Improved fog detection from satellite in the presence of clouds

This study analyzes radiative effect of the higher clouds over the fog layer and presents the improvement of fog detection over the Korean peninsula, utilizing satellite data of the Multi-functional Transport SATellite (MTSAT)-1R and the MODerate resolution Imaging Spectroradiometer (MODIS) and the Look-Up Table (LUT) based on Radiative Transfer Model (RTM) simulations. Fog detection utilizing the satellite data from visible (0.68 µm) and infrared (3.75 µm and 10.8 µm) channels has been evaluated in comparison with ground-based observations over 52 meteorological stations in the Korean Peninsula from March 2006 to February 2007. The threshold values for fog sensing have been derived from the difference (i.e., T_3.7–11) in brightness temperature between 3.75 µm (T_3.7) and 10.8 µm (T₁₁) during day and night, and also from the reflectivity at 0.68 µm (R_0.68) in the daytime. In the twilight, however, the difference between the temperature values at 10.8 µm and their maximum within previous 15 days (i.e., T_11max-11) are used instead, because the 3.75 µm channel is inaccurate for the fog detection at dawn/dusk. The sensitivity of the T_3.7–11 values with respect to the clouds is investigated based on the cloud variables such as its height, optical thickness, and amount. The values of T_3.7–11 are the most sensitive to cloud height, followed by cloud optical thickness and effective radius, while R0.68 is insensitive to cloud height. The sensitivity is examined with various conditions of cloud phases and day/night. Sixteen cases among eighteen fog occurrences, which have been unable to be sensed by using only the conventional threshold values, are successfully detected with the additional LUT corrections, indicating a significant improvement. The method of fog detection in this study can be useful to the Communication, Ocean, and Meteorological Satellite (COMS) Meteorological Data Processing System (CMDPS) by reducing the cloud effect on fog sensing.     

用于空间对地遥感的一个参数化辐射传输计算模式

一个足够精度的参数化向上亮度模式对空间对地遥感的大气订正应用是很有意义的。依据向上天空亮度对大气散射相函数、光学厚度、太阳天顶角、空间方位和天顶角等有关参数的依赖特性,基于大量的天空亮度数值模拟计算资料,本文发展了一个关于向上天空亮度的参数化模式。39440组的数值模拟对比资料表明,在气溶胶光学厚度#Aτt≤1,太阳天顶角小于72°,μ≤0.432 （天顶角小于65°）以及无云的条件下,由现在的参数化的模式所计算的向上亮度的误差一般小于6%。地表反射率越大,参数化模式的精度越高。参数化模式的精度一般随着光学厚度和太阳天顶角的增大而变差。     

Ultraviolet sky radiance distributions of translucent overcast skies

Summary The diffuse sky radiation component in the ultraviolet wavelengths is often at least 50% of the global irradiance under clear skies, and is the dominant component of ultraviolet global radiation under translucent overcast skies. The distribution of sky radiance was measured in a rural area and modeled for wavelength bands of ultraviolet-B (UVB, 280–320 nm) and ultraviolet-A (UVA, 320–400 nm). Sky radiance measurements were made during the summer of 1993 over a wide range of solar zenith angles using radiance sensors mounted on a hand-operated hemispherical rotation mount. UVB irradiance measurements were also made during each scan. Since the ratio of measured irradiance under overcast skies and that predicted for clear skies was not correlated with cloud base height, opaque cloud fraction, or solar zenith angle, it was concluded that the scattering from the clouds dominated the global irradiance, and this scattering was relatively unaffected by the scattering off opaque clouds in the translucent atmosphere.Analysis of the translucent overcast sky UVA and UVB radiance measurements using a semi-empirical distribution model showed that the spectral influences on multiple scattering, single scattering, and horizon brightening components of the distributions agreed with basic atmospheric radiation theory. The best model used solar zenith, the sky zenith, and the scattering angle with resultant coefficient of determination values of 0.62 and 0.25 for the UVA and UVB respectively. The developed equations can be applied directly to the diffuse sky irradiance on the horizontal to provide radiance distributions for the sky.With 6 Figures     

