Coupling Ensemble Kalman Filter with Four-dimensional Variational Data Assimilation

This study examines the performance of coupling the deterministic four-dimensional variational assimilation system (4DVAR) with an ensemble Kalman filter (EnKF) to produce a superior hybrid approach for data assimilation. The coupled assimilation scheme (E4DVAR) benefits from using the state-dependent uncertainty provided by EnKF while taking advantage of 4DVAR in preventing filter divergence: the 4DVAR analysis produces posterior maximum likelihood solutions through minimization of a cost function about which the ensemble perturbations are transformed, and the resulting ensemble analysis can be propagated forward both for the next assimilation cycle and as a basis for ensemble forecasting. The feasibility and effectiveness of this coupled approach are demonstrated in an idealized model with simulated observations. It is found that the E4DVAR is capable of outperforming both 4DVAR and the EnKF under both perfect- and imperfect-model scenarios. The performance of the coupled scheme is also less sensitive to either the ensemble size or the assimilation window length than those for standard EnKF or 4DVAR implementations.     

基于贝叶斯方案的雷达反射率反演水汽及其同化试验

雷达资料是目前为数不多有能力为高分辨率预报模式提供高分辨率信息资料的资料之一。为充分利用该资料所包含的中小尺度信息,文中基于雷达反射率,利用贝叶斯方法反演出大气相对湿度;将质控后的资料引入3Dvar系统进行同化分析,为高分辨率模式提供初值场。以台风“妮妲”登陆为例,通过一维反演及三维变分系统分析,有效地订正了实况有回波而模式预报无回波区域的大气湿度趋于合理,增加背景场的湿度,减小模拟回波比观测偏强的区域的大气湿度;同化大气湿度后模式在前6小时报出的台风外围回波分布、演变更合理,改进了降水雨带的分布与强度。1个月的批量试验反映1D+3Dvar同化雷达资料后,大气对流层中低层（850~400 hPa）增湿明显,其增湿影响程度可延续12小时以上。其逐时降水预报在前12小时的TS均比控制试验高,而大于5 mm以上降水预报偏差则与控制试验的大略一致或更接近1。      

基于约束性分析的数据与作物模型同化方法

同化观测数据可为作物生长模型的区域应用提供支持。该文定义了观测数据对模型参数的约束性,研究发现华北夏玉米观测数据对WOFOST模型的可约束参数主要包括初始总干物重、不同发育阶段的比叶面积、初始最大CO₂同化速率、叶片衰老系数、初始土壤有效水、最大根深日增量以及初始根深的初始土壤水分含量等。建立了基于参数约束性分析的观测数据与作物生长模型同化方法和流程, 利用优化算法进行作物生长模型所有参数和变量初值的敏感性分析,遴选出各状态变量的敏感参数;根据拟合度与优化结果之间关系进行敏感参数的约束性分析,获得不同变量的可约束参数;组合优化可约束参数得到各参数最优值,由此实现了观测数据与作物生长模型的同化。约束性体现了观测数据对模型参数或变量初值的控制能力,可约束参数作为待优化参数使数据模型同化获得了最优结果。     

全球定位系统的可降水量资料在北京地区快速更新循环系统中的同化试验

针对发生于2006年8月和2007年6-8月的28次降水过程,在3 h快速更新循环同化预报系统平台上分别进行了有无同化局地GPS可降水量资料的试验.结果表明,局地GPS可降水量的同化可以使模式对大阈值降水的时段、强度和落区的预报性能均获得全面的提高,此现象在模式积分最初的0-6小时表现得更为明显.由于同化所用的GPS可降水量观测资料主要集中在极为局地的京津冀地区,而且GPS-PWV资料的同化仅能对模式中的水汽含量发生影响,因此该资料的同化与否并不能为模式总体预报性能带来明显差异.但从以54511测站为代表的北京地区局地常规要素预报效果来看,快速更新循环的同化方式能使高时空分辨率的GPS可降水量在北京局地的同化效果得以累积,并为气象要素在局地的预报效果带来显著的正面影响.通过对一次强对流个例进行分析可以发现,以快速更新循环的方式可以同化较高时间频率的GPS可降水量观测,从而对模式大气的初始湿度条件起到持续累积的影响.在一定的动力和热力条件下,由GPS可降水量同化带来的累积的模式大气增湿也有望在模式中形成利于局地对流形成和发展的环境,提高模式对局地对流的预报水平.     

Parameterized Forward Operators for Simulation and Assimilation of Polarimetric Radar Data with Numerical Weather Predictions

Many weather radar networks in the world have now provided polarimetric radar data(PRD)that have the potential to improve our understanding of cloud and precipitation microphysics,and numerical weather prediction(NWP).To realize this potential,an accurate and efficient set of polarimetric observation operators are needed to simulate and assimilate the PRD with an NWP model for an accurate analysis of the model state variables.For this purpose,a set of parameterized observation operators are developed to simulate and assimilate polarimetric radar data from NWP model-predicted hydrometeor mixing ratios and number concentrations of rain,snow,hail,and graupel.The polarimetric radar variables are calculated based on the T-matrix calculation of wave scattering and integrations of the scattering weighted by the particle size distribution.The calculated polarimetric variables are then fitted to simple functions of water content and volumeweighted mean diameter of the hydrometeor particle size distribution.The parameterized PRD operators are applied to an ideal case and a real case predicted by the Weather Research and Forecasting(WRF)model to have simulated PRD,which are compared with existing operators and real observations to show their validity and applicability.The new PRD operators use less than one percent of the computing time of the old operators to complete the same simulations,making it efficient in PRD simulation and assimilation usage.     

