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。     

COSMIC资料在GRAPES全球三维变分同化系统的初步研究

利用COSMIC获得的GPS无线电掩星数据所反演得到的大气温湿度廓线资料,在GRAPES全球三维变分同化系统中做连续循环同化试验,研究了加入COSMIC资料后得到的GRAPES分析场及其5天中期预报是否有所改善。研究结果表明：通过加入COSMIC资料连续循环同化得到的分析场,相对原来没有加COSMIC资料连续同化出来的GRAPES分析场有明显改善,由此GRAPES全球模式全球5天中期预报水平有明显提高,其中加入没有稀疏化后COSMIC资料对南半球GRAPES分析场的改善以及GRAPES全球模式5天中期预报水平的提高都比较显著;加入稀疏化后的COSMIC资料对北半球GRAPES分析场的改善以及GRAPES全球模式5天中期预报水平的提高都有比较显著的效果。     

CORRECTION OF ASYMMETRIC STRENGTHENING OF QUIKSCAT WIND FIELD AND ASSIMILATION APPLICATION IN TYPHOON SIMULATION

As an approach to the technological problem that the wind data of QuikSCAT scatterometercannot accurately describe the zone of typhoon-level strong wind speed, some objective factors such as thetyphoon moving speed, direction and friction are introduced in this study to construct the asymmetricstrengthening of the QuikSCAT wind field. Then by adopting a technology of four-dimensional dataassimilation, an experiment that includes both the assimilation and forecasting phases is designed tosimulate Typhoon Rananim numerically. The results show that with model constraints and adjustment, thistechnology can incorporate the QuikSCAT wind data to the entire column of the model atmosphere,improve greatly the simulating effects of the whole-column wind, pressure field and the track as well as thesimulated typhoon intensity covered by the forecast phase, and work positively for the forecasting oflandfall locations.     

台风模拟中QuikSCAT风场的非对称加强订正及同化应用

针对QuikSCAT 散射计风场资料对台风级别的强风速区还无法准确刻画这个技术问题,引入台风移速、移向、摩擦等客观因素对该风场产品资料进行非对称加强订正;然后利用四维同化技术,并设计了先同化后预报两个阶段的试验方案,对“云娜”台风进行了数值模拟试验。试验结果表明：通过模式约束调整,四维同化技术可以把QuikSCAT 海面风场资料引入到整层模式大气中,并对整层风场和气压场的强度及路径模拟都有很大的改进,而且该资料还提高了预报阶段台风强度的模拟效果,并对登陆点的预报起到了积极作用。     

云迹风在热带气旋路径数值预报中的应用研究

通过一系列四维变分同化试验对GMS5卫星资料反演的云迹风资料在西北太平洋热带气旋的初始化及路径数值预报中的作用进行研究,同化资料为中国国家卫星气象中心提供的GMS5水汽和红外云迹风资料,其中70%在400hPa以上,50%集中在200～300hPa。应用美国NCAR/PSU中尺度模式MM5及其四维变分同化系统,同化窗口为6h,对初始时刻和6h后的云迹风进行同化。同化前对云迹风资料进行了简单的类似ECMWF初值检验方法的质量控制。对2002年8个西北太平洋热带气旋共进行了22组试验。结果表明,采用四维变分同化技术同化云迹风对热带气旋路径预报有一定改善,12,24,36和48h预报的平均距离误差分别降低5%,12%,10%和7%,但同化云迹风的作用与初始气旋强度有关。选择初始中心海平面气压960hPa作为强、弱气旋的分类标准,则11个较强气旋平均路径误差12h减小了13%,12h以后的预报误差减小率维持在20%以上。而对于11个较弱气旋,平均路径误差反而略有增加,说明同化云迹风资料对不同初始强度的气旋作用也有所不同。其主要原因是由于强度较强的热带气旋往往具有较为深厚的垂直结构,因此受高层大气流场的影响更明显;同时,较弱热带气旋的云迹风观测相对稀少且凌乱,并且更容易受环境气流的影响,因此对于较弱的热带气旋,当模式变量与模式或变量之间在同化后不够协调的话,就会产生负效应。     