晴空与有云大气辐射分布的数值模拟及其对全天空图像云识别的应用

全天空图像自动云识别的研究相对来说是一项较新的研究领域。当前国际上较为通用的全天空图像云识别方法主要依靠图像蓝红灰度值对比的阈值方法进行判断。然而非洁净大气中气溶胶的增多给云识别增加了难度,同时太阳高度角不同天空色度分布情况也不同。文中利用Libradtran辐射传输模式,计算了不同能见度不同太阳高度角情况下3个典型波长(450,550,650nm;蓝/绿/红)无云及有云大气的天空辐亮度分布情况,并进行了比较分析。结果表明,相同太阳高度角情况下,无云及有云大气中蓝红比值随能见度的下降呈单调下降趋势。在特定的云光学厚度和能见度情况下,天空色度彼此呈现出类似的分布状况。全天空图像阈值判断云识别自适应算法的建立需要与太阳高度角、地面能见度联系起来。当前尚无法建立一个判断阈值随太阳高度角以及能见度变化的函数关系式。较为可行的办法是建立典型能见度、典型太阳高度角情况下的辐射信息库,在具体云识别时,首先确定太阳高度角,而后根据天空辐射比情况确定天空能见度,并利用辐射信息标准库做云或非云判别。该工作为全天空云识别算法提供判别依据,同时建立云识别随能见度和太阳高度角变化的判别信息库。     

双波段大气向下红外辐射云遥感数值模拟

为了将卫星多通道遥感技术应用到地基测云红外遥感中,利用SBDART(平面平行辐射传输模式)模式对双波段(10.3～11.3μm,11.5～12.5μm)大气向下红外辐射进行计算。分析了双波段辐射亮温差的特点,讨论了不同能见度、不同天空状况对两通道亮温差的影响,同时将利用实际测量的亮温差值所判断出来的天空状况与实际天空状况进行了对比。结果表明,在能见度较好时天顶方向辐射率变化比天边方向明显;在高能见度时一定天顶角范围内,可以利用不同天空状况亮温差不同的特点对云进行识别。     

A novel approach to fog/low stratus detection using Meteosat 8 data

A method is presented for fog and low stratus detection from daytime satellite imagery based on Meteosat 8 SEVIRI (Spinning-Enhanced Visible and Infra-Red Imager) data. With its excellent spatial, spectral and temporal resolutions, this imagery is an ideal basis for operational fog monitoring. The scheme utilizes a range of pixel-based and novel object-oriented techniques to separate fog and low stratus clouds from other cloud types. Fog and low stratus are identified by a number of tests which explicitly and implicitly address fog/low stratus spectral, spatial and microphysical properties. The scheme's performance is evaluated using ground-based measurements of cloud height over Europe. The algorithm is found to detect low clouds very accurately, with probabilities of detection (POD) ranging from 0.632 to 0.834 (for different inter-comparison approaches), and false alarm ratios (FAR) between 0.059 and 0.021. The retrieval of sub-pixel and temporal effects remain issues for further investigation.     

云辐射效应在华北持续性大雾维持和发展中的作用

观测研究发现华北地区的持续性大雾天气通常伴随高层云的存在,具有云-雾共存结构特征,为揭示云在持续性大雾维持和发展中的作用,利用中尺度数值模式WRF,结合华北雾霾观测试验期间的卫星、探空、地面观测、系留气艇、微波辐射计等观测资料,研究了2011年12月3—6日和2013年1月28—31日两次华北持续性大雾天气形成和发展演变过程。在模拟与观测对比检验研究的基础上,重点开展了云辐射效应在大雾维持和发展中作用的探讨。研究结果表明:两次大雾过程持续时间超过48 h,近地面具有偏南暖湿平流,在持续性大雾发展过程中,均出现了由单层雾发展为云-雾共存结构,一般是雾形成24 h以后有中高云移到雾层之上,云底高度在3 km以上,云厚超过3.5 km,云中以冰晶和雪晶为主。白天云-雾共存结构出现后,云-雾的反照率效应使地表接收的短波辐射减少71%—84%,地面增温效应显著减小,从而阻碍了大雾的消散过程,使大雾天气得以维持,同时由于云-雾产生的温室效应,湍流过程加强,使地面雾向上扩展,雾在稳定层内维持;夜晚云-雾共存时,由于云-雾温室效应使地表净长波辐射增大超过70 W/m2,导致地面长波辐射冷却过程减弱,并不利于雾的加强,但云对雾的增温效应有利于混合层内的湍流扩散过程,促使雾在更高的空间内得以维持。可见,在云-雾共存结构中,云辐射效应有利于低层大雾的长时间维持,对持续性大雾的形成和发展产生了重要作用。      

