Optimal Perturbations with Zero Potential Vorticity in the Eady Model

The problem of finding optimal perturbations, which are perturbations with a maximum ratio of the final energy to the initial energy, is considered in the Eady model of baroclinic instability. The solution to the problem uses explicit expressions for the energy functional, which are functions of parameters of an initial perturbation. For perturbations with zero potential vorticity, the basic parameters are the amplitudes of the initial buoyancy distributions at the boundaries of the atmospheric layer and a phase shift between these distributions. Dependences of the optimal phase shift and maximum energy ratio on the wave number and time optimization are determined using an analysis for extremum. The parameters of the optimal perturbations are compared with those of the growing normal modes. It is found that only one exponentially growing mode is an optimal perturbation.     

直接同化卫星辐射数据的黄海海雾数值试验研究

基于中尺度大气模式WRF及其3DVAR.模块,采用循环3DVAR数据同化方案,针对6次明显的黄海海雾过程,实施了一系列直接同化ATOVS卫星辐射数据数值试验.在试验中设计了不同化任何观测数据、仅同化GTS常规数据、仅直接同化辐射数据,同时同化二者,以及同化不同疏密程度辐射数据的对比研究方案.利用地面水平能见度与卫星云图对模拟的海雾雾区进行了评估,并比较了各种同化方案所形成初始场的差异.对试验结果的统计分析表明:同化试验较好地再现了影响海雾的天气系统,模拟雾区与实际观测较为吻合,并且初始温度场和湿度场对比不同化任何观测数据的试验有明显的改善;仅同化辐射数据的结果略优于仅同化常规数据的结果,疏化或者只同化海上辐射数据几乎不影响模拟的雾区,但却可以大幅节约计算资源;同时同化常规数据与辐射数据的结果为单独同化它们所得结果的综合体现,总体效果最好.     

Optimal precursors of double-gyre regime transitions with an adjoint-free method

In this paper, we find the optimal precursors which can cause double-gyre regime transitions based on conditional nonlinear optimal perturbation (CNOP) method with Regional Ocean Modeling System (ROMS). Firstly, we simulate the multiple-equilibria regimes of double-gyre circulation under different viscosity coefficient and obtain the bifurcation diagram, then choose two equilibrium states (called jet-up state and jet-down state) as reference states respectively, propose Principal Component Analysis-based Simulated Annealing (PCASA) algorithm to solve CNOP-type initial perturbations which can induce double-gyre regime transitions between jet-up state and jet-down state. PCASA algorithm is an adjoint-free method which searches optimal solution randomly in the whole solution space. In addition, we investigate CNOP-type initial perturbations how to evolve with time. The results show:(1) the CNOP-type perturbations present a two-cell structure, and gradually evolves into a three-cell structure at predictive time;(2) by superimposing CNOP-type perturbations on the jet-up state and integrating ROMS, double-gyre circulation transfers from jet-up state to jet-down state, and vice versa, and random initial perturbations don't cause the transitions, which means CNOP-type perturbations are the optimal precursors of double-gyre regime transitions;(3) by analyzing the transition process of double-gyre regime transitions, we find that CNOP-type initial perturbations obtain energy from the background state through both barotropic and baroclinic instabilities, and barotropic instability contributes more significantly to the fast-growth of the perturbations. The optimal precursors and the dynamic mechanism of double-gyre regime transitions revealed in this paper have an important significance to enhance the predictability of double-gyre circulation.     

Numerical experiments with data assimilation in the simplest dynamic-stochastic model for the ocean's synoptic variability closed at the level of the second moments

The results of numerical experiments with data assimilation in a non-linear model of the synoptic dynamics of the world's oceans are reported. The model's response to the data assimilation is studied at different values of the relative error in determining the initial fields and assimilation discreteness. The results of application of the algorithms of optimum filtration and optimum interpolation are compared. The advantages of the filtration algorithm employed are discussed only for the case of small errors in determining the initial fields and low assimilation discreteness. The possibility of deterioration of the prediction estimates in data assimilation with discreteness less than the typical time of variability of the average fields is noted.Translated by Mikhail M. Trufanov.     

