All Results

Subjects: Geosciences >> Space Physics

小流域是研究小微尺度水文水资源系统演变规律的理想对象，是用于计算河流产水产沙 的最小单元，是水文及水土流失研究与管理的最佳地域尺度。通过遥感技术，气候模式获得降水 数据，并驱动分布式水文模型，模拟和预测水文过程，是流域水文水资源研究的必然趋势。以 NO? AA-CPC-US 降水作为参照，在美国不同地区的 9 个小流域，评估卫星降水产品 PERSIANN，PERSI? ANN-CDR，TRMM-3B42V7，GPM-IMERG，雷达降水 StageIV 以及气候模式 ERA5 降水产品的精度， 并用这 7 种降水产品驱动 CREST 分布式水文模型，评估了 7 种降水产品的水文模拟效用。研究表 明：各降水产品与 NOAA-CPC-US 降水吻合程度从高到低，依次是 StageIV 雷达降水，PERSIANN- CDR 和 GPM-IMERG 次之，再次是 PERSIANN 和 ERA5，最后是 TRMM-3B42V7。各降水产品在美 国北部高纬度地区和西部山地等区域的小流域降水估算精度略低，在美国中部，南部，东部的小流 域有较好的降水精度。在水文模拟效用评估中，设定相同率定期，分别使用 7 种降水产品率定 CREST 模型参数，得到率定参数集后，在相同验证期对流域日径流过程进行模拟。结果表明：NO? AA-CPC-US 和 Stage IV 雷达降水在各小流域水文模拟中效果较好，在美国北部和西部地区，使用 PERSIANN，PERSIANN- CDR，GPM- IMERG，ERA5 降 水 进 行 水 文 模 拟 时 需 要 谨 慎 。 TRMM- 3B42V7 的小流域水文模拟效果不理想。 |

submitted time
2021-01-06
From cooperative journals:《干旱区地理》
Hits*757*，
Downloads*500*，
Comment
*0*

Response of the temperature of cold-point-mesopause to solar activity based on SABER dataset

魏合理; 唐超礼Subjects: Geosciences >> Space Physics

The thermal structure and energy balance of upper atmosphere are dominated by solar activity. The response of Cold-Point-Mesopause(CPM) to solar activity is an important form. This article presents the response of Temperature-of-CPM(T-CPM) to solar activity using fourteen-year SABER data series over 80°S–80°N regions. These regions are divided into 16 latitude zones with 10° interval, and the spatial areas of 80°S—80°N,180°W—180°E are divided into 96 lattices with 10°(latitude)×60°(longitude) grid. The annual-mean values of T-CPM and F10.7 are calculated. The least squares regression method and correlation analysis are applied to these annual-mean series. First, the results show that the global T-CPM is significantly correlated to solar activity at the 0.05 level of significance with correlation coefficient of 0.90. The global solar response of T-CPM is 4.89±0.67K/100 Solar-Flux-Units(SFU). Then, for each latitude zone, the solar response of T-CPM and its fluctuation are obtained. The solar response of T-CPM becomes stronger with increasing latitude. The fluctuation ranges of solar response at middle latitude regions are smaller than those of equator and high latitude regions, and the global distribution takes on W-shape. The co-relationship analysis shows that the T-CPM is significantly correlated to solar activity at the 0.05 level of significance for each latitude zone. The correlation coefficients at middle latitude regions are higher than those of equator and high latitude regions, and the global distribution takes on M-shape. At last, for each grid cell, the response of T-CPM to solar activity and their correlation coefficient are presented. |