Evaluation on penetration rate of cloud for incoming solar radiation using geostationary satellite data

Solar surface insolation (SSI) represents how much solar radiance reaches the Earth??s surface in a specified area and is an important parameter in various fields such as surface energy research, meteorology, and climate change. This study calculates insolation using Multi-functional Transport Satellite (MTSAT-1R) data with a simplified cloud factor over Northeast Asia. For SSI retrieval from the geostationary satellite data, the physical model of Kawamura is modified to improve insolation estimation by considering various atmospheric constituents, such as Rayleigh scattering, water vapor, ozone, aerosols, and clouds. For more accurate atmospheric parameterization, satellite-based atmospheric constituents are used instead of constant values when estimating insolation. Cloud effects are a key problem in insolation estimation because of their complicated optical characteristics and high temporal and spatial variation. The accuracy of insolation data from satellites depends on how well cloud attenuation as a function of geostationary channels and angle can be inferred. This study uses a simplified cloud factor that depends on the reflectance and solar zenith angle. Empirical criteria to select reference data for fitting to the ground station data are applied to suggest simplified cloud factor methods. Insolation estimated using the cloud factor is compared with results of the unmodified physical model and with observations by ground-based pyranometers located in the Korean peninsula. The modified model results show far better agreement with ground truth data compared to estimates using the conventional method under overcast conditions.     

A New Algorithm for Sea Fog／Stratus Detection Using GMS-5 IR Data

A new algorithm for the detection of fog/stratus over the ocean from the GMS-5 infrared (IR) channel data is presented. The new algorithm uses a clear-sky radiance composite map (CSCM) to compare the hourly observations of the IR radiance. The feasibility of the simple comparison is justified by the theoretical simulations of the fog effect on the measured radiance using a radiative transfer model. The simulation results show that the presence of fog can be detected provided the visibility is worse than 1 km and the background clear-sky radiances are accurate enough with known uncertainties. For the current study, an accurate CSCM is constructed using a modified spatial and temporal coherence method, which takes advantage of the high temporal resolution of the GMS-5 observations. The new algorithm is applied for the period of 10-12 May 1999, when heavy sea fog formed near the southwest coast of the Korean Peninsula. Comparisons of the fog/stratus index, defined as the difference between the measured and clear-sky brightness temperature, from the new algorithm to the results from other methods, such as the dual channel difference of NOAA/AVHRR and the earth albedo method, show a good agreement. The fog/stratus index also compares favorably with the ground observations of visibility and relative humidity. The general characteristics of the fog/stratus index and visibility are relatively well matched, although the relationship among the absolute values, the fog/stratus index, visibility, and relative humidity, varies with time. This variation is thought to be due to the variation of the atmospheric conditions and the characteristics of fog/stratus, which affect the derived fog/stratus index.     

Cloud mapping from the earth's surface using infrared radiance contrasts*

Abstract

Radiance criteria for distinguishing low, middle, and high clouds in the 9.5–11.5 μm band of the infrared are developed and used to produce local cloud maps. The performance of this radiance contrast method for mapping clouds from the earth's surface is evaluated with a view to using the technique for objective observation of cloud amount and distribution in the sky hemisphere.

Discrimination radiance formulas are developed using a multilayer, wavenumber‐specific infrared radiative transfer model including cloud parameters measured by other workers and atmospheric conditions measured by radiosonde. The clear sky radiance (N₀) is the dominant independent variable in the discrimination formulas. The variation of N₀ with time (primarily due to changes in atmospheric water vapour content) and zenith angle are found to be important in distinguishing cirriform clouds from clear sky and other clouds.