扩展水凝物控制变量的雷达资料同化对台风“天鸽”数值模拟的影响

利用WRF(Weather Research and Forecast)模式及WRFDA(WRF model data assimilation system)系统,针对2017年台风“天鸽”个例通过同化雷达径向速度(Vr)和反射率因子(RF),研究水凝物控制变量的雷达资料同化对台风分析预报的影响。研究表明：雷达径向速度的直接同化有效地改进了模式初始场中台风涡旋区的中小尺度信息,分析场中产生了气旋性的风场增量,对模式背景场中的台风有显著增强作用。通过在传统控制变量中扩展针对水凝物的控制变量可有效地同化雷达反射率因子资料,对初始场的水物质进行调整,并对随后确定性预报的台风路径和强度都有一定的正效果。此外,相比没有水凝物控制变量的雷达同化试验,加入了水凝物控制变量的雷达资料同化试验降水预报效果更好。这为将我国近海的地基多普勒天气雷达用于台风初始化分析和预报提供了一定的技术支撑和保障。     

雷达资料的3DVAR和EnSRF直接同化方法对一次暴雨预报的影响分析

利用合肥站雷达资料和NCEP再分析资料,采用WRF模式、WRF-3DVAR和WRF-En SRF同化系统设计了直接同化雷达径向风和反射率的试验,对2003年7月4—5日发生在江淮流域的一次梅雨锋暴雨过程进行数值模拟分析。结果表明:(1)同化雷达径向风资料可以增加暴雨区低层的扰动,增强对流运动,同时可以减少模拟的过量降水;(2)3DVAR同化方法对水平风场调整明显,同化反射率改进了模式中的微物理量和动力场,同时同化径向风和反射率可以更好地调整风场,改善模拟雷达回波带的强度和位置;(3)En SRF同化方法可以将更多小尺度的风场信息同化进入模式,单独同化反射率可以较准确预报出降水中心,同时同化径向风和反射率可以改进模拟雷达回波带的位置;(4)使用3DVAR和En SRF同化雷达径向风和反射率可以提高降水预报,改善雨带的范围,对降水中心位置和强度的预报也有改进。     

雷达TREC风的三维变分同化应用与试验

介绍了雷达TREC风场在中国气象科学研究院开发的三维变分同化系统GRAPeS-3DVar中的应用,研究了GRAPES-3DVar系统同化雷达TREC风场资料的一些基本特性,发现雷达TREC风场在GRAPES-3DVar中有较好的应用价值,能有效地提高分析产品的质量,并通过一个热带气旋登陆变性过程的数值试验,说明了在GRAPES-3DVar中加入雷达TREC风场也能改善预报效果。     

ANALYSIS OF THE EFFECT OF 3DVAR AND ENSRF DIRECT ASSIMILATION OF RADAR DATA ON THE FORECAST OF A HEAVY RAINFALL EVENT

The present study designs experiments on the direct assimilation of radial velocity and reflectivity data collected by an S-band Doppler weather radar (CINRAD WSR-98D) at the Hefei Station and the reanalysis data produced by the United States National Centers for Environmental Prediction using the Weather Research and Forecasting (WRF) model, the WRF model with a three-dimensional variational (3DVAR) data assimilation system and the WRF model with an ensemble square root filter (EnSRF) data assimilation system. In addition, the present study analyzes a Meiyu front heavy rainfall process that occurred in the Yangtze -Huaihe River Basin from July 4 to July 5, 2003, through numerical simulation. The results show the following. (1) The assimilation of the radar radial velocity data can increase the perturbations in the low-altitude atmosphere over the heavy rainfall region, enhance the convective activities and reduce excessive simulated precipitation. (2) The 3DVAR assimilation method significantly adjusts the horizontal wind field. The assimilation of the reflectivity data improves the microphysical quantities and dynamic fields in the model. In addition, the assimilation of the radial velocity and reflectivity data can better adjust the wind fields and improve the intensity and location of the simulated radar echo bands. (3) The EnSRF assimilation method can assimilate more small-scale wind field information into the model. The assimilation of the reflectivity data alone can relatively accurately forecast the rainfall centers. In addition, the assimilation of the radial velocity and reflectivity data can improve the location of the simulated radar echo bands. (4) The use of the 3DVAR and EnSRF assimilation methods to assimilate the radar radial velocity and reflectivity data can improve the forecast of precipitation, rain-band areal coverage and the center location and intensity of precipitation.     

