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1.
A data mining approach for understanding topographic control on climate-induced inter-annual vegetation variability over the United States 总被引:3,自引:0,他引:3
The complex feedback relationship between climate variability and vegetation dynamics is a subject of intense investigation for its implications in furthering our understanding of the global biogeochemical cycle. We address an important question in this context: “How does topography influence the vegetation's response to natural climate fluctuations?” We explore this issue through the analysis of inter-annual vegetation variability over a very large area (continental United States) using long-term (13-year period of 1989-2001), monthly averaged, biweekly maximum value composite normalized difference vegetation index (NDVI) data. These data are obtained from satellite remote sensing at 1-km resolution. Through the novel implementation of data mining techniques, we show that the Northern Pacific climate oscillation and the ENSO phenomena influence the year-to-year vegetation variability over an extensive geographical domain. Further, the vegetation response to these fluctuations depends on a variety of topographic attributes such as elevation, slope, aspect, and proximity to moisture convergence zones, although the first two are the predominant controls. Therefore, the dynamic response of terrestrial vegetation to climate fluctuations, which shows tremendous spatial heterogeneity, is closely linked to the variability induced by the topography. These findings suggest that the representation of vegetation dynamics in existing climate models, which do not incorporate such dependencies, may be inadequate. Therefore, climate models that are regularly employed to guide policy decisions need to better incorporate these dependencies for the assessment of terrestrial carbon sequestration under evolving climate scenarios. 相似文献
2.
Six years (1998-2003) of SeaWiFS multispectral satellite data are used to document the seasonal and interannual variability of the Columbia River plume on the North American west coast. A supervised classification scheme using 5 channels of normalized water-leaving radiance (nLw at 412, 443, 490, 510 and 555 nm), with training pixels adjusted temporally to optimize the signature of plume core characteristics, quantifies the climatological seasonal location of 4 spectrally defined classes of surface water and provides estimates of variability in position as a probability. Winter plume orientation was northward and close to the shore, with infrequent adjustments to the south. Summer plume orientation was offshore and to the south, dissociated from the coast, with more frequent (> 20%) occurrences of plume water and peripheral plume water (> 50%) to the north. An effective characterization of interannual variability in plume dynamics is provided by time series of temporally averaged nLw at 555 nm, used as an estimate of suspended particulate material. Monthly means during maxima and minima in annual river discharge show the plume to be weakest both in spatial extent as well as absolute nLw values in 2001, a year of minimum river discharge. Time series of both (a) nLw 555 values at the river mouth and (b) Mode 2 of an empirical orthogonal function decomposition of the 6-year nLw 555 time series variance that isolates winter patterns are strongly correlated with river discharge. Interannual differences in monthly mean wind forcing are evident as changes in plume position during the winter, but at the 8-day and longer time scales examined here, summer interannual differences are dominated by differences in discharge volume. 相似文献
3.
Determinants of the interannual relationships between remote sensed photosynthetic activity and rainfall in tropical Africa 总被引:6,自引:0,他引:6
The response of photosynthetic activity to interannual rainfall variations in Africa South of the Sahara is examined using 20 years (1981-2000) of Normalised Difference Vegetation Index (NDVI) AVHRR data. Linear correlations and regressions were computed between annual NDVI and annual rainfall at a 0.5° latitude/longitude resolution, based on two gridded precipitation datasets (Climate Prediction Center Merged Analysis of Precipitation [CMAP] and Climatic Research Unit [CRU]). The spatial patterns were then examined to detect how they relate to the mean annual rainfall amounts, land-cover types as from the Global Land Cover 2000 data set, soil properties and soil types. Yearly means were computed starting from the beginning of the vegetative year (first month after the minimum of the NDVI mean regime), with a one-month lead for rainfall.One third of tropical Africa displays significant (95% c.l.) correlations between interannual NDVI variations and those of rainfall. At continental scale, soil types and soil properties are only minor factors in the overall distribution of the correlations. Mean annual rainfall amounts and land-cover types are much more discriminating. The largest correlations, mostly over 0.60, are distinctly found in semi-arid (200-600 mm annual rainfall) open grassland and cropland areas. The presence of one of these two determinants (semi-aridity, and favourable land-cover type, i.e. open grassland and cropland) in the absence of the other does not systematically result in a significant correlation between rainfall and NDVI. By contrast, NDVI variations are independent from those of rainfall in markedly arid environments and in most forest and woodland areas. This results from a low signal-to-noise ratio in the former, and the fact that precipitation is generally not a limiting factor in the latter.The marginal response of NDVI to a given increase/decrease in rainfall, as described by the slope of the regression, displays a similar pattern to that of the correlation, with maximum slopes in semi-arid regions, except that a weaker response is noted in more densely populated areas, suggesting an incidence of particular land-use and agricultural practises.One-year lag relationships between annual rainfall and NDVI in the next year were also considered. Ten percent of the grid-points show significant correlations, but the spatial patterns remain difficult to interpret. 相似文献
4.