The local cloud maps are produced by applying the discrimination radiances (in voltage form) to the output from a narrow‐view infrared radiometer pointed at a sky‐scanning mirror. It is necessary to assume that the radiance from a cloud observed at the surface decreases unambiguously with an increase in cloud base height. Cloud maps for five days in July 1978 indicate the mapping technique shows promise under a wide range of sky conditions. Cloud motions rapidly degrade the maps’ quality over time‐scales that are much less than the current manual cloud sampling period of 1 h.     

Post-flight radiometric calibration of the Korean geostationary satellite COMS meteorological imager

The first Korean geostationary satellite, the Communication, Ocean, Meteorological Satellite (COMS) carries the Meteorological Imager (MI) that measures solar radiance at 0.675 μm and infrared (IR) brightness temperatures at four spectral bands centered at 3.8, 6.7, 10.8, and 12.0 μm. This study reports the calibration status of the COMS MI solar and four IR channels, based mainly on a comparison with Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. The results obtained from four months of COMS MI solar channel measurements demonstrate that the solar channel has a dark bias of about 9–10%. On the other hand, the four IR channels appear to be well-calibrated as evidenced by a high correlation and near-unity slope between COMS and MODIS data. Nevertheless, existing biases of tenths of a kelvin are still considered to be substantial. Overall, the interpretation of COMS-derived meteorological products should take into account some uncertainty caused by possible calibration errors.     

雾的气象卫星遥感光谱特征

从大气辐射传输理论入手，用频谱分析方法研究了气象卫星遥感图像上云雾的可见光和红外光光谱特征，阐述了雾在NOAA各通道的光谱响应特征及相关指标。根据云雾的不同顶部结构，分析了云雾反射可见光的非朗伯特性。根据瑞利(Rayleigh)准则，解释了低太阳高度角时可见光对云雾具有更强的识别作用，指出雾与云相比，具有更强的方向性反射特点，当卫星处于云雾层的镜面反射方向时，雾区具有更强的亮度，即反射率高于云区；反之．雾区则相对较暗，指出可见光在云雾识别方面具有重要作用。通过分析云雾层的不同红外光谱特点，给出了一些云雾识别与分类的指标，说明了通道组合方法对云雾识别的重要作用，为云雾分类提供了依据。通过时间动态的频谱分析，给出了判断大雾成熟期的光谱特征，对判定大雾的发展与消散具有重要意义。     

基于多灰度共生矩阵特征值的黄渤海白天海雾识别算法

本文基于多灰度共生矩阵特征值,即相关性、对比度、同质性和能量,进行联合海雾遥感判识,提出一种高准确率黄渤海白天海雾识别算法。采用第二代静止气象卫星FY-4A可见光、近红外和红外数据,将该算法应用于黄渤海区域白天海雾判识,并利用2019—2020年沿黄渤海气象站点能见度实测数据及CALIPSO卫星数据产品对本算法识别结果进行精度验证。结果表明：海雾识别平均检测率(POD)为92%,误报率(FAR)为27%,临近成功指数(CSI)为69%,可以实现对海雾的动态监测,为海上交通等领域提供较好的数据支持。     

Spectral radiance and sky luminance in Antarctica: a case study

Summary Sky luminance and spectral radiance has been characterised at Neumayer, Antarctica for selected situations during the austral summer 2003/04. Luminance has also been measured at Boulder, Colorado, USA in June 2003. The high reflectivity of the surface (albedo) in Antarctica, reaching values up to 100% in the ultraviolet (UV) and visible part of the solar spectrum due to snow cover, modifies the radiation field considerably when compared to mid-latitudes. A dependence of luminance and spectral radiance on solar zenith angle (SZA) and surface albedo has been identified. For snow and cloudless sky, the horizon luminance exceeds the zenith luminance by as much as a factor of 8.2 and 7.6 for a SZA of 86° and 48°, respectively. In contrast, over grass this factor amounts to 4.9 for a SZA of 86° and a factor of only 1.4 for a SZA of 48°. Thus, a snow surface with high albedo can enhance horizon brightening compared to grass by a factor of 1.7 for low sun at a SZA of 86° and by a factor of 5 for high sun at a SZA of 48°. For cloudy cases, zenith luminance and radiance exceed the cloudless value by a factor of 10 due to multiple scattering between the cloud base and high albedo surface. Measurements of spectral radiance show increased horizon brightening for increasing wavelengths and generally confirm the findings for luminance. Good agreement with model results is found for some cases; however there are also large deviations between measured and modelled values especially in the infrared. These deviations can only partly be explained by measurement uncertainties; to completely resolve the differences between model and measurement further studies need to be performed, which will require an improvement of modelling the spectral radiance. From the present study it can be concluded that a change in albedo conditions, which is predicted as a consequence of climate change, will significantly change the radiation conditions in polar regions as well.     