Generation of internal waves in the sea by local perturbations of wind and density flows on the surface

On the basis of a nonlinear model taking into account viscosity, density diffusion, and currents, we perform the numerical analysis of propagation of perturbations formed in the abyssal part of the Black Sea as a result of the action of tangential wind stresses or density flows in the form of a strip on the sea surface. It is shown that, in the course of time, in the region of the bottom slope, these perturbations generate a train of waves in the sea, which causes oscillations of the fields of density and currents. There are some differences between the generation of internal waves by the flows of density and the wind. Unlike the wind action, in the presence of flows of density in the upper layer in the region of the strip, the initial period of generation is characterized by the formation of intense perturbations without train structure. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

微型浮游动物摄食实验——稀释法中浮游植物负生长的可能原因分析

稀释法(dilution technique)是研究微型浮游动物摄食和浮游植物生长的常用方法之一,负值浮游植物生长率是稀释实验中常见的现象。分析了造成负值生长率出现的因素,以及这些因素对实验结果的影响,并提出了防止不利影响产生的措施。负值生长率的出现不能简单地视为实验失败的标志,培养光照和温度条件、取样误差、无颗粒水污染、营养盐污染和限制等都可能造成负生长率的出现,且对实验结果的影响不同。同时,根据实验结果,演示浮游植物光适应、取样误差、无颗粒水污染和加富营养盐对稀释实验的影响。结果显示,光照条件可以改变细胞色素含量,且不同浮游植物类群对光照条件的响应不同,从而导致基于色素分析的稀释实验结果出现误差;取样混合不均,可造成取值偏低,导致浮游植物生长率估值偏低,甚至为负值,但可能不影响对摄食率的估算。另外,实验污染(无颗粒水和加富营养盐污染)往往会抑制浮游植物生长,甚至造成浮游植物死亡。因此,培养条件模拟和人为干扰控制是稀释实验成功的关键。     

Mesoscale SST perturbation-induced impacts on climatological precipitation in the Kuroshio-Oyashio extension region,as revealed by the WRF simulations

Mesoscale coupling between perturbations of mesoscale sea surface temperature (SST) and lowlevel winds has been extensively studied using available high-resolution satellite observations. However, the climatological impacts of mesoscale SST perturbations (SST meso ) on the free atmosphere have not been fully understood. In this study, the rectified eff ect of SSTmeso on local climatological precipitation in the Kuroshio- Oyashio Extension (KOE) region is investigated using the Weather Research and Forecasting (WRF) Model;two runs are performed, one forced by low-resolution SST fields (almost no mesoscale signals) and another by additional high-resolution SSTmeso fields extracted from satellite observations. Climatological precipitation response to SST meso is characterized mainly by enhanced precipitation on the warmer flank of three oceanic SST fronts in this region. The results show that the positive correlation between the 10-m wind speed perturbations and SSTmeso is well captured by the WRF model with a reasonable spatial pattern but relatively weak strength. The addition of SSTmeso improves the climatological precipitation simulated by WRF with a better representation of fine-scale structures compared with satellite observations. A closer examination on the underlying mechanism suggests that while the pressure adjustment mechanism can explain the climatological precipitation enhancement along the fronts and the relatively high contribution of the convective precipitation, other factors such as synoptic events should also be taken into consideration to account for the seasonality of the precipitation response.     

Comparative analysis of the North Atlantic surface circulation reproduced by three different methods

Calculation results are presented for long-term mean annual surface currents in the North Atlantic based on direct drifter measurements and numerical experiments with the ocean general circulation model using both climatic arrays of hydrological data World Ocean Atlas 2009 and Argo profiling data. The calculations show that the technique suggested for model calculations of oceanographic characteristics of the World Ocean with the use of Argo data significantly improves the climatic fields of the temperature and salinity even on a coarse grid. The comparison of the model calculation results with drifter data showed that the temperature and salinity fields found from Argo data with the use of data variational interpolation on a regular grid allow the calculation of realistic currents and can be successfully used as initial conditions in hydrodynamic models of the ocean dynamics.     