云迹风资料的三维变分同化及对台风预报的影响试验

主要研究云迹风资料对三维变分同化分析场及台风预报的影响。首先分析了不同通道和层次的静止气象卫星云迹风产品的误差特征；然后，把云迹风资料放到GRAPES三维变分同化系统中进行同化，通过将不同观测类型资料的同化结果进行对比，探讨了红外通道和水汽通道云迹风资料对数值预报分析场的影响；最后，把同化后的分析场作为初始场，用WRF模式对2002年“森拉克”台风做24小时预报实验。结果表明，不同通道和层次的云迹风资料具有不同的误差特征；云迹风资料可以提高分析场和预报场中风压场的质量；而且在台风预报试验中可以更准确地预报台风的路径和降水。     

ASSIMILATION METHOD FOR RETRIEVING HORIZONTAL WIND FIELD FROM SINGLE-DOPPLER DATA

Doppler weather radar can provide the wind information such as the radial velocity,and hasbecome one of the most powerful means for studies of severe mesoscale and microscale weathersystems.Usually the radar observation provides only plane position indicator(PPI)scanning dataand range height indicator(RHI)scanning data.and this makes some retrieval methods usingthree-dimensional volume scanning data impractical in operation.In order to use these data moreefficiently,a plane assimilation retrieval(PAR)method is developed using data assimilationtechnique on the PPI scanning plane.The PAR method is tested with the observation data ofChinese Academy of Meteorological Sciences(CAMS)Doppler weather radar and the result isencouraging.     

Improvement of the Surface Pressure Operator in GRAPES and Its Application in Precipitation Forecasting in South China

In this study we investigated the problems involved in assimilating surface pressure in the current global and regional assimilation and prediction system, GRAPES. A new scheme of assimilating surface pressure was proposed, including a new interpolation scheme and a refreshed background covariance. The new scheme takes account of the differences between station elevation and model topography, and it especially deals with stations located at elevations below that of the first model level. Contrast experiments were conducted using both the original and the new assimilation schemes. The influence of the new interpolation scheme and the updated background covariance were investigated. Our results show that the new interpolation scheme utilized more observations and improved the quality of the mass analysis. The background covariance was refreshed using statistics resulting from the technique proposed by Parrish and Derber in 1992. Experiments show that the updated vertical covariance may have a positive influence on the analysis at higher levels of the atmosphere when assimilating surface pressure. This influence may be more significant if the quality of the background field at high levels is poor. A series of assimilation experiments were performed to test the validity of the new scheme. The corresponding simulation experiments were conducted using the analysis of both schemes as initial conditions. The results indicated that the new scheme leads to better forecasting of sea level pressure and precipitation in South China, especially the forecast of moderate and heavy rain.     

Developments of the Three-Dimensional Variational Data Assimilation System for the Nonhydrostatic GRAPES

Based on the original GRAPES (Global/Regional Assimilation and PrEdiction System) 3DVAR (p3DAR), which is defined on isobaric surface, a new three-dimensional variational data assimilation system (m3DVAR) is constructed and used exclusively with the nonhydrostatic GRAPES model in order to reduce the errors caused by spatial interpolation and variable transformation, and to improve the quality of the initial value for operational weather forecasts. Analytical variables of the m3DVAR are fully consistent with predictands of the GRADES model in terms of spatial staggering and physical definition. A different vertical coordinate and the nonhydrostatic condition are taken into account, and a new scheme for solving the dynamical constraint equations is designed for the m3DVAR. To deal with the difficulties in solving the nonlinear balance equation at σ levels, dynamical balance constraints between mass and wind fields are reformulated, and an effective mathematical scheme is implemented under the terrain-following coordinate. Meanwhile, new observation operators are developed for routine observational data, and the background error covariance is also obtained. Currently, the m3DVAR system can assimilate all routine observational data. Multi-variable idealized experiments with single point observations are performed to validate the m3DVAR system. The results show that the system can describe correctly the multi-variable analysis and the relationship of the physical constraints. The difference of innovation and the analysis residual for ∏ also show that the analysis error of the m3DVAR is smaller than that of the p3DVAR. The Ts scores of precipitation forecasts in August 2006 indicate that the m3DVAR system provides reduced errors in the model initial value than the p3DVAR system. Therefore, the m3DVAR system can improve the analysis quality and initial value for numerical weather predictions.     

