A Precipitation Detection Method for MWTS-Ⅱ Radiance Assimilation in Typhoon Simulation

FY-3C Microwave Temperature Sounder Ⅱ(MWTS-Ⅱ) lacks observations at 23.8 GHz, 31 GHz and 89 GHz,making it difficult to remove the data contaminated by precipitation in assimilation. In this paper, a fast forward operator based on the Community Radiative Transfer Model(CRTM) was used to analyze the relationship between the observation minus background simulation(O-B) and the cloud fractions in different MWTS-Ⅱ channels. In addition,based on the community Gridpoint Statistical Interpolation(GSI) system, the radiation brightness temperature of the MWTS-Ⅱ was assimilated in the regional Numerical Weather Prediction(NWP) model. In the process of assimilation,Visible and Infrared Radiometer(VIRR) cloud detection products were matched to MWTS-Ⅱ pixels for precipitation detection. For typhoon No. 18 in 2014, impact tests of MWTS-Ⅱ data assimilation was carried out. The results show that,though the bias observation minus analysis(O-A) of assimilated data can be reduced by quality control only with | O-B| 3 K; however, the O-A becomes much smaller while the precipitation detection is performed with F_(virr) 0.9(VIRR cloud fraction threshold of 0.9). Besides, the change of the environmental field around the typhoon is more conducive to make the simulated track closer to the observation. The 72-hour typhoon track simulation error also shows that, after the precipitation detection, the error of simulated typhoon track is significantly reduced, which reflects the validity of a precipitation detection method based on a double criterion of | O-B | 3 K and F_(virr) 0.9.     

The WRF 3DVar System Combined with Physical Initialization forAssimilation of Doppler Radar Data

The three-dimensional variational data assimilation (3DVar) system of the Weather Research and Forecasting (WRF) model (WRF-Var) is further developed with a physical initialization (PI)procedure to assimilate Doppler radar radial velocity and reflectivity observations. In this updated 3DVar system, specific humidity, cloud water content, and vertical velocity are first derived from reflectivity with PI, then the model fields of specific humidity and cloud water content are replaced with the modified ones, and finally, the estimated vertical velocity is added to the cost-function of the existing WRF-Var (version 2.0) as a new observation type, and radial velocity observations are assimilated directly by the method afforded by WRF-Var. The new assimilation scheme is tested with a heavy convective precipitation event in the middle reaches of Yangtze River on 19 June 2002 and a Meiyu front torrential rain event in the Huaihe River Basin on 5 July 2003. Assimilation results show that the increments of analyzed variables correspond well with the horizontal distribution of the observed reflectivity. There are positive increments of cloud water content, specific humidity, andvertical velocity in echo region and negative increments of vertical velocity in echo-free region where the increments of horizontal winds present a clockwise transition. Results of forecast experiments show that the effects of adjusting cloud water content or vertical velocity directly with PI on forecast are not obvious. Adjusting specific humidity shows better performance in forecasting the precipitation than directly adjusting cloud water content or vertical velocity. Significantimprovement in predicting precipitation as well as in reducing the model's spin-up time are achieved when radial velocity and reflectivity observations are assimilated with the new scheme.     

利用非常规观测资料对上海特大暴雨过程的模拟研究

2001年8月5～6日,上海市区出现了建国以来破记录的一次特大暴雨.天气形势分析表明,停滞在上海的热带低压和主体位于海上的西太平洋副热带高压为此次特大暴雨的发生提供了有利的环流背景,而低压内部特有的动力、热力结构及在其内发生发展的一系列中尺度对流云团与此次暴雨的发生有直接的关系.作者利用非静力中尺度数值模式MM5V3对此次暴雨过程进行了数值模拟研究,结果表明:在非静力中尺度模式全物理过程的模拟中,采用牛顿张弛逼近法的四维资料同化方案来同化分析场和云顶亮温及上海地面气象自动站等非常规观测资料,模拟结果不仅基本上再现出大尺度及天气尺度系统的发展演变过程,而且还较好地模拟出了此次城市强暴雨过程的雨量空间分布及时序变化.因此,将较高时空密度的非常规观测资料用于高分辨率MM5中尺度数值模式,对于有效地预报城市灾害性暴雨是必要和有意义的.研究还揭示:与热带低压相伴随的偏南暖湿气流不仅是此次暴雨过程的强水汽及热量输送带,而且也是热带低压和其内中尺度对流系统维持发展的必要条件;造成暴雨的中尺度对流系统垂直向上是一具有相对暖心、高湿且低空湿对流不稳定结构的气旋性涡旋,其内强上升运动与涡柱内低空辐合、高空强辐散密切联系.     

