Statistics of cloud heights over the Tibetan Plateau and its surrounding region derived from CloudSat data

Cloud-radiation interaction has a large impact on the Earth＇s weather and climate change, and clouds with different heights cause different radiative forcing. Thus, the information on the statistics of cloud height and its variation in space and time is very important to global climate change studies. In this paper, cloud top height （CTH）, cloud base height （CBH） and cloud thickness over regions of the Tibetan Plateau, south slope of the plateau and South Asian Monsoon are analyzed based on CloudSat data during the period from June 2006 to December 2007. The results show that frequency of CTH and CBH in unit area over the studied regions have certain temporal-spatial continuity. The CTH and CBH of different cloud types have different variation scopes, and their seasonal variations are distinct. Cloud thickness is large （small） in summer （winter）, and the percentages of different cloud types also have certain regularity.

Climatology of Cloud-base Height from Long-term Radiosonde Measurements in China

Clouds are critical to the global radiation budget and hydrological cycle, but knowledge is still poor concerning the observed climatology of cloud-base height （CBH） in China. Based on fine-resolution sounding observations from the China Radiosonde Network （CRN）, the method used to estimate CBH was modified, and uncertainty analyses indicated that the CBH is good enough. The accuracy of CBH estimation is verified by the comparison between the sounding-derived CBHs and those estimated from the micro-pulse lidar and millimeter-wave cloud radar. As such, the CBH climatology was compiled for the period 2006-16. Overall, the CBH exhibits large geographic variability across China, at both 0800 Local Standard Time （LST） and 2000 LST, irrespective of season. In addition, the summertime cloud base tends to be elevated to higher altitudes in dry regions [i.e., Inner Mongolia and the North China Plain （NCP）]. By comparison, the Tibetan Plateau （TP）, Pearl River Delta （PRD） and Sichuan Basin （SCB） have relatively low CBHs （〈 2.4 km above ground level）. In terms of seasonality, the CBH reaches its maximum in summer and minimum in winter. A low cloud base tends to occur frequently （〉 70%） over the TP, PRD and SCB. In contrast, at most sites over the Yangtze River Delta （YRD） and the NCP, about half the cloud belongs to the high-cloud category. The CBH does not exhibit marked diurnal variation in summer, throughout all CRN sites, probably due to the persistent cloud coverage caused by the East Asia Summer Monsson. To the best of our knowledge, this is the first CBH climatology produced from sounding measurements in China, and provides a useful reference for obtaining observational cloud base information.

Assessment of FY-4A and Himawari-8 Cloud Top Height Retrieval through Comparison with Ground-Based Millimeter Radar at Sites in Tibet and Beijing

The accuracy of passive satellite cloud top height (CTH) retrieval shows regional dependence. This paper assesses the CTH derived from the FY-4A and Himawari-8 satellites through comparison with those from the ground-based millimeter radar at two sites: Yangbajing, Tibet, China (YBJ), and the Institute of Atmospheric Physics (IAP), Beijing, China. The comparison shows that Himawari-8 missed more CTHs at night than FY-4A, especially at YBJ. It is found that the CTH difference (CTHD;radar CTH minus satellite CTH) for FY-4A and Himawari-8 is 0.06 ± 1.90 km and −0.02 ± 2.40 km at YBJ respectively, and that is 0.93 ± 2.24 km and 0.99 ± 2.37 km at IAP respectively. The discrepancy between the satellites and radar at IAP is larger than that at YBJ. Both satellites show better performance for mid-level and low-level clouds than for high-level clouds at the two sites. The retrievals from FY-4A agree well with those from Himawari-8, with a mean difference of 0.08 km at YBJ and 0.06 km at IAP. It is found that the CTHD decreases as the cloud depth increases at both sites. However, the CTHD has no obvious dependence on cloud layers and fractions. Investigations show that aerosol concentration has little impact on the CTHD. For high and thin clouds, the CTHD increases gradually with the increase of the surface temperature, which might be a key factor causing the regional discrepancy between IAP and YBJ.

