the one that would be expected from the sole radiative changes. This results from direct impacts on the total turbulent energy flux (related to changes in land-cover properties other than albedo, such as evapotranspiration efficiency or surface roughness) that decreases at all seasons, and thereby induces a relative warming in all models. The magnitude of those processes varies significantly from model to model, resulting on different climate responses to LULCC. To address this uncertainty, we analyzed the LULCC impacts on surface albedo, latent heat and total turbulent energy flux, using a multivariate statistical analysis to mimic the models' responses. The differences are explained by two major ‘features’ varying from one model to another: the land-cover distribution and the simulated sensitivity to LULCC. The latter explains more than half of the inter-model spread and resides in how the land-surface functioning is parameterized, in particular regarding the evapotranspiration partitioning within the different land-cover types, as well as the role of leaf area index in the flux calculations. This uncertainty has to be narrowed through a more rigorous evaluation of our land-surface models.
A laboratory goniometer system for measuring reflectance and emittance anisotropy
Roosjen, P.P.J. ; Clevers, J.G.P.W. ; Bartholomeus, H. ; Schaepman, M.E. ; Schaepman-Strub, G. ; Jalink, H. ; Schoor, R. van der; Jong, A. de - \ 2012
Sensors 12 (2012)12. - ISSN 1424-8220 - p. 17358 - 17371.
bidirectional reflectance - radiance data - albedo - field - brdf - algorithm - surface - model - acquisition - vegetation
In this paper, a laboratory goniometer system for performing multi-angular measurements under controlled illumination conditions is described. A commercially available robotic arm enables the acquisition of a large number of measurements over the full hemisphere within a short time span making it much faster than other goniometers. In addition, the presented set-up enables assessment of anisotropic reflectance and emittance behaviour of soils, leaves and small canopies. Mounting a spectrometer enables acquisition of either hemispherical measurements or measurements in the horizontal plane. Mounting a thermal camera allows directional observations of the thermal emittance. This paper also presents three showcases of these different measurement set-ups in order to illustrate its possibilities. Finally, suggestions for applying this instrument and for future research directions are given, including linking the measured reflectance anisotropy with physically-based anisotropy models on the one hand and combining them with field goniometry measurements for joint analysis with remote sensing data on the other hand. The speed and flexibility of the system offer a large added value to the existing pool of laboratory goniometers.
Estimating forest variables from top-of-atmosphere radiance satellite measurements using coupled radiative transfer models
Laurent, V.C.E. ; Verhoef, W. ; Clevers, J.G.P.W. ; Schaepman, M.E. - \ 2011
Remote Sensing of Environment 115 (2011)4. - ISSN 0034-4257 - p. 1043 - 1052.
leaf-area index - bidirectional reflectance - biophysical variables - global products - canopy - vegetation - inversion - validation - simulation - albedo
Traditionally, it is necessary to pre-process remote sensing data to obtain top of canopy (TOC) reflectances before applying physically-based model inversion techniques to estimate forest variables. Corrections for atmospheric, adjacency, topography, and surface directional effects are applied sequentially and independently, accumulating errors into the TOC reflectance data, which are then further used in the inversion process. This paper presents a proof of concept for demonstrating the direct use of measured top-of-atmosphere (TOA) radiance data to estimate forest biophysical and biochemical variables, by using a coupled canopy–atmosphere radiative transfer model. Advantages of this approach are that no atmospheric correction is needed and that atmospheric, adjacency, topography, and surface directional effects can be directly and more accurately included in the forward modelling. In the case study, we applied both TOC and TOA approaches to three Norway spruce stands in Eastern Czech Republic. We used the SLC soil–leaf–canopy model and the MODTRAN4 atmosphere model. For the TOA approach, the physical coupling between canopy and atmosphere was performed using a generic method based on the 4-stream radiative transfer theory which enables full use of the directional reflectance components provided by SLC. The method uses three runs of the atmosphere model for Lambertian surfaces, and thus avoids running the atmosphere model for each new simulation. We used local sensitivity analysis and singular value decomposition to determine which variables could be estimated, namely: canopy cover, fraction of bark, needle chlorophyll, and dry matter content. TOC and TOA approaches resulted in different sets of estimates, but had comparable performance. The TOC approach, however, was at its best potential because of the flatness and homogeneity of the area. On the contrary, the capacities of the TOA approach would be better exploited in heterogeneous rugged areas. We conclude that, having similar performance, the TOA approach should be preferred in situations where minimizing the pre-processing is important, such as in data assimilation and multi-sensor studies.