Wind Speed and Altitude Dependent AMDAR Observational Error and Its Impacts on Data Assimilation and Forecasting

Aircraft Meteorological Data Relay (AMDAR) observations have been widely used in numerical weather prediction (NWP) because of its high spatiotemporal resolution. The observational error of AMDAR is influenced by aircraft flight altitude and atmospheric condition. In this study, the wind speed and altitude dependent observational error of AMDAR is estimated. The statistical results show that the temperature and the observational error in wind speeds slightly decrease as altitude increases, and the observational error in wind speed increases as wind speed increases. Pseudo single AMDAR observation assimilation tests demonstrate that the wind speed and altitude dependent observational error can provide more reasonable analysis increment. Furthermore, to assess the performance of wind speed and altitude dependent observational error on data assimilation and forecasting, two-month 3-hourly cycling data assimilation and forecast experiments based on the Weather Research and Forecasting Model (WRF) and its Data Assimilation system (WRFDA) are performed for the period during 1 September-31 October, 2017. The results of the two-month 3-hourly cycling experiments indicate that new observational error improves analysis and forecast of wind field and geo-potential height, and has slight improvements on temperature. The Fractions Skill Score (FSS) of the 6-h accumulated precipitation shows that new wind speed and altitude dependent observational error leads to better precipitation forecast skill than the default observational error in the WRFDA does.     

Estimating the 4DVAR analysis error of GODAE products

We explore the ocean circulation estimates obtained by assimilating observational products made available by the Global Ocean Data Assimilation Experiment (GODAE) and other sources in an incremental, four-dimensional variational data assimilation system for the Intra-Americas Sea. Estimates of the analysis error (formally, the inverse Hessian matrix) are computed during the assimilation procedure. Comparing the impact of differing sea surface height and sea surface temperature products on both the final analysis error and difference between the model state estimates, we find that assimilating GODAE and non-GODAE products yields differences between the model and observations that are comparable to the differences between the observation products themselves. While the resulting analysis error estimates depend on the configuration of the assimilation system, the basic spatial structures of the standard deviations of the ocean circulation estimates are fairly robust and reveal that the assimilation procedure is capable of reducing the circulation uncertainty when only surface data are assimilated.     

三维变分同化雷达视风速的改进方案及其数值试验

用雨滴末速度改进了万齐林等2005年提出的一种直接同化多普勒雷达径向风和回波强度时空变化所包含的风场信息——视风速的变分同化方法。通过理论推导和尺度分析发现,中尺度系统中雨滴末速度具有较大的量级,故该项在雷达回波守恒方程中起着重要作用。在原方案的基础上考虑雨滴末速度的影响,可以避免云团的局地增长作用对视风速的干扰,视风速能更准确地表示雷达回波的平流信息。台风风神的雷达观测资料分析表明,改进方案计算的视风速分布紧密、有规则,能较好地表现台风云系内的中尺度辐合辐散结构。运用中国气象科学研究院研发的三维变分同化系统(GRAPES-3D-Var)进行了一组模拟观测试验表明,改进方案的水平风场增量和位势高度增量都与模拟观测减自由积分的差值场相似,呈偶极型环流,径向风的垂直变化也得到明显改进。改进的视风速与径向风共同反演出的大气水平风场更准确。原方案的模拟观测试验则不能表现出以上特征。使用改进方案同化广州雷达观测资料后,敏感性试验的初始场中台风的中心位置与实况观测更接近,初始风场的气旋性环流更明显。敏感性试验模拟的6h降水分布与实况观测更相似,降水中心强度较小,但优于控制试验。     

