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Quantitative comparison of the variability in observed and simulated shortwave reflectance
Y. L. Roberts, P. Pilewskie, B. C. Kindel, D. R. Feldman,W. D. Collins
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2013,
Abstract: The Climate Absolute Radiance and Refractivity Observatory (CLARREO) is a climate observation system that has been designed to monitor the Earth's climate with unprecedented absolute radiometric accuracy and SI traceability. Climate Observation System Simulation Experiments (OSSEs) have been generated to simulate CLARREO hyperspectral shortwave imager measurements to help define the measurement characteristics needed for CLARREO to achieve its objectives. To evaluate how well the OSSE-simulated reflectance spectra reproduce the Earth's climate variability at the beginning of the 21st century, we compared the variability of the OSSE reflectance spectra to that of the reflectance spectra measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). Principal component analysis (PCA) is a multivariate decomposition technique used to represent and study the variability of hyperspectral radiation measurements. Using PCA, between 99.7% and 99.9% of the total variance the OSSE and SCIAMACHY data sets can be explained by subspaces defined by six principal components (PCs). To quantify how much information is shared between the simulated and observed data sets, we spectrally decomposed the intersection of the two data set subspaces. The results from four cases in 2004 showed that the two data sets share eight (January and October) and seven (April and July) dimensions, which correspond to about 99.9% of the total SCIAMACHY variance for each month. The spectral nature of these shared spaces, understood by examining the transformed eigenvectors calculated from the subspace intersections, exhibit similar physical characteristics to the original PCs calculated from each data set, such as water vapor absorption, vegetation reflectance, and cloud reflectance.
Quantitative comparison of the variability in observed and simulated shortwave reflectance
Y. L. Roberts,P. Pilewskie,B. C. Kindel,D. R. Feldman
Atmospheric Chemistry and Physics Discussions , 2012, DOI: 10.5194/acpd-12-28305-2012
Abstract: The Climate Absolute Radiance and Refractivity Observatory (CLARREO) is a climate observation system that has been designed to monitor the Earth's climate with unprecedented absolute radiometric accuracy and SI traceability. Climate Observation System Simulation Experiments (OSSEs) have been generated to simulate CLARREO hyperspectral shortwave imager measurements to help define the measurement characteristics needed for CLARREO to achieve its objectives. To evaluate how well the OSSE-simulated reflectance spectra reproduce the Earth's climate variability at the beginning of the 21st century, we compared the variability of the OSSE reflectance spectra to that of the reflectance spectra measured by the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). Principal component analysis (PCA) is a multivariate spectral decomposition technique used to represent and study the variability of hyperspectral radiation measurements. Using PCA, between 99.7% and 99.9% of the total variance the OSSE and SCIAMACHY data sets can be explained by subspaces defined by six principal components (PCs). To quantify how much information is shared between the simulated and observed data sets, we spectrally decomposed the intersection of the two data set subspaces. The results from four cases in 2004 showed that the two data sets share eight (January and October) and seven (April and July) dimensions, which correspond to about 99.9% of the total SCIAMACHY variance for each month. The spectral nature of these shared spaces, understood by examining the transformed eigenvectors calculated from the subspace intersections, exhibit similar physical characteristics to the original PCs calculated from each data set, such as water vapor absorption, vegetation reflectance, and cloud reflectance.
The influence of cloud structure and droplet concentration on the reflectance of shortwave radiation  [cached]
P. F. Coley,P. R. Jonas
Annales Geophysicae (ANGEO) , 2003,
Abstract: The effects of cloud shadowing, channelling, cloud side illumination and droplet concentration are investigated with regard to the reflection of shortwave solar radiation. Using simple geometric clouds, coupled with a Monte Carlo model the transmission properties of idealized cloud layers are found. The clouds are illuminated with direct solar radiation from above. The main conclusion reached is that the distribution of the cloud has a very large influence on the reflectivity of a cloud layer. In particular, if the cloud contains vertical gaps through the cloud layer in which the liquid water content is zero, then, smaller more numerous gaps are more influential on the radiation than fewer, larger gaps with equal cloud fraction. At very low solar zenith angles channelling of the radiation reduces the reflection expected on the basis of the percentage cloud cover. At high solar zenith angles the illumination of the cloud edges significantly increases the reflection despite the shadowing of one cloud by another when the width of the gaps is small. The impact of droplet concentration upon the reflection of cloud layers is also investigated. It is found that at low solar zenith angles where channelling is important, the lower concentrations increase the transmission. Conversely, when cloud edge illumination is dominant the cloud distribution is found to be more important for the higher concentrations.
