Quality Assessment of FY-4A Lightning Data in Inland China

The Lightning Mapping Imager(LMI)equipped on the FY-4 A(Feng Yun-4 A)geostationary satellite achieves lightning positioning through optical imaging and has the advantages of high temporal resolution,high stability,and continuous observation.In this study,FY-4 A LMI lightning event,group and flash data from April to August 2018 are selected,and their quality are assessed through qualitative and quantitative comparison with the ground-based Advanced Time of Arrival and Direction system(ADTD)lightning observation network data and the American International Space Station(ISS)lightning imaging sensor(LIS)data.The results show that the spatial distributions of FY-4 A lightning are consistent with those of the ground-based ADTD and ISS LIS.The temporal variation in FY-4 A lightning group frequency is consistent with that of ADTD stroke,which reflects that FY-4 A LMI can capture the lightning occurrence in inland China.Quantitative statistics show that the consistency rate of FY-4 A LMI and ISS LIS events is relatively high but their consistency rate is lower in terms of lightning group and flash data.Compared with the lightning observations by the ISS LIS and the ground-based ADTD,FY-4 A LMI reports fewer lightning events in the Tibetan Plateau.The application of Tibetan Plateau lightning data requires further processing and consideration.

FY-4 Meteorological Satellite

FY-4 is the second generation of Chinese geostationary satellite for quantitative remote sensing meteorological application. The detection efficiency, spectral bands, spatial and time resolution have been greatly improved with respect to those of first generation, as well as the radiometric calibration and sensitivity. The combination of multichannel detection and vertical sounding was first realized on FY-4, because both the Advanced Geostationary Radiation Imager(AGRI) and Geostationary Interferometric Infrared Sounder(GIIRS) are on the same spacecraft. The main performance of the payloads including AGRI, GIIRS and Lightning Mapping Imager, and the spacecraft bus are presented, the performance being equivalent to the level of the third generation meteorological satellites in Europe and USA. The acquiring methods of remote sensing data including multichannel and high precision quantitative observing, imaging collection of the ground and cloud, vertical observation of atmospheric temperature and moisture, lightning imaging observation and space environment detection are shown. Several innovative technologies including high accuracy rotation angle detection and scanning control, high precision calibration, micro vibration suppression, unified reference of platform and payload and on-orbit measurement, real-time image navigation and registration on-orbit were applied in FY-4.

Instantaneous Real-Time Detection Technology of GLI on FY-4 Geostationary Meteorological Satellite

Lightning is a typical example of an instantaneous random point source target. It has close connection with severe convective phenomena such as a thunderstorm, whose distribution, variation, position and forecasting can be acquired through lightning observation. In this paper, we discuss the way to achieve instantaneous lightning signal intensification and detection from geostationary orbit by using the differences between the lightning signal and the slowly changing background noise such as that of cloud, land and ocean, combining three methods, spectral filtering, spatial filtering and background noise, enabling removal between frames. After six months of operation in orbit, lightning within the coverage of the Geostationary Lightning Imager was effectively detected, strongly supporting the case for shorttime and real-time early warning, forecasting and tracking of severe convective phenomena in China.

Assimilation of the FY-4A AGRI Clear-Sky Radiance Data in a Regional Numerical Model and Its Impact on the Forecast of the“21·7”Henan Extremely Persistent Heavy Rainfall