Nishant Kumar Naveen Garg Ravinder Agarwal D. Saha Mahavir Singh 《International journal of remote sensing》2017,38(11):3466-3482
The variability of the atmospheric boundary layer together with meteorological parameters has been investigated over the semi-arid region Delhi. Two sources of the dataset have been used: sound detection and ranging (SODAR) and automatic weather station during the period from December 2013 to November 2014. A Laboratory Virtual Instrument Engineering Workbench (LabVIEW)-based programme has been developed to plot the stability class from A to F directly from the mixing height dataset. Based on the SODAR echograms and mixing height, temporal and seasonal variability of stability classes has been estimated. It is observed that the convective boundary layer height advances and decreases during the daytime depending on the increase and decrease of surface temperature due to solar heating of the ground. From seasonal classification of the stability class, it is observed that the class A and class E are dominated in convection and nocturnal periods in all seasons, whereas class F is not found during the winter and pre-monsoon seasons. Impact of meteorological parameters, that is, wind speed, temperature, and relative humidity on mixing height during different seasons has also been studied. 相似文献
5.
本文主题的KVM over IP就是采用OVER IP技术的一个热门例子。本文介绍了KVM的基本概念和发展历程,并通过KVM over IP技术在市气象局机房和雷达站机房的应用实例,指出KVM over IP的应用可以大大提升气象业务网络的管理效率。 相似文献
6.
Monsoon rainfall distribution over the Indian sub‐continent is inconsistent every year. Due to uncertainty and dependence on the monsoon onset and weather conditions, estimation of crop yield in India is difficult. In this paper, analyses of the crop yield, normalized difference vegetation index, soil moisture, surface temperature and rainfall data for 16 years (from 1984 to 1999) have been carried out. A non‐linear iterative multivariate optimization approach (quasi‐Newton method with least square loss function) has been used to derive an empirical piecewise linear crop yield prediction equation (with a break point). The derived empirical equation (based on 1984 to 1998 data) has been used to predict 1999 crop yield with R2>0.90. The model has been validated for the three years 1997, 1998 and 1999. A crop yield prediction equation has been obtained for each province in India (for wheat and rice) that accounts for>90% of the variance in the dataset. 相似文献
7.
Retrieval of bare soil and vegetation parameters from wind scatterometer measurements over three different climatic regions 总被引:1,自引:0,他引:1
Medium to low resolution (1-50 km) active microwave sensors such as spaceborne scatterometers and wide-swath mode synthetic aperture radars have great potential as tools for long term monitoring over land and ice. To optimise the use of this kind of data, the heterogeneity of the target and its effects on the radar measurements need to be investigated and modelled, particularly in the view of retrieving geophysical parameters. In this paper, wind scatterometer measurements over three different test sites, the NOPEX region in Sweden, the HAPEX-Sahel site in Niger and the Niger delta area in Nigeria, are analysed. For these regions, a forward model is developed by considering the backscatter contributions of the bare surface, the seasonal and evergreen vegetation and the open water areas. Colocated high spatial resolution SAR data and ground information are used to characterise the target scene. The model is then inverted to retrieve monthly soil roughness, dielectric properties and vegetation parameters. It is shown that the measurements contain enough information to characterise these three different regions and to monitor their temporal evolution. The retrieved values obtained for the bare surface and the vegetation parameters are consistent with ground measurements collected in these areas. Further improvements are achieved by incorporating the time scale variability of the variables investigated into the retrieval scheme. 相似文献
8.
