Satellite All-sky Infrared Radiance Assimilation:Recent Progress and Future Perspectives

Satellite infrared(IR)sounder and imager measurements have become one of the main sources of data used by data assimilation systems to generate initial conditions for numerical weather prediction(NWP)models and atmospheric analysis/reanalysis.This paper reviews the development of satellite IR data assimilation in NWP in recent years,especially the assimilation of all-sky satellite IR observations.The major challenges and future directions are outlined and discussed.

Assimilating All-sky Infrared Radiances from Himawari-8 Using the 3DVar Method for the Prediction of a Severe Storm over North China

Although radar observations capture storm structures with high spatiotemporal resolutions, they are limited within the storm region after the precipitation formed. Geostationary satellites data cover the gaps in the radar network prior to the formation of the precipitation for the storms and their environment. The study explores the effects of assimilating the water vapor channel radiances from Himawari-8 data with Weather Research and Forecasting model data assimilation system(WRFDA) for a severe storm case over north China. A fast cloud detection scheme for Advanced Himawari imager(AHI)radiance is enhanced in the framework of the WRFDA system initially in this study. The bias corrections, the cloud detection for the clear-sky AHI radiance, and the observation error modeling for cloudy radiance are conducted before the data assimilation. All AHI radiance observations are fully applied without any quality control for all-sky AHI radiance data assimilation. Results show that the simulated all-sky AHI radiance fits the observations better by using the cloud dependent observation error model, further improving the cloud heights. The all-sky AHI radiance assimilation adjusts all types of hydrometeor variables, especially cloud water and precipitation snow. It is proven that assimilating all-sky AHI data improves hydrometeor specifications when verified against the radar reflectivity. Consequently, the assimilation of AHI observations under the all-sky condition has an overall improved impact on both the precipitation locations and intensity compared to the experiment with only conventional and AHI clear-sky radiance data.

Assimilation of All-sky Geostationary Satellite Infrared Radiances for Convection-Permitting Initialization and Prediction of Hurricane Joaquin(2015)

Intensity forecasting is one of the most challenging aspects of tropical cyclone(TC) forecasting. This work examines the impact of assimilating high-resolution all-sky infrared radiance observations from geostationary satellite GOES-13 on the convection-permitting initialization and prediction of Hurricane Joaquin(2015) with an ensemble Kalman filter(EnKF)based on the Weather Research and Forecasting(WRF) model. Given that almost all operational global and regional models struggled to capture Hurricane Joaquin(2015)'s intensity, this study examines the potential in improving Joaquin's prediction when assimilating all-sky infrared radiances from GOES-13's water vapor channel. It is demonstrated that, after a few 3-hour cycles assimilating all-sky radiance, the WRF model was able to forecast reasonably well Joaquin's intensity,including its rapid intensification(RI). The improvement was largely due to a more realistic initial hurricane structure with a stronger, warmer, and more compact inner-core. Ensemble forecasts were used to further explore the important physical mechanisms driving the hurricane's RI. Results showed that the RI forecasts were greatly impacted by the initial inner-core vortex structure.

Evaluation of All-Sky Assimilation of FY-3C/MWHS-2 on Mei-yu Precipitation Forecasts over the Yangtze-Huaihe River Basin

All-sky (i.e., clear, cloudy, and precipitating conditions) assimilation of microwave observations shows potentially positive impacts on the improvement of the forecasts of cloud-associated weather processes. In this study, a typical mei-yu heavy precipitation event that occurred in 2017 was investigated, and the Weather Research and Forecasting data assimilation (WRFDA) as well as its 3D-Var assimilation scheme (excluding cloud and precipitation control variables) were applied to assimilate the Fengyun-3C (FY-3C) Microwave Humidity Sounder-2 (MWHS-2) observations under clear- sky (excluding the observations that are strongly affected by ice clouds and precipitation) and all-sky conditions. Three experiments including a control experiment without assimilating any observations, clear-sky, and all-sky experiments with only FY-3C/MWHS-2 observations assimilated were carried out. The results show that the all-sky assimilation approach that provides more cloud and precipitation information and increased more than 10% of the satellite data usage than the clear-sky experiment. Meanwhile, as compared with the control experiment, the all-sky assimilation reduced nearly 0.5% of the root mean square errors in the humidity fields, leading to more accurate forecast performances regarding the distribution and intensity of heavy rainfall;but it exhibited a neutral to negative impacts on the wind and temperature. Although the system used to conduct all-sky assimilation is only able to adjust control variables for moisture-, wind-, and temperature- related variables in the presence of cloud and does not benefit directly from cloud or precipitation information, the positive effects on heavy rainfall forecasts achieved in this study indicate a potential future benefit regarding disaster prevention and mitigation.