Evaluation of a method to reduce uncertainty in wind hindcasts performed with regional atmosphere models

For more and more applications in coastal and offshore engineering, numerical simulations of waves and surges are required. An important input parameter for such simulations are wind fields. They represent one of the major sources for uncertainties in wave and surge simulations. Wind fields for such simulations are frequently obtained from numerical hindcasts with regional atmospheric models (RAMs). The skill of these atmospheric hindcasts depends, among others, on the quality of the forcing at the boundaries. Furthermore, results may vary due to uncertainties in the initial conditions. By comparing different existing approaches for forcing a regional atmospheric model, it is shown that the models' sensitivity to uncertainties in the initial conditions may be reduced when a more sophisticated approach is used that has been suggested recently. For a specific, although somewhat brief test period, it is demonstrated that an improved hindcast skill for near surface wind fields is obtained when this approach is adopted. Consequences of the reduced uncertainty in wield fields for the hindcast skill of subsequent wave modelling studies are demonstrated. Recently, this new approach has been used together with a regional atmosphere model to produce a 40-year wind hindcast for the Northeast Atlantic, the North Sea and the Baltic Sea. The hindcast is presently extended to other areas and the wind fields are used to produce 40-year high-resolution hindcasts of waves and surges for various European coastal areas.     

Objective analysis of the structure of three-dimensional atmospheric fronts

A computational method for diagnosing three-dimensional atmospheric fronts from temperature, wind, and geopotential fields on a three-dimensional regular grid is proposed. The criterion, which serves for the diagnosis of atmospheric fronts, is discussed. The weights of the input information about the mentioned fields are optimized based on the maximal difference between the correlation functions for (a) pairs of particles separated by the front and (b) pairs from one synoptic mass. These weights were different for different baric levels. The correlation functions and the optimization of weights were estimated on the basis of the archive of fields of the NCEP objective analysis on the half-degree latitude-longitude grid and data from aerological observations. The results of numerical experiments on the construction of atmospheric fronts are presented. Applying the described method to fields predicted for a term of up to 36 h showed that errors in the prognostic models introduce a relatively weak distortion into the geometry of atmospheric fronts.     

Short-Range Prediction Experiments with Operational Data Assimilation System for the Kuroshio South of Japan

The short-range (one month) variability of the Kuroshio path was predicted in 84 experiments (90-day predictions) using a model in an operational data assimilation system based on data from 1993 to 1999. The predictions started from an initial condition or members of a set of initial conditions, obtained in a reanalysis experiment. The predictions represent the transition from straight to meander of the Kuroshio path, and the results have been analyzed according to previously proposed mechanisms of the transition with eddy propagation and interaction acting as a trigger of the meander and self-sustained oscillation. The reanalysis shows that the meander evolves due to eddy activity. Simulation (no assimilation) shows no meander state, even with the same atmospheric forcing as the prediction. It is suggested therefore that the initial condition contains information on the meander and the system can represent the evolution. Mean (standard deviation) values of the axis error for all 84 cases are 13, 17, and 20 (10, 10, and 12) km, in 138.5°E, in the 30-, 60-, and 90-day predictions respectively. The observed mean deviation from seasonal variation is 30 km. The predictive limit of the system is thus about 80 days. The time scale of the limit depends on which stage in the transition is adopted as the initial condition. The gradual decrease of the amplitude in a stage from meander to straight paths is also predicted. The predictive limit is about 20 days, which is shorter than the prediction of the opposite transition. This revised version was published online in July 2006 with corrections to the Cover Date.     

An analysis of the error space of a high-resolution implementation of the GHER hydrodynamic model in the Mediterranean Sea

An ensemble of 250 model setups covering the Mediterranean Sea is built by perturbing various parameters: the bathymetry, the initial conditions, atmospheric forcing fields (air temperature, cloud coverage, wind), and internal model parameters (diffusion coefficients). The ensemble is then forwarded in time using the GHER hydrodynamic model, allowing to obtain information about the expected error associated with the forecast in a natural way. The evolution of this error is analyzed. In particular, we examine the time evolution and stationarity of its spatial average, and the spatial distribution of the error at different instants, by means of its first to fourth order moments, and of empirical orthogonal functions. We verify whether the a posteriori error distribution is Gaussian using the Anderson-Darling test. From these results, we are able to assess what parameters and forcing fields are most critical for the forecast. Qualitative conclusions are obtained throughout the text, in accordance with our expectations. Moreover, quantitative estimations of the expected error are also given.     