In order to prioritize the measurement requirements and accuracies of the two new lidar missions, a physical model is required for a fundamental understanding of the impact of surface topography, footprint size and off-nadir pointing on vegetation lidar waveforms and vegetation height retrieval. In this study, we extended a well developed Geometric Optical and Radiative Transfer (GORT) vegetation lidar model to take into account for the impacts of surface topography and off-nadir pointing on vegetation lidar waveforms and vegetation height retrieval and applied this extended model to assess the aforementioned impacts on vegetation lidar waveforms and height retrieval.Model simulation shows that surface topography and off-nadir pointing angle stretch waveforms and the stretching effect magnifies with footprint size, slope and off-nadir pointing angle. For an off-nadir pointing laser penetrating vegetation over a slope terrain, the waveform is either stretched or compressed based on the relative angle. The stretching effect also results in a disappearing ground peak return when slope or off-nadir pointing angle is larger than the “critical slope angle”, which is closely related to various vegetation structures and footprint size. Model simulation indicates that waveform shapes are affected by surface topography, off-nadir pointing angle and vegetation structure and it is difficult to remove topography effects from waveform extent based only on the shapes of waveform without knowing any surface topography information.Height error without correction of surface topography and off-nadir pointing angle is the smallest when the laser beams at the toward-slope direction and the largest from the opposite direction. Further simulation reveals within 20° of slope and off-nadir pointing angle, given the canopy height as roughly 25 m and the footprint size as 25 m, the error for vegetation height (RH100) ranges from − 2 m to greater than 12 m, and the error for the height at the medium energy return (RH50) from − 1 m to 4 m. The RH100 error caused by unknown surface topography and without correction of off-nadir pointing effect can be explained by an analytical formula as a function of vegetation height, surface topography, off-nadir pointing angle and footprint size as a first order approximation. RH50 is not much affected by topography, off-nadir pointing and footprint size. This forward model simulation can provide scientific guidance on prioritizing future lidar mission measurement requirements and accuracies. 相似文献
9.
Assessing vegetation response to drought in the northern Great Plains using vegetation and drought indices 总被引:17,自引:0,他引:17
The Normalized Difference Vegetation Index (NDVI) derived from the Advanced Very High Resolution Radiometer (AVHRR) has been widely used to monitor moisture-related vegetation condition. The relationship between vegetation vigor and moisture availability, however, is complex and has not been adequately studied with satellite sensor data. To better understand this relationship, an analysis was conducted on time series of monthly NDVI (1989-2000) during the growing season in the north and central U.S. Great Plains. The NDVI was correlated to the Standardized Precipitation Index (SPI), a multiple-time scale meteorological-drought index based on precipitation. The 3-month SPI was found to have the best correlation with the NDVI, indicating lag and cumulative effects of precipitation on vegetation, but the correlation between NDVI and SPI varies significantly between months. The highest correlations occurred during the middle of the growing season, and lower correlations were noted at the beginning and end of the growing season in most of the area. A regression model with seasonal dummy variables reveals that the relationship between the NDVI and SPI is significant in both grasslands and croplands, if this seasonal effect is taken into account. Spatially, the best NDVI-SPI relationship occurred in areas with low soil water-holding capacity. Our most important finding is that NDVI is an effective indicator of vegetation-moisture condition, but seasonal timing should be taken into consideration when monitoring drought with the NDVI. 相似文献
10.
NDVI (Normalized Difference Vegetation Index) has been widely used to monitor vegetation changes since the early eighties. On the other hand, little use has been made of land surface temperatures (LST), due to their sensitivity to the orbital drift which affects the NOAA (National Oceanic and Atmospheric Administration) platforms flying AVHRR sensor. This study presents a new method for monitoring vegetation by using NDVI and LST data, based on an orbital drift corrected dataset derived from data provided by the GIMMS (Global Inventory Modeling and Mapping Studies) group. This method, named Yearly Land Cover Dynamics (YLCD), characterizes NDVI and LST behavior on a yearly basis, through the retrieval of 3 parameters obtained by linear regression between NDVI and normalized LST data. These 3 parameters are the angle between regression line and abscissa axis, the extent of the data projected on the regression line, and the regression coefficient. Such parameters characterize respectively the vegetation type, the annual vegetation cycle length and the difference between real vegetation and ideal cases. Worldwide repartition of these three parameters is shown, and a map integrating these 3 parameters is presented. This map differentiates vegetation in function of climatic constraints, and shows that the presented method has good potential for vegetation monitoring, under the condition of a good filtering of the outliers in the data. 相似文献
11.
