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11.
Summary In addition to global solar radiationE
g
, the hourly diffuse componentE
d
incident on a horizontal surface has been measured from February 1993 to January 1995 at a meteorological station in tropical West Africa. The measured diffuse solar irradiance data was corrected for shadow band effects. The monthly mean diurnal variations of diffuse solar irradiance obtained for identical months in the two years have been compared and found to be generally consistent. The corresponding monthly mean hourly values ofE
d
for identical months in 1993 and 1994 agreed to within 9% while yielding correlation coefficients greater than 0.960. In addition, the monthly mean daily totals ofE
d
for identical months were found to agree mostly to within 6% and showed virtually the same annual variations in both years. The monthly mean daily total values of diffuse solar radiation for most months in the two years ranged between 7.94 MJm–2d–1 and 10.50 MJm–2d–1. The monthly mean of daily hourly maximum values ofE
d
obtained for identical months in the two years have been discussed in relation to the dominant atmospheric conditions during these months. The results been presented here have been compared with those of some investigators within and outside the Africa region.With 8 Figures 相似文献
12.
Changes in the duration of the photosynthetically active period strongly influence the changes in the carbon sequestration potential of boreal forests under climatic warming. In this paper, current theories on the effects of environmental variables such as spring air and soil temperature, photoperiod and chilling temperatures on the timing and initiation of photosynthesis in boreal deciduous and coniferous trees are discussed. Different dynamic phenological modeling approaches are reviewed, and model simulations are utilized to demonstrate model predictions under changing climatic conditions. A process-based forest ecosystem model is applied to estimate the relative importance of the duration of the photosynthetically active period on the amount of annual gross primary production and net primary production of boreal coniferous forests. All applied modeling approaches predict an increasing duration of the photosynthetically active period as a result of climatic warming. However, the magnitude of the response to increasing temperature varies between models and therefore affects the predictions of the changes in production. 相似文献
13.
14.
Raimo Nikkilä Edward Nash Jens Wiebensohn Ilkka Seilonen Kari Koskinen 《International journal of geographical information science》2013,27(6):1210-1226
Rule interchange between information systems is expanding as new interoperable rule formats are emerging from research. However, existing spatial inference systems generally operate on locally stored data with an internal rule format. Consequently, their design offers little support or facilities for rule interchange. This article presents the requirements, components and design for a spatial inference system with rule interchange. Computational efficiency and overall functionality of the design are considered separately, with the latter demonstrated using encoded agricultural legislation and data. A spatial inference system with rule interchange is based on three primary components: rule representation, spatial functionality and data integration. Of these, the interoperable rule representation and data integration distinctly differ from existing spatial inference systems. The presented inference system combines a spatial superset of the W3C Rule Interchange Format (RIF) with full Open Geospatial Consortium simple feature access (OGC SFA) functionality and on-demand data integration utilising Resource Deception Framework (RDF). The design was found to be effective with a computational efficiency depending predominantly on the spatial operations. This design could be further adapted to implement spatial extensions for existing inference systems. Considerable benefits were also discovered when RIF was used as the native language for the inference engine, thereby removing the need for rule transformations and facilitating on-demand data integration with the GML. 相似文献
15.
Abstract We present trial calculations for a simple nonlinear ‘‘thick disk'’ galaxy dynamo. The nonlinearity is a simple α-quenching. Our strictly axisymmetric solution demonstrates the possibility of a nonlinear interaction between modes of opposite parity. We suggest that a three dimensional model might exhibit a similar persistent interaction between axisymmetric and non-axisymmetric modes. 相似文献
16.
