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1.
The accumulation of MF radar wind and hydroxyl temperature measurements at Davis from 1997 to 2005 has enabled the compilation of a climatology of long-period (period >1 day) wave activity. A time domain filtering technique that makes allowance for the differing sampling characteristics of the measurements is described and wave amplitudes are presented for 1.7–4, 4–8 and 8–16 day period bands. Product averages of the time series yield horizontal heat and momentum fluxes for the height of the hydroxyl layer (approximately 86 km). The climatology is then discussed in terms of current knowledge of planetary wave characteristics and forcing. Heat and momentum fluxes during the year of the southern hemisphere stratospheric warming (2002) are also presented.  相似文献   

2.
Pulse trains of irregular pulse characteristics and erratic amplitudes are observed in lightning flashes. These are termed “chaotic pulse trains” in this study. The average width of the individual pulses of these pulse trains is in the range of a few microseconds, with the lower limit of individual measurements extending into the sub-microsecond region. The typical pulse separation lies in the range of 2–. The pulse duration shows a consistent distribution with the most probable value at 400–. Chaotic pulse trains occur most often immediately prior to subsequent strokes. However, they occur also a few milliseconds to several tens of milliseconds prior to return strokes and sometimes do not precede return strokes at all. Twenty-six percent of the total subsequent strokes recorded in this study were preceded by chaotic pulse trains. A few of these pulse trains have occurred in a sequence of several tens of millisecond intervals in between return strokes. We have also found them to occur within very intense cloud flashes. These chaotic pulse trains have features that are different from those of other known pulse trains reported in the lightning literature. Hence, they may be caused by a physical process that has not yet been identified.  相似文献   

3.
Site amplification defined as the peak value of spectrum ratio was investigated using surface and base accelerations recorded in a number of KiK-net down-hole arrays in Japan during three major earthquakes. An important task was to determine the spectral amplifications relative to outcropping motions with the aid of the down-hole array records. Based on soil data available for individual arrays, theoretical amplifications were calculated and adjusted to coincide with the peak amplifications of the array records. A good and unique correlation was found between the peak amplifications thus obtained and S-wave velocity ratios, defined by S-wave velocity in base layer divided by average S-wave velocity , for different sites and different earthquakes. The value of was evaluated from fundamental mode frequency and the thickness of an equivalent surface layer in which peak amplification is exerted. The conventional parameter Vs30; averaged shear wave velocity in the top 30 m used in current design codes, did not correlate well with the obtained amplifications. It is suggested that may be determined not only from Vs-logging data but also from microtremor measurements.  相似文献   

4.
The new LIMA/ice model is used to study interhemispheric temperature differences at the summer upper mesosphere and their impact on the morphology of ice particle related phenomena such as noctilucent clouds (NLC), polar mesosphere clouds (PMC), and polar mesosphere summer echoes (PMSE). LIMA/ice nicely reproduces the mean characteristics of observed ice layers, for example their variation with season, altitude, and latitude. The southern hemisphere (SH) is slightly warmer compared to the NH but the difference is less than 3 K at NLC/PMC/PMSE altitudes and poleward of 70N/S. This is consistent with in situ temperature measurements by falling spheres performed at 69N and 68S. Earth's eccentricity leads to a SH mesosphere being warmer compared to the NH by up to approximately 85 km and fairly independent of latitude. In general, NH/SH temperature differences in LIMA increase with decreasing latitude and reach at 50. The latitudinal variation of NH/SH temperature differences is presumably caused by dynamical forcing and explains why PMSE are basically absent at midlatitudes in the SH whereas they are still rather common at similar colatitudes in the NH. The occurrence frequency and brightness of NLC and PMC are larger in the NH but the differences decrease with increasing latitude. Summer conditions in the SH terminate earlier compared to NH, leading to an earlier weakening and end of the ice layer season. The NLC altitude in the SH is slightly higher by 0.6–1 km, whereas the NLC altitudes itself depend on season in both hemispheres. Compared to other models LIMA/ice shows smaller interhemispheric temperature differences but still generates the observed NH/SH differences in ice layer characteristics. This emphasizes the importance of temperature controlling the existence and morphology of ice particles. Interhemispheric differences in NLC/PMC/PMSE characteristics deduced from LIMA/ice basically agree with observations from lidars, satellites, and radars.  相似文献   