Rapid changes of land use and land cover (LULC) in urban areas have become a major environmental concern due to environmental impacts, such as the reduction of green spaces and development of urban heat islands (UHI). Monitoring and management plans are required to solve this problem effectively. The Tabriz metropolitan area in Iran, selected as a case study for this research, is an example of a fast growing city. Multi-temporal images acquired by Landsat 4, 5 TM and Landsat 7 ETM+ sensors on 30 June 1989, 18 August 1998, and 2 August 2001 respectively, were corrected for radiometric and geometric errors, and processed to extract LULC classes and land surface temperature (LST). The relationship between temporal dynamics of LST and LULC was then examined. The temperature vegetation index (TVX) space was constructed in order to study the temporal variability of thermal data and vegetation cover. Temporal trajectory of pixels in the TVX space showed that most changes due to urbanization were observable as the pixels migrated from the low temperature-dense vegetation condition to the high temperature-sparse vegetation condition in the TVX space. The uncertainty analysis revealed that the trajectory analysis in the TVX space involved a class-dependant noise component. This emphasized the need for multiple LULC control points in the TVX space. In addition, this research suggests that the use of multi-temporal satellite data together with the examination of changes in the TVX space is effective and useful in urban LULC change monitoring and analysis of urban surface temperature conditions as long as the uncertainty is addressed. 相似文献
12.
Combining vegetation index and model inversion methods for the extraction of key vegetation biophysical parameters using Terra and Aqua MODIS reflectance data 总被引:11,自引:0,他引:11
Accurate estimates of vegetation biophysical variables are valuable as input to models describing the exchange of carbon dioxide and energy between the land surface and the atmosphere and important for a wide range of applications related to vegetation monitoring, weather prediction, and climate change. The present study explores the benefits of combining vegetation index and physically based approaches for the spatial and temporal mapping of green leaf area index (LAI), total chlorophyll content (TCab), and total vegetation water content (VWC). A numerical optimization method was employed for the inversion of a canopy reflectance model using Terra and Aqua MODIS multi-spectral, multi-temporal, and multi-angle reflectance observations to aid the determination of vegetation-specific physiological and structural canopy parameters. Land cover and site-specific inversion modeling was applied to a restricted number of pixels to build multiple species- and environmentally dependent formulations relating the three biophysical properties of interest to a number of selected simpler spectral vegetation indices (VI). While inversions generally are computationally slow, the coupling with the simple and computationally efficient VI approach makes the combined retrieval scheme for LAI, TCab, and VWC suitable for large-scale mapping operations. In order to facilitate application of the canopy reflectance model to heterogeneous forested areas, a simple correction scheme was elaborated, which was found to improve forest LAI predictions significantly and also provided more realistic values of leaf chlorophyll contents.The inversion scheme was designed to enable biophysical parameter retrievals for land cover classes characterized by contrasting canopy architectures, leaf inclination angles, and leaf biochemical constituents without utilizing calibration measurements. Preliminary LAI validation results for the Island of Zealand, Denmark (57°N, 12°E) provided confidence in the approach with root mean square (RMS) deviations between estimates and in-situ measurements of 0.62, 0.46, and 0.63 for barley, wheat, and deciduous forest sites, respectively. Despite the independence on site-specific in-situ measurements, the RMS deviations of the automated approach are in the same range as those established in other studies employing field-based empirical calibration.Being completely automated and image-based and independent on extensive and impractical surface measurements, the retrieval scheme has potential for operational use and can quite easily be implemented for other regions. More validation studies are needed to evaluate the usefulness and limitations of the approach for other environments and species compositions. 相似文献
13.
Effects of different physical workload parameters on mental workload and performance 总被引:1,自引:0,他引:1
Angela DiDomenico Maury A. Nussbaum 《International Journal of Industrial Ergonomics》2011,41(3):255-260
The design and evaluation of an occupational task should include an assessment of mental workload, since excessive levels of mental workload can cause errors or delayed information processing. Physically demanding work that is performed concurrently with a cognitive task may impact mental workload by impairing mental processing or decreasing performance. The primary objective of this study was to determine whether there is a differential effect of various types of physical activity on both mental workload and cognitive performance. Objective and subjective assessment tools (heart rate variability and visual analog scale) were used as indicators of mental workload, while correct responses during an arithmetic task reflected levels of performance. Thirty participants (ages 18-24 years) performed a combination of tasks inducing both physical and mental workload. Type of physical effort, frequency of movement, and force exertion level were manipulated to alter the workload associated with the physical activity. Changes in subjective ratings generally corresponded to changes in both performance on the arithmetic task and objective mental workload assessment. Some discrepancies occurred at the highest physical force exertion level as participants perceived an increase in effort to maintain the same level of performance. Further research is needed to determine the force exertion threshold, beyond which the physical effort required interferes with mental workload and/or cognitive performance.