Increasing River Flows in the Sahel?  [PDF]
Okechukwu Amogu,Luc Descroix,Kadidiatou Souley Yéro,Eric Le Breton,Ibrahim Mamadou,Abdou Ali,Théo Vischel,Jean-Claude Bader,Ibrahim Bouzou Moussa,Emmanuèle Gautier,Stéphane Boubkraoui,Philippe Belleudy
Water , 2010, DOI: 10.3390/w2020170
Abstract: Despite the drought observed since 1968 in most of the West African Sahel, runoff and rivers discharges have been increasing in the same region. This trend is related with land use change rather than climate change. This paper aims to describe the regional extension of such a phenomenon and to demonstrate that the increase in runoff is observed from the point scale up to the regional scale. It highlights the opposition of functioning between a Sahelian zone, where the Sahel’s paradox applies, and the Sudanian and Guinean areas, where runoff has been logically decreasing with the rainfall. The current trend is evidenced using experimental runoff plots and discharge data from the local to the regional scales.
Impacts of the Sahel-Sahara Interface Reforestation on West African Climate: Intra-Annual Variability and Extreme Temperature Events  [PDF]
Ibrahima Diba, Moctar Camara, Arona Diedhiou
Atmospheric and Climate Sciences (ACS) , 2019, DOI: 10.4236/acs.2019.91003
Abstract: The impacts of the reforestation of the Sahel-Sahara interface on the seasonal distribution of the surface temperature and thermal extremes are studied in the Sahel (West African region lying between 11°N and 18°N). We performed a simulation with the standard version of the RegCM4 model followed by another one using the altered version of the same model taking into account an incorporated forest. The impacts of the vegetation change are assessed by analyzing the difference between the two runs. The reforestation may influence strongly the frequency of warm days (TG90P) and very warm days (TX90P) by decreasing it over the reforested zone from March to May (MAM) and the entire Sahel during the June-August (JJA) period. These TG90P and TX90P indices decrease may be due to the strengthening of the atmospheric moisture content over the whole Sahel region and the weakening of the sensible heat flux over the reforested zone. The analysis also shows a decrease of the TN90P indice (warm nights) over the Sahel during the wet season (JJA) which could be partly associated with the strengthening of the evapotranspiration over the whole Sahel domain. The analysis of additional thermal indices shows an increase of the tropical nights over the entire Sahel from December to February (DJF) and during the warm season (MAM). The strengthening of the tropical night is partly associated with an increase of the surface net downward shortwave flux over the reforested zone. When considering the heat waves, an increase (a decrease) of these events is recorded over the southern Sahel during JJA and SON periods (over the whole Sahelian region during DJF), respectively. Changes in latent heat flux appear to be largely responsible for these extreme temperatures change. This work shows that the vegetation change may impact positively some regions like the reforested area by reducing the occurrence of thermal extremes; while other Sahel regions (eastern part of the central Sahel) could suffer from it because of the strengthening of thermal extremes.
Relationship of Sahel Precipitation and Atmospheric Centers of Action  [PDF]
Sultan Hameed,Nicole Riemer
Advances in Meteorology , 2012, DOI: 10.1155/2012/953853
Abstract: The dynamics associated with drought in the Sahel have attracted considerable attention in the recent literature. This paper evaluates Sahel precipitation using the paradigm of the atmospheric centers of action, that is, the extended semipermanent highs and lows that dominate regional circulations and are evident in sea level pressure patterns. We find that Sahel precipitation is significantly influenced by changes in the Azores High and the South Asia Low. Specifically, about 50 percent of the variance of July to September rainfall over the Sahel is explained by changes in the Azores High Longitude position and South Asia Low pressure. In contrast, the contribution of the Southern Oscillation to Sahel precipitation is smaller in comparison. Results presented in this paper suggest that a key test for a climate model in simulating variability of Sahel rainfall is the accuracy with which the model simulates the dynamics of South Asia Low and the Azores High.