Influence of future air pollution mitigation strategies on total aerosol radiative forcing
Kloster, S. ; Dentener, F. ; Feichter, J. ; Raes, F. ; Aardenne, J.A. van; Roeckner, E. ; Lohmann, U. ; Stier, P. ; Swart, R.J. - \ 2008
Atmospheric Chemistry and Physics 8 (2008)21. - ISSN 1680-7316 - p. 6405 - 6437.
climate model echam5-ham - global sulfate distribution - cloud microphysics - greenhouse-gas - sulfur cycle - emissions - impact - simulation - albedo - flux
We apply different aerosol and aerosol precursor emission scenarios reflecting possible future control strategies for air pollution in the ECHAM5-HAM model, and simulate the resulting effect on the Earth's radiation budget. We use two opposing future mitigation strategies for the year 2030: one in which emission reduction legislation decided in countries throughout the world are effectively implemented (current legislation; CLE 2030) and one in which all technical options for emission reductions are being implemented independent of their cost (maximum feasible reduction; MFR 2030). We consider the direct, semi-direct and indirect radiative effects of aerosols. The total anthropogenic aerosol radiative forcing defined as the difference in the top-of-the-atmosphere radiation between 2000 and pre-industrial times amounts to -2.00 W/m2. In the future this negative global annual mean aerosol radiative forcing will only slightly change (+0.02 W/m2) under the "current legislation" scenario. Regionally, the effects are much larger: e.g. over Eastern Europe radiative forcing would increase by +1.50 W/m2 because of successful aerosol reduction policies, whereas over South Asia it would decrease by -1.10 W/m2 because of further growth of emissions. A "maximum feasible reduction" of aerosols and their precursors would lead to an increase of the global annual mean aerosol radiative forcing by +1.13 W/m2. Hence, in the latter case, the present day negative anthropogenic aerosol forcing could be more than halved by 2030 because of aerosol reduction policies and climate change thereafter will be to a larger extent be controlled by greenhouse gas emissions. We combined these two opposing future mitigation strategies for a number of experiments focusing on different sectors and regions. In addition, we performed sensitivity studies to estimate the importance of future changes in oxidant concentrations and the importance of the aerosol microphysical coupling within the range of expected future changes. For changes in oxidant concentrations caused by future air pollution mitigation, we do not find a significant effect for the global annual mean radiative aerosol forcing. In the extreme case of only abating SO2 or carbonaceous emissions to a maximum feasible extent, we find deviations from additivity for the radiative forcing over anthropogenic source regions up to 10% compared to an experiment abating both at the same time.
Diurnal temperature fluctuations in an artificial small shallow water body
Jacobs, A.F.G. ; Heusinkveld, B.G. ; Kraai, S. ; Paaijmans, K.P. - \ 2008
International Journal of Biometeorology 52 (2008)4. - ISSN 0020-7128 - p. 271 - 280.
ponds - lakes - albedo
For aquatic biological processes, diurnal and annual cycles of water temperature are very important to plants as well as to animals and microbes living in the water. An existing one-dimensional model has been extended to simulate the temperature profile within a small water body. A year-round outdoor experiment has been conducted to estimate the model input parameters and to verify the model. Both model simulations and measurements show a strong temperature stratification in the water during daytime. Throughout the night, however, a well-mixed layer starting at the water surface develops. Because the water body is relatively small, it appears that the sediment heat flux has a strong effect on the behaviour of the water temperature throughout the seasons. In spring, the water temperature remains relatively low due to the cold surrounding soil, while in autumn the opposite occurs due to the relatively warm soil. It appears that, in small water bodies, the total amount of incoming long wave radiation is sensitive to the sky view factor. In our experiments, the intensity of precipitation also appears to have a small effect on the stratification of the water temperature.
Effect of smoke and clouds on the transmissivity of photosynthetically active radiation inside the canopy
Yamasoe, M.A. ; Randow, C. von; Manzi, A.O. ; Schafer, J.S. ; Eck, T.F. ; Holben, B.N. - \ 2006
Atmospheric Chemistry and Physics 6 (2006). - ISSN 1680-7316 - p. 1645 - 1656.