Relevance to industry
Technological advancements have increased the requirement for many workers to execute cognitive tasks concurrently with physical activity. When designing and evaluating such situations it is important to determine the interactive effects of these activities. A simple, uni-dimensional tool is suggested as a screening tool to identify situations requiring excessive or increased mental workload that many degrade performance or place additional stress on the individual. 相似文献14.
Annual and interannual (ENSO) variability of spatial scaling properties of a vegetation index (NDVI) in Amazonia 总被引:1,自引:0,他引:1
The space-time variability of the Normalized Difference Vegetation Index (NDVI) over the Amazon River basin is quantified through the bi-dimensional Fourier spectrum, and moment-scaling analysis of monthly imagery at 8 km resolution, for the period July 1981-November 2002. Monthly NDVI fields exhibit power law Fourier spectra, E(k)=ck−β, with k denoting the wavenumber, c the prefactor, and β the scaling exponent. Fourier spectra exhibit two scaling regimes separated at approximately 29 km, above which NDVI exhibit long-range spatial correlations (0<β<2), and below which NDVI behaves like white noise in space (β?0). Series of monthly values of c(t) and β(t) exhibit high negative correlation (−0.88, P>0.99), which suggest their linkages in power laws, but also that Et(k)=c(t)k−β(t), with t the time index. Results show a significant negative simultaneous correlation (−0.82, P>0.95) between monthly series of average precipitation over the Amazon, 〈P(t)〉, and scaling exponents, β(t); and high positive lagged correlation (0.63, P>0.95), between 〈P(t)〉 and 〈NDVI(t+3)〉. Parameters also reflect the hydrological seasonal cycle over Amazonia: during the wet season (November-March), β(t) ranges between 0.9 and 1.15, while during the dry season (May-September), β(t)?1.30. These results reflect the more (less) coherent spatial effect of the dry (wet) season over Amazonia, which translates into longer (shorter)-range spatial correlations of the NDVI field, as witnessed by higher (lower) values of β(t). At interannual timescales, both phases of ENSO reflect on both parameters, as β(t) is higher during El Niño than during La Niña, due to the more coherent effects of El Niño-related dryness, whereas NDVI spatial variability is enhanced during La Niña, due to positive rainfall anomalies. Results from the moment-scale analysis indicate the existence of multi-scaling in the spatial variability of NDVI fields. Departures from single scaling exhibit also annual and interannual variability, which consistently reflect the effects from both phases of ENSO. Furthermore, departures from single scaling are independent of the order moment, q, as the PDF of departures scaled by the mean collapse to a unique distribution. These results point out that ideas of spatial scaling constitute a promising framework to synthesize important hydro-ecological processes of Amazonia. 相似文献
15.
N. GAUTAM SUJIT BASU C. M. KISHTAWAL R. M. GAIROLA P. C. PANDEY 《International journal of remote sensing》2013,34(18):3649-3664
Ten years (1982–1991) of upper air and ocean surface observations over the Indian Ocean from Sagar Kanya and Monex–79 data have been used to examine the relationship between the precipitable water (W) and surface level humidity (Q) on an instantaneous scale. Our analysis of Q and W over the Indian Ocean shows that Q is correlated with Won an instantaneous scale with r=O·44, which is a statistically significant correlation. A regression between Q and W has been fitted and it has been found that a fifth order polynomial yields a lowest root-mean-square (rms) error of I-4gkg- 1 when compared with observed Q using an independent observation. The validity of earlier derived global relation between Q and W has been examined over the Indian Ocean. It has been found that Liu's global Q-W relation gave a large rms error of 4·1 gkg -1 when compared with the observed instantaneous Q values over the Indian Ocean. The usefulness of the above derived Q-W relation and an earlier derived relation between the monthly mean Q and W has been examined for the estimation of latent heat fluxes (LHF) over the Indian Ocean using an independent observation. The LHF estimated from the bulk aerodynamic method using all quantities available from ship observations, called the direct method (M1), has been compared with the LHF computed by using a derived Q-W relation (M2). The rms error between MI versus M2 is found to be 56 Wm-2. The LHF estimated by Liu's Q -W relation, when compared with MI gave an rms error of 155Wm -2, which is suggestive of its unsuitability for the estimation of LHF over the Indian Ocean on an instantaneous basis. The difference between the sea surface humidity (Q,) and surface level humidity (Q) has been parametrized in terms of sea surface temperature (SST) and W, both obtainable from satellite sensors. This relation has then been used to compute LHF (M3) and was compared with MI, where it was found that M1 versus M3 gave an rms error of 58Wm-2. Thus, this study indicates that methods M2 and M3 are found to be more consistent and accurate in nature, and also suggests that it can be further applied to the LHF estimation using satellite based microwave/IR measurements for Wand SST on an instantaneous basis. 相似文献
16.
