Analysis of a cryolava flow-like feature on Titan

This paper reports on the analysis of the highest spatial resolution hyperspectral images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft during its prime mission. A bright area matches a flow-like feature coming out of a caldera-like feature observed in Synthetic Aperture Radar (SAR) data recorded by the Cassini radar experiment [Lopes et al., 2007. Cryovolcanic features on Titan's surface as revealed by the Cassini Titan Radar Mapper. Icarus 186, 395-412, doi:10.1016/j.icarus.2006.09.006]. In this SAR image, the flow extends about 160 km east of the caldera. The contrast in brightness between the flow and the surroundings progressively vanishes, suggesting alteration or evolution of the composition of the cryolava during the lifetime of the eruptions. Dunes seem to cover part of this flow on its eastern end. We analyze the different terrains using the Spectral Mixing Analysis (SMA) approach of the Multiple-Endmember Linear Unmixing Model (MELSUM, Combe et al., 2008). The study area can be fully modeled by using only two types of terrains. Then, the VIMS spectra are compared with laboratory spectra of known materials in the relevant atmospheric windows (from 1 to 2.78 μm). We considered simple molecules that could be produced during cryovolcanic events, including H₂O, CO₂ (using two different grain sizes), CH₄ and NH₃. We find that the mean spectrum of the cryoflow-like feature is not consistent with pure water ice. It can be best fitted by linear combinations of spectra of the candidate materials, showing that its composition is compatible with a mixture of H₂O, CH₄ and CO_2.     

都安一次特大暴雨过程卫星和雷达图像特征分析

利用彩色增强显示的卫星云图、雷达和自动气象站资料对2010年5月31日22时至6月1日11时都安县出现的特大暴雨进行分析,发现这次暴雨过程由2个强降雨阶段组成,造成都安县特大暴雨的原因是1个形成于滇黔桂交界处的低涡在南移过程中,低涡东边缘对流强烈发展,对流云团主体移过都安而产生了强降雨,对应着这2个强降雨阶段,在卫星云图上分析出是由2个对流云团前后移过所造成的,在雷达回波图上可分析出对流云团中又先后有多个对流单体移过都安县上空,所以造成了这次特大暴雨过程     

超级单体引发的龙卷天气过程分析

利用营口市多普勒天气雷达资料,对2005年8月10日16时10-20分左右营口市东南部六个乡出现的龙卷天气过程进行了简要分析,该龙卷发生前的主要天气形势是:一个东移的东北低涡引导高空槽,沿高空等高线冷干气流与低空的暖湿气流产生对流不稳定层结,超低空南支急流与低空西南风急流以及高空西北风产生的较大垂直风切变,有利于龙卷天气的产生.产生该龙卷的对流系统是由渤海湾生成的片状层状云和积状云混合降水回波.自东向偏北方向移动,15:50以后低层反射率因子的强降水回波移入大连北部与营口南部临近区域,在层状云降水中含有一些零散的和有组织的对流降水回波,主体为一个近似团状的对流系统,而龙卷产生自该系统南端的一个超级单体.最初的中气旋形成于8月10日15:56,相应对流单体的反射率因子还没有呈现出超级单体的特征,随后中气旋迅速发展加强,在16:02-16:08反射率因子形态呈现出经典超级单体的特征:明显的低层入流缺口,入流缺口位于超级单体移动方向(偏东南方向)的右侧,低层的弱回波区和中高层的回波悬垂结构,最大反射率因子超过56 dRz.在龙卷产生前几分钟和龙卷进行过程中,中气旋保持较强,而后迅速减弱,低层入流缺口渐渐消失.在龙卷进行过程中,相应45 dBz超级单体的反射率因子区局限在6 km以下,此系统为低质心的对流系统,产生的天气是龙卷,伴随有大风短时强降水,与冰雹的高质心对流系统有明显区别.同时也初步探讨了引发此次龙卷的生成机制.     

Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a three-dimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h.     

Radar imaging of Asteroid 7 Iris

Arecibo radar images of Iris obtained in November 2006 reveal a topographically complex object whose gross shape is approximately ellipsoidal with equatorial dimensions within 15% of 253 × 228 km. The radar view of Iris was restricted to high southern latitudes, precluding reliable estimation of Iris’ entire 3D shape, but permitting accurate reconstruction of southern hemisphere topography. The most prominent features, three roughly 50-km-diameter concavities almost equally spaced in longitude around the south pole, are probably impact craters. In terms of shape regularity and fractional relief, Iris represents a plausible transition between ∼50-km-diameter asteroids with extremely irregular overall shapes and very large concavities, and very much larger asteroids (Ceres and Vesta) with very regular, nearly convex shapes and generally lacking monumental concavities.     