Numerical analysis and visualization of fine structures of the fields of two-dimensional attached internal waves

In the linear approximation, we compute the patterns of two-dimensional perturbations formed in a viscous exponentially stratified fluid in the process of motion of a plate at an arbitrary angle to the horizon. The exact solution of the problem obtained in quadratures and satisfying the physically meaningful boundary conditions is numerically analyzed. The properties of the fields are computed and described in broad ranges of all parameters of the problem, including the length and velocity of motion of the plate, the characteristics of stratification and viscosity of the medium, and the slope of the path. In the picture of currents, we distinguish two groups of waves and compact nonwave singularities near the edges of a source of generation. The results of comparison with the available data of independently performed calculations and experiments reveal the existing agreement between the computed and observed pictures of the currents.     

Flow-dependent versus flow-independent initial perturbations for ensemble prediction

Ensemble prediction relies on a faithful representation of initial uncertainties in a forecasting system. Early research on initial perturbation methods tested random perturbations by adding 'white noise' to the analysis. Here, an alternative kind of random perturbations is introduced by using the difference between two randomly chosen atmospheric states (i.e. analyses). It yields perturbations (random field, RF, perturbations) in approximate flow balance.
The RF method is compared with the operational singular vector based ensemble at European Centre for Medium Range Weather Forecasts (ECMWF) and the ensemble transform (ET) method. All three methods have been implemented on the ECMWF IFS-model with resolution T _L 255L40. The properties of the different perturbation methods have been investigated both by comparing the dynamical properties and the quality of the ensembles in terms of different skill scores. The results show that the RF perturbations initially have the same dynamical properties as the natural variability of the atmosphere. After a day of integration, the perturbations from all three methods converge. The skill scores indicate a statistically significant advantage for the RF method for the first 2–3 d for the most of the evaluated parameters. For the medium range (3–8 d), the differences are very small.     

不同初始场对台风Khanun模拟效果的影响及其暴雨过程的螺旋度分析

利用中尺度非静力MM5模式,在NCEP和JMA两种初始场方案下,对2005年严重影响我国的15号台风Khanun的登陆过程进行了数值模拟,模拟结果的对比分析及其与观测的比较表明:台风对初始场十分敏感,JMA初始场方案在台风路径、强度和暴雨预报上较NCEP初始场方案都有显著改善。利用JMA方案高分辨模拟结果,对台风暴雨过程的螺旋度特征进行了诊断分析,结果表明:暴雨区上空螺旋度呈高层负值区,低层正中心分布,低层700hPa和850hPa螺旋度高值中心位置和强度演变对未来6h暴雨落区与强度有一定指示意义。     

Revealing the energy sources of alternating intensity regimes of the evolving Alberto tropical cyclone using microwave satellite sensing data

The trajectory characteristics and the intensity evolution of the Alberto tropical cyclone (TC) in the integral water-vapor field are analyzed in detail using a software-algorithmic complex designed for constructing highly detailed global radio-thermal fields of the ocean-atmosphere system based on microwave satellite measurements. This complex was developed by the authors earlier and allows one to obtain global animated radio-thermal fields with a time step of 1.5 h and a spatial resolution of 0.2°. Special attention is paid to the problem of revealing the energy sources of three consecutive intensifications of this TC. The analysis of satellite data with the use of the animation method developed by the authors shows for the first time that these sources are regions with an increased (exceeding the critical level) integral water-vapor content located in different geographical areas lying along the complex trajectory of the TC with a variable intensity. The first region was located in the equatorial intertropical convergence zone, the second was located above the Gulf Stream water area, and the third resided in the area affected by the Azores High.     

基于不同背景场条件的雷达资料同化在登陆台风“桑美”中的应用研究

本文针对2006年登陆我国的超强台风“桑美”,分别采用美国国家环境预报中心的全球预报系统(Global Forecasting System, GFS)再分析资料和日本气象厅(Japan Meteorological Agency, JMA)区域客观再分析资料作为背景场,利用中尺度数值模式WRF(Weather Research and Forecasting Model)及其三维变分同化系统进行多普勒雷达资料同化和数值模拟试验,考察不同的背景场条件下雷达资料同化对台风初始场、内部结构及其随后确定性预报的影响。结果表明:GFS试验和JMA试验在同化了雷达资料之后分析出的台风700 hPa风场和500 hPa高度场相比其初始场均有所增强,JMA试验在3 h同化窗内的均方根误差和最小海平面气压的改进效果均比GFS试验显著,同时对台风动力和热力结构的改进效果也优于GFS试验;JMA试验对台风降水、路径、强度的预报均优于GFS试验,且能预报出台风前沿的降水,更加接近观测实况。     