Verification and Correction of Cloud Base and Top Height Retrievals from Ka–band Cloud Radar in Boseong,Korea

In this study,cloud base height（CBH） and cloud top height（CTH） observed by the Ka-band（33.44 GHz） cloud radar at the Boseong National Center for Intensive Observation of Severe Weather during fall 2013（September-November） were verified and corrected.For comparative verification,CBH and CTH were obtained using a ceilometer（CL51） and the Communication,Ocean and Meteorological Satellite（COMS）.During rainfall,the CBH and CTH observed by the cloud radar were lower than observed by the ceilometer and COMS because of signal attenuation due to raindrops,and this difference increased with rainfall intensity.During dry periods,however,the CBH and CTH observed by the cloud radar,ceilometer,and COMS were similar.Thin and low-density clouds were observed more effectively by the cloud radar compared with the ceilometer and COMS.In cases of rainfall or missing cloud radar data,the ceilometer and COMS data were proven effective in correcting or compensating the cloud radar data.These corrected cloud data were used to classify cloud types,which revealed that low clouds occurred most frequently.

A STUDY ON THE APPLICATION OF FY-2E CLOUD DRIFT WIND HEIGHT REASSIGNMENT IN NUMERICAL FORECAST OF TYPHOON CHANTHU(1003) TRACK

In this paper, we first analyzed cloud drift wind(CDW) data distribution in the vertical direction, and then reassigned the height of every CDW in the research domain in terms of background information, and finally, conducted contrast numerical experiments of assimilating the CDW data before and after reassignment to examine the impacts on the forecast of the track of Typhoon Chanthu(1003) from 00:00(Coordinated Universal Time) 21 July to 00:00 UTC23 July, 2010. The analysis results of the CDW data indicate that the number of CDWs is mainly distributed in the midand upper-troposphere above 500 h Pa, with the maximum number at about 300 h Pa. The height reassigning method mentioned in this work may update the height effectively, and the CDW data are distributed reasonably and no obvious contradiction occurs in the horizontal direction after height reassignment. After assimilating the height-reassigned CDW data, especially the water vapor CDW data, the initial wind field around Typhoon Chanthu(1003) became more reasonable, and then the steering current leading the typhoon to move to the correct location became stronger. As a result, the numerical track predictions are improved.

The Significant Role of Radiosonde-measured Cloud-base Height in the Estimation of Cloud Radiative Forcing

The satellite-based quantification of cloud radiative forcing remains poorly understood,due largely to the limitation or uncertainties in characterizing cloud-base height(CBH).Here,we use the CBH data from radiosonde measurements over China in combination with the collocated cloud-top height(CTH) and cloud properties from MODIS/Aqua to quantify the impact of CBH on shortwave cloud radiative forcing(SWCRF).The climatological mean SWCRF at the surface(SWCRFSUR),at the top of the atmosphere(SWCRFTOA),and in the atmosphere(SWCRFATM) are estimated to be-97.14,-84.35,and 12.79 W m^(-2),respectively for the summers spanning 2010 to 2018 over China.To illustrate the role of the cloud base,we assume four scenarios according to vertical profile patterns of cloud optical depth(COD).Using the CTH and cloud properties from MODIS alone results in large uncertainties for the estimation of SWCRFATM,compared with those under scenarios that consider the CBH.Furthermore,the biases of the CERES estimation of SWCRFATM tend to increase in the presence of thick clouds with low CBH.Additionally,the discrepancy of SWCRFATM relative to that calculated without consideration of CBH varies according to the vertical profile of COD.When a uniform COD vertical profile is assumed,the largest SWCRF discrepancies occur during the early morning or late afternoon.By comparison,the two-point COD vertical distribution assumption has the largest uncertainties occurring at noon when the solar irradiation peaks.These findings justify the urgent need to consider the cloud vertical structures when calculating the SWCRF which is otherwise neglected.

Cloud Top Height Retrieval Using Polarizing Remote Sensing Data of POLDER

A retrieval method of cloud top heights using polarizing remote sensing is proposed in this paper. Using the vector radiative transfer model in a coupled atmosphere-ocean system, the factors influencing the upwelling linear polarizing radiance at top-of-atmosphere are analyzed, which show that the upwelling linear polarizing radiance varies remarkably with the cloud top height, but has negligible sensitivity with cloud albedo and aerosol scattering above the cloud layer. Based on this property, a cloud top height retrieval algorithm using polarizing remote sensing was developed. The algorithm has been applied to the polarizing remote sensing data of Polarization and Directionality of the Earth's Reflectances-2 (POLDER-2). The retrieved cloud top height from POLDER-2 compares well with the Moderate Resolution Imaging Spectroradiometer (MODIS) operational product with a bias of 0.83 km and standard deviation of 1.56 km.