Calibration and Validation of an All-Sky Imager

The automatic all-sky imager developed by the Institute of Atmospheric Physics,Chinese Academy of Sciences,provides all-sky visible images in the red,green,and blue channels.This paper presents three major cali-bration experiments of the all-sky imager,geometric an-gular calibration,optical calibration,and radiometric calibration,and then infers an algorithm to retrieve rela-tive radiance from the all-sky images.Field experiments show that the related coefficient between retrieved radi-ance and measured radiance is about 0.91.It is feasible to use the algorithm to retrieve radiance from images.The paper sets up a relationship between radiance and the im-age,which is useful for using the all-sky image in nu-merical-simulations that predict more meteorological pa-rameters.

Preliminary Retrieval of Aerosol Optical Depth from All-sky Images

The relationship between the radiance ratio （radiance at wavelength 450 nm to 650 nm） and aerosol optical depth （AOD） is analyzed in this paper by numerical simulation and a ＂LUT＂ （look-up table） approach is then presented for the retrieval of AOD from the radiance ratio. In this LUT approach, the AOD retrieval error depends mainly on the assumption of aerosol types. From the preliminary simulation, a typical error of 15%–20% in AOD obtained from the radiance ratio is estimated, due to neglecting changes in the ground albedo and background atmosphere. At its worst, the AOD error reached a maximum of around 50%, which will be refined in the future. In the latter part of the paper, comparisons are made between AOD from the imager and from the CE-318 sun photometer, both located at Xianghe observatory in Hebei Province （39.75° N, 116.96° E）. This field experiment shows that AOD from the imager is highly correlated with AOD from the sun photometer, with a correlation coefficient of 0.95 and an average retrieval error of around 7%. A contrast experiment confirms the feasibility of retrieving AOD from all-sky images, but more analysis and future research are required to improve the accuracy.

Occurrence characteristics of branching structures in equatorial plasma bubbles:a statistical study based on all-sky imagers in China

Branching structure(BS)is a very important phenomenon in the evolution of equatorial plasma bubbles(EPBs),the mechanism of which is widely studied from observation and from simulation.However,occurrence characteristics of branching structure of equatorial plasma bubbles(BSEPBs)have not been well addressed.In this work,we used seven-years(2012-2018)of observations from two all-sky imagers to study occurrence of BSEPBs in detail.These data reveal a high incidence of BS in EPB cases;in particular,most EPBs occurring on days with geomagnetic disturbances exhibited BS.Periods when all EPBs exhibited BS increased significantly in the 2014 solar maximum.Occurrence times of BSEPBs varied with local time;most of the BSEPBs began to appear between 21:00 and 22:00 LT.During the solar maximum,some BSEPBs were observed after midnight.The data also reveal that BSEPBs are characterized primarily by two branches or three branches.Multi-branching appeared only in the solar maximum.EPB events with different coexisting branching structures increased from 2012 to 2014 and decreased from 2014 to 2018.These results strongly suggest that BSEPB occurrence is related to solar activity and geomagnetic activity,and thus provide a new perspective for future studies of EPBs as well as enriching our understanding of ionospheric irregularity.

Impact of EnKF assimilating Himawari-8 all-sky infrared radiance on the forecasting of a warm-sector rainstorm event

Warm-sector rainstorms are highly localized events, with weather systems and triggering mechanisms are not obvious,leading to limited forecasting capabilities in numerical models. Based on the ensemble Kalman filter(PSU-En KF) assimilation system and the regional mesoscale model WRF, this study conducted a simulation experiment assimilating all-sky infrared(IR)radiance for a warm-sector rainstorm in East China and investigated the positive impact of assimilating the Himawari-8 moisture channel all-sky IR radiance on the forecast of the rainstorm. Results indicate that hourly cycling assimilation of all-sky IR radiance can significantly improve the forecast accuracy of this warm-sector rainstorm. There is a notable increase in the Threat Score(TS), with the simulated location and intensity of the 3-hour precipitation aligning more closely with observations. These improvements result from the assimilation of cloud-affected radiance, which introduces more mesoscale convective information into the model's initial fields. The adjustments include enhancements to the moisture field, such as increased humidity and moisture transport, and modifications to the wind field, including the intrusion of mid-level cold air and the strengthening of lowlevel convergent shear. These factors are critical in improving the forecast of this warm-sector rainstorm event.

