System Optimisation Of Multi-Beam Aperture Synthesis Arrays For Survey Performance     

Jaap D. Bregman 《Experimental Astronomy》2004,17(1-3):365-380

Investigating the requirements for an aperture synthesis array that optimise the performance for surveying shows that, next to collecting area and system temperature, the field-of-view (FoV) is key parameter. However, the effective sensitivity not only depends on bandwidth and integration time but could be seriously limited by side lobe confusion and by gain errors that determine the effective dynamic range. From the basic sensitivity equation for a radiometric system we derive an optimum cost ratio between collecting area and processing electronics, where the latter should be less than a third of the total cost. For an instrument that has to cover a fraction of sky larger than its field-of-view, the FoV enters the equation for survey sensitivity and we identify the number of independent feed systems per unit collecting area as a key parameter. Then the optimum cost distribution allows the electronics to account for up to half the total cost. Further analysis of the sensitivity formula shows that there is an optimum design frequency for a survey instrument below 1 GHz. We discuss the impact of station size and array configuration on self-calibration, side lobe confusion and effective sensitivity and conclude that a minimum station size of 20 m diameter is required at 0.3 GHz as long as multi-patch self-calibration procedures need, per baseline, a signal-to-noise ratio of more than two for each ionospheric coherence patch.  相似文献   

Urban field classification by “local climate zones” in a medium-sized Central European city: the case of Olomouc (Czech Republic)     

Michal Lehnert  Jan Geletič  Jan Husák  Miroslav Vysoudil 《Theoretical and Applied Climatology》2015,122(3-4):531-541

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SfM-MVS Photogrammetry for Splash Erosion Monitoring under Natural Rainfall     

Tomáš Laburda  Josef Krása  David Zumr  Jan Devátý  Michal Vrána  Nives Zambon  Lisbeth Lolk Johannsen  Andreas Klik  Peter Strauss  Tomáš Dostál 《地球表面变化过程与地形》2021,46(5):1067-1082

An understanding of splash erosion is the basis to describe the impact of rain characteristics on soil disturbance. In typical splash cup experiments, splashed soil is collected, filtered, and weighed. As a way to collect additional data, our experiments have been supplemented by a photogrammetric approach. A total of three soils were tested across three sites, one in the Czech Republic and two in Austria, all equipped with rain gauges and disdrometers to measure rainfall parameters. The structure from motion multiview stereo (SfM-MVS) photogrammetric method was used to measure the raindrops impact on the soil surface. The images were processed using Agisoft PhotoScan, resulting in orthophotos and digital elevation models (DEMs) with a resolution of 0.1 mm/pix. The surface statistics included the mean surface height (whose standard deviation was used as a measure of surface roughness), slope, and other parameters. These parameters were evaluated depending on soil texture and rainfall parameters. The results show a linear correlation between consolidation and splash erosion with a coefficient of determination (R²) of approximately 0.65 for all three soils. When comparing the change in soil volume with rainfall parameters, the best correlation was found with the maximum 30-minintensity (I₃₀), resulting in R² values of 0.48 (soil A, silt loam, 26% clay), 0.59 (soil B, silt loam, 18% clay), and 0.68 (soil C, loamy sand, 12% clay). The initial increase in the sample volume for the lowest splashed mass corresponds with the increase in the clay content of each of the soils. Soil A swells the most. Soil B swells less. Soil C does not swell at all and consolidates the most. We derived the relationship between the photogrammetrically measured change in surface height and the splash erosion (measured by weight) by accounting for the effect of the clay content.  相似文献   

Distribution of Crenarchaeota tetraether membrane lipids in surface sediments from the Red Sea     

Gabriele Trommer  Michael Siccha  Marcel T.J. van der Meer  Stefan Schouten  Jaap S. Sinninghe Damsté  Hartmut Schulz  Christoph Hemleben  Michal Kucera 《Organic Geochemistry》2009,40(6):724-731

The Red Sea represents an extreme marine environment, with high salinity, high temperature and low level of nutrients, complicating the application of standard geochemical palaeotemperature proxies. In order to investigate the applicability of the TEX₈₆ (TetraEther indeX of GDGTs with 86 carbons) proxy for sea surface temperature (SST) in the Red Sea, the distribution of glycerol dialkyl glycerol tetraether membrane lipids (GDGTs) in sediments from the Red Sea and the Gulf of Aden were examined. Against expectations, TEX₈₆ values for the Red Sea do not show a simple linear relationship with SST and deviate from the global core top calibration. In the northern Red Sea, at temperatures between 25 and 28 °C, the values increase linearly with SST, whereas in the southern Red Sea, at temperatures above 28 °C, TEX₈₆ decreases with increasing temperature. Factors like seasonality and depth of production, salinity and nutrient availability, as well as diagenetic overprint or influence of allochtonous terrestrial lipids, cannot explain this pattern. However, the observed TEX₈₆ relationship with SST could be explained by the presence of a hypothetical endemic Crenarchaeota population in the Red Sea with a specific TEX₈₆ vs. SST relationship. In the Southern Red Sea, a two-component mixing model implies an exponential decrease in the endemic population towards the Gulf of Aden. Thus, the application of the TEX₈₆ as a palaeotemperature proxy in the Red Sea is likely only possible for the northern Red Sea area with the specific SST vs. TEX₈₆ relationship determined in this study and potentially for the whole Red Sea basin during glacials, when water exchange with the Indian Ocean was more restricted than today and the endemic archaeal lipid distribution was not affected by transport from the Indian Ocean. Our results suggest that distinct populations of Crenarchaeota in extreme environments such as evaporitic basins may have different membrane composition, necessitating application of another calibration than the global core top calibration for TEX₈₆ palaeothermometry.  相似文献   

