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831.
The transient planetary waves in the atmosphere and ionosphere seem to occur in the form of bursts of a couple of waves with limited persistence. To study persistence of planetary wave events in the lower ionosphere, data from two radio paths from Central Europe are used, Luxembourg – Panská Ves (f = 6.09 MHz, f
eq = 2.1-2.2 MHz) and Deutschlandfunk – Panská Ves (f = 1539 kHz, f
eq = 650-700 kHz). The absorption along the former radio paths is formed very predominantly at altitudes of about 90-100 km, whereas the latter absorption is formed mostly at altitudes of about 85-90 km. The persistence of planetary wave type oscillations is studied in three period bands centred at 5, 10 and 16 days. Waves with period T near 5 days reveal a typical persistence of wave events around 5 cycles. Waves with T = 10 days are less persistent with a typical persistence of 3-4 cycles. The typical persistence of waves T = 16 days is no more than 3 cycles. In terms of number of cycles, the persistence of oscillations evidently decreases with increasing period. On the other hand, in terms of number of days, the persistence seems rather to increase with increasing period. 相似文献
832.
Johana Brokešová 《Pure and Applied Geophysics》1996,148(3-4):503-538
The seismic wave field, in its high-frequency asymptotic approximation, can be interpolated from a low- to a high-resolution spatial grid of receivers and, possibly, point sources by interpolating the eikonal (travel time) and the amplitude. These quantities can be considered as functions of position only. The travel time and the amplitude are assumed to vary in space only slowly, otherwise the validity conditions of the theory behind would be violated. Relatively coarse spatial sampling is then usually sufficient to obtain their reasonable interpolation. The interpolation is performed in 2-D models of different complexity. The interpolation geometry is either 1-D, 2-D, or 3-D according to the source-receiver distribution. Several interpolation methods are applied: the Fourier interpolation based on the sampling theorem, the linear interpolation, and the interpolation by means of the paraxial approximation. These techniques, based on completely different concepts, are tested by comparing their results with a reference ray-theory solution computed for gathers and grids with fine sampling. The paraxial method holds up as the most efficient and accurate in evaluating travel times from all investigated techniques. However, it is not suitable for approximation of amplitudes, for which the linear interpolation has proved to be universal and accurate enough to provide results acceptable for many seismological applications. 相似文献
833.
Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic.
The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and
shaledominated Přídolí and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 μm), inclusion-rich,
xenotopic matrix dolomite (δ
18O=−4.64 to −3.40‰ PDB;δ
13C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite
type. Coarse crystalline saddle dolomite (δ
18O=−8.04 to −5.14‰ PDB;δ
18C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization
event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides
which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal.
Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization
of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably
migrated through the more permeable upper portion of the Přídolí sequence appears as a viable explanation for this dolomitization
overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during
deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated
quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C.
Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account
for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian
strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that
the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought. 相似文献
834.
Summary The gravitational potential energy of the Sun and its corresponding internal energy have been estimated on the basis of the
standard model of the Sun's internal structure. It is demonstrated that the principal moment of inertia of the Sun, computed
from the model, does not fit the hydrostatic equilibrium limit. 相似文献
835.
Luděk Klimeš 《Pure and Applied Geophysics》1996,148(3-4):539-563
A new computational scheme for calculating the first-arrival travel times on a rectangular grid of points is proposed. The new proposed method is of second-order accuracy. This means that the error of the calculated travel time is proportional to the second power of the grid spacing. The method should be sufficiently accurate for all applications in smooth seismic models. On the other hand, the method is not, in its present form, proposed for models with structural interfaces which make the method unstable and generate travel-time errors of the first order. Equations are also presented for the appropriate evaluation of the errors of calculated travel times to check their accuracy, and the proposed method is compared with other numerical methods. The method is developed, described and demonstrated in 2-D, but may also be extended to 3-D models and to general models with structural interfaces. 相似文献
836.
Summary Predictor vectors, including upper air as well as surface data, were used for categorical forecasting convective events over a subregion of the Czech territory, and the effect of including surface variables in the predictor vector was examined. While upper air data were considered as Perfect Prognosis, the surface data were successively included according to the time of their origin. The forecasting technique was based on linear multiple regression with learning, and the accuracy of the forecast was measured by the Critical Success Index. The input data from the three May-September periods in 1989–91 were used, and the first year served as the learning set. The aerological data from TEMP 12 UTC, simulating Perfect Prognosis, were the source of the upper air predictors. The performance of all, upper air, surface and combined, predictors were evaluated and compared. It turned out that the improvement of prediction accuracy due to the inclusion of surface variables was not negligible. Significant improvements were made in the forecasts of thunderstorm occurrence between 18 and 24 UTC. 相似文献
837.
838.
839.
Summary In the study, urban-suburban/rural vapour pressure and relative humidity differences at fixed hours in the Belgrade area
were analysed and compared. The data from one urban, two suburban and one rural stations (0700, 1400 and 2100 hours LTC),
for the period 1976–1980 were used. It has been found, on the basis of the vapour pressure differences that the atmosphere
in urban areas is more humid than that in the suburban and rural areas at 0700 and 2100 hours, in the period from September
to February, while from February to September the urban atmosphere is drier. At 1400 hours, urban atmosphere is drier throughout
the year. However, relative humidity differences indicate that urban air is less humid than the air in suburban and rural
areas throughout the year. Relationships between urban-suburban/rural vapour pressure differences and heat island intensity
revealed that the local effects could be significant.
Received December 23, 1999 Revised September 27, 2000 相似文献
840.
Ivana Nemešová 《Studia Geophysica et Geodaetica》1991,35(3):244-249
Summary The paper deals with the role of penetrative downdrafts in the convective adjustment suggested by Betts[1]. Three different types of downdraft sinking levels are used in the assessment of the reference profiles of temperature and humidity in cumulus cloud layers and these are compared with the profiles with no downdraft. It is shown that the depths of downdraft penetration may significantly influence the reference profiles mainly for clouds of vertical extents larger than one kilometre. 相似文献