利用CloudSat资料分析青藏高原、高原南坡及南亚季风区云高度的统计特征量

云与辐射的相互作用对全球的天气和气候变化过程有着重要的影响,不同高度的云有着不同的辐射强迫,获得云体高度及其在时空上的变化对研究全球气候的变化有着重要意义。本文利用云卫星上的云廓线雷达(CloudSat/CPR)2006年6月—2007年12月期间的资料,对比分析了青藏高原、高原南坡和南亚季风区域不同云类的云顶、云底高度和云厚统计量。结果表明,在所研究区域单位面积上的云顶和云底高度变化具有一定的时空连续性,不同云类的云顶和云底高度存在不同的变化范围,且随着季节的改变均有明显的变化;同时各区域不同云类的云体厚度在夏季较大,冬季较小;各区域不同云类所占的比例(云量)也具有一定的季节变化规律。

基于CloudSat卫星资料的新疆三大山区不同类型云高特征研究

作为空中水资源的重要组成部分,云在地球水循环过程和气候系统中扮演着重要角色,不同高度的云因其物理特性和动力过程的不同而对人工增水作业具有不同的指示意义.采用2007年1月至2008年12月的美国宇航局(NASA)云卫星(CloudSat)2B-CLDCLASS资料,从不同类型云的高度分布特征分析了新疆阿尔泰山、天山和昆仑山区的云水资源情况.结果表明:各个季节三大山区高层云所占比例均较大,在20%以上,其中,天山山区和昆仑山区雨层云所占比例也较大,在15%以上.三大山区不同云的云顶和云底高度年变化趋势基本一致,昆仑山区各类型云的平均云顶和云底的高度最大,阿尔泰山区的最低.

利用地面观测评估ERA5云底高度产品

云底高度对天气预报和飞行保障有着重要作用,但目前十分缺乏高时空分辨率的云底高度资料。本文利用中国东部地区云底高度的地面观测对ERA5再分析资料的云底高度进行评估,基于地面观测和ERA云底高度的小时平均偏差对ERA进行了初步订正和验证,综合分析了中国地区的云底高度分布特征。分析发现,ERA5再分析数据中的云底高度比观测值整体偏低,其中高估了低云的云底高度,低估了中、高云云底高度,且中、高云云底高度的偏差较大。另外,在有降水的条件下,ERA5数据中低云云底高度的准确性比无降水条件下的相对较高,但中、高云云底高度的准确性却更低。综合地面观测和ERA5数据云底高度的日变化和月变化分析发现,在我国东部地区,白天的云底高度整体低于夜间,峰值出现在夜间22:00(北京时间,下同),谷值出现在上午09:00,秋季和冬季的ERA5云底高度月变化与地面观测资料较为一致。基于日变化分析获得的小时平均偏差对ERA5数据的云底高度进行订正和验证,分析发现,2010~2019年间,中国地区的云底高度呈现自西北向东南逐渐降低的分布特征;中国南部、东北地区和青藏高原区域的云底高度相对较低,其中,青藏高原夏季和秋季的云底高度较低,春季和冬季较高,而沙漠地区在四个季节的云底高度都较高。

基于云高仪探测数据的湖北省襄阳市云宏观特征分析

利用湖北省襄阳市2019—2021年3a云高仪观测资料,统计云出现频率、云层数和云底高度等指标,对该地区云的宏观特征进行分析,结果表明:(1)襄阳市全年云系覆盖率较高,3a平均云出现频率为61.3%。云出现频率呈现夏季高于冬季的季节变化特征,以及白天低、夜间高的日变化特征。(2)云层数以单层云为主(78.3%),尤其在冬季(80.1%)。多层云分布中,双层云所占比例最大(18.5%),且多层云出现频率随云层数增加而降低;多层云出现频率夏季高、冬季低,主要与夏季水汽相对充足、温度较高使得对流旺盛等因素有关。(3)云底高度在1.0km以下的云,1月份出现频率为26%,云系分布较为分散,7月份达40%,云系相对集中;而云底高度在1.0~3.5km之间的云,1月份出现频率达57%,云系分布较为集中,7月份为24%,云系相对分散。(4)所有的对流性降水云,出现在6—8月的占83.3%,傍晚前后出现对流性降水的概率最大;云底高度上,对流性降水云平均云底高度较所有降水云平均结果更低,低云所占比例更大。