Dynamic migration of rotating neutron stars due to a phase transition instability     

Harald Dimmelmeier  Michal Bejger  Pawel Haensel  J. Leszek Zdunik 《Monthly notices of the Royal Astronomical Society》2009,396(4):2269-2288

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Imprint of the 11-year solar cycle in reanalyzed and radiosonde datasets: a spatial frequency analysis approach   总被引：1，自引：0，他引：1  

Petr Pi?oft  Eva Holtanová  Peter Huszár  Ji?í Mik?ovsky  Michal ?ák 《Climatic change》2012,110(1-2):85-99

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Optical observations of transient luminous events associated with winter thunderstorms near the coast of Israel   总被引：1，自引：0，他引：1  

Yoav Yair  Colin Price  Michal Ganot  Eran Greenberg  Roy Yaniv  Baruch Ziv  Yosef Sherez  Adam Devir  Jo'zsef Br  Gabriella Stori 《Atmospheric Research》2009,91(2-4):529-537

We report the results of two observation campaigns conducted during the Northern Hemisphere winters of 2005–6 and 2006–7 aiming to detect transient luminous events (TLEs) above winter thunderstorms in the vicinity of Israel and the eastern coastline of the Mediterranean Sea. In 10 out of 31 different observation nights we detected 66 events: 56 sprites and 10 Elves. The detection ranges varied from 250 to 450 km. Sprites were found to be produced by active cells with a vertical dimension of 5–9 km and cloud top temperature ~ − 40 °C, embedded in a much larger matrix of stratiform precipitating cloudiness. This configuration closely resembles the conditions for winter sprites in the Hokuriku region of Japan. Synchronized with the optical observations, ELF data were recorded at two observation stations in Israel and Hungary in order to qualify and quantify parameters of the parent lightning discharge associated with the TLEs. These stations are located 500 km and 2100 km respectively from the Eastern Mediterranean Sea, where most TLEs occur. Among the optically observed TLE events, we found that all the ELF signals were produced solely by positive cloud-to-ground flashes (+ CGs), most of which were recorded in Israel (88%) and Hungary (77%). Calculation of the Charge Moment Change showed average values of 1400 ± 600 C km, with some extreme events exceeding 3500 C km. The average time delay between the ELF transient of the parent + CG and the observed sprites was 55 ms, with shorter delays for column sprites (42 ± 34 ms) compared to carrot sprites (68 ± 34). Furthermore, based on the ELF data, there were no early identifiable precursors to TLE occurrence in the regional lightning activity. From the spatial formation of the observed columniform sprites, we propose that columniform sprites are sometimes arranged in a 3-dimensional circular pattern, thus mapping the instantaneous electric field in the mesosphere.  相似文献   

A search for fragmentation debris near Ursa Major Stream stars     

F.C. Witteborn  J.D. Bregman  D.F. Lester  D.M. Rank 《Icarus》1982,50(1):63-71

Accretion models of planet formation and the early cratering history of the solar system suggest that planet formation is accompanied by a cloud of debris resulting from accumulation and fragmentation. A rough estimate of the infrared luminosities of debris clouds is presented for comparison with measured 10-μm luminosities of young stars. New measurements of 13 F, G, and K main-sequence stars of the Ursa Major Stream, which is thought to be about 2.7 x 10⁸ years old, place constraints on the amount of debris which could be present near these stars.  相似文献   

Snow and climate trends and their impact on seasonal runoff and hydrological drought types in selected mountain catchments in Central Europe     

Andrea Blahušiaková  Milada Matoušková  Michal Jenicek  Ondřej Ledvinka  Zdeněk Kliment  Jana Podolinská 《水文科学杂志》2020,65(12):2083-2096

ABSTRACT

This study investigates changes in seasonal runoff and low flows related to changes in snow and climate variables in mountainous catchments in Central Europe. The period 1966–2012 was used to assess trends in climate and streamflow characteristics using a modified Mann–Kendall test. Droughts were classified into nine classes according to key snow and climate drivers. The results showed an increase in air temperature, decrease in snowfall fraction and snow depth, and changes in precipitation. This resulted in increased winter runoff and decreased late spring runoff due to earlier snowmelt, especially at elevations from 1000 to 1500 m a.s.l. Most of the hydrological droughts were connected to either low air temperatures and precipitation during winter or high winter air temperatures which caused below-average snow storages. Our findings show that, besides precipitation and air temperature, snow plays an important role in summer streamflow and drought occurrence in selected mountainous catchments.  相似文献   

Land-air boundary environment as recorded by the¹⁸O/¹⁶O and¹³C/¹²C isotope ratios in the shells of land snails     

Mordeckai Magaritz  Jossef Heller  Michal Volokita 《Earth and Planetary Science Letters》1981,52(1):101-106

A yearly cycle of carbon and oxygen isotope composition of shells of the Israeli land snailXeropicta vestalis is presented. The¹⁸O/¹⁶O values indicate that the snails use water from the land-air boundary zone. The¹⁸O/¹⁶O ratio of the shells is in isotopic equilibrium with the water condensate from the vapour during the winter months. During the summer months a contribution to the above water from soil water migrating upwards due to evaporation is noticeable. The δ¹³C values indicate that as in marine molluscs, the carbon isotopic composition in land snails is controlled mainly by the aqueous carbonate compound which is in equilibrium with the land-air boundary CO₂.  相似文献