5.
The mechanisms driving the air–sea exchange of carbon dioxide (CO2) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO2 flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO2 uptake is high () due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO2 partial pressure (pCO2) during the productive season. In contrast, the permanently mixed southern region is a weak net CO2 source (). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO2 appears to be controlled by temperature.  相似文献   

6.
Recent studies in various settings across eastern North America have examined the movement of volatile organic compound (VOC) plumes from groundwater to streams, but few studies have addressed focused discharge of such plumes in unlithified sediments. From 1999 through 2002, we monitored concentrations of trichloroethene (TCE) and the non-volatile co-contaminant technetium-99 (99Tc) along Little Bayou Creek, a first-order perennial stream in the Coastal Plain of western Kentucky. Spring flow contributed TCE and 99Tc to the creek, and TCE concentrations tended to vary with 99Tc in springs. Contaminant concentrations in stream water fluctuated seasonally, but not always synchronously with stream flow. However, contaminant influxes varied seasonally with stream flow and were dominated by a few springs. Concentrations of O2, , and , values of δ37ClDOCl in groundwater, and the lack of less-chlorinated ethenes in groundwater and stream water indicated that anaerobic biodegradation of TCE was unlikely. Losses of TCE along Little Bayou Creek resulted mainly from volatilization, in contrast to streams receiving diffuse contaminated discharge, where intrinsic bioremediation of VOCs appears to be prevalent.  相似文献   

7.
Monitoring the water balance of Lake Victoria, East Africa, from space   总被引：3，自引：0，他引：3  
Sean Swenson  John Wahr   《Journal of Hydrology》2009,370(1-4):163-176
Using satellite gravimetric and altimetric data, we examine trends in water storage and lake levels of multiple lakes in the Great Rift Valley region of East Africa for the years 2003–2008. GRACE total water storage estimates reveal that water storage declined in much of East Africa, by as much as , while altimetric data show that lake levels in some large lakes dropped by as much as 1–2 m. The largest declines occurred in Lake Victoria, the Earth’s second largest freshwater body. Because the discharge from the outlet of Lake Victoria is used to generate hydroelectric power, the role of human management in the lake’s decline has been questioned. By comparing catchment water storage trends to lake level trends, we confirm that climatic forcing explains only about 50decline. This analysis provides an independent means of assessing the relative impacts of climate and human management on the water balance of Lake Victoria that does not depend on observations of dam discharge, which may not be publically available. In the second part of the study, the individual components of the lake water balance are estimated. Satellite estimates of changes in lake level, precipitation, and evaporation are used with observed lake discharge to develop a parameterization for estimating subsurface inflows due to changes in groundwater storage estimated from satellite gravimetry. At seasonal timescales, this approach provides closure to Lake Victoria’s water balance to within . The third part of this study uses the water balance of a downstream water body, Lake Kyoga, to estimate the outflow from Lake Victoria remotely. Because Lake Kyoga is roughly 20 times smaller in area than Lake Victoria, its water balance is strongly influenced by inflow from Lake Victoria. Lake Kyoga has been shown to act as a linear reservoir, where its outflow is proportional to the height of the lake. This model can be used with satellite altimetric lake levels to estimate a time series of Lake Victoria discharge with an rms error of about .  相似文献   