AMDAR资料质量分析及对GRAPES系统同化和预报影响

为了研究AMDAR(Aircraft Meteorological Data Relay)资料对GRAPES(Global Regional Assimilation and Prediction System)系统的影响,针对从国家气象信息中心实时库检索的业务AMDAR资料,分析了该资料的偏差和标准差等质量特性;利用GRAPES数值预报系统,做了两个月的同化预报循环试验。选取资料和试验的时间是2013年5月、6月,同化窗口6 h,预报时效7 d。结果表明,与背景场该资料的温度要素的平均偏差在0.2°C,标准差在2°C,风的平均偏差在0.2 m/s,标准差在3 m/s,偏差基本符合正态分布;AMDAR资料对同化和预报效果均有改善。     

雷达资料在云南一次强降水过程中的三维变分同化试验

鉴于云南观测信息相对不足、局地强降水突出的现状,利用WRF(Weather Research and Forecasting)模式及其变分同化系统进行雷达反射率因子和反演风场的三维变分同化试验。通过对2012年9月12日00:00—13日00:00发生在云南的一次强降水过程进行数值模拟和对比分析,结果表明,同时同化雷达反演风场和基本反射率因子,对区域模式同化系统中风矢量、相对湿度、位势高度几个基本分析量都有明显影响。雷达资料的同化,有利于区域模式初始场中强降水区域的上游中低层空气湿度增加、水汽输送增强和强降水发生区域的风场辐合加强,从而改善区域模式对强降水落区、强度的预报质量。对于切变线等天气尺度系统影响下的强降水过程,雷达资料的同化持续时间选取3 h、同化间隔为1 h较适宜。另外,雷达反演风场和基本反射率因子的同化均对降水预报改善有明显贡献,且多种资料的同化效果好于单一资料同化。     

IMPACTS OF ATOVS DATA ASSIMILATION ON PREDICTION OF A RAINSTORM OVER SOUTHEAST CHINA

Using the Advanced Research WRF (ARW WRF) model and the Gridpoint StatisticalInterpolation (GSI) three-dimensional variational analysis (3DVAR) system, the impact of assimilatingATOVS (Advanced TIROS Operational Vertical Sounder) radiance through the prototype CommunityRadiative Transfer Model (pCRTM) is evaluated on the forecasting of a heavy rainstorm occurring over thecentral Guangdong province in the southeast of China on 20-21 June 2005. A pair of comparisonexperiments (NODA and DA) for this case is conducted with multiple configurations, including nestingdomains with 4-km and 12-km grid distances. The results showed that by changing the initial conditionthrough data assimilation, a modified divergence and moisture field with the structure of dipoles has beenadded to the axis of the rainband with a southwest-northeast orientation. When more moisture carried by asouthwesterly low level jet (LLJ) was converged into the northeast portion of the rainband around theobservatory station of Longmen, the amplitude of moisture static energy (MSE) increased substantially atlow levels much more than at middle levels, resulting in the enlarging of differences in MSE between 500hPa and 850 hPa; the atmosphere became more unstable. Consequently, the convective rainfall increased inthe northeast part of the province around the Longmen station, which was consistent with the observeddistribution of rainfall.     

Performance Evaluation of the GRAPES Model in Wind Simulations Over South China

In the present study, the performance of the GRAPES model in wind simulation over south China was assessed. The simulations were evaluated by using surface observations and two sounding stations in south China. The results show that the GRAPES model could provide a reliable simulation of the distribution and diurnal variation of the wind. It showed a generally overestimated southerly wind speed especially over the Pearl River Delta region and the south of Jiangxi Province as well as the coastal region over south China. GRAPES also exhibited a large number of stations with the opposite surface wind directions over the east of Guangxi and the south of Jiangxi during the nocturnal- to-morning period, as well as an overall overestimation of surface wind over the coastal regions during the afternoon. Although GRAPES could simulate the general evolutional characteristics of vertical wind profile, it underestimated wind speed above 900 hPa and overestimated wind speed below 900 hPa. Though the parameterization scheme of gravity wave drag proved to be an effective method to alleviate the systematic deviation of wind simulation, GRAPES still exhibited large errors in wind simulation, especially in the lower and upper troposphere.     