submitted time
2017-10-31
Hits*2974*，
Downloads*1573*，
Comment
*0*

Subjects: Geosciences >> Space Physics

Low temperature thermochronology plays a key role in the study of tectonic evolution of the upper crust. The general application of thermal history modelling of apatite fission-track analysis requires both the parameters of the apparent age together with the confined track-length distribution of the spontaneous tracks. However, obtaining length data is relatively easy and does not require either irradiation or LA-ICP-MS commonly used for determining the uranium content of the grains for age dating. This leads to a shorter laboratory process. For this purpose, based on apatite U-Th/He method, this paper attempts to decouple apatite fission-track age from apatite fission-track length, and then combine the lengths with the respective apatite U-Th/He age to model the thermal history. Therefore, experiments were designed and conducted using a new program "Low-T Thermo". Results of this modelling are presented from the following experiments: apatite fission-track age combined with apatite U-Th/He age; apatite fission-track confined track-length distribution plus apatite U-Th/He age. The modelling precision using this method is related to the relative errors of the apatite U-Th/He ages and the helium diffusion model. This combination of apatite fission-track length and apatite U-Th/He ages has not been implemented before but is presented here as an alternative way of determining thermal histories without the addition of apatite fission-track ages. |

submitted time
2017-06-13
Hits*18524*，
Downloads*1770*，
Comment
*0*

Radial variations of outward and inward Alfvnic fluctuations based on Ulysses observations

Yang, L.; Lee, L. C.; Li, J. P.; Luo, Q. Y.; Kuo, C. L.; Shi, J. K.; Wu, D. J.Subjects: Geosciences >> Space Physics

Ulysses magnetic and plasma data are used to study hourly-scale Alfv{\'e}nic fluctuations in the solar polar wind. The calculated energy ratio $R^2_{v_A}$(cal) of inward to outward Alfv{\'e}n waves is obtained from the observed Wal{\'e}n slope through an analytical expression, and the observed $R^2_{v_A}$(obs) is based on a direct decomposition of original Alfv{\'e}nic fluctuations into outward- and inward-propagating Alfv{\'e}n waves. The radial variation of $R^2_{v_A}$(cal) shows a monotonically increasing trend with heliocentric distance $r$, implying the increasing local generation or contribution of inward Alfv{\'e}n waves. The contribution is also shown by the radial increase in the occurrence of dominant inward fluctuations. We further pointed out a higher occurrence ($\sim 83\%$ of a day in average) of dominant outward Alfv{\'e}nic fluctuations in the solar wind than previously estimated. Since $R^2_{v_A}$(cal) is more accurate than $R^2_{v_A}$(obs) in the measurement of the energy ratio for dominant outward fluctuations, the values of $R^2_{v_A}$(cal) in our results are likely more realistic in the solar wind than previously estimated and than $R^2_{v_A}$(obs) in our results. The duration ratio $R_T$ of dominant inward to all Alfv{\'e}nic fluctuations increases monotonically with $r$, and is about two or more times that from {\it Voyager 2} observations at $r \geq 4$~AU. Finally, from the variation trend in our results, a higher (lower) occurrence rate is expected at $r < 1$~AU ($r > 4$~AU) for dominant outward Alfv{\'e}nic fluctuations, and opposite variations are expected for dominant inward fluctuations. Simultaneously, $R^2_{v_A}$(cal) and $R_T$ will be expected to be smaller at $r < 1$~AU and larger at $r > 4$~AU. These results reveal new qualitative and quantitative features of Alfv{\'e}nic fluctuations therein compared with previous studies and put constraints on modelling the variation of solar wind fluctuations. |