用三通道合成彩色图像进行云的分类解释判读

为更好地利用FY-3A气象卫星上中分辨率光谱成像仪(MERSI)资料高空间分辨率及多光谱的优势,细致分析云系在宏、微观方面的多重特征,首先利用平面平行辐射传输模式(SBDART)证明了MERSI的0.65,1.6 μm和11.25 μm通道能够分别反映云光学厚度、云粒子大小、云顶高度的信息,然后采用三通道合成彩色图像的...     

用GMS—5气象卫星资料遥感监测白天雾的研究

用日本GMS－5静止气象卫星的可见光通道和长波红外通道的资料，采用多通道合成和图像增强技术生成云雾彩色图像，分析了云雾的纹理特征、运动规律和消散规律，探讨了白天雾和低层云的遥感监测和识别方法，对沪宁高速公路沿线大雾进行了实时监测。     

Intercomparison of height assignment methods for opaque clouds over the tropics

Several methods of determining the height of opaque clouds over the tropics were compared using geostationary satellite measurements. The possible use of ozone channel measurements around the 9.7-μm ozone absorption band was examined in conjunction with the infrared window (IRW; 10.8 μm), H₂O (6.3 μm), and CO₂ (13.4 μm) channels, which are generally used for the assignment of cloud heights. Cloud top heights were retrieved from Meteosat-8 measurements with the aid of radiative transfer calculations using reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) as inputs. By using cloud top heights from collocated CloudSat observations as a reference, cloud top heights were determined from the one-channel radiance, two-channel brightness temperature difference (BTD), and two-channel radiance ratio methods, and the respective results were then compared for clouds with geometrical thicknesses of > 4 km. Overall, the retrievals from the CO₂-IRW ratio and O₃-CO₂ ratio methods are in substantial agreement with CloudSat observations, while the other methods either underestimate cloud top heights or demonstrate a lower ratio of successful height assignment. The O₃-CO₂ ratio method appears to be less practical than the CO₂-IRW ratio method because it requires two absorption channels. Our comparison also shows that the BTD between the ozone and IRW channels yields information that is similar to that of the IRW channel alone. It further shows that the O₃-IRW combination is not appropriate for the two-channel radiance ratio method. These results suggest that the inclusion of the ozone channel in BTD and ratio methods may not offer any significant improvement in convective cloud height retrieval over the tropics. In conclusion, the CO₂-IRW ratio method appears to provide the most accurate retrievals for opaque clouds.     

夜间云观测辅助方法

在夜间灯光很弱或者没有灯光、星光、月光的情况下,云的目测是比较困难的。常规夜间观测云的方法是,根据视觉,结合星光疏密、空气清晰程度、云体的颜色、移动速度以及伴见的天气现象和实测云高,参照傍晚时云的状况来判别云状。为了能够在夜间快速准确地识别所在测站云状,以现代化探测设备为手段,对夜间云的观测方法进行了总结,介绍了用大气辐射、大气电场、气象卫星云图、多普勒天气雷达回波资料来辅助观云,可较好地弥补夜间目测云的不足。     

基于MODIS多通道资料的白天雾监测

根据云雾微物理性质的差别,用SBDART辐射传输模式模拟了MODIS通道1、6、20、31的云雾光谱辐射特性。模拟结果表明,这4个通道的辐射值中都包含有云雾的信息,能体现出云雾的微物理性质差异,可以用来区分中高云、雾和无云地表,从而实现白天雾的监测。在模拟分析的基础上提出一种白天雾监测的方案,并进行了实例分析。实例分析结果表明,这种多通道雾监测方法提高了白天卫星识别低云和大雾的能力。