基于机载LiDAR数据的单木分割研究

基于机载LiDAR数据进行单木分割,分别讨论了基于冠层高度模型的单木提取方法(FCHM)以及基于归一化点云的单木提取算法(FNPC)。FCHM的分离单木的总正确率的均方根值为0.93,F_Score的均方根值为0.62,FNPC算法分别为0.90和0.56,FCHM算法的精度优于FNPC算法。通过从不同的林相、林型、林分密度的分析得出,在林分密度小于400棵/hm^(2)的单层针叶林中提取效果最好。

基于多线激光的大尺寸水冷板高度测量系统

为了满足现代工业非接触、高精度以及快速对大尺寸工件表面高度测量的需求,设计了一种基于多线激光的水冷板高度测量系统,该系统由运动控制、图像采集和图像处理3部分组成。首先对系统的硬件进行了选型;其次,根据线激光获取的点云数据,提出了水冷板高度测量方法;为了减少干扰点云的影响,采用高斯滤波对模板匹配后的点云去噪,并基于统计的方法筛选参与高度计算的点云数据。实验数据表明:系统重复测量误差小于30μm,测量时间少于1 min,满足企业生产需求。

CloudSat云底高度外推估计的可行性分析

云底高度对于全球辐射平衡以及航空飞行均具有重要影响。针对CloudSat与MODIS主、被动观测的优缺点,本文提出了利用MODIS云分类信息进行CloudSat云底高度外推估计的技术。首先使用MODIS和CloudSat数据,利用回归分析方法比较了基于云类型(CSAT)与基于距离(MSAT)的云底高度估计方法的优劣。此外,分析了中国及周边地区CloudSat各类云云底高度的均一性特征。最后,利用CloudSat各类云云底高度的统计特征,建立了一种基于云类型和距离权重的云底高度估计方法,并对该方法进行了验证和分析。结果表明,利用该方法得到的MODIS各类云云底高度估计误差的标准差均小于1.5 km,除了积雨云在观测点与待测点距离大于400 km的估计误差均值稍大于1.5 km外,各种情况下其他各类云的云底高度估计误差的均值均小于1.5 km。

基于机载激光雷达和机器学习的林分平均胸径遥感估测

为探究不同模型对林分平均胸径的预测精度,使用贵州省桂花国有林场马厂工区同步获取的机载激光雷达点云数据和地面实测样地数据,通过提取样地水平的点云特征变量,采用方差膨胀因子分析和皮尔逊相关性检验进行自变量选择,建立机器学习模型估测样地平均胸径。结果表明:1)点云特征变量,如平均冠层高度和高度偏态与林分平均胸径有很强的相关性。2)机器学习模型(随机森林、支持向量机、最近邻算法)优于多元线性回归模型,其中,随机森林的拟合效果最好。随机森林的决定系数(R^(2))为0.71,均方根误差(RMSE)为2.50。3)通过柳杉纯林、针叶混交林、针阔混交林、马尾松纯林4种森林类型的林分平均胸径预测值与实际值差值,进一步证实随机森林模型精度最高,拟合效果最好。利用机载激光雷达点云数据提取点云特征变量,并构建基于机器学习算法的林分平均胸径估测模型是可行的,该方法的精度能满足森林资源调查的应用需求,可作为辅助林业调查工作的技术手段。

Synergistic Use of AIRS and MODIS for Dust Top Height Retrieval over Land

It is nontrivial to extract the dust top height（DTH） accurately from passive instruments over land due to the complexity of the surface conditions. The Moderate Resolution Imaging Spectroradiometer（MODIS） deep blue（DB） algorithm can be used to infer the aerosol optical depth（AOD） over high-reflective surfaces. The Atmospheric Infrared Sounder（AIRS） can simultaneously obtain the DTH and optical depth information. This study focuses on the synergistic use of AIRS observations and MODIS DB results for improving the DTH by using a stable relationship between the AIRS infrared and MODIS DB AODs. A one-dimensional variational（1DVAR） algorithm is applied to extract the DTH from AIRS. Simulation experiments indicate that when the uncertainty of the dust optical depth decreases from 50% to 20%, the improvement of the DTH retrieval accuracy from AIRS reaches 200 m for most of the assumed dust conditions. For two cases over the Taklimakan Desert, the results are compared against Cloud-Aerosol Lidar with Orthogonal Polarization（CALIOP） measurements. The results confirm that the MODIS DB product could help extract the DTH over land from AIRS.