Relationship of Sahel Precipitation and Atmospheric Centers of Action  [PDF]
Sultan Hameed,Nicole Riemer
Advances in Meteorology , 2012, DOI: 10.1155/2012/953853
Abstract: The dynamics associated with drought in the Sahel have attracted considerable attention in the recent literature. This paper evaluates Sahel precipitation using the paradigm of the atmospheric centers of action, that is, the extended semipermanent highs and lows that dominate regional circulations and are evident in sea level pressure patterns. We find that Sahel precipitation is significantly influenced by changes in the Azores High and the South Asia Low. Specifically, about 50 percent of the variance of July to September rainfall over the Sahel is explained by changes in the Azores High Longitude position and South Asia Low pressure. In contrast, the contribution of the Southern Oscillation to Sahel precipitation is smaller in comparison. Results presented in this paper suggest that a key test for a climate model in simulating variability of Sahel rainfall is the accuracy with which the model simulates the dynamics of South Asia Low and the Azores High. 1. Introduction Since the 1950s, the semiarid African Sahel zone has undergone a persistent drought, although there is some amelioration observed since the 1980s. Several theories on the cause of this phenomenon have been discussed, with a final conclusion and a quantification of human versus natural causes still to be reached. The Sahel drought does not only have devastating consequences for the Sahel region itself [1], but it also influences regions far away because of the increased output of soil-derived aerosol particles into the atmosphere [2, 3]. Soon after the drought in Sahel became known as a major climatic phenomenon, Angell and Korshover [4] analyzed the trends in the atmospheric centers of action and suggested that “the southward or southeastward movement of the Azores High after 1945 is related to the recent drought just south of the Sahara (Sahelian Zone).” Later other more local processes were put forth as influencing Sahel rainfall such as changes in albedo due to land use change [5], the amount of moisture in the soil [6], and the surface roughness [7]. These can be influenced by overgrazing or deforestation, which led to the hypothesis that human activity contributes to the continuing drought by impacting the land surface processes, and which in turn affect atmospheric circulation. More recently, several global climate models have been used to study Sahel precipitation by using the paradigm that the Sahel rainfall responds to sea surface temperature changes in certain ocean basins [8, 9]. Some models have been used to make projections for Sahel precipitation in the future under
Detrended fluctuation analysis of intertrade durations  [PDF]
Zhi-Qiang Jiang,Wei Chen,Wei-Xing Zhou
Quantitative Finance , 2008, DOI: 10.1016/j.physa.2008.10.028
Abstract: The intraday pattern, long memory, and multifractal nature of the intertrade durations, which are defined as the waiting times between two consecutive transactions, are investigated based upon the limit order book data and order flows of 23 liquid Chinese stocks listed on the Shenzhen Stock Exchange in 2003. An inverse $U$-shaped intraday pattern in the intertrade durations with an abrupt drop in the first minute of the afternoon trading is observed. Based on the detrended fluctuation analysis, we find a crossover of power-law scaling behaviors for small box sizes (trade numbers in boxes) and large box sizes and strong evidence in favor of long memory in both regimes. In addition, the multifractal nature of intertrade durations in both regimes is confirmed by a multifractal detrended fluctuation analysis for individual stocks with a few exceptions in the small-duration regime. The intraday pattern has little influence on the long memory and multifractaility.
Reflectance Hashing for Material Recognition  [PDF]
Hang Zhang,Kristin Dana,Ko Nishino
Computer Science , 2015,
Abstract: We introduce a novel method for using reflectance to identify materials. Reflectance offers a unique signature of the material but is challenging to measure and use for recognizing materials due to its high-dimensionality. In this work, one-shot reflectance is captured using a unique optical camera measuring {\it reflectance disks} where the pixel coordinates correspond to surface viewing angles. The reflectance has class-specific stucture and angular gradients computed in this reflectance space reveal the material class. These reflectance disks encode discriminative information for efficient and accurate material recognition. We introduce a framework called reflectance hashing that models the reflectance disks with dictionary learning and binary hashing. We demonstrate the effectiveness of reflectance hashing for material recognition with a number of real-world materials.
Detrended Fluctuation Analysis of Traffic Data
ZHU Xiao-Yan,LIU Zong-Hua,TANG Ming,
朱晓妍
,刘宗华,唐明

中国物理快报 , 2007,
Abstract: Different routing strategies may result in different behaviour of traffic on internet. We analyse the correlation of traffic data for three typical routing strategies by the detrended fluctuation analysis (DFA) and find that the degree of correlation of the data can be divided into three regions, i.e. weak, medium, and strong correlation. The DFA scalings are constants in both the regions of weak and strong correlations but monotonically increase in the region of medium correlation. We suggest that it is better to consider the traffic on complex network as three phases, i.e. the free, buffer, and congestion phase, than just as two phases believed before, i.e. the free andcongestion phase.
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