Intensity correction in all-sky auroral image projection transform

BY using all-sky(fish-eye)lens and highly sensitive TV camera,the effective monitoring ra-dius and time resolution of ground aurora observation have now been raised to 600 km and1/30 s(for instance,SIT-TV camera recording in NTSC format).By incorporating high-sen-sitivity imaging device(integrated from image intensifier and CCD(charge coupled device)

Assimilation of All-Sky Radiance from the FY-3 MWHS-2 with the Yinhe 4D-Var System

Compared with traditional microwave humidity sounding capabilities at 183 GHz,new channels at 118 GHz have been mounted on the second generation of the Microwave Humidity Sounder(MWHS-2)onboard the Chinese FY-3C and FY-3D polar orbit meteorological satellites,which helps to perform moisture sounding.In this study,as the allsky approach can manage non-linear and non-Gaussian behavior in cloud-and precipitation-affected satellite radiances,the MWHS-2 radiances in all-sky conditions were first assimilated in the Yinhe four-dimensional variational data assimilation(YH4DVAR)system.The data quality from MWHS-2 was evaluated based on observation minus background statistics.It is found that the MWHS-2 data of both FY-3C and FY-3D are of good quality in general.Six months of MWHS-2 radiances in all-sky conditions were then assimilated in the YH4DVAR system.Based on the forecast scores and observation fits,we conclude that the all-sky assimilation of the MWHS-2 at 118-and 183-GHz channels on FY-3C/D is beneficial to the analysis and forecast fields of the temperature and humidity,and the impact on the forecast skill scores is neutral to positive.Additionally,we compared the impacts of assimilating the 118-GHz channels and the equivalent Advanced Microwave Sounding Unit-A(AMSUA)channels on global forecast accuracy in the absence of other satellite observations.Overall,the impact of the 118-GHz channels on the forecast accuracy is not as large as that for the equivalent AMSUA channels.Nevertheless,all-sky radiance assimilation of MWHS-2 in the YH4DVAR system has indeed benefited from the 118-GHz channels.

Two methods for separating the magnetospheric solar wind charge exchange soft X-ray emission from the diffuse X-ray background

Solar wind charge exchange(SWCX)is the process of solar wind high-valence ions exchanging charges with neutral components and generating soft X-rays.Recently,detecting the SWCX emission from the magnetosphere is proposed as a new technique to study the magnetosphere using panoramic soft X-ray imaging.To better prepare for the data analysis of upcoming magnetospheric soft X-ray imaging missions,this paper compares the magnetospheric SWCX emission obtained by two methods in an XMM-Newton observation,during which the solar wind changed dramatically.The two methods differ in the data used to fit the diffuse X-ray background(DXB)parameters in spectral analysis.The method adding data from the ROSAT All-Sky Survey(RASS)is called the RASS method.The method using the quiet observation data is called the Quiet method,where quiet observations usually refer to observations made by the same satellite with the same target but under weaker solar wind conditions.Results show that the spectral compositions of magnetospheric SWCX emission obtained by the two methods are very similar,and the changes in intensity over time are highly consistent,although the intensity obtained by the RASS method is about 2.68±0.56 keV cm^(-2)s^(-1)sr^(-1)higher than that obtained by the Quiet method.Since the DXB intensity obtained by the RASS method is about 2.84±0.74 keV cm^(-2)s^(-1)sr^(-1)lower than that obtained by the Quiet method,and the linear correlation coefficient between the difference of SWCX and DXB obtained by the two methods in diffe rent energy band is close to-1,the diffe rences in magnetospheric SWCX can be fully attributed to the diffe rences in the fitted DXB.The difference between the two methods is most significant when the energy is less than 0.7 keV,which is also the main energy band of SWCX emission.In addition,the difference between the two methods is not related to the SWCX intensity and,to some extent,to solar wind conditions,because SWCX intensity typically va ries with the solar wind.In summary,both methods are robust and reliable,and should be considered based on the best available options.