8.
Hydrogeochemical investigations along an E–W transect in the middle Meghna basin show groundwater chemistry and redox condition vary considerably with the change in geology. Groundwater in the Holocene shallow (<150 m bgl) alluvial aquifer in western part of the transect is affected by high arsenic concentration (As > 10 μg/l) and salinity. On the other hand, groundwater from the Pliocene Dupi Tila sandy aquifer in the eastern part is fresh and low in As (<10 μg/l). The Holocene shallow aquifers are high in dissolved As, , Fe and dissolved organic carbon (DOC), but generally low in and . High concentrations (250–716 mg/l) together with high DOC concentrations (1.4–21.7 mg/l) in these aquifers reflect active sources of degradable natural organic matter that drives the biogeochemical process. There is generally de-coupling of As from other redox-sensitive elements. In contrast, the Pliocene aquifers are low in As, and DOC. Molar ratio of /H4SiO4 suggests that silicate weathering is dominant in the deeper Holocene aquifers and in the Pliocene aquifers. Molar ratios of Cl−/ and Na+/Cl− suggest mixing of relict seawater with the fresh water as the origin of groundwater salinity. Speciation calculations show that saturation indices for siderite and rhodochrosite vary significantly between the Holocene and Pliocene aquifers. Stable isotopes (δ2H and δ18O) in groundwater indicate rapid infiltration without significant effects of evaporation. The isotopic data also indicates groundwater recharge from monsoonal precipitation with some impact of altitude effect at the base of the Tripura Hills in the east. The results of the study clearly indicate geological control (i.e. change in lithofacies) on groundwater chemistry and distribution of redox-sensitive elements such as As along the transect.  相似文献   

9.
The brief interaction of precipitation with a forest canopy can create a high spatial variability of both throughfall and solute deposition. We hypothesized that (i) the variability in natural forest systems is high but depends on system-inherent stability, (ii) the spatial variability of solute deposition shows seasonal dynamics depending on the increase in rainfall frequency, and (iii) spatial patterns persist only in the short-term. The study area in the north-western Brazilian state of Rondônia is subject to a climate with a distinct wet and dry season. We collected rain and throughfall on an event basis during the early wet season (n = 14) and peak of the wet season (n = 14) and analyzed the samples for pH and concentrations of , Na+, K+, Ca2+, Mg2+, Cl−, , and DOC. The coefficient of variation for throughfall based on both sampling intervals was 29%, which is at the lower end of values reported from other tropical forest sites, but which is higher than in most temperate forests. Coefficients of variation of solute deposition ranged from 29% to 52%. This heterogeneity of solute deposition is neither particularly high nor particularly low compared with a range of tropical and temperate forest ecosystems. We observed an increase in solute deposition variability with the progressing wet season, which was explained by a negative correlation between heterogeneity of solute deposition and antecedent dry period. The temporal stability of throughfall patterns was low during the early wet season, but gained in stability as the wet season progressed. We suggest that rapid plant growth at the beginning of the rainy season is responsible for the lower stability, whereas less vegetative activity during the later rainy season might favor the higher persistence of “hot” and “cold” spots of throughfall quantities. The relatively high stability of throughfall patterns during later stages of the wet season may influence processes at the forest floor and in the soil. Solute deposition patterns showed less clear trends but all patterns displayed a short-term stability only. The weak stability of those patterns is apt to impede the formation of solute deposition-induced biochemical microhabitats in the soil.  相似文献   

10.
Nearly 900 nocturnal temperature profiles (85–105 km) from the Colorado State University Na lidar at Fort Collins, CO (40.59N, 105.14W) from 1990 to 2007. After the removal of an episodic warming attributable to Mt. Pinatubo eruption, the time series is analyzed as the sum of the climatological mean, annual and semiannual oscillation, solar cycle effect and trends along with possible annual/semiannual modulation of the latter two. The direct seasonal variation is consistent with the concept of the two-level mesopause. The trends in summer and winter are comparable 90–96 km at −0.15±0.1 K/year. The summer trend turns positive above 96 km. The winter trend is negative with minimum of −0.3 K/year at 100 km but positive at 104 km. The negative trend values are a factor of five smaller than an earlier analysis of the early part of this data due to removal of an episodic event.  相似文献   

