排序方式： 共有116条查询结果，搜索用时 15 毫秒
91.
92.
Boniface O Oindo Rolf A de By Andrew K Skidmore 《International Journal of Applied Earth Observation and Geoinformation》2000,2(3-4)
Species richness, or simply the number of species in a given area, is commonly used as an important indicator of biological diversity. Spatial variability in species richness has been postulated to depend upon environmental factors such as climate and climatic variability, which in turn may affect net primary productivity. The Advanced Very High Resolution Radiometer (AVHRR) derived Normalized Difference Vegetation Index (NDVI) has been shown to be correlated with climatic variables including rainfall, actual evapotranspiration and net primary productivity. To determine factors favoring high species richness, we examined the relationship between interannual NDVI variables and species richness of birds at a quarter degree scale (55 × 55 km). Results revealed a strong positive correlation between species richness and maximum average NDVI. Conversely, species richness showed negative correlation with standard deviation of maximum NDVI and the coefficient of variation. Though these relationships are indirect, they apparently operate through the green vegetation cover. Understanding such relationships can help in mapping and monitoring biological diversity, as well as in estimating changes in species richness in response to global climatic change. 相似文献
93.
Assimilation of image sequences in numerical models 总被引：1，自引：0，他引：1
By OLIVIER TITAUD ARTHUR VIDARD INNOCENT SOUOPGUI FRANÇOIS-XAVIER LE DIMET 《地球，A辑:动力气象学与海洋学》2010,62(1):30-47
Understanding and forecasting the evolution of geophysical fluids is a major scientific and societal challenge. Forecasting algorithms should take into account all the available information on the considered dynamic system. The variational data assimilation (VDA) technique combines all these informations in an optimality system (O.S.) in a consistent way to reconstruct the model inputs. VDA is currently used by the major meteorological centres. During the last two decades about 30 satellites were launched to improve the knowledge of the atmosphere and of the oceans. They continuously provide a huge amount of data that are still underused by numerical forecast systems. In particular, the dynamic evolution of certain meteorological or oceanic features (such as eddies, fronts, etc.) that the human vision may easily detect is not optimally taken into account in realistic applications of VDA. Image Assimilation in VDA framework can be performed using 'pseudo-observation' techniques: they provide apparent velocity fields, which are assimilated as classical observations. These measurements are obtained by certain external procedures, which are decoupled with the considered dynamic system. In this paper, we suggest a more consistent approach, which directly incorporates image sequences into the O.S. 相似文献
94.
This study simulates optical depth of marine warm clouds for year 2001 based on interactively predicted aerosol concentrations with a global chemical transport model (CTM) driven by the ERA-40 re-analysis meteorological data. The simulated aerosol and cloud droplet number concentrations (CDNC) largely reproduce the variations between polluted and pristine marine environment as revealed by surface and aircraft measurements. By constraining cloud liquid water path (CLWP) with satellite microwave measurements, the simulated global and southern hemispheric aerosol optical depth (AOD) and cloud optical depth (COD) are well within 10% of the observed values. As a result of larger anthropogenic aerosol loadings over the northern oceans, the simulated CDNC and COD are, respectively, by 51 and 18% higher than those over the southern oceans, while the column-averaged droplet effective radius is 13% smaller. These simulated interhemispheric differences, while qualitatively consistent with satellite observations, are larger than the observations. Inclusion of drizzle effect improved the disparities but not entirely. The constrained CTM generally captures the seasonality in AOD and CLWP observations, and demonstrates that annual cycle of COD is dominated by CLWP. During winter monsoon the simulated and observed COD correlate more strongly with changes in AOD over the N. Indian Ocean. 相似文献
95.
Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006 总被引：1，自引：0，他引：1
By ANDREAS PETZOLD KATHARINA RASP BERNADETT WEINZIERL MICHAEL ESSELBORN THOMAS HAMBURGER REAS DÖRNBRACK KONRAD KANDLER LOTHAR SCHÜTZ PETER KNIPPERTZ MARKUS FIEBIG AKI VIRKKULA 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):118-130
During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10−3 to 5.2 × 10−3 at 450 nm and from 0.3 × 10−3 to 2.5 × 10−3 at 700 nm. Groupings of k values could be attributed to different source regions. 相似文献
96.
By MATTHIAS TESCHE ALBERT ANSMANN DETLEF MÜLLER DIETRICH ALTHAUSEN INA MATTIS BIRGIT HEESE VOLKER FREUDENTHALER MATTHIAS WIEGNER MICHAEL ESSELBORN GIANLUCA PISANI PETER KNIPPERTZ 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):144-164
Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated during SAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Ångström exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300 Mm−1 with a mean value of 100 Mm−1 in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm. 相似文献
97.