Assimilation and Simulation of Typhoon Rusa （2002） Using the WRF System

Using the recently developed Weather Research and Forecasting (WRF) 3DVAR and the WRF model, numerical experiments are conducted for the initialization and simulation of typhoon Rusa (2002).The observational data used in the WRF 3DVAR are conventional Global Telecommunications System (GTS) data and Korean Automatic Weather Station (AWS) surface observations. The Background Error Statistics (BES) via the National Meteorological Center (NMC) method has two different resolutions, that is, a 210-km horizontal grid space from the NCEP global model and a 10-km horizontal resolution from Korean operational forecasts. To improve the performance of the WRF simulation initialized from the WRF 3DVAR analyses, the scale-lengths used in the horizontal background error covariances via recursive filter are tuned in terms of the WRF 3DVAR control variables, streamfunction, velocity potential, unbalanced pressure and specific humidity. The experiments with respect to different background error statistics and different observational data indicate that the subsequent 24-h the WRF model forecasts of typhoon Rusa‘s track and precipitation are significantly impacted upon the initial fields. Assimilation of the AWS data with the tuned background error statistics obtains improved predictions of the typhoon track and its precipitation.     

Euler–Lagrange equations for the spectral element shallow water system

We present the derivation of the discrete Euler–Lagrange equations for an inverse spectral element ocean model based on the shallow water equations. We show that the discrete Euler–Lagrange equations can be obtained from the continuous Euler–Lagrange equations by using a correct combination of the weak and the strong forms of derivatives in the Galerkin integrals, and by changing the order with which elemental assembly and mass averaging are applied in the forward and in the adjoint systems. Our derivation can be extended to obtain an adjoint for any Galerkin finite element and spectral element system.We begin the derivations using a linear wave equation in one dimension. We then apply our technique to a two-dimensional shallow water ocean model and test it on a classic double-gyre problem. The spectral element forward and adjoint ocean models can be used in a variety of inverse applications, ranging from traditional data assimilation and parameter estimation, to the less traditional model sensitivity and stability analyses, and ensemble prediction. Here the Euler–Lagrange equations are solved by an indirect representer algorithm.     

RTTOV散射模块在FY-3 MWHS资料同化中的应用

一直以来,受同化系统中观测算子的限制,云区的卫星微波观测资料在使用时都常会做晴空假设,只考虑大气的吸收作用而忽略散射作用,从而会造成对大气中水汽含量的高估。为了克服这一缺点,文中设计了RTTOV微波散射模块在WRF 3DVAR系统中的应用方案,使得计算辐射能衰减的同时考虑大气的散射作用,并在此基础上通过4组试验进行了风云3 A星微波湿度计亮温以及多普勒雷达反射率因子的同化研究对比。结果表明：RTTOV微波散射模块的引入,能有效抑制晴空辐射模块对大气水汽含量的高估。若先同化雷达反射率因子后再继续同化卫星资料,会发现同化雷达后,同化区域内大气水凝物含量明显增加,受此影响同化卫星资料时大气的散射作用也会随之增大。并且雷达同化区域覆盖率越大,对降水预报的改善效果也愈明显。     

面向WRF/Chem模式MOSAIC气溶胶方案的资料同化实现

CMA-CUACE-Haze化学天气模式是中国研发的气溶胶过程模拟和评估工具,但目前还没有配套的大气化学天气耦合同化分析系统。文中在CMA-MESO三维变分分析的基础上建立了区域化学天气耦合同化系统,该系统把变量之间不相关的PM_2.5 和PM_2.5-10作为控制变量,采用模型化的背景误差协方差,初步实现气溶胶观测PM_2.5和PM₁₀的同化分析。通过单站气溶胶观测的理想同化试验验证耦合同化系统设计的合理性,并针对2016年12月的重污染天气过程进行GTS传输的常规观测与气溶胶PM_2.5和PM₁₀观测资料同化与预报试验。试验结果表明,大气化学天气耦合同化系统可对气溶胶观测和天气变量观测同时进行极小化分析,大气化学变量和天气变量分析场互不影响;气溶胶观测资料的同化合理修正了大气化学背景场,PM_2.5和PM₁₀变量分析场更接近观测;气溶胶观测资料同化对污染物预报的影响可持续72 h。搭建的区域化学天气耦合同化系统能为CMA-CUACE-Haze化学天气模式提供更准确的化学初始场。