biomass-burning aerosols - optical-properties - global radiation - direct component - amazon smoke - rain-forest - carbon - scattering - diffuse - albedo
Biomass burning activities emit high concentrations of aerosol particles to the atmosphere. Such particles can interact with solar radiation, decreasing the amount of light reaching the surface and increasing the fraction of diffuse radiation through scattering processes, and thus has implications for photosynthesis within plant canopies. This work reports results from photosynthetically active radiation ( PAR) and aerosol optical depth (AOD) measurements conducted simultaneously at Reserva Biologica do Jaru (Rondonia State, Brazil) during LBA/SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia/Smoke, Aerosols, Clouds, Rainfall, and Climate) and RaCCI ( Radiation, Cloud, and Climate Interactions in the Amazon during the Dry-to-Wet Transition Season) field experiments from 15 September to 15 November 2002. AOD values were retrieved from an AERONET ( Aerosol Robotic Network) radiometer, MODIS ( Moderate Resolution Spectroradiometer) and a portable sunphotometer from the United States Department of Agriculture - Forest Service. Significant reduction of PAR irradiance at the top of the canopy was observed due to the smoke aerosol particles layer. This radiation reduction affected turbulent fluxes of sensible and latent heats. The increase of AOD also enhanced the transmission of PAR inside the canopy. As a consequence, the availability of diffuse radiation was enhanced due to light scattering by the aerosol particles. A complex relationship was identified between light availability inside the canopy and net ecosystem exchange ( NEE). The results showed that the increase of aerosol optical depth corresponded to an increase of CO2 uptake by the vegetation. However, for even higher AOD values, the corresponding NEE was lower than for intermediate values. As expected, water vapor pressure deficit (VPD), retrieved at 28 m height inside the canopy, can also affect photosynthesis. A decrease in NEE was observed as VPD increased. Further studies are needed to better understand these findings, which were reported for the first time for the Amazon region under smoky conditions.
Reflectance quantities in optical remote sensing - definitions and case studies
Schaepman-Strub, G. ; Schaepman, M.E. ; Painter, T.H. ; Dangel, S. ; Martonchik, J. - \ 2006
Remote Sensing of Environment 103 (2006)1. - ISSN 0034-4257 - p. 27 - 42.
bidirectional reflectance - directional reflectance - land-surface - radiative-transfer - grain-size - brdf model - vegetation - albedo - snow - variability
The remote sensing community puts major efforts into calibration and validation of sensors, measurements, and derived products to quantify and reduce uncertainties. Given recent advances in instrument design, radiometric calibration, atmospheric correction, algorithm development, product development, validation, and delivery, the lack of standardization of reflectance terminology and products becomes a considerable source of error. This article provides full access to the basic concept and definitions of reflectance quantities, as given by Nicodemus et al. [Nicodemus, F.E., Richmond, J.C., Hsia, J.J., Ginsberg, I.W., and Limperis, T. (1977). Geometrical Considerations and Nomenclature for Reflectance. In: National Bureau of Standards, US Department of Commerce, Washington, D.C. URL: http://physics.nist.gov/Divisions/Div844/facilities/specphoto/pdf/geoConsid.pdf.] and Martonchik et al. [Martonchik, J.V., Bruegge, C.J., and Strahler, A. (2000). A review of reflectance nomenclature used in remote sensing. Remote Sensing Reviews, 19, 9¿20.]. Reflectance terms such as BRDF, HDRF, BRF, BHR, DHR, black-sky albedo, white-sky albedo, and blue-sky albedo are defined, explained, and exemplified, while separating conceptual from measurable quantities. We use selected examples from the peer-reviewed literature to demonstrate that very often the current use of reflectance terminology does not fulfill physical standards and can lead to systematic errors. Secondly, the paper highlights the importance of a proper usage of definitions through quantitative comparison of different reflectance products with special emphasis on wavelength dependent effects. Reflectance quantities acquired under hemispherical illumination conditions (i.e., all outdoor measurements) depend not only on the scattering properties of the observed surface, but as well on atmospheric conditions, the object's surroundings, and the topography, with distinct expression of these effects in different wavelengths. We exemplify differences between the hemispherical and directional illumination quantities, based on observations (i.e., MISR), and on reflectance simulations of natural surfaces (i.e., vegetation canopy and snow cover). In order to improve the current situation of frequent ambiguous usage of reflectance terms and quantities, we suggest standardizing the terminology in reflectance product descriptions and that the community carefully utilizes the proposed reflectance terminology in scientific publications
The structure of the atmospheric surface layer subject to local advection
Bink, N.J. - \ 1996
Agricultural University. Promotor(en): L. Wartena; H.F. Vugts; L.J.M. Kroon. - S.l. : Bink - ISBN 9789054855132 - 206
atmosfeer - luchttemperatuur - fluctuaties - zonnestraling - albedo - reflectie - wolken - microklimaat - bodem - landschap - grenslaag - aardoppervlak - aarde - atmosphere - air temperature - fluctuations - solar radiation - albedo - reflection - clouds - microclimate - soil - landscape - boundary layer - land surface - earth
For many applications in agriculture, hydrology and meteorology simple methods are needed to determine the surface-atmosphere exchange of momentum, heat and water vapour, i.e to determine the fluxes of momentum, heat and water vapour. Most methods to calculate these fluxes are only valid for horizontal, homogeneous terrain with sufficient large dimensions. It is thus assumed that so-called advective effects can be neglected because wind speed, temperature and humidity do not change in the horizontal direction. In practice, the surface is hardly ever homogeneous. It was the objective of this study to investigate the effects of advection of heat and moisture on the fluxes. It appeared that only a few sets related to advection were available, and the available sets yielded contradictive results. Therefore an experiment was carried out in La Crau, France around a step-change from a dry and bare terrain to irrigated grass in order to measure the influence of advection on the structure of the flow and the exchange processes near the earth's surface.