11.
We present the results of a probabilistic seismic hazard assessment and disaggregation analysis aimed to understand the dominant magnitudes and source-to-site distances of earthquakes that control the hazard at the Celano site in the Abruzzo region of central Italy. Firstly, we calculated a peak ground acceleration map for the central Apennines area, by using a model of seismogenic sources defined on geological-structural basis. The source model definition and the probabilistic seismic hazard evaluation at the regional scale (central Apennines) were obtained using three different seismicity models (Gutenberg–Richter model; characteristic earthquake model; hybrid model), consistent with the available seismological information. Moreover, a simplified time-dependent hypothesis has been introduced, computing the conditional probability of earthquakes occurrence by Brownian passage time distributions.Subsequently, we carried out the disaggregation analysis, with a modified version of the SEISRISK III code, in order to separate the contribution of each source to the total hazard.The results show the percentage contribution to the Celano hazard of the various seismogenic sources, for different expected peak ground acceleration classes. The analysis was differentiated for close (distance from Celano <20 km) and distant (distance from Celano >20 km) seismogenic sources. We propose three different “scenario earthquakes”, useful for the site condition studies and for the seismic microzoning study: (1) large (M=6.6) local (Celano-epicentre distance 16 km) earthquake, with mean recurrence time of 590 years; (2) moderate (M=5.5) local (Celano-epicentre distance 7.5 km) earthquake, with mean recurrence time of 500 years; and (3) large (M=6.6) distant (Celano-epicentre distance 24 km) earthquake, with mean recurrence time of 980 years.The probabilistic and time-dependent approach to the definition of the “scenario earthquakes” changes clearly the results in comparison to traditional deterministic analysis, with effects in terms of engineering design and seismic risk reduction.  相似文献   

12.
The Earth’s gravity field observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission shows variations due to the integral effect of mass variations in the atmosphere, hydrosphere and geosphere. Several institutions, such as the GeoForschungsZentrum (GFZ) Potsdam, the University of Texas at Austin, Center for Space Research (CSR) and the Jet Propulsion Laboratory (JPL), Pasadena, provide GRACE monthly solutions, which differ slightly due to the application of different reduction models and centre-specific processing schemes. The GRACE data are used to investigate the mass variations in Fennoscandia, an area which is strongly influenced by glacial isostatic adjustment (GIA). Hence the focus is set on the computation of secular trends. Different filters (e.g. isotropic and non-isotropic filters) are discussed for the removal of high frequency noise to permit the extraction of the GIA signal. The resulting GRACE based mass variations are compared to global hydrology models (WGHM, LaDWorld) in order to (a) separate possible hydrological signals and (b) validate the hydrology models with regard to long period and secular components. In addition, a pattern matching algorithm is applied to localise the uplift centre, and finally the GRACE signal is compared with the results from a geodynamical modelling. The GRACE data clearly show temporal gravity variations in Fennoscandia. The secular variations are in good agreement with former studies and other independent data. The uplift centre is located over the Bothnian Bay, and the whole uplift area comprises the Scandinavian Peninsula and Finland. The secular variations derived from the GFZ, CSR and JPL monthly solutions differ up to 20%, which is not statistically significant, and the largest signal of about 1.2 Gal/year is obtained from the GFZ solution. Besides the GIA signal, two peaks with positive trend values of about 0.8 Gal/year exist in central eastern Europe, which are not GIA-induced, and also not explainable by the hydrology models. This may indicate that the recent global hydrology models have to be revised with respect to long period and secular components. Finally, the GRACE uplift signal is also in quite good agreement with the results from a simple geodynamical modelling.  相似文献   

13.
The effect of different organic-rich sewage concentration (0%, 20% and 60% diluted in seawater) and absence or presence of mangrove trees on the survival, bioturbation activities and burrow morphology of fiddler crabs species was assessed. After 6 months, males of both species always showed higher survival (80%) when compared to females (20%). Crabs inhabiting pristine conditions achieved higher survival (67–87%) than those living in sewage-exposed mesocosms (40–71%). At 60% sewage loading, fiddler crabs processed less sediment (34–46%) during feeding and excavated slightly more sediment (45–80%) than at pristine conditions. While percent volume of the burrow chambers increased (13–66%) at contaminated mesocosms for both vegetation conditions, burrows were shallower (33%) in bare cells loaded with sewage. The results show that fiddler crabs presented moderate mortality levels in these artificial mangrove wetlands, but mainly in sewage impacted cells. However, they still function as ecosystem engineers through bioturbation activities and burrow construction.  相似文献   

