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241.
242.
Study of sporadic-E clouds by backscatter radar 总被引:1,自引:0,他引:1
It is shown that swept-frequency backscatter ionograms covering a range of azimuths can be used to study the dynamics of sporadic-E clouds. A simple technique based on analytic ray tracing can be used to simulate the observed narrow traces associated with Es patches. This enables the location and extent of the sporadic-E clouds to be determined. The motion of clouds can then be determined from a time sequence of records. In order to demonstrate the method, results are presented from an initial study of 5 days of backscatter ionograms from the Jindalee Stage B data base obtained during March-April 1990. Usually 2–3 clouds were observed each day, mainly during the evening and up to midnight. The clouds lasted from 1–4 h and extended between 30°–80° in azimuth and 150–800 km in range. The clouds were mostly stationary or drifted generally westward with velocities of up to 80 ms–1. Only one cloud was observed moving eastward. 相似文献
243.
The validity of the Winkler foundation model is investigated by predicting the experimentally measured displacement transfer functions and strain spectra of a single pile embedded in a sandbox and supporting a single-degree-of-freedom superstructure. The foundation-superstructure system is a scale model and was subjected to shake table excitations. The distributed springs and dashpots of the Winkler foundation model are frequency dependent and the calibrated model predicts satisfactorily the displacement transfer function at different depths for both fixed- and free-tip pile conditions. On the other hand, the pile-axial-strains are substantially underestimated when expressed in terms of the second derivative of the computed elastic line of the pile. It is shown that a much more dependable prediction is achieved when pile-axial-strains are expressed in terms of the inertial forces acting along the pile-superstructure system. 相似文献
244.
着重介绍了间隔式钻孔灌注桩在挡土护坡技术中的应用方法,设计时的主要参考依据,计算土压力理论的应用原则以及施工中需注意的几个问题,为进行深基础挡土护坡的工程提供了部分参考依据。 相似文献
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A reexamination of the valley wind system in the Alpine Inn Valley with numerical simulations 总被引:2,自引:0,他引:2
G. Zängl 《Meteorology and Atmospheric Physics》2004,87(4):241-256
Summary This paper presents idealized numerical simulations of the valley wind circulation in the Alpine Inn Valley, which are compared with existing data and are used to improve our dynamical understanding of the valley wind. The simulations have been performed with the Penn State/NCAR mesoscale model MM5. They use a high-resolution realistic topography but idealized large-scale conditions without any synoptic forcing to focus on the thermally induced valley wind system. The comparison with the available observations shows that this simplified set-up is sufficient to reproduce the essential features of the valley wind.The results show that the tributaries of the Inn Valley have a considerable impact on the along-valley mass fluxes associated with the valley wind circulation. The upvalley mass flux is found to increase where tributaries enter the Inn Valley from the north, that is, from the direction where the Alpine foreland is located. On the other hand, the upvalley mass flux is reduced at the junctions with southern tributaries because part of the upvalley flow is deflected into these tributaries. For the downvalley flow, the situation is essentially reversed, but the influence of the valley geometry on the flow structure is larger than for the upvalley flow. The most important feature is a lateral valley contraction near the valley exit into the Alpine foreland. It reduces the downvalley mass flux at low levels, so that the wind maximum in the interior of the valley is shifted to a fairly large distance from the ground. North of the valley contraction, however, the downvalley flow strongly accelerates and forms a pronounced low-level jet. A dynamical analysis indicates that this acceleration can be interpreted as a transition from subcritical to supercritical hydraulic flow. Another interesting feature is that the low-level jet maintains its structure for several tenths of kilometres into the Alpine foreland. This appears to be related to the fact that the lateral wind shear on the flanks of the jet is associated with a strong dipole of potential vorticity (PV). Due to the conservation properties of the PV, the downstream advection of the PV dipole leads to the formation of a band-like feature that decays fairly slowly. 相似文献
247.
Global monsoons in the mid-Holocene and oceanic feedback 总被引:10,自引:3,他引:10
The response of the six major summer monsoon systems (the North American monsoon, the northern Africa monsoon, the Asia monsoon, the northern Australasian monsoon, the South America monsoon and the southern Africa monsoon) to mid-Holocene orbital forcing has been investigated using a coupled ocean–atmosphere general circulation model (FOAM), with the focus on the distinct roles of the direct insolation forcing and oceanic feedback. The simulation result is also found to compare well with the NCAR CSM. The direct effects of the change in insolation produce an enhancement of the Northern Hemisphere monsoons and a reduction of the Southern Hemisphere monsoons. Ocean feedbacks produce a further enhancement of the northern Africa monsoon and the North American monsoon. However, ocean feedbacks appear to weaken the Asia monsoon, although the overall effect (direct insolation forcing plus ocean feedback) remains a strengthened monsoon. The impact of ocean feedbacks on the South American and southern African monsoons is relatively small, and therefore these regions, especially the South America, experienced a reduced monsoon regime compared to present. However, there is a strong ocean feedback on the northern Australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region. A new synthesis is made for mid-Holocene paleoenvironmental records and is compared with the model simulations. Overall, model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations. 相似文献
248.
Broadband solar irradiance data obtained in the spectral range 400–940 nm at Kwangju, South Korea from 1999–2000 have been analyzed to investigate the effects of cloud cover and atmospheric optical depth on solar radiation components. Results from measurements indicate that the percentage of direct and diffuse horizontal components of solar irradiance depend largely on total optical depth (TOD) and cloud cover. During summer and spring, the percentages of diffuse solar irradiance relative to the global irradiance were 5.0% and 4.9% as compared to 2.2% and 3.0% during winter and autumn. The diffuse solar irradiance is higher than the direct in spring and summer by 24.2%, and 40.6%, respectively, which may largely be attributed to the attenuation (scattering) of radiation by heavy dust pollution and large cloud amount. In cloud-free conditions with cloud cover ≤2/10, the fraction of the direct and diffuse components were 66.0% and 34.0%, respectively, with a mean daily global irradiance value of 7.92±2.91 MJ m−2 day−1. However, under cloudy conditions (with cloud cover ≥8/10), the diffuse and direct fractions were 97.9% and 2.2% of the global component, respectively. The annual mean TOD under cloudless conditions (cloud cover≤2/10) yields 0.74±0.33 and increased to as much as 3.15±0.67 under cloudy conditions with cloud amount ≥8/10. An empirical formula is derived for estimating the diffuse and direct components of horizontal solar irradiance by considering the total atmospheric optical depth (TOD). Results from statistical models are shown for the estimation of solar irradiance components as a function of TOD with sufficient accuracy as indicated by low standard error for each solar zenith angle (SZA). 相似文献
249.
250.