Hydrochemical monitoring and heavy metal speciation by sequential extraction techniques indicate direct relationships among enrichment of the heavy metals (copper, lead, zinc, iron, manganese, and cadmium), soil acidification, and salinization in Kedron Brook floodplain area of Brisbane, Australia. Assessment of modes of occurrence and distribution pattern of the heavy metals in soil, sediment, and water environments of this coastal plain indicates that the total concentrations and reactive fractions of these metals are elevated in soil and channel bed sediments. Such geochemical signatures reflect the complex influence of sources and a combination of natural and anthropogenic processes on concentration and dispersion within the coastal zone. According to a working model presented, the enrichment of the heavy metals, like cadmium, in the soil and sediment profiles is triggered by capillary pumping during low groundwater standing levels, when the metals are in a stable form associated with dry gels. During higher groundwater levels and occasional flood events, these metals become mobilized when the gel material is transformed into soluble colloidal phase.This study indicates that the potential impacts of heavy metal pollution on the coastal ecosystems can not be assessed and managed in isolation solely by considering the natural cause-effect relationships. The complex nature of sediment-soil-water interactions in the coastal hydrodynamic zones can produce a manifold of effects, including mobilization, concentration, and/or dispersion of heavy metals at both short and longer time scales. In the case of Kedron Brook, chemical erosion due to variation in natural climatic and hydrodynamic conditions contributes significantly to concentration of heavy metals in the coastal environments. Therefore, a sound understanding of the prevailing hydrogeochemical processes is essential for prediction of the fate of heavy metals and establishment of meaningful coastal zone management strategies. 相似文献
17.
If the assumption of a catastrophic explosion during the formation of a neutron star is correct, the parent systems for klovsky's model of SCO XR-1 seem to have been very short-period white-dwarf binaries. A white dwarf originally in contact with its Roche limit is forced to lose mass. During the ejection of matter the primary may pass the white-dwarf mass limit and become a neutron star. The mass transfer time-scale can change from pulsational to thermal, and a mass flow of 10–9
M
per year needed for SCO XR-1 can be understood, while at the same time the orbital period will increase. 相似文献
18.
Dinosaur eggshells from the Cretaceous red beds of southern China were selected for element geochemical study.The concentrations of the major elements indicate that the eggshells did not change much in their chemical composition after they were formed.Variations in the contents of trace elements and REEs imply that a palaeoclimatic anomaly might occur during Late Cretaceous. 相似文献
19.
Summary The annual and monthly mean diurnal variations of the diffuse fraction of global solar irradiance arriving on the ground
at a tropical station in Sub-Sahel Africa is here been reported. The monthly mean hourly values of the diffuse fraction (K
d
) for such clear-sky months as February, March and November at this location, which approach a minimum at about local noon,
are observed to lie generally below 0.50 during the period from 11:00 to 15:00 hrs (LST). Consequently, solar concentrators
utilising parabolic mirrors are expected to have high performance during these months in this region. Like the mainly-cloudy
and wet months (June to August) in which monthly mean hourly values of K
d
higher than 0.62 have been recorded, the corresponding diffuse fraction for dust-haze months (mostly December and January)
with high turbidity coefficients were generally above 0.50. Monthly mean hourly values of K
d
for less cloudy months (April, May, September and October) ranged between 0.48 and 0.77 during the period from 11:00 to 15:00 hrs
(LST). The effects of atmospheric dust-haze, clouds and albedo on the monthly mean diurnal variation of the diffuse fraction
has been discussed. Also reported are the characteristic values of K
d
for sets of months with relatively similar atmospheric and sky conditions at this location. The annual variations of the
monthly mean daily values of K
d
which exhibit strong seasonal dependence showed a peak in August for both years. Except for the months of February and March,
the monthly mean daily totals of K
d
exhibited similar annual marches during both years. The major discrepancy in the values of the monthly mean daily totals
of K
d
in both years were recorded in the months of February, November and December, with the corresponding K
d
values for these months in both years agreeing only to within 32.9% in February, 15.4% in November and 16.2% in December.
Apart from the aforementioned months, the corresponding monthly mean daily totals of K
d
for the remaining nine months in both years agreed mostly to within less than 8.4%. The least monthly mean daily ratios of
K
d
were obtained in the relatively clear month of November for both years being 0.43 in 1993 and 0.49 in 1994. On an annual
average, the diffuse component was found to constitute 59.6% of the global solar irradiance arriving on the ground at this
region in 1993 and 60.9% in 1994. The results been reported here have been compared with a few others emanating from other
tropical stations.
Received February 5, 1998 Revised May 12, 1998 相似文献