Assimilation of the Advanced Geostationary Radiance Imager(AGRI)clear-sky radiance in a regional model is performed.The forecasting effectiveness of the assimilation of two water vapor(WV)channels with conventional observations for the“21·7”Henan extremely heavy rainfall is analyzed and compared with a baseline test that assimilates only conventional observations in this study.The results show that the 24-h cumulative precipitation forecast by the assimilation experiment with the addition of the AGRI exceeds 500 mm,compared to a maximum value of 532.6 mm measured by the national meteorological stations,and that the location of the maximum precipitation is consistent with the observations.The results for the short periods of intense precipitation processes are that the simulation of the location and intensity of the 3-h cumulative precipitation is also relatively accurate.The analysis increment shows that the main difference between the two sets of assimilation experiments is over the ocean due to the additional ocean observations provided by FY-4A,which compensates for the lack of ocean observations.The assimilation of satellite data adjusts the vertical and horizontal wind fields over the ocean by adjusting the atmospheric temperature and humidity,which ultimately results in a narrower and stronger WV transport path to the center of heavy precipitation in Zhengzhou in the lower troposphere.Conversely,the WV convergence and upward motion in the control experiment are more dispersed;therefore,the precipitation centers are also correspondingly more dispersed.

Characteristics of Fengyun-4A Satellite Atmospheric Motion Vectors and Their Impacts on Data Assimilation

The high observation efficiency,scanning speed and observation frequency of the Fengyun-4A(FY-4A)satellite indicates the progress of Chinese geostationary meteorological satellites.The characteristics of FY-4A atmospheric motion vectors(AMVs)derived from the high-level water vapor(WV-High)channel,mid-level water vapor(WV-Mid)channel,and infrared(IR)channel of FY-4A are analyzed,and their corresponding observation errors estimated.Then,the impacts of single-channel and multi-channel FY-4A AMVs on RMAPS-ST(the Rapid-refresh Multi-scale Analysis and Prediction System-Short Term)are evaluated based on one-month data assimilation cycling and forecasting experiments.Results show that the observation errors of FY-4A AMVs from the three channels have an explicit vertical structure.Results from the cycling experiments indicate that the assimilation of AMVs from WV-High produces more apparent improvement of the wind in the upper layer,while a more positive effect in the lower layer is achieved by the assimilation of AMVs from IR.Furthermore,the assimilation of AMVs from IR is more skillful for medium and moderate precipitation than from other channels owing to the good quality of data in the lower layer in the AMVs from IR.Assimilation of FY-4A AMVs from the three channels could combine the advantages of assimilation from each individual channel to improve the wind in the upper,middle and lower layers simultaneously.

Quantitative Characteristics Analysis of FY4A Satellite Products during a Thunderstorm Weather Process

Based on the lightning monitoring and FY4A satellite data in 12 periods during a thunderstorm,the relationship between lightning activity and four satellite digital products:blackbody radiation brightness temperature(TBB),cloud top temperature(CTT),cloud top height(CTH)and cloud top pressure(CTP)was quantitatively analyzed.The following conclusions were obtained:(1)at lightning location,90.5%of TBB values were less than 214.1 K;88.5%of CTT values were less than 207.7 K;88.5%of CTP values were less than 137.7 hPa,and 88.5%of CTH values were greater than 14872 m.At location without lightning,92.5%of TBB values were greater than 214.1 K;90.4%of CTT values were greater than 207.7 K;89%of CTP values were greater than 137.7 hPa,and 92%of CTH values were less than 14872 m.(2)Lightning activity was concentrated in the cloud area with TBB between 190-210 K,CTT between 185-210 K,CTP between 50-150 hPa and CTH between 12-18 km.Lightning intensity was roughly positively correlated with TBB,CTT and CTP,and negatively correlated with CTH.With the increase of CTH,lightning intensity decreased.(3)TBB,CTT,CTP and CTH can well indicate the location and activity frequency of lightning in thunderstorm weather.