云迹风资料同化对东亚海域风场数值模拟的影响

为了揭示云迹风资料同化对于东亚海域内各层模拟风场的影响,采用由日本GMS卫星红外和水汽通道资料反演得到的1999年6月底至7月初的云迹风资料,首先对这些云迹风资料进行质量评估,评估结果表明云迹风u,v分量的均方根误差和偏差都在所容许的误差范围之内.然后针对10个个例进行5种不同的四维同化模拟试验,多种同化试验和无同化控制试验的检验对比表明:在经过初始12 h常规高空地面资料和云迹风资料同化过程以后,模拟结果质量有了比较明显的提升;各种同化试验与控制试验间的最大差异发生在12h,也即同化结束的时刻,此后随着模拟时间的增长,两者间的差异逐渐变小;但24~36h同化试验对控制试验的改进效果又开始提升,36h后效果再次下降.由于海面上缺少常规测站,因此无法实现海面部分的测站同化,而这必将影响到海面部分的模拟质量,而云迹风资料恰可以祢补这个不足.试验表明云迹风资料对u,v风场的改进则主要集中在对流层高层(300~200hPa),对中低层的影响不很明显,这与云迹风资料主要集中在高层是相一致的.另外,敏感性试验表明经过质量控制的云迹风同化模拟的性能相对不采用质量控制的同化模拟有了一定程度的提高.     

Predicting the East Australian Current

Results are presented from an ensemble prediction study (EPS) of the East Australian Current (EAC) with a specific focus on the examination of the role of dynamical instabilities and flow dependent growing errors. The region where the EAC separates from the coast, is characterized by significant mesoscale eddy variability, meandering and is dominated by nonlinear dynamics thereby representing a severe challenge for operational forecasting. Using analyses from OceanMAPS, the Australian operational ocean forecast system, we explore the structures of flow dependent forecast errors over 7 days and examine the role of dynamical instabilities. Forecast ensemble perturbations are generated using the method of bred vectors allowing the identification of those perturbations to a given initial state that grow most rapidly. We consider a 6 month period spanning the Austral summer that corresponds to the season of maximum eddy variability. We find that the bred vector (BV) structures occur in areas of instability where forecast errors are large and in particular in regions associated with the Tasman Front and EAC extension. We also find that very few BVs are required to identify these regions of large forecast error and on that basis we expect that even a small BV ensemble would prove useful for adaptive sampling and targeted observations. The results presented also suggest that it may be beneficial to supplement the static background error covariances typically used in operational ocean data assimilation systems with flow dependent background errors calculated using a relatively cheap EPS.     

Physical modeling of long-range infrasonic propagation in the atmosphere

The results of experiments on the physical modeling of long-range infrasonic propagation in the atmosphere are given. Such modeling is based on the possible coincidence between the forms of the vertical profiles of the effective sound speed stratification in the atmospheric boundary layer (between 0 and 600 m for the case under consideration) and in the atmosphere as a whole (from the land surface up to thermospheric heights (about 150 km)). The source of acoustic pulses was an oscillator of detonation type. Owing to the detonation of a gas mixture of air (or oxygen) and propane, this generator was capable of producing short, powerful (the maximum acoustic pressure was on the order of 30 to 60 Pa at a distance of 50 to 100 m from the oscillator), and sufficiently stable acoustic pulses with a spectral maximum at frequencies of 40 to 60 Hz and a pulsing period of 20 to 30 s. The sites of acoustic-signal recording were located at different distances (up to 6.5 km) from the source and in different azimuthal directions. The temperature and wind stratifications were monitored in real time during the experiments with an acoustic locator—a sodar—and a temperature profiler. The data on the physical modeling of long-range sound propagation in the atmosphere are analyzed to verify the physical and mathematical models of predicting acoustic fields in the inhomogeneous moving atmosphere on the basis of the parabolic equation and the method of normal waves. A satisfactory agreement between calculated and experimental data is obtained. One more task was to compare the theoretical relations between variations in the azimuths and angles of tilting of sound rays about the horizon and the parameters of anisotropic turbulence in the lower troposphere and stratosphere with the experimental data. A theoretical interpretation of the experimental results is proposed on the basis of the theory of anisotropic turbulence in the atmosphere. The theoretical and experimental results are compared, and a satisfactory agreement between these results is noted.