Before this study there was confusion about the behaviour of the flux-gradient ratios or eddy diffusivities under conditions of local advection. The flux-gradient ratios or eddy diffusivities were estimated using the calculated surface fluxes and the gradients from the profile measurements. It was found that the flux-gradient ratio for heat was smaller than that for water vapour in the lower part of the surface layer after the step-change. This was in agreement with the ratio of the observed transfer efficiencies. Higher up in the surface layer after the step-change and for weak advective conditions it was found that the flux-gradient ratio for heat was larger than that for water vapour.
Also, flux determination methods were tested using a second-order closure model which was found to compare favourably with the measurements. It was found that for situations similar to that in the Crau, the so-called gradient Bowen ratio can be used at fetch-to-height ratios up to z/x = 0.02 which is high compared to what was accepted untill now. It was also found that the difference between the Bowen ratio at the surface and at some level above the surface is compensated for by the ratio of the eddy diffusivities at the height where the gradient is measured. For the Bowen ratio from standard deviations in the thermally stable surface layer the error was found to be below 10%, up to z/x = 0.02. The ratio of the transfer efficiencies was below unity which compensated the error due to the flux divergence. For the unstable surface layer z/x must be below 0.008 to achieve the same accuracy because now the ratio of the transfer efficiencies amplifies the error due to flux divergence. For the cases studied here, a number of factors may have cooperated in a favourable manner due to which the error sources cancel out. However, it is significant to note that the model can be used to forecast which conditions are favourable and which are not for the application of micro- meteorological methods to determine the surface fluxes under conditions of local advection.
A simple surface radiation budget model for a point in snow covered mountainous terrain
Uijlenhoet, R. - \ 1992
Wageningen : Landbouwuniversiteit Wageningen (Rapport / Landbouwuniversiteit, Vakgroep Waterhuishouding 20) - 125
albedo - atmosfeer - wolken - gebergten - straling - reflectie - sneeuw - zonnestraling - temperatuur - aarde - albedo - atmosphere - clouds - mountains - radiation - reflection - snow - solar radiation - temperature - earth
The energy balance of the earth's surface : a practical approach
Bruin, H.A.R. de - \ 1982
Landbouwhogeschool Wageningen. Promotor(en): L. Wartena, co-promotor(en): H. Tennekes. - Wageningen : De Bruin - 177
luchttemperatuur - albedo - atmosfeer - wolken - evaporatie - fluctuaties - warmte - hydrologie - land - mechanica - meteorologie - reflectie - bodem - bodemtemperatuur - zonnestraling - structuur - oppervlakten - temperatuur - theorie - thermische eigenschappen - thermodynamica - water - waterbalans - aarde - hydrologische cyclus - aardoppervlak - air temperature - albedo - atmosphere - clouds - evaporation - fluctuations - heat - hydrology - land - mechanics - meteorology - reflection - soil - soil temperature - solar radiation - structure - surfaces - temperature - theory - thermal properties - thermodynamics - water - water balance - earth - hydrological cycle - land surface
This study is devoted to the energy balance of the earth's surface with a special emphasis on practical applications. A simple picture of the energy exchange processes that take place at the ground is the following. Per unit time and area an amount of radiant energy is supplied to the surface. This radiation originates partly from the sun, but an~ other fraction is coming from the atmosphere (= infra-red radiation emitted by clouds, water vapour and CO 2 ). From these gain terms the following losses must be subtracted: (a) the reflected solar radiation and (b) the infra-red radiation emitted by the surface itself. The final result is that a net amount of radiant energy is received by the surface, simply denoted as net radiation. At the ground net radiation is used to heat the ground (soil heat flux), to evaporate liquid water (evaporation), and to heat the atmosphere (sensible heat flux). In this simple picture we have neglected minor terms such as the energy used by the plants for their photosynthesis.Due to the high value of the latent heat of vaporization, the energy needed for evaporation is often an important term in the energy balance. In addition the energy balance of the earth's surface is linked with the water budget of both the atmosphere and the earth's surface, through the evaporation at the ground.Several practical questions in agriculture, hydrology and meteorology require information m the energy balance of the surface. It is the purpose of this study to find solutions for some of these problems.In hydrology one is mainly concerned in evaporation averaged over 1 day or more on a regional scale. Generally, this refers to land surfaces, but the evaporation of inland lakes or reservoirs is also of interest. In this context we also mention