FY-4卫星闪电成像仪设计与实现

介绍了我国第一台探测闪电的空间光学遥感仪器——风云四号(FY-4)卫星装载的闪电成像仪的设计与实现。给出了空间光学闪电探测原理。FY-4A星闪电成像仪采用小F数透射光学系统,用双镜头拼接实现大视场覆盖;采用高稳定度超窄带多腔干涉滤光片实现闪电信号滤波,以高速多抽头大光敏元CCD器件为敏感元件获取闪电与背景图像,由实时事件处理器在积分时间内按像元完成焦面数据的多帧背景评估、背景去除、阈值比较和闪电事件编码;采用与闪电事件一致的视场分辨率进行空间滤波,降低云层、陆地和海洋等背景信号对闪电信号的影响。研制的闪电成像仪由闪电成像仪主体、闪电信息处理盒、闪电管理与温控盒和闪电配电盒组成,设计了闪电探测、地标观测和FPGA程序上注三种工作模式。FY-4A星闪电成像仪研制中突破了闪电成像仪分系统总体、超窄带滤光片应用、高帧率CCD器件、实时事件处理器,以及闪电成像仪实验室标定与验证等关键技术。闪电成像仪的指标与国际同类仪器相当。至目前为此在轨测试表明:该闪电成像仪能实现对不同强度闪电事件实时探测,具备对强对流天气过程完整监测和跟踪能力。展望了后续我国闪电成像仪技术的发展和应用需求。

FY-4卫星应用和发展

介绍了我国新一代静止气象卫星风云四号(FY-4)卫星应用及其发展。给出了FY-4卫星装载的先进静止轨道辐射成像仪、静止轨道干涉式红外探测仪、闪电成像仪和空间环境监测仪4种主要观测仪器,以及卫星的观测能力和提供的定量化产品,并与我国现有的业务卫星风云二号(FY-2)卫星和国际同期在轨静止气象卫星性能进行了比较。FY-4光学卫星系列与美国GOES-R、日本Himawari-8/9卫星和欧洲MTG卫星性能相似,属于与国际同期发展的先进静止气象卫星。给出了FY-4A星获得的图像和数据。列出了FY-4A星的基本定量产品,给出了使用的仪器、数据特性、物理意义,以及应用领域、方法和范例。描述FY-4卫星提供的定量化产品及其在数值天气预报、气候、生态环境、专业气象服务、人工影响天气、空间天气监测预警等领域的应用,并介绍了FY-4A星在轨测试期间的部分应用。对FY-4后续业务卫星发展进行了展望。

FY-4A LMI Observed Lightning Activity in Super Typhoon Mangkhut(2018)in Comparison with WWLLN Data

Using lightning observations from the Fengyun-4A Lightning Mapping Imager(FY-4A LMI),best-track data from the Shanghai Typhoon Institute,bright temperature(TBB)data from Himawari-8 satellite,and composite reflectivity(CR)data from the South China radar network,we investigate the spatiotemporal distribution of lightning activity and convective evolution during the landfall of Super Typhoon Mangkhut,the strongest landing typhoon in China in2018.Three stages of active total lightning are observed,and differences of lightning characteristics between the inner core and the outer rainbands are present.The onset of inner-core lightning outbreak is about 4 h ahead of the maximum intensity of the storm,providing indicative information on the change of typhoon intensity.Lightning rates in the outer rainbands increase rapidly 12 h before the landfall,and lightning activity is mainly confined in the outer rainbands after the landfall.A good correlation in hourly variation is shown between lightning rates from the LMI and TBBs from the satellite.The averaged TBB within the inner core reaches its minimum(–80℃)when the innercore lightning outbreak occurs,indicating the occurrence and enhancement of deep convection there.Lightning locations observed by the LMI has a good spatial correspondence with regions of low TBBs and high CRs,revealing the monitoring capability of the LMI to lightning activity and deep convection in landing typhoons.Comparisons between the World Wide Lightning Location Network(WWLLN)and the LMI reveal that the spatial distribution,temporal evolution,and radial pattern of lightning activity in Mangkhut observed by the two systems are consistent.Furthermore,due to the detection capability of total lightning,the LMI has advantages in revealing the higher ratio of intra-cloud lightning within the inner core in typhoon.The continuous and real-time observation ofFY-4ALMI provides an unprecedented platform for monitoring total lightning and deep convection in landing typhoons in China,which will promote the generation of new research and applications in the future.