A COMPARISON OF THE BLENDING AND CONSTRAINING METHODS TO INTRODUCE LARGE-SCALE INFORMATION INTO GRAPES MESOSCALE ANALYSIS

To solve the problem of mesoscale analysis error accumulation after a period of continuous cycle data assimilation (CCDA), a blending method and a constraining method are compared to introduce global analysis information into the Global/Regional Assimilation and Prediction Enhanced System mesoscale three-dimensional variational data assimilation system (GRAPES-Meso 3Dvar). Based on a spatial filter used to obtain a blended analysis, the blending method is weighted toward the T639 global analysis for scales larger than the cutoff wavelength of 1,200 km and toward the GRAPES mesoscale analysis for wavelengths below that. The constraining method considers the T639 global analysis data as an extra source of information to be added in the 3DVar cost function. The cloud-resolving GRAPES-Meso system (3 km resolution) with a 3 h analysis cycle update is chosen, and forecast experiments on an extreme precipitation event over the eastern part of China are presented. The comparison shows that the inclusion of large-scale information with both methods has a positive impact on the regional model, in which the 3 h background forecasts are slightly closer to the radiosonde observations. The results also show that both methods are effective in improving large-scale analysis while reserving the well-featured mesoscale information, leading to an enhancement in the balance and accuracy of the analysis. Subjective verification reveals that the introduction of large-scale information has a visible beneficial impact on the forecast of precipitation location and intensity. The methodologies and experiences presented in this paper could serve as a reference for ongoing efforts toward the development of multi-scale analysis in GRAPES-Meso.     

WIND SHEAR IDENTIFICATION WITH THE RETRIEVED WIND OF DOPPLER WEATHER RADAR

Wind shear reflects that the wind field is not uniform, which is one of the primary factors which make the retrieval of the wind field difficult. Based on volume velocity process(VVP) wind field retrieval technique, the intensity of wind shear is identified in this paper. After analyzing the traditional techniques that rely on the difference of radial velocity to identify wind shear, a fixed difference among radial velocities that may cause false identification in a uniform wind field was found. Because of the non-uniformity in wind shear areas, the difference of retrieved results between surrounding analysis volumes can be used as a measurement to show how strong the wind shear is. According to the analysis of a severe convective weather process that occurred in Guangzhou, it can be found that the areas of wind shear appeared with the strength significantly larger than in other regions and the magnitude generally larger than4.5 m/(s·km). Besides, by comparing the variation of wind shear strength during the convection, it can be found that new cells will be more likely to generate when the strength is above 3.0 m/(s·km). Therefore, the analysis of strong wind shear's movement and development is helpful to forecasting severe convections.     

ATOVS 不同卫星资料在台风模拟中的同化试验研究

利用美国国家大气研究中心(NCAR)开发的中尺度模式WRF(ARW)V3.2 及其三维变分同化系统WRF-3DVAR,以1011 号超强台风“ 凡亚比” 为个例,采用连续循环同化的方法对ATOVS 卫星资料进行同化试验,探讨了同化ATOVS 不同卫星资料对“ 凡亚比” 模拟的影响。结果表明,强度影响方面:同化ATOVS不同资料均可有效改善台风强度,台风中心海平面气压平均偏差从42 hPa 下降到18 hPa,但不同资料间的差异并不显著,平均在6 hPa 以内,这表明仅同化ATOVS 资料对台风强度的改善相对有限。路径影响方面:(1)不同卫星的同一种传感器资料效果略有不同,同化NOAA-18 和NOAA-15 的AMSU-A 资料效果较好,NOAA-16 的AMSU-A 效果较差;同化NOAA-15 和NOAA-16 的AMSU-B 资料效果相当,且均优于AMSU-A 资料。(2) 同一颗卫星不同传感器资料的差异较大,同化AMSU-B 资料的改善较为明显,HIRS-3 次之,AMSU-A较差,而同时同化不同资料并没有带来更为明显的改善。(3) 同时同化多颗卫星ATOVS 资料的试验表明,将多种资料引入到同化系统的同时,也带来相应的累积误差,因而仅同化一颗卫星可能比同时同化两颗或三颗卫星ATOVS 资料的效果要好。