Constraints on Titan’s topography through fractal analysis of shorelines

Titan’s north polar hydrocarbon lakes offer a unique opportunity to indirectly characterize the statistical properties of Titan’s landscape. The complexity of a shoreline can be related to the complexity of the landscape it is embedded in through fractal theory. We mapped the shorelines of 290 of the north polar titanian lakes in the Cassini synthetic aperture radar dataset. Out of these, we used a subset of 190 lake shorelines for our analysis. The fractal dimensions of the shorelines were calculated via two methods: the divider/ruler method and the box-counting method, at length scales of (1-10) km and found to average 1.27 and 1.32, respectively. The inferred power-spectral exponent of Titan’s topography (β) from theoretical and empirical relations is found to be ?2, which is lower than the values obtained from the global topography of the Earth or Venus. Some of the shorelines exhibit multi-fractal behavior (different fractal dimensions at different scales), which we interpret to signify a transition from one set of dominant surface processes to another. We did not observe any spatial variation in the fractal dimension with latitude; however we do report significant spatial variation of the fractal dimension with longitude. A systematic difference between the dimensions of orthogonal sections of lake shorelines is also noted, which signifies possible anisotropy in Titan’s topography. The topographic information thus gleaned can be used to constrain landscape evolution modeling to infer the dominant surface processes that sculpt the landscape of Titan.     

新一代天气雷达故障诊断技术简介

简单介绍了新一代天气雷达故障诊断技术的必要性、基本概念、实现方法及故障诊断软件的原理。     

地质雷达在乌拉山山前断裂探测中的应用

地质雷达是利用电磁波对地下不同电性介质进行探测的地球物理仪器，其探测速率快、分辨率高，可弥补探槽和其他地球物理方法存在探测盲区的缺陷，正在越来越多地应用于活动断层探测领域。本文以乌拉山山前断裂为例开展地质雷达探测工作，使用无人机正射影像技术对测线进行地形校正，获得断层浅部地质雷达图像。研究结果表明，本文研究方法能有效反映探槽揭露的地层单元和断层分布。本次探测中，雷达波形图像特征为:浅地表的土壤层反射波总体较弱；粗粒沉积为主的砾石层反射波总体较强，同相轴连续性好；细粒沉积为主的砂层反射波弱于砾石层，波形以中、高频为主，同相轴具有弱连续性；对于洪冲积地区，地质雷达能分辨具有一定特征的地层单元，这为剖面图像的断层识别提供了标志；通过无人机正射影像技术对地质雷达测线进行地形校正，有利于获得更为准确的探测结果。     

Areal extent and vertical structure of radar weather echoes at Montreal

A large sample of radar data from the period May–August 1976 was analyzed to provide information on the total areal coverage of precipitation echoes at 1-km steps in altitude ranging from 2 km up to 10 km. A transformation based on known statistics of surface rainfall rate was employed to express this information in terms of the probability of certain reflectivities being exceeded at a randomly chosen point at a given altitude. The same data base was used to construct a family of average vertical profiles of precipitation reflectivity. The profiles were conditioned by the reflectivity value at the lowest level (2 km), thus providing a basis for estimating the reflectivity aloft, given the low level or surface rainfall rate. Analyses of this kind have been reported for other geographical locations and for a variety of applications, such as hydrology, rain effects on earth-space radio propagation, and precipitation modeling. Similarities in the results from these two quite different analysis procedures, possibly not hitherto recognized, were found in the present study.     

On the application of SAR interferometry to geomorphological studies: estimation of landform attributes and mass movements

This paper presents two examples of application of Synthetic Aperture Radar (SAR) interferometry (InSAR) to typical geomorphological problems. The principles of InSAR are introduced, taking care to clarify the limits and the potential of this technique for geomorphological studies. The application of InSAR to the quantification of landform attributes such as the slope and to the estimation of landform variations is investigated. Two case studies are presented. A first case study focuses on the problem of measuring landform attributes by interferometric SAR data. The interferometric result is compared with the corresponding one obtained by a Digital Elevation Model (DEM). In the second case study, the use of InSAR for the estimation of landform variations caused by a landslide is detailed.