High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T(sub eff) = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models.

关键词：白矮星；X射线光学；紫外光谱；X射线天体物理设施；磁悬浮;

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The motivation for this project was to advance the science of climate change and prediction in the Arctic region. Its primary goals were to (i) develop a state-of-the-art Regional Arctic Climate system Model (RACM) including high-resolution atmosphere, land, ocean, sea ice and land hydrology components and (ii) to perform extended numerical experiments using high performance computers to minimize uncertainties and fundamentally improve current predictions of climate change in the northern polar regions. These goals were realized first through evaluation studies of climate system components via one-way coupling experiments. Simulations were then used to examine the effects of advancements in climate component systems on their representation of main physics, time-mean fields and to understand variability signals at scales over many years. As such this research directly addressed some of the major science objectives of the BER Climate Change Research Division (CCRD) regarding the advancement of long-term climate prediction.

关键词：气候变化；北极地区；预测；大气环流

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关键词：主成分分析法；大气；资料同化

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Local, one-dimensional (1-D) simulations of the upper-ocean structure observed at the Hawaii Ocean Time Series (HOTS) mooring located 100 km north of Oahu are compared with output from the operational Global NCOM ocean forecast system at the location of the mooring. The observations from the mooring indicate that the upper-ocean density structure and mixed-layer depth (MLD) in this area are significantly affected by the extensive eddy field that exists there. Local, 1-D simulations do not account for the modulation of the upper-ocean density structure by the eddies. However, Global NCOM, along with the data assimilation that it uses, provides a reasonably accurate simulation of the eddies and does a fairly good job of accounting for the effect of the eddies on the upper-ocean density structure and MLD. Comparison of atmospheric fields from NOGAPS, which are used by Global NCOM, with observations from the mooring show fairly good agreement, which contributes to the skill shown by Global NCOM in simulating the sea-surface temperature and MLD observed at the mooring.

关键词：海洋模式;空气与水的相互作用;大气;表面混合层

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The present study considers direct ballistic entries into the atmosphere of Venus using a 45deg sphere-cone rigid aeroshell, a legacy shape that has been used successfully in the past in the Pioneer Venus Multiprobe Mission. For a number of entry mass and heatshield diameter combinations (i.e., various ballistic coefficients) and entry velocities, the trajectory space in terms of entry flight path angles between skip out and -30deg is explored with a 3DoF trajectory code, TRAJ. From these trajectories, the viable entry flight path angle space is determined through the use of mechanical and thermal performance limits on the thermal protection material and science payload; the thermal protection material of choice is entry-grade carbon phenolic, for which a material thermal response model is available. For mechanical performance, a 200 g limit is placed on the peak deceleration load experienced by the science instruments, and 10 bar is assumed as the pressure limit for entry-grade carbon-phenolic material. For thermal performance, inflection points in the total heat load distribution are used as cut off criteria. Analysis of the results shows the existence of a range of critical ballistic coefficients beyond which the steepest possible entries are determined by the pressure limit of the material rather than the deceleration load limit.

关键词：大气层;金星大气; Aeroshells;隔热;先锋金星飞船;热保护

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Individual datastreams from instrumentation at the Atmospheric Radiation Measurement (ARM) Climate Research Facility fixed and mobile research sites are collected and routed to the Data Management Facility (DMF) for processing in near-real-time. Instrument and processed data are then delivered approximately daily to the ARM Data Archive, where they are made freely available to the research community. For each instrument, we calculate the ratio of the actual number of processed data records received daily at the Data Archive to the expected number of data records. The results are tabulated by (1) individual datastream, site, and month for the current year and (2) site and fiscal year dating back to 1998.

关键词：大气辐射；辐射测量；气候；数据库管理；数据采集

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A new approach approach to tunable mid-infrared lasers, an optically pumped, type-II, InGaSb/InAs gain medium with a chirped distributed feedback grating, has been developed. The chirped grating is patterned using an interferometric lithography (IL) technique with spherical wave fronts and etched into the top cladding of the laser slab waveguide structure. A reduced longitudinal chirp grating fabrication technique has been developed that dramatically extends the single frequency tuning range. Continuous tuning of 80nm around 3.1 um with 320 mW single facet output power at 80K and a 1.6nm FWHM is reported. The present device is designed in the 3-to-4 um range which matches a low loss atmospheric transmission window, and covers an important region of molecular vibration spectra, in particular, the hydrocarbon C-H stretch at 3.3 um, making it suitable for atmospheric pressure remote gas sensing of industrially important small molecules such as methane, hydrogen chloride and ammonia.

关键词：红外激光器;指纹;光刻;光抽运;波导

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关键词：电;计算机模型;错误;预测;测量;大气

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The resolutions of interests in atmospheric simulations require prohibitively large computational resources. Adaptive mesh refinement (AMR) tries to mitigate this problem by putting high resolution in crucial areas of the domain. We investigate the performance of a tree-based AMR algorithm for the high order discontinuous Galerkin method on quadrilateral grids with non- conforming elements. We perform a detailed analysis of the cost of AMR by comparing this to uniform reference simulations of two standard atmospheric test cases: density current and rising thermal bubble. The analysis shows up to 15 times speed-up of the AMR simulations with the cost of mesh adaptation below 1of the total runtime. We pay particular attention to the implicit-explicit (IMEX) time integration methods and show that the ARK2 method is more robust with respect to dynamically adapting meshes than BDF2. Preliminary analysis of preconditioning reveals that it can be an important factor in the AMR overhead. The compiler optimizations provide signi cant runtime reduction and positively a ect the e ectiveness of AMR allowing for speed-ups greater than it would follow from the simple performance model.

关键词：大气模式;可压缩流动; Euler方程;伽辽金法;算法;

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For conventional imaging systems, GEO space objects cannot be resolved due to their 40 Mm distance. There exists a strong need to obtain high resolution images of GEO objects and to accomplish this task, investigation into the suitability of ISAL is currently underway. A critical component in determining this suitability is to accurately model the atmospheric impacts on LADAR pulses. Conventional knowledge says that while the atmosphere churns, wind is the predominant cause of temporal evolution which simplifies all modeling and simulation into the frozen flow hypothesis. The concern is that the frozen flow hypothesis based phase screen generation techniques fail to accurately predict the temporal development of optical phase. This thesis proposes a new approach and provides a detailed derivation of a new temporally evolving Zernike polynomial based atmospheric phase screen generation model. This new model is experimentally verified, and utilized to analyze atmospheric impacts on mixing efficiencies. It is shown that this new turbulent flow model more accurately predicts mixing efficiency than that of the basic frozen flow approximation.

关键词：光雷达;地球轨道;效率;高分辨率;激光束;大气

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