submitted time
2017-03-31
Hits*21007*，
Downloads*1744*，
Comment
*0*

Subjects: Geosciences >> Space Physics

海洋沉积物中天然气水合物的存在与分布与地温条件紧密相关。南海北部陆坡大部分区域更、全新世沉积速率较高，同时局部地区(如神狐天然气水合物钻探区)发育有大量的海底冲蚀谷。这些快速沉积或侵蚀作用可以对地温场及天然气水合物的形成与赋存产生重要影响，然而目前注意到这些问题的研究相对较少。本文中我们结合南海北部陆坡的实际情况，建立了针对地温场发生动态变化时的水合物成藏数值模型。我们给出两个成藏模拟案例，其一是快速沉积的情形，其二是地形遭受侵蚀、产生冲蚀谷时的情形。结果表明，在这两种情形下地温场将发生不同的瞬态调整，并对天然气水合物在地层中聚集及分布产状产生重要影响。在快速沉积发生时，地温梯度将变小，天然气水合物稳定带（HSZ）将增厚；然而当其底界将随着热传递而上升，天然气水合物将不断在原稳定带底界分解，而后在新的稳定带底界处重新生成水合物。这样长时间的快速沉积作用可以造成水合物主要富集在稳定带底界处。而当快速侵蚀作用发生时，地温梯度将变大，天然气水合物稳定带的厚度将变薄，原来位于冲蚀谷处的水合物藏将被侵蚀作用破坏。而后，随着热平衡的发展，逐渐形成新的、类似于似海底反射层的稳定带底界。单纯考虑甲烷扩散作用下，高丰度水合物的形成需要较长的时间，相对而言脊下的水合物成藏较好，形成矿点的概率较大。 The occurrence and distribution of gas hydrate in marine sediments is closely related to the state of geothermal field. The sedimentation rate is relatively high during the Pleistocene and Holocene in the northern margin of the South China Sea. Moreover, there are many erosional canyons at the sea bottom in some local areas. Such fast sedimentation and erosion can greatly affect the geothermal regime and the formation and occurrence of gas hydrate deposits. However, there are few studies that consider such scenarios. In this study, considering the geological reality, we build a numerical reservoir model aimed to simulate the evaluation of the gas hydrate system upon two cases: the fast sedimentation and the development of erosional canyons. The results indicate the geothermal filed and the accumulation and distribution of gas hydrate can be adjusted dynamically when the fast sedimentation or erosion occurs. Under fast sedimentation, the geothermal gradient becomes smaller and the thickness of gas hydrate stability zone (HSZ) becomes larger. However, due to the role of heat transfer, the bottom of HSZ rises upward. This process leads to the hydrate near the previous bottom of HSZ being dissociated and depositing near the bottom of new HSZ. After a long duration, the fast sedimentation will result the formation of hydrate being mainly enriched near the bottom of HSZ. By contrast, when fast erosion occurs, the geothermal gradient increases and the HSZ turn thinner. The gas hydrate deposit system can be partially destroyed by the formation of canyons. A long time is necessary for the recovery of a gas hydrate deposit with significant abundance. Relatively, the deposits under the residual ridges are more possible to be potential “sweet spots”. |

submitted time
2017-03-23
Hits*40667*，
Downloads*3589*，
Comment
*0*

Deep space communication channel characteristics under solar scintillation

Wu, Tong; Yan, Yi; Li, Yongcheng; Wang, Chunmei; Yao, Xiujuan; Li, XueSubjects: Geosciences >> Space Physics

Electromagnetic waves used for deep space communications are mainly affected by the charged particles ejected by the sun. These effects may result in degradation of communication quality or communication interruption. This paper discusses the effects of solar scintillation on electro-magnetic waves, including the scintillation index which is a measure of the intensity scintillation, the coherence bandwidth and the coherence time of deep space communication channel. The deep space communication channel under solar scintillation is modeled by using Rician fading channel according to the scintillation index. The coherence bandwidth will determine whether the channel is flat fading or frequency selective fading and the coherence time will determine whether the channel is slow fading or fast fading. The approach of choosing signal band width is determined by the coherence bandwidth and the coherence time with the change of the solar elongation angle. The simulation results show the bit error rate of the signal bandwidth chosen by the proposed approach is lower than a random choice. |

submitted time
2017-03-10
Hits*23363*，
Downloads*2226*，
Comment
*0*

A broadband KU-band microstrip reflectarray antenna using single-layer fractal elements

Xue, Fei; Wang, Hong-Jian; Yi, Min; Liu, GuangSubjects: Geosciences >> Space Physics