Objective
This study tested the relationship between individual differences and Situation Awareness (SA) during training in a navigation simulator.Background
Simulators have become an important tool in the training and education of maritime personnel with a view to improving decisions and performance. There are a few, inconclusive empirical studies that have focused on mapping personality characteristics and their link to the ability to generate and maintain SA during simulator training.Method
Thirty-six first-year students from the Royal Norwegian Navy Officer Candidate School participated in the study. SA was measured using both subjective and observer ratings.Results
The results indicated that low scores on Neuroticism and high scores on Extraversion and Conscientiousness (resilient personality type) predicted both subjective and observer-rated SA. Furthermore, participants with high SA were able to modulate their Heart Rate Variability (HRV), with suppression of HRV during navigation training as well showing recovery of HRV.Application
The potential applications of this research include the assessment of personality differences as a tool in selecting navigators, and the use of HRV as an objective index of adaptability to environmental demands. 相似文献17.
Differences in Landsat-based trend analyses in drylands due to the choice of vegetation estimate 总被引:3,自引:0,他引:3
Ruth Sonnenschein Tobias Kuemmerle Thomas Udelhoven Patrick Hostert 《Remote sensing of environment》2011,115(6):1408-1420
Drylands cover about 41% of the globe's surface and provide important ecosystem services, but land use and climate change exert considerable pressure on these ecosystems. Both of these drivers frequently result in gradual vegetation change and landscape-scale trend analysis based on yearly vegetation estimates can capture such changes. Such trend analyses based on high-resolution time series of satellite imagery have so far not widely been used and existing studies in drylands relied on different vegetation measures. Spectral mixture analysis (SMA) has been chosen due to its superiority to simpler vegetation estimates in quantifying vegetation cover in single-date studies, however SMA can be challenging to implement for large areas. Here, we quantify the trade-off involved when using simple vegetation estimates instead of SMA fractions for subsequent trend analyses. We calculated NDVI, SAVI and Tasseled Cap Greenness, as well as SMA green vegetation fractions for a time series of Landsat images from 1984-2005 for a study region in Crete. Linear trend analysis showed that trend coefficients and the spatial patterns of trends were similar across all vegetation estimates and the entire study region, especially for areas where vegetation changed gradually. On average, trends based on simple measures differed less than 5% from SMA-based trends with decreasing similarity in trend results from Tasseled Cap Greenness to SAVI and NDVI. Vegetation estimates differed markedly in their response to disturbance events such as fires. Trend analyses based on qualitative measures can easily be applied across very large areas and using multi-sensor time series based on high-resolution data. While the subtle differences between vegetation estimates may still be important for some applications, the robustness of trend analyses regarding the choice of vegetation estimate bears considerable promise to reconstruct fine-scale vegetation dynamics and land use histories and to assess climate change impacts on the world's drylands. 相似文献
18.
