大气能量有效性的研究进展

大气能量有效性一直是大气科学研究中的重要组成部分，在过去的几十年中，得到了长足发展。对大气能量学特别是能量有效性问题的研究进展进行了全面回顾。为了将能量有效性的研究从全球大气整体向局地或区域尺度拓展，针对经典有效位能理论中存在的问题提出了扰动位能的新概念，并初步探讨了其理论和应用问题。在已开展的工作中，利用NCEP/NCAR再分析资料研究了扰动位能的时空结构以及与大气动能之间的联系，取得了很有意义的结果。     

Monitoring the three-dimensional ionospheric electron density distribution using GPS observations over China

In this paper, an IRI model assisted GPS-based Computerized Ionospheric Tomography (CIT) technique is developed to inverse the ionospheric electron density (IED) distribution over China. Essentially, an improved algebraic reconstruction technique (IART) is first proposed to reconstruct the ionospheric images with high resolution and high efficiency. A numerical experiment is used to validate the reliability of the method and its advantages to the classical algebraic reconstruction technique (ART). This is then used to reconstruct the IED images using the GPS data in China. The variations of the IED during magnetically quiet and disturbed days are reported and analyzed here. Reconstructed results during magnetically quiet days show some prominent ionospheric features such as the development of equatorial anomaly and the tilt of ionization crest. Meanwhile, ionospheric storm phase effects and disturbed features can also be revealed from the reconstructed IED image under storm conditions. Research shows that the positive storm phase effects usually happen in southern China, and the negative storm phase effects mainly occur in northern China. The equatorial anomaly crest moved to the north in the main phase of the storm. Ionosonde data recorded at Wuhan station provides the verification for the reliability of GPS-based CIT technique.     

重复轨道雷达干涉测量中的大气影响及其研究展望

在研究大气延迟物理机制的基础上，分析了大气效应对雷达干涉测量产生的影响，并对重复轨道雷达干涉测量中大气延迟的分析方式进行了归纳与探讨； 回顾了国内外关于InSAR大气影响的研究方法，着重对现有的大气误差改正方法进行了分类和比较，并对存在的问题进行了分析，对今后的研究方向提出了建议。     

Medium-scale traveling ionospheric disturbances observed by GPS receiver network in Japan: a short review

Medium-scale traveling ionospheric disturbances (MSTID) are wave-like perturbations of the ionospheric plasma with wavelengths of several hundred kilometres and velocities of several hundred metres per second. MSTID is one of the most common ionospheric phenomena that generally induce the perturbations of ionospheric total electron content (TEC) by ∼10¹⁶ electron/m², which corresponds to ∼54 ns (16.2 cm) delay at GPS L1 signal. In the past decade, several new characteristics on MSTIDs have been revealed by the TEC observations using the dense GPS receiver network in Japan. In this paper, we provide a short review of these new observations and summarize the morphological characteristics of MSTIDs in Japan.     

东经120°E中间层和低热层大气潮汐及其季节变化特征

利用为期一年的卫星遥感温度(SABER/TIMED)资料重建了120°E子午圈内中间层和低热层大气潮汐各主要频率分量(周日、半日和8小时潮汐).这些主要频率分量随高度振幅增大,在97km高度达到显著的振幅;其中迁移性周日潮汐在97km高度出现极大振幅,然后随高度衰减.本文从考察迁移性成分和非迁移性成分各自在总潮汐中贡献角度出发,着重讨论了那些对形成该子午圈中97km高度上整体潮汐扰动起控制作用的潮汐成分.结果显示,对周日和半日频率这两种潮汐而言,迁移性成分控制了它们的总体时空分布.在春分季节,迁移性周日潮的控制作用最显著,决定了赤道和两半球热带的活动中心;其中北半球副热带地区的季节变化形势与以往利用武汉(30°N,114°E)流星雷达风测量资料开展分析得到的结果是一致的;其他季节受非迁移性成分明显影响,例如,在本文关注的2005年中,夏至季节受(1,0)模、(1,-3)模和(1,-2)模的共同影响形成了从赤道向南延伸的活动中心,极值中心位于赤道附近,振幅达到了20K以上,是全年的最大值.受迁移性成分控制,半日潮活动主要出现在两半球热带地区,北半球活动中心位于秋分季节(振幅达到13K),南半球活动中心位于春分和夏至之间.其他季节受非迁移成分的影响,形成若干分布在两半球的活动中心.在本文关注的40°S~40°N范围内,与周日潮和半日潮相比,8小时潮汐具有显著较低的振幅;另外,虽然迁移性成分在一年中的大部分时间系统地分布在两半球热带地区,但是非迁移成分具有与迁移性成分相当或更大的振幅,在整体上控制了这种潮汐的时空分布.     