the problem of thermal pollution of open water bodies by industry or power plants. For this the so-called natural water temperature must be known, which is the temperature of the water in the hypothetical case that there is no artificial heating. It appears that this temperature depends mainly m the energy balance at the surface. In Chapter VI a model dealing with this problem is discussed.In agriculture one is interested also in evaporation. Now time intervals ranging from half an hour to several days are of interest.The relation between evaporation an the one side and plant diseases and pest control an the other can be mentioned.Furthermore, the yield of several agricultural crops is the greatest when the evapotranspiration is potential (= a maximum under the given weather conditions). When the crop transpires less than the potential rate, because the soil is too dry, the yield can be augmented by artificial precipitation. For applications such as these cheap and simple techniques are required for measuring the actual and potential evaporation. This applies also to agricultural research projects, e.g. to determine yield-water use relationships.In Chapter II simple measurement techniques are considered.Recent developments in meteorology have led to an increase of the interest in the energy balance of the earth's surface, especially in the input of heat and humidity at ground level into the atmosphere. Examples are models for the atmospheric boundary layer and related models for short range weather forecasts (12-18 h ahead). These models require simple parameterizations of the surface fluxes. This applies also to weather forecast models on a medium time range (3-10 days ahead).Since the height of the boundary layer is related to the heat input at the ground information an the surface energy balance is needed also for air pollution problems.In Chapter III a simple parameterization for evaporation and sensible heat flux is described that can be used for these type of problems.Usually, the only available data are standard weather observations. For that reason, many of the practical questions, mentioned above, can be formulated as: "How can the surface energy balance be estimated from standard weather data only'?" In Chapters III and VI possible answers to that question are discussed.Chapter II is devoted to simple measuring techniques that, in principle, can be used on an operational base. These methods will be compared with the so-called energy-balance method, using Bowen's ratio.In Chapter III two models for evaporation and sensible heat flux during daytime are compared. Both require standard weather data as input and an indication of the surface wetness. The first model needs more data, but contains more physics. The second is less complete, but requires less input data.Chapter IV has a mainly theoretical character. A model is presented that couples the evolution of the atmospheric boundary layer to the surface energy balance. It describes the course of the height, temperature and humidity of the boundary layer, together with the surface fluxes, when the initial profiles of temperature and humidity the radiative forcing and the surface wetness are known. It is restricted to convective conditions. Model output will be compared with observations.In Chapter V an empirical evaporation model for open water is considered. Comparisons with observations of evaporation of the former Lake Flevo will be made; the annual and the diurnal cycle will be considered.In Chapter VI a model for the (natural) temperature and energy balance of inland lakes and water reservoirs is discussed that requires standard weather data only. A comparison between the calculated and measured water temperature will be given. This concerns two adjacent water reservoirs, which have about the same size, but which differ in depth (5 and 15 m). This is of importance, since the water temperature also depends on water depth.At some places we made new modifications, but most of the theoretical concepts applied in this study are adopted from literature. This is inherent in our practical approach. Some of the theories used have been available for many years. But, e.g. because no suitable instruments were available, they were not usefull for practical applications. Recent developments in the field of instrumentation and data handling have changed the situation-to our advantage. A good example is the temperature fluctuation method for measuring the sensible heat flux (discussed in 11.4). The theoretical basis for this approach was given by Prandtl already in 1932. But for an experimental verification we had to wait until the sixties and early seventies. In that period instruments were developed to measure turbulent surface fluxes and fast temperature fluctuations, while also the data handling techniques were improved significantly. Finally) the method wouldn't be operationally until quite recently.For the verification of the parameterizations, measuring techniques and models treated in this study, we used data collected at the 200 m mast at Cabauw, and at the nearby micrometeorological field, of the Royal Netherlands Meteorological Institute.