基于深度学习的全天空相机成像日间云量计算研究

云量是天文领域中地基光电望远镜站址选择的重要评价参数之一。针对全天空相机成像的日间云量计算存在的问题,提出一种基于深度学习的全天空相机成像日间云量计算模型。云检测层,模型通过构建通道加权-特征融合(channel weighting-feature fusion,CWFF)结构,从而加强对云层记忆能力和深层特征的提取能力以完成云检测任务;云量计算层,模型提出一种基于云检测模型的云量计算方法,有效提高云量计算的误差率。实验表明,该方法在云检测任务中的综合准确率超过95%,在云量计算任务中的平均绝对误差不超过5%。

GOES-16 ABI观测和ERA5全天空模拟亮温揭示的热带对流水平特征尺度对比分析

本研究通过将GOES-16 ABI观测亮温和ERA5再分析资料的全天空模拟亮温进行对比分析,发现观测亮温和模拟亮温对流低值区位置大体接近,也能够定性反映出对流从发展到减弱的日变化过程,但亮温低值区域的强度存在较大差异。即使把高分辨率ABI通道13观测亮温平均到ERA5分辨率(0.25°×0.25°),在热带对流区的低亮温强度仍然高于ERA5全天空模拟亮温。因此利用主成分分析(Principal Component Analysis,PCA)和离散傅里叶变换(Discrete Fourier Transform,DFT)对任意选择的两对流区域内的观测和模拟亮温进行了尺度分析和对比。在其中对流较强的区域内,当ABI观测亮温的主成分分量从1增加到9时,水平特征尺度从700 km逐渐减小到150 km。ERA5全天空模拟亮温从主成分1增加到4时,水平特征尺度从950 km减小到270 km空间尺度,但当主成分4增加到9时,特征尺度几乎不变。在对流较弱的另一区域也能够发现ERA5模拟亮温对对流水平特征尺度有明显高估。ERA5模拟亮温各主成分的相位和观测亮温存在2 h以内的误差。由于ERA5全天空模拟亮温低值区与ERA5云冰路径大值区吻合度较高,由此可以推测ERA5云冰路径误差是造成ERA5全天空模拟亮温与ABI观测亮温差别的主要原因之一。

多波段鱼眼镜头相机实验室几何定标方法

鱼眼镜头因其独特的曲率和光学特性,使用非相似成像模型拟合后,仍有接近10像素的径向残余畸变。研究了多波段鱼眼相机的定标原理、定标流程,在非相似成像模型基础上,提出了基于分离式精密二维转台的鱼眼镜头定标方法,并通过相机姿态位置精密调整与方位转台360°旋转,光斑像素位置始终不变的方法,确定相机主点。为分析色差对鱼眼镜头径向畸变的影响,采取不同波段单独定标的方法。利用不同时刻的太阳角度数据来验证镜头定标系数,结果显示天顶角与方位角的均方根误差分别为0.226°和0.487°。该方法有效实现了鱼眼镜头相机在不同波段的几何定标,提高了像素点与入射光线角度之间的匹配精度。

风云三号C星微波湿度计资料全天候同化对台风玛莉亚预报的影响

随着风云三号系列卫星的成功发射,越来越多的卫星微波直接观测资料应用于数值天气预报的资料同化系统。并且由于卫星微波全天候同化技术可以充分利用晴天及云雨区微波观测资料,在增加同化使用的观测数据的基础上,有效提高数值天气预报准确率,该技术在卫星资料同化领域也颇受瞩目。本研究选取2018年7月的台风玛莉亚,利用WRF(Weather Research and Forecasting)模式及其同化系统WRFDA(WRF Data Assimilation)中三维变分方法,探讨风云三号C星微波湿度计观测资料的全天候同化技术在区域模式中的适用性,以及其在不同模式驱动场中的预报表现。通过对比仅同化晴空区域卫星资料的试验和全天候同化的试验结果发现,全天候条件下更多的云雨区域观测资料被有效利用,能够更好地模拟出台风玛莉亚核心区域的暖心和对称风速结构,有效改善湿度场的预报,对台风路径的预报误差平均降低了大约34%~62%,且这种正面影响均能在不同模式驱动场中得到体现。

全固态中波发射机天调网络的运用以及技术创新

全固态中波发射机天调网络作为连接发射机与天线的关键环节,其技术创新对于确保发射机高效稳定运行至关重要。通过探讨全固态中波发射机天调网络的应用及其技术创新,重点分析数字信号处理技术如何通过自动频率识别与动态调谐、高级算法与控制逻辑以及智能调谐技术的集成等关键技术点提高发射机的性能和可靠性。