Comparison between MODIS-derived Day and Night Cloud Cover and Surface Observations over the North China Plain

Satellite and human visual observation are two of the most important observation approaches for cloud cover. In this study, the total cloud cover （TCC） observed by MODIS onboard the Terra and Aqua satellites was compared with Synop meteorological station observations over the North China Plain and its surrounding regions for 11 years during daytime and 7 years during nighttime. The Synop data were recorded eight times a day at 3-h intervals. Linear interpolation was used to interpolate the Synop data to the MODIS overpass time in order to reduce the temporal deviation between the satellite and Synop observations. Results showed that MODIS-derived TCC had good consistency with the Synop observations; the correlation coefficients ranged from 0.56 in winter to 0.73 in summer for Terra MODIS, and from 0.55 in winter to 0.71 in summer for Aqua MODIS. However, they also had certain differences. On average, the MODIS-derived TCC was 15.16% higher than the Synop data, and this value was higher at nighttime （15.58%-16.64%） than daytime （12.74%-14.14%）. The deviation between the MODIS and Synop TCC had large seasonal variation, being largest in winter （29.53%-31.07%） and smallest in summer （4.46%-6.07%）. Analysis indicated that cloud with low cloud-top height and small cloud optical thickness was more likely to cause observation bias. Besides, an increase in the satellite view zenith angle, aerosol optical depth, or snow cover could lead to positively biased MODIS results, and this affect differed among different cloud types.

基于相干激光雷达气象多要素探测

相干激光测风雷达具有体积小、探测效率高、信噪比强等特点,在气象、航空保障以及风电等行业得到了广泛的应用,但是目前雷达数据产品大多是利用回波多普勒频移信息反演的风场数据产品,未对雷达回波信号强度信息进行数据挖掘。针对当前相干激光测风雷达数据产品开发不足的问题,介绍了一种新型基于相干探测的激光雷达数据产品生成技术,该技术在探测风场的同时,实现云高、消光系数及能见度等多种气象要素数据产品的生成。首先,通过分析回波信号特性,推导出雷达回波信号功率的计算方法,获取探测范围内各距离位置的回波信号强度信息。在此基础上,利用阈值法和微分零叉法反演出云底高度,同时采用改进型Klett算法实现大气消光系数的反演,继而实现能见度的测量。最后,与气象站能见度仪、激光云高仪以及532 nm气溶胶激光雷达进行数据对比验证试验,试验数据结果显示:云高对比精度可达5.0%,能见度对比精度可达12.2%,消光系数对比有较好的一致性,并可长期在多种气象环境下连续测量。

机载激光点云单木分割方法对比及精度分析

通过对机载激光点云基于冠层高度模型、基于点云以及基于层堆叠种子点分割方法,对针叶林、阔叶林、针阔混交林3种不同类型林分进行单木分割,并通过计算单木分割的检测率、正确率、F-score等精度指标,探究不同分割方法在不同类型林分的适用性。实验结果表明,对于针叶林,基于点云分割方法的分割精度最高,基于层堆叠种子点分割方法对树木分割的正确率最高;对于阔叶林和针阔混交林,基于层堆叠种子点分割方法的分割精度和检测率较高,优于其他两种分割方法。

基于三维点云模型获取单木树高和胸径的研究

使用无人机和智能手机分别从空中和地面拍摄的样地林分影像构建三维点云模型,并从三维点云模型中获取样地内单木树高和胸径参数。本研究以池杉人工林为研究对象,利用PhotoScan Agisoft软件对无人机倾斜摄影和智能手机近景摄影的样地影像进行三维重建,通过对齐照片、控制点刺点、对齐优化、建立密集点云等步骤,构建出与样地实景相符的三维点云模型;通过LiDAR 360软件从样地三维点云模型中获取单木的树高和胸径参数,将其与实地测量获取的单木树高和胸径参数进行对比分析。利用无人机和智能手机影像构建的三维模型可以满足《数字航空摄影测量测图规范》的精度要求。通过实测数据和点云数据获取的树高和胸径的平均差值分别为-0.9 m和-0.8 cm,平均相对误差分别为5.4%和7.1%。以实测数据作为自变量x,以点云数据作为因变量y,树高和胸径回归模型的R2分别为0.809 5和0.918 4。将倾斜摄影和近景摄影的点云模型统一在同一空间参考基准下可构建出与样地实景相匹配的三维点云模型,从样地三维点云模型中获取的单木树高和胸径与实地测量结果具有较好的线性相关性,本研究所使用的方法可以代替传统林业调查手段应用于人工林。