14.
This baseline reports daily variations of physico-chemical parameters, nutrients and metals in water of two sites [eutrophic conditions (BB) versus reference (LL)] in Óbidos coastal lagoon (Portugal). Dissolved oxygen in BB varied between 40% saturation levels (night) and 190% (daylight). , and ratios to Al of Mn, Fe and Pb in suspended particulate matter increased during the period of lower oxygenation, indicating exportation of nutrients and metals from the sediment to water at night. At LL, O2 values oscillated always around 100%. and Mn/Al ratio at LL were always lower than at BB and no increases were found during the night. The input of and Mn provided from the sediment at BB during the night could surpass the contribution of freshwater discharges. PCA confirmed the separation of most samples from BB and LL. These results pointed to the importance of assessing chemical conditions over day-night cycles.  相似文献   

15.
In this paper the chemistry of major and trace elements and, for the first time, strontium isotopic ratios measured in running waters from the Arno River Basin (Tuscany, central-northern Italy) and thermal springs discharging in the same hydrographic system are presented and discussed. Classical graphical methods (e.g. mixing diagrams) have here been improved to identify, in a correct statistical sample space, extreme chemical compositions attributable to the action of geochemical processes and/or inherited from specific lithologies (namely contributions or components 1, 2 and 3) to be used in inverse modeling procedures, due to the absence of clear end-members. A linear least squares problem, with non-negativity constraints and distances, as required for compositional data (convex linear mixing) was solved by considering the contribution of the most discriminant variables given by the 87Sr/86Sr isotopic ratios and the concentration of Ca2+, Mg2+, Sr and Rb. Following such approach, it can be assumed (p < 0.05) that component 1, characterized by a Ca2+(Mg2+)– radiogenic-rich (87Sr/86Sr = 0.71274; Rb/Sr = 0.039), represents a dominant feature at basin scale, with a weight ranging from 69% to 100%. Much lower percentages are related to component 2, represented by a Ca2+(Mg2+)– facies with intermediate 87Sr/86Sr (0.70874) and low Rb/Sr (2.8 × 10−4) ratios and component 3, identified by Ca2+– facies with less radiogenic 87Sr/86Sr (0.70827) and low Mg2+/Ca2+ (0.011) ratios. These components are mainly dominated by the dissolution of evaporitic rocks and/or mixing with thermal waters in the southern part of the Arno River Basin and by dissolution of the carbonatic fraction, kinetically favored with respect to that of the silicatic minerals, in the upper reaches of the main course and its tributaries, respectively.  相似文献   

16.
Ionospheric topside sounders can be considered to act as mobile ionospheric heating facilities. They stimulate a wide variety of plasma phenomena that suggests that significant plasma heating can be produced in the vicinity of the spacecraft following the short duration (0.1 ms) high-power (hundreds of watts) sounder pulse. Most of these phenomena are sensitive to the ambient plasma conditions, particularly to the ratio of the plasma frequency to the gyro frequency . Certain stimulated phenomena only occur, or are greatly enhanced, when where n is an integer. One example is a diffuse signal return that appears at a frequency just above the Z mode wave cutoff frequency (the L = 0 condition in the notation of Stix). This signal, which is the subject of the present paper, is present only for moderately large near-integer values for (e.g., n > 3). It is a relatively short-duration echo—usually less than about 10 ms. These characteristics are quite different from the Z mode echoes often observed between or (whichever is greater) and the upper hybrid frequency . These latter echoes occur for smaller values (not necessarily near-integer values) of and are received during the entire 30 ms listening time period following the sounder pulse. They have been attributed to the scattering of sounder Z mode signals from naturally occurring electron density fieldaligned irregularities (FAI). The short-duration echoes observed just above , on the other hand, are here attributed to the ducting of sounder-generated Z mode waves in sounder-stimulated (or sounderenhanced) FAI. These FAI are believed to be generated (or enhanced) on a very short time scale (? 1 s) by the efficient absorption of sounder energy when the plasma/gyro frequency ratio is nearly an integer value significantly greater than one. The most likely generation process is the filamentation instability driven by the ponderomotive force due to the high-power sounder pulse.  相似文献   