By DANIEL ROSENFELD JIM PETERSON ARON GINGIS 《Tellus. Series B, Chemical and physical meteorology》2009,61(4):694-700
Rosenfeld (2000, hereafter R00), in applying new satellite methodology to analyse case studies in Southeastern Australia and elsewhere, provided evidence that urban and industrial air pollution can suppress precipitation from shallow clouds. He concluded that 'Air pollution must be an important factor in determining the precipitation amounts in the Snowy Mountains'. These satellite observations were the impetus for our proposed detailed follow-on research program to further validate and quantify these inferences, publicly offered in Rosenfeld et al. (2006, hereafter R06) and repeated here, thereby recognizing the remaining large uncertainties. In response, Ayers (2009, hereafter A09) attempts to deny the significance and validity of the observations of R00. His scientific arguments are refuted here. Furthermore, A09 wrote erroneously that 'a hypothesis that air pollution in the form of small particles has caused a secular decrease in precipitation over SE Australia was advanced by Rosenfeld (2000) , who concluded that the hypothesis was proven.' But R00 did not make such a claim, although this is a viable hypothesis that warrants testing (R06). In fact, R00 wrote: 'trend analyses of snow, winter temperature, and total winter rainfall for the period 1910–1991 showed statistically insignificant decreases… ( Duus, 1992 )'. 相似文献
98.
The El Niño/Southern Oscillation (ENSO) constitutes a major source of potential predictability in the tropics. The majority of past seasonal prediction studies have concentrated on precipitation anomalies at the seasonal mean timescale. However, fields such as agriculture and water resource management require higher time frequency forecasts of precipitation variability. Regional climate models (RCMs), with their increased resolution, may offer one means of improving general circulation model forecasts of higher time frequency precipitation variability.
Part I of this study evaluated the ability of the Rossby Centre regional atmospheric model (RCA), forced by analysed boundary conditions, to simulate seasonal mean precipitation anomalies over the tropical Americas associated with ENSO variability. In this paper the same integrations are analysed, with the focus now on precipitation anomalies at subseasonal (pentad) timescales.
RCA simulates the climatological annual cycle of pentad-mean precipitation intensity quite accurately. The timing of the rainy season (onset, demise and length) is well simulated, with biases generally of less than 2 weeks. Changes in the timing and duration of the rainy season, associated with ENSO forcing, are also well captured. Finally, pentad-mean rainfall intensity distributions are simulated quite accurately, as are shifts in these distributions associated with ENSO forcing. 相似文献
Part I of this study evaluated the ability of the Rossby Centre regional atmospheric model (RCA), forced by analysed boundary conditions, to simulate seasonal mean precipitation anomalies over the tropical Americas associated with ENSO variability. In this paper the same integrations are analysed, with the focus now on precipitation anomalies at subseasonal (pentad) timescales.
RCA simulates the climatological annual cycle of pentad-mean precipitation intensity quite accurately. The timing of the rainy season (onset, demise and length) is well simulated, with biases generally of less than 2 weeks. Changes in the timing and duration of the rainy season, associated with ENSO forcing, are also well captured. Finally, pentad-mean rainfall intensity distributions are simulated quite accurately, as are shifts in these distributions associated with ENSO forcing. 相似文献
99.
An aliasing operator is introduced to mimic the effect of aliasing that causes discontinuities in radial-velocity observations, and to modify the observation term in the costfunction for direct assimilations of aliased radar radial-velocity observations into numerical models. It is found that if the aliasing operator is treated as a part of the observation operator and applied to the analysed radial velocity in a conventional way, then the analysis is not ensured to be aliased (or not aliased) in consistency with the aliased (or not aliased) observation at every observation point. Thus, the analysis-minus-observation term contains a large alias error whenever an inconsistency occurs at an observation point. This causes fine-structure discontinuities in the costfunction. An unconventional approach is thus introduced to apply the aliasing operator to the entire analysis-minus-observation term at each observation point in the observation term of the costfunction. With this approach, the costfunction becomes smooth and concave upwards in the vicinity of the global minimum. The usefulness of this approach for directly assimilating aliased radar radial-velocity observations under certain conditions is demonstrated by illustrative examples. 相似文献
100.
By BARBARA MAY DIETMAR WAGENBACH SAMUEL HAMMER PETER STEIER HANS PUXBAUM CASIMIRO PIO 《Tellus. Series B, Chemical and physical meteorology》2009,61(2):464-472
To constrain the relatively uncertain anthropogenic impact on the organic aerosol load, radiocarbon analyses were performed on aerosol samples, collected year-round, at six non-urban sites including a maritime background and three remote mountain stations, lying on a west-east transect over Western Europe. From a crude three component model supported by TOC and levoglucosan filter data, the fossil fuel, biomass burning and biogenic TOC fraction are estimated, showing at all stations year-round, a relatively constant fossil fuel fraction of around (26 ± 6)% , a dominant biogenic contribution of on average (73 ± 7)% in summer and the continental as well as the maritime background TOC to be only about 50% biogenic. Assuming biomass burning as completely anthropogenic, the carbonaceous aerosol concentration at the mountain sites was found to have increased by a factor of up to (1.4 ± 0.2) in summer and up to (2.5 ± 1.0) in winter. This figure is significantly lower, however, than the respective TOC change since pre-industrial times seen in an Alpine ice core. Reconciling both observations would require an increase, since pre-industrial times, of the background biogenic aerosol load, which is estimated at a factor of 1.3–1.7. 相似文献