Numerical simulation and data assimilation of the water-energy cycle over semiarid northeastern China

The default fractional vegetation cover and terrain height were replaced by the estimated fractional vegetation cover, which was calculated by the Normalized Difference Vegetation Index(NDVI) of Earth Observing System Moderate-Resolution Imaging Spectroradiometer(EOS-MODIS) and the Digital Elevation Model of the Shuttle Radar Topography Mission(SRTM) system. The near-surface meteorological elements over northeastern China were assimilated into the three-dimensional variational data assimilation system(3DVar) module in the Weather Research and Forecasting(WRF) model. The structure and daily variations of air temperature, humidity, wind and energy fields over northeastern China were simulated using the WRF model. Four groups of numerical experiments were performed, and the simulation results were analyzed of latent heat flux, sensible heat flux, and their relationships with changes in the surface energy flux due to soil moisture and precipitation over different surfaces. The simulations were compared with observations of the stations Tongyu, Naiman, Jinzhou, and Miyun from June to August, 2009. The results showed that the WRF model achieves high-quality simulations of the diurnal characteristics of the surface layer temperature, wind direction, net radiation, sensible heat flux, and latent heat flux over semiarid northeastern China in the summer. The simulated near-surface temperature, relative humidity, and wind speed were improved in the data assimilation case(Case 2) compared with control case(Case 1). The simulated sensible heat fluxes and surface heat fluxes were improved by the land surface parameterization case(Case 3) and the combined case(Case 4). The simulated temporal variations in soil moisture over the northeastern arid areas agree well with observations in Case 4, but the simulated precipitation should be improved in the WRF model. This study could improve the land surface parameters by utilizing remote sensing data and could further improve atmospheric elements with a data assimilation system. This work provides an effective attempt at combining multi-source data with different spatial and temporal scales into numerical simulations. The assimilation datasets generated by this work can be applied to research on climate change and environmental monitoring of arid lands, as well as research on the formation and stability of climate over semiarid areas.     

Modelling the global ocean tides: modern insights from FES2004

During the 1990s, a large number of new tidal atlases were developed, primarily to provide accurate tidal corrections for satellite altimetry applications. During this decade, the French tidal group (FTG), led by C. Le Provost, produced a series of finite element solutions (FES) tidal atlases, among which FES2004 is the latest release, computed from the tidal hydrodynamic equations and data assimilation. The aim of this paper is to review the state of the art of tidal modelling and the progress achieved during this past decade. The first sections summarise the general FTG approach to modelling the global tides. In the following sections, we introduce the FES2004 tidal atlas and validate the model against in situ and satellite data. We demonstrate the higher accuracy of the FES2004 release compared to earlier FES tidal atlases, and we recommend its use in tidal applications. The final section focuses on the new dissipation term added to the equations, which aims to account for the conversion of barotropic energy into internal tidal energy. There is a huge improvement in the hydrodynamic tidal solution and energy budget obtained when this term is taken into account.     

一阶有限元浅水模式的变分资料同化

一阶有限元浅水方程的伴随模式应用于气象资料同化场,并利用最优化方法对具有随机扰动的初始场进行优化处理。通过极小化算法,使所定义的目标函数达到最小,从而得到最优的气象要素初始场。数值试验结果表明这种处理是很有效的     

云迹风资料同化对暴雨预报影响的数值模拟

文中运用MM5对 1998年 6月 13～ 14日长江中下游一次暴雨强降水进行了数值模拟及云迹风资料同化试验研究。结果表明 ,云迹风资料同化能有效地改善高空风场质量 ,修正控制试验中模式高空风场对中纬度高空西风急流前沿西风强度的描述 ,使得模拟的暴雨强度与实际降水分布更加接近。分析认为在暴雨预报业务和科研中 ,云迹风资料的使用可有效地改善高空风场分布 ,对提高降水预报质量是一个有效途径 ,值得进一步尝试     

GRAPES地面风场同化方案研究

随着预报准确度要求以及同化分析循环周期的缩短,具有高时空分辨率的地面观测资料在同化系统中也越来越受到重视,而且一个好的资料同化系统应该能够给出模式近地面动力、热力和湿度状况的合理分析。利用GRAPES区域同化预报系统(GRAPES——Global and Regional Assimilation and Prediction Enhanced System),在近地层Monin_Obukhov相似理论的基础上,考虑了边界层的动力和湿热力约束,建立新的地面风场观测算子。将此新方案与目前GRAPES系统使用的三次样条插值的地面同化方案进行高度场预报试验、统计试验以及降水预报的TS评分比较,结果表明,加入新方案地面风场资料同化后,预报效果和对降水的预报能力都比旧方案有所提高。