17.
In solar cycles 22–23, all solar indices showed maxima near 1990 and 2000 and minima in 1996. The maximum to minimum variation was only 1–2% in the UV range 240–350 nm. Dobson ozone intensities did not show any clear relationship with solar cycle and ozone variations were less than 10%. The UV-B (295–325 nm) observed at ground by Brewer spectrophotometers at some locations had variations of 50–100% for 295–300 nm, and 20–50% for 305–325 nm. The maxima were in different years at different locations (even with separations of only 300 km), did not match with the solar cycle, and were far too large to be explained on the basis of ozone changes (1% decrease of ozone is expected to cause 2% increase of UV-B). Thus, if the data are not bad, the UV-B changes do not match with solar activity or ozone changes and must be mostly due to other local effects (clouds, etc.?). When data are averaged over wide geographical regions, UV-B variation ranges are smaller (10–20%, probably because localised, highly varying cloud effects get filtered out), and are roughly as expected from ozone variations.  相似文献   

18.
Using 3 years of high-quality temperature measurements (2002–2004) recorded from Maui, HI (20.8°N), we have investigated the characteristics of mesospheric seasonal oscillations at low-latitudes. Measurements of the near-infrared OH (6,2) and O2 (0,1) nightglow emission layers (centered at 87 and 94 km) independently reveal a distinct semi-annual oscillation (SAO) and annual oscillation (AO) with amplitudes of 3.8 and 2.0 K, respectively. An observed asymmetry in the seasonal variation of the nocturnal mean, previously reported by Taylor et al. [2005. Characterization of the semi-annual-oscillation in mesospheric temperatures at low-latitudes. Advances in Space Research 35, doi:10.1016/j.asr.2005.05.111] from this site is shown to be due to a superposed AO of amplitude 50% of the SAO signature. Detailed investigations of the local-time variation of the SAO amplitude and phase combined with TIME-GCM simulations of the seasonal variation of the diurnal tide strongly suggest a large local-time dependence of the amplitude (but not phase) of the observed SAO. These data indicate that the true mean temperature SAO amplitude could be as high as 7 K at this latitude.  相似文献   

19.
The aim of this paper is to report some periodicities observed in the ionospheric parameter foF2 measured at Tucuman (26.9°S; 65.4°W), station placed near the southern crest of the equatorial anomaly. For that, monthly medians of foF2 at several hours of LT for the period 1958–1987 are used. The data are run with Fast Fourier Transform (FFT). Data gaps (4–5 months) are filled by means of linear interpolation. Several periodicities are present. Besides the solar cycle dominant dependence (11 years), semi-annual, annual, five years and quasi-biennial periodicities are also observed. A marked quasi-biennial periodicity is observed at daytime and nighttime hours being their greater amplitude at local noon and midnight. Different mechanisms or combined effects possibly cause them. It is suggested that the solar activity by means of extreme ultraviolet radiation (EUV), which present a quasi-biennial oscillation (QBO) and it is responsible for the ionization, could be the dominant mechanism for the diurnal quasi-biennial periodicity of foF2. At night, since the photoionization by extreme ultraviolet radiation is not significant and the F2 layer is lower than during daytime (100 km) other mechanism may be operative for the quasi-biennial periodicity observed. Possibly the stratospheric QBO contributes to the modulation of the observed behaviour in foF2 at night. This result is preliminary because it needs to be extended to other stations so as to extract definite conclusions. Moreover, we cannot dismiss the possibility of a combined effect of both these mechanisms mainly at daytime and/or QBO influence of geomagnetic parameters.  相似文献   

20.
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