日侧极光卵的可见光多波段观测特征——中国北极黄河站首次极光观测初步分析

中国在斯瓦尔巴特(Svalbard)群岛新奥尔松(Ny-Alesund)地区建立了一个永久性科学考察站——北极黄河站。北极黄河站(78．92°N,11．93 °E)的修正磁纬为76．24°。2003年11月黄河站建立了一套多波段(427．8nm,557．7nm,630．0nm)单色极光全天空CCD成像观测系统,并于2003年12月11日-2004年2月25日极夜期开展了首次极光越冬观测,获得了1200多小时的连续观测数据。利用这1200多小时观测数据制作了以时间为横轴的南北向极光活动图,对日侧极光卵的极光活动特征进行了研究。初步分析发现,沿日侧极光卵不论磁静日还是磁扰日,在三个波段上都同时观测到了四个重复出现的极光活动区,即0900MLT(磁地方时)附近的 A区、1330MLT的B区和1630MLT的C区以及1200MLT附近的G区。A区内的极光活动由低纬向高纬运动,强度逐渐变弱,形态由较宽的弧状极光向冕状极光演变;B区内的极光活动由高纬向低纬方向运动,形态呈现为冕状极光向较窄的极光弧的演变,极光强度逐渐增强;C区极光活动向高纬方向展宽,强度逐渐变弱,极光由单弧向多重弧演化;而在A与B之间存在一个纬度范围相对较窄的极光活动区G区,该区的极光呈现暗弱的冕状极光。日侧极光活动随时间从午前、正午到午后呈现出一个系统的光谱变化:绿色-红色-绿色。A区的位置出现在卫星探测的午前极光峰之中;而正午附近极光强度暗弱的G区,可部分地解释卫星观测的所谓“正午极光空白区”;卫星观测到的1500MLT附近的“极光热点”区在地面观测中可以区分为以红色极光为主的B区和以绿色极光为主的C区。

折反式红外全景天空相机光学系统设计

利用红外全景天空相机可以获得天空中云的分布和温度信息,从而可对天气状况做出判断。采用等距离投影成像方式,可根据简单的数据转换关系从天空全景图像中任意点的位置坐标得到其对应的空间角度。为了实现等距离投影红外全景相机的设计,提出了一种简洁高效的计算等距离投影反射镜面型的数值方法,并在此基础上设计了8～12μm长波红外波段全景天空相机。该相机的视场角为360°×(5°～80°),校准畸变量在±0.02%以下,MTF值在探测器Nyquist频率处高于0.4,非常适用于全景天空观测。

基于全天空F-P干涉仪反演热层垂直中性风

由于测量与计算的难度,对热层垂直中性风的观测还很不够,这影响了人们对热层及热层-电离层耦合的认识.本文基于全天空法布里-珀罗干涉仪(FPI)对热层风场的观测,提出了一种反演垂直中性风的方法.利用该方法,对北极黄河站全天空FPI观测数据进行了垂直中性风的反演计算,结果表明,高热层与低热层的垂直风平均幅值分别在40m·s-1和15m·s-1,且垂直风日变化表现出明显的时间演变特性,且与地磁ap指数的变化有一定的相关性,在地磁活动强烈时,低热层垂直风会出现高达100m·s-1的扰动,高热层甚至会达到300m·s-1的扰动,这些特征与其他学者的观测结果相一致.本文方法不需要假设垂直风均值为零,也不用限制FPI的观测方位,可用于垂直风的反演.

AMSU-A全空辐射率资料同化对台风“天鸽”的预报影响研究

利用美国NCEP(National Centers for Environmental Prediction)发展的GSI(Gridpoint Statistical Interpolation)同化系统和GSM(Global Spectral Model)全球频谱预报模式作为循环同化系统,选用2017年第13号台风为研究个例,采用全空和晴空两种同化方案对AMSU-A(The Advanced Microwave Sounding Unit-A)辐射率观测开展同化对比试验,并对台风“天鸽”个例进行5 d预报,研究AMSU-A全空辐射率同化对台风天鸽发展过程预报的影响。结果表明,全空同化方案相比晴空同化方案预报的台风路径、台风中心气压以及台风最大风速预报误差更小;全空同化方案对台风“天鸽”生命周期的模拟更加准确,更接近中国气象局发布的台风天鸽最佳路径的最低气压,而晴空同化方案预报的台风发展较弱,无法预报出成熟期的台风强度;全空同化方案能够增加低层通道海上厚云覆盖区域辐射率资料同化量,增幅占AMSUA同化观测总量10%,从而改进海洋区域天气系统的热力场结构。