A Godunov-Type Scheme for Atmospheric Flows on Unstructured Grids: Euler and Navier-Stokes Equations

In recent years there has been a growing interest in using Godunov-type methods for atmospheric flow problems. Godunov's unique approach to numerical modeling of fluid flow is characterized by introducing physical reasoning in the development of the numerical scheme (van Leer, 1999). The construction of the scheme itself is based upon the physical phenomenon described by the equation sets. These finite volume discretizations are conservative and have the ability to resolve regions of steep gradients accurately, thus avoiding dispersion errors in the solution. Positivity of scalars (an important factor when considering the transport of microphysical quantities) is also guaranteed by applying the total variation diminishing condition appropriately. This paper describes the implementation of a Godunov-type finite volume scheme based on unstructured adaptive grids for simulating flows on the meso-, micro- and urban-scales. The Harten-Lax-van Leer-Contact (HLLC) approximate Riemann solver used to calculate the Godunov fluxes is described in detail. The higher-order spatial accuracy is achieved via gradient reconstruction techniques after van Leer and the total variation diminishing condition is enforced with the aid of slope-limiters. A multi-stage explicit Runge-Kutta time marching scheme is used for maintaining higher-order accuracy in time. The scheme is conservative and exhibits minimal numerical dispersion and diffusion. The subgrid scale diffusion in the model is parameterized via the Smagorinsky-Lilly turbulence closure. The scheme uses a non-staggered mesh arrangement of variables (all quantities are cell-centered) and requires no explicit filtering for stability. A comparison with exact solutions shows that the scheme can resolve the different types of wave structures admitted by the atmospheric flow equation set. A qualitative evaluation for an idealized test case of convection in a neutral atmosphere is also presented. The scheme was able to simulate the onset of Kelvin-Helmholtz type instability and shows promise in simulating atmospheric flows characterized by sharp gradients without using explicit filtering for numerical stability.     

Sounding-derived indices for neural network based short-term thunderstorm and rainfall forecasts

A neural network-based scheme to do a multivariate analysis for forecasting the occurrence and intensity of a meteo event is presented. Many sounding-derived indices are combined together to build a short-term forecast of thunderstorm and rainfall events, in the plain of the Friuli Venezia Giulia region (hereafter FVG, NE Italy).For thunderstorm forecasting, sounding, lightning strikes and mesonet station data (rain and wind) from April to November of the years 1995–2002 have been used to train and validate the artificial neural network (hereafter ANN), while the 2003 and 2004 data have been used as an independent test sample. Two kind of ANNs have been developed: the first is a “classification model” ANN and is built for forecasting the thunderstorm occurrence. If this first ANN predicts convective activity, then a second ANN, built as a “regression model”, is used for forecasting the thunderstorm intensity, as defined in a previous article.The classification performances are evaluated with the ROC diagram and some indices derived from the Table of Contingency (like KSS, FAR, Odds Ratio). The regression performances are evaluated using the Mean Square Error and the linear cross correlation coefficient R.A similar approach is applied to the problem of 6 h rainfall forecast in the Friuli Venezia Giulia plain, but in this second case the data cover the period from 1992 to 2004. Also the forecasts of binary events (defined as the occurrence of 5, 20 or 40 mm of maximum rain), made by classification and regression ANN, were compared. Particular emphasis is given to the sounding-derived indices which are chosen in the first places by the predictor forward selection algorithm.     

Uncertainty Analysis in Atmospheric Dispersion Modeling

The concentration of a pollutant in the atmosphere is a random variable that cannot be predicted accurately, but can be described using quantities such as ensemble mean, variance, and probability distribution. There is growing recognition that the modeled concentrations of hazardous contaminants in the atmosphere should be described in a probabilistic framework. This paper discusses the various types of uncertainties in atmospheric dispersion models, and reviews sensitivity/uncertainty analysis methods to characterize and/or reduce them. Evaluation and quantification of the range of uncertainties in predictions yield a deeper insight into the capabilities and limitations of atmospheric dispersion models, and increase our confidence in decision-making based on models.     

Stability of jets on Jupiter and Saturn

The linear stability of a zonal jet that decays with depth is investigated under the assumption that the thermal stratification is very small. A westerly cosine jet is found to be more stable than it is in a thin fluid shell with two-dimensional flow. This is in agreement with observations of Jupiter and Saturn, where jet curvature exceeds the barotropic stability criterion. This result constitutes an alternative hypothesis to that of Ingersoll and Pollard [Icarus 52 (1982) 62], who showed that deep jets extending through the interior are also more stable than thin shell jets. The flow regime assumed in the present work requires that a small stratification can exist and persist even in the presence of horizontal temperature gradients. Further work will be needed to test whether this is realistic.