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991.
本文利用卫星遥感资料以及常规气象资料驱动基于地表净辐射、植被指数、平均气温和日温差的蒸散模型来估测日实际蒸散量,并与栾城站涡动相关法测量的实际蒸散作对比验证。定性分析了实际蒸散与各相关影响因子的时空变化规律;通过蒸散干旱指数(EDI)分析华北地区的干旱分布特点,并分别与PDSI指数和降水距平百分率作对比。结果表明:ET模型估测的蒸散值与实测值的相关性很好,其模拟精度对于大面积干旱监测的空间尺度上是可用的;EDI距平指数表征干旱分布的空间分辨率较高,且对旱情的指示和干旱程度的判定比较可靠。 相似文献
992.
目的:比较16层螺旋CT的4种不同胸部的扫描技术参数,探讨胸部检查最适合的扫描参数。资料与方法:回顾性分析2003年6月至2011年4月间CT检查采用此4种扫描参数的患者100例,肺部低剂量检查和常规剂量检查均分别采用1.5mm×16(24mm)及0.75mm×16(12mm)宽探测器进行扫描;有效mAs分别为20mAs及100mAs;管电压、扫描周期及重建层厚等均相同,为120kV、0.5s、5mm。再分别重叠50%重组2mm及1mm的最薄层图像用于三维后处理用,对照分析其影像及后处理重建图像的质量。结果:4种扫描检查方法所得的肺部影像正常结构及所显示的病灶均无明显差异,图像质量优良;采用较宽探测器和低剂量扫描方案的扫描时间及X线剂量均明显减少,所产生的X线辐射也大幅减少。结论:胸部CT扫描采用宽(1.5mm)探测器和低剂量扫描方案进行时,其辐射剂量(CTDIvol)只为窄(0.75mm)探测器和常规剂量扫描方案的20%,但能提供相近的诊断信息。 相似文献
993.
《Geomechanics and Geoengineering》2013,8(2):65-75
A simplified method to analyse diffuse and localized bifurcations of sand under drained and undrained conditions is presented in this paper. This method utilizes results from bifurcation analysis and critical state plasticity theory to detect the onset of pure and dilatant shear band formation, static liquefaction and drained shear failures systematically. To capture the soil collapse observed in experiments, the instability state line concept originated by Chu, Lo and Lee in 1993 is adopted. Emphasis is given to examine how the presence of pore-fluid may facilitate or delay instability after yielding occurs. The predictions of instabilities are compared with experimental data from triaxial compression tests on Toyoura and Changi sands. 相似文献
994.
995.
Based on idealized numerical simulations, the impacts of the diurnal cycle of solar radiation on the diurnal variation of outer rainbands in a tropical cyclone are examined. It is found that cold pools associated with precipitation-driven downdrafts are essential for the growth and propagation of spiral rainbands. The downdrafts result in surface outflows, which act as a lifting mechanism to trigger the convection cell along the leading edge of the cold pools. The diurnal cycle of solar radiation may modulate the diurnal behavior of the spiral rainbands. In the daytime, shortwave radiation will suppress the outer convection and thus weaken the cold pools. Meanwhile, the limited cold pool activity leads to a strong modification of the moisture field, which in turn inhibits further convection development. 相似文献
996.
广州市雷暴日异常的基本气候特征及其与近海海温关系初探 总被引:2,自引:3,他引:2
以低纬高原城市昆明为研究对象,利用城市区域内4月、8月、12月的太阳辐射观测资料,对城市区域内不同下垫面性质和位置差异的辐射平衡特征和变化规律进行了研究。结果表明:昆明地区4月的太阳辐射各分量均较高,而多云天气的8月太阳辐射分量较小;屋顶的总辐射量和净辐射量均维持较高的数值,而受周围环境、建筑物遮蔽等影响的草地和球场则数值变化较大;随时间和下垫面的不同,反射辐射呈现较大差异;有效辐射则在多云天的8月呈现较小值(分配率趋于0)。所得结果可为探讨城市气候形成机制及城市气候的深入研究提供依据,并为其它地区的研究提供参考。 相似文献
997.
Temperature gradients in dust beds embedded in a low pressure gaseous environment induce a lift of particles under certain conditions. This effect can erode planetesimals and enables entrainment of dust into the martian atmosphere. Here, we consider a numerical model to calculate the temperature profile in a dust bed which is subject to illumination. We consider the situation when the illumination is switched on and heats the dust bed’s surface and when it is switched off again after a certain time. The calculations focus on the heat transfer by infrared radiation within the dust layer. We find that radiative transfer within the dust bed modifies the absolute temperatures and temperature gradients significantly. This is important for effects which are sensitive to absolute temperatures, i.e. ice sublimation or melting of solids. For low thermal conductivity dust beds of 0.001 W m−1 K−1 it determines the temperature structure of the dust. For higher thermal conductivities the modifications are moderate with respect to dust eruptions as the order of magnitude of temperature gradients stays the same. 相似文献
998.
Xiao WuZhi-Min Liu Da-Hai YanKun-Sen Bao Ben-Zhong Dai 《Chinese Astronomy and Astrophysics》2011,35(1):10
From the original observed data of RXTE (Rossi X-ray timing explorer), the data of 3C 273 in the X-ray band of 2-10 keV from February 2, 1996 (MJD 50115) to August 27, 2007 (MJD 54339) are analyzed. The photon spectral indexes and corresponding fluxes of the observational data from 1010 observations are obtained in total. The average time of each observation is 1666.76 seconds. By analyzing the spectral variation and behavior of the light variation of 3C 273 in the hard X-ray band of 2-10 keV during observational periods, it is found that there was a significant anti-correlation between the photon spectral index Γ and flux lgF2∼10 keV in February 2000, March and April 2003, February, March and July 2004, as well as 2006 and 2007. During the observational periods in 1999 and from 2000 to 2007, there was also a significant anti-correlation between the monthly average behaviors of Γ and lgF2∼10 keV. From the quantitative analysis of the behavior of light variation in the entire observational period, several relatively large light variations are found. The evidence of the existence of the Fe emission line with average width of 93.85±21.49 eV is also found by fitting a part of the spectra. Through analyzing the intensity of the light variation and features of Fe emission line, it is found that 3C 273 has some characteristics of blazars and Seyfert galaxies simultaneously. 相似文献
999.
某些伽玛射线暴(简称伽玛暴)的中心致密天体可能是一颗具有强磁场的毫秒脉冲星,它通过磁偶极辐射可对伽玛暴外激波注入能量,从而导致早期余辉光变曲线的变平.近年来,从Swift卫星观测到的大量伽玛暴X射线余辉中发现,很多X射线余辉光变曲线在暴后10~2~10~4s期间的确存在明显的变平现象.利用周期为毫秒量级的磁星能量注入模型对11个加玛暴的X射线余辉光变曲线进行了拟合,显示该模型在解释余辉变平现象上的有效性和广泛性,通过对余辉光变曲线的拟合,同时也给出了相关中心磁星的磁场强度和旋转周期. 相似文献
1000.
We demonstrate, using a sample of high quality spectra, that the average equivalent width ratio of the unsaturated R(0) and R(1) lines of the CN B2Σ+ – X2Σ+ (0, 0) band near 3875 Å equals π instead of 3.73 – calculated for the CMBR temperature T = 2.725 K and the already published oscillator strengths of both transitions, i.e. the excitation temperature of CN is slightly higher than it should be expected if CMBR is the only excitation mechanism. Taking into account the saturation effects by means of profile fitting during the column density calculations does not remove the revealed temperature excess. Possible interpretations of the effect are discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献