Variation of lightning flash distribution, rain, and normalized difference vegetation index (NDVI) and their dependency on meteorological parameters were studied along the Indo-Gangetic Plain from 21°N to 35°N, dividing it into four regions of equal area (R1, R2, R3 and R4). Time series and correlation analysis were used to study the variations and relations among the parameters. Maximum lightning flashes were observed in region R1 where rain, surface temperature, convective available potential energy (CAPE), cloud cover, water vapour, and NDVI were found to be minimum. Lightning flashes and rain are positively correlated with surface temperature, CAPE, and aerosol optical depth (AOD), on both an annual and seasonal basis, over the whole region. Seasonal variation in NDVI showed a double-peak distribution, which can be understood by considering its dependence on water vapour, soil moisture, accumulated rain and surface temperature, and their variation from season to season and region to region. The effects of the atmospheric pollutants (AOD, carbon dioxide (CO2), nitrogen dioxide (NO2), and ozone (O3)) on NDVI showed a mixed response. A small variation in atmospheric CO2 showed no statistically significant impact in any season or region. AOD had a negative effect in general except that a small positive tendency was observed in R3 and R4 during summer months. NO2 showed a negative impact in all regions and seasons. O3 had a negative effect in R3 and R4 during the monsoon months and in R2–R4 during the summer months, and a positive effect in the other regions and seasons. When all four regions are merged, correlation analysis on an annual basis showed that the effects in localized regions are lost and the impact from the larger area predominates. To understand the regional localized effect, analysis of the smaller regions seems to be important. 相似文献
19.
Most ocean general circulation models (OGCMs) do not take into account the effect of space- and time-varying phytoplankton on solar radiation penetration, or do it in a simplistic way using a constant attenuation depth, even though one-dimensional experiments have shown potential significant effect of phytoplankton on mixed-layer dynamics. Since some ocean basins are biologically active, it is necessary for an OGCM to take water turbidity into account, even if it is not coupled with a biological model. Sensitivity experiments carried out with the Massachusetts Institute of Technology (MIT) OGCM with spatially and temporally-varying pigment concentration from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data during 1998-2003 reveal the effect of ocean turbidity on tropical Indian Ocean circulation. Variations of light-absorbing phytoplankton pigments change the vertical distribution of solar heating in the mixed layer, thereby affecting upper-ocean circulation. A simulation was performed from 1948 to 2003 with a constant minimum pigment concentration of 0.02 mg m− 3 while another simulation was performed from September 1997 to December 2003 with variable pigment concentration, and the differences between these two simulations allow us to quantify the effects of phytoplankton on solar radiation penetration in the ocean model. Model results from a period of 6 years (1998-2003) show large seasonal variability in the strength of the meridional overturning circulation (MOC), meridional heat transports (MHT), and equatorial under current (EUC). The MOC mass transport changes by 2 to 5 Sv (1 Sv = 106 m3 s− 1) between boreal winter (January) and boreal summer (July), with a corresponding change in the MHT of ∼ 0.05 PW (1 PW = 1015 W) in boreal winter, which is close to the expected change associated with a significant climate change [Shell, K., Frouin, R., Nakamoto, S., & Somerville, R.C.J. (2003): Atmospheric response to solar radiation absorbed by phytoplankton. Journal of Geophysical Research, 108(D15), 4445. doi:10.1029/2003JD003440.]. In addition, changes in phytoplankton pigments concentration are associated with a reduction in the EUC by ∼ 3 cm s− 1. We discuss the possible physical mechanisms behind this variability, and the necessity of including phytoplankton forcing in the OGCM. 相似文献
20.
Aerosol and cloud data from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the Earth Observing System (EOS) Aqua are used to investigate interannual variability of smoke and warm cloud relationships during the dry-to-wet transition season (August-October) over the Amazon for two years and its association with meteorological conditions. In one year (2003), smoke aerosols are associated with an increase of cloud fraction and a decrease of cloud effective radius. These effects amplify the cooling at the surface and at the top of the atmosphere (TOA) caused by the aerosol extinction. However, in another year (2002) the cloud fraction decreases with increasing aerosol optical depth. Such a decrease of cloud fraction could offset the effect of increased reflection of solar radiation by the aerosols both at the surface and at TOA. The changes in radiative fluxes between these years would contribute to interannual changes of surface energy fluxes and radiative balance at the top of the atmosphere and influence variability of the wet season onset in the basin. In 2003, the atmosphere was more humid and less stable. These conditions may be relatively favorable for the activation of aerosol particles into cloud condensation nuclei and hence cloud droplets. In 2002, the clouds were less extensive and thinner in a relatively dry atmosphere and presumably dissipated more easily. This study suggests that the aerosol-cloud relation can be influenced by atmospheric structure and convective motions, in addition to changes in aerosols properties. An adequate characterization of aerosol-cloud relationship would require a longer time series of data that includes a variety of climate conditions. The caveat of this analysis is that differences in aerosol absorption and its vertical distribution may have contributed to the observed interannual change of smoke-cloud relationship but could not be determined due to lack of adequate measurements. 相似文献