A novel single-layer microstrip reflectarray element with fractal structure is proposed. Ansoft HFSS is used to analyze the reflect phase for the fractal element in honeycomb lattice. A 469-element prime focus microstrip reflectarray antenna composed of the proposed fractal elements is designed, manufactured, and measured. The measured gain level of 29.8 dB is obtained at the center frequency of 13.58 GHz with 1-dB gain bandwidth of 15.3%. |

submitted time
2017-03-10
Hits*16907*，
Downloads*1938*，
Comment
*0*

Subjects: Geosciences >> Space Physics

Dual-frequency polarized scatterometer (DFPSCAT) is a pencil-beam rotating scatterometer which is designed for snow water equivalent (SWE) measurement, and Doppler beam sharpening (DBS) technique is proposed for DFPSCAT to achieve the azimuth resolution. However, the DBS technique is inapplicable for the forward-looking and afterward-looking regions. Based on an approximate aperiodic model of scatterometer echo signal, an improved adaptive regularization deconvolution algorithm with gradient histogram preservation (GHP) constraint is implemented to settle the problem. To investigate its performance of resolution enhancement and resulted accuracy, both a synthetic backscattering coefficient (σ0field reconstruction and SWE σ0reconstruction are carried out. The results show that the proposed method can recover the truth signal and achieve azimuth resolution of 2 km with the designed scatterometer system, which is required by the SWE retrieval. Moreover, the relative errors of reconstructed σ0are less than 0.5 dB that satisfy the accuracy requirement for SWE retrieval, and comparisons with observed results show that the error reduction is more than 0.03 dB. Meanwhile, a comparison between the proposed algorithm and some existing resolution enhancement methods is analyzed, which concludes that the proposed method can obtain a comparable resolution enhancement as L1method and has less noise. The technique is also verified with advanced scatterometer (ASCAT) scatterometer data. |

submitted time
2017-03-10
Hits*16416*，
Downloads*1897*，
Comment
*0*

Subjects: Geosciences >> Space Physics

Target brightness temperature maps can be used in geostationary interferometric microwave sounder (GIMS) system simulation. System simulation with accurate target brightness temperature maps can evaluate system performance in near real case and thus help adjust design parameters for the sensor before it is finally put into use. In this paper, method of simulating target brightness temperature using Weather Research and Forecasting Model and Radiative Transfer for TOVS (RTTOV) has been discussed. Target brightness temperature simulation results at oxygen absorption band and water-vapor absorption band have been presented. Some preliminary analyses of simulated brightness temperature for GIMS' observation have also been given. |

submitted time
2017-03-10
Hits*18125*，
Downloads*1921*，
Comment
*0*

Subjects: Geosciences >> Space Physics

With a standard deviation as large as 2 cm, the sea state bias (SSB) has become the dominant source of error in satellite altimetry. The operational SSB correction models are two-dimensional (2-D) empirical (parametric or nonparametric) models based on the altimeter-measured wind speed (U) and significant wave height (SWH). However, these 2-D SSB models cannot entirely parameterize the range bias variability. The SSB uncertainty may be lowered through improved SSB models including additional measurable or predictable correlatives. This paper presents a method to estimate the SSB from crossover differences by using a three-dimensional (3-D) nonparametric model. The model is based on U, SWH from Jason-2 altimeter ocean observations, and the mean wave period from the European Centre for Medium-Range Weather Forecasts reanalysis project ERA-Interim (The SSB model developed with the method presented in this paper is called “3-D SSB model” and the SSB estimated with the 3-D SSB model is called “3-D SSB estimate”). Simulations indicate that the wave period can greatly affect the SSB. Evaluated by the separate annual datasets from 2009 to 2011, the 3-D SSB estimates can increase the explained variance by 1.32 cm2, or 1.15-cm RMS relative to the traditional 2-D SSB estimates based on U and SWH. Spatial evaluation of improvement shows that the 3-D SSB estimates are better than the traditional 2-D SSB estimates at all latitudes. The enhancement from 2-D to 3-D SSB estimates is of great significance to improve the precision of the altimeter product.[COMP]: Please set math TYPE gin the sentence below (40) as per the authors PDF. |

submitted time
2017-03-10
Hits*17512*，
Downloads*1975*，
Comment
*0*