Evaluation of Cloud Top Height Retrievals from China’s Next-Generation Geostationary Meteorological Satellite FY-4A

To evaluate the validity of cloud top height (CTH) retrievals from FY-4A, the first of China's next-generation geostationary meteorological satellite series, the retrievals are compared to those from Himawari-8, CloudSat, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and Moderate Resolution Imaging Spectroradiometer (MODIS) operational products from August to October 2017. Regarding CTHs from CloudSat, CALIPSO, and MODIS as truth, the results show that the performance of FY-4A CTH retrievals is similar to that of Himawari-8. Both FY-4A and Himawari-8 retrieve reasonable CTH values for single-layer clouds, but perform poorly for multi-layer clouds. The mean bias error (MBE) shows that the mean value of FY-4A CTH retrievals is smaller than that of Himawari-8 for single-layer clouds but larger for multi-layer clouds. For ice crystal clouds, both FY-4A and Himawari-8 obtain the underestimated CTHs. However, there is a tendency for FY-4A and Himawari-8 to overestimate the CTH values of CloudSat and CALIPSO mainly for low level liquid water clouds. The temperature inversion near the tops of water clouds may result in an overestimation of CTHs. According to the MBE change with altitude, FY-4A and Himawari-8 overestimate the CTHs mainly for clouds below 3 km, and the overestimation is slightly more apparent in Himawari-8 data than that in FY-4A values. As the cloud optical thickness (COT) increases, the CTH bias of FY-4A CTH retrievals gradually decreases. Two typical cases are analyzed to illustrate the differences between different satellites' CTH retrievals in detail.

Developing the Science Product Algorithm Testbed for Chinese Next-Generation Geostationary Meteorological Satellites：Fengyun-4 Series

Fengyun-4A（FY-4A）, the first of the Chinese next-generation geostationary meteorological satellites, launched in2016, offers several advances over the FY-2： more spectral bands, faster imaging, and infrared hyperspectral measurements. To support the major objective of developing the prototypes of FY-4 science algorithms, two science product algorithm testbeds for imagers and sounders have been developed by the scientists in the FY-4 Algorithm Working Group（AWG）. Both testbeds, written in FORTRAN and C programming languages for Linux or UNIX systems, have been tested successfully by using Intel/g compilers. Some important FY-4 science products, including cloud mask, cloud properties, and temperature profiles, have been retrieved successfully through using a proxy imager, Himawari-8/Advanced Himawari Imager（AHI）, and sounder data, obtained from the Atmospheric Infra Red Sounder, thus demonstrating their robustness. In addition, in early 2016, the FY-4 AWG was developed based on the imager testbed—a near real-time processing system for Himawari-8/AHI data for use by Chinese weather forecasters.Consequently, robust and flexible science product algorithm testbeds have provided essential and productive tools for popularizing FY-4 data and developing substantial improvements in FY-4 products.

FY-4闪电资料在厦门强降水监测预警中的应用

FY-4上的闪电成像仪(Lightning Mapping Imager,LMI)从静止轨道平台对视场覆盖范围内的闪电进行连续不间断的观测,闪电资料为监测预警强对流天气提供了重要信息。为研究FY-4闪电资料监测预警强对流的能力,以2018年5月7日厦门暴雨为研究个例,利用FY-4闪电资料、FY-4亮温资料、厦门市自动气象站降水资料以及地面闪电定位网监测数据,研究闪电数据在强降水监测预警中的应用。结果表明:FY-4闪电资料与地基闪电进行数据融合,有效减少了天基与地基闪电产品各自数据的不完整性、不确定性和误差;闪电的移动轨迹与对流云团的移动轨迹相符,且在云团移动轨迹的前方;温度梯度较大的区域和深对流内,闪电强度较强;闪电强度与雨强成正比,且闪电频数峰值多出现在降水峰值前45 min左右。