An Algorithm for Detecting Ice Cloud at Different Altitudes by Combining Dual CrIS Full Spectrum Resolution CO2 Channels

Using infrared sensors to detect ice clouds in different atmospheric layers is still a challenge. The different scattering and absorption properties of longwave and shortwave infrared channels can be utilized to fulfill this purpose.In this study, the release of Suomi-NPP Cross-track Infrared Sounder(Cr IS) full spectrum resolution is used to select and pair channels from longwave(～ 15 μm) and shortwave(～4.3 μm) CO_2 absorption bands under stricter conditions, so as to better detect ice clouds. Besides, the differences of the weighting function peaks and cloud insensitive level altitudes of the paired channels are both within 50 h Pa so that the variances due to atmospheric conditions can be minimized. The training data of clear sky are determined by Visible Infrared Imaging Radiometer Suite(VIIRS) cloud mask product and used to find the linear relationship between the paired longwave and shortwave CO_2 absorption channels. From the linear relationship, the so-called cloud emission and scattering index(CESI) is derived to detect ice clouds. CESI clearly captures the center and the ice cloud features of the Super Typhoon Hato located above 415 h Pa. Moreover, the CESI distributions agree with cloud top pressure from the VIIRS in both daytime and nighttime in different atmospheric layers.     

基于模糊纹理光谱的全天空红外图像云分类

为了对全天空红外测云系统获得的红外图像进行云类自动识别, 提出了基于模糊纹理光谱结合云物理属性的全天空云类识别方法。首先根据不同滤波窗口的模糊纹理光谱图像特征, 确定了滤波窗口大小, 然后通过分析不同天空类型下的FUTS谱 (fuzzy uncertainty texture spectrum) 以及同一种天空类型下的FUTS谱, 考察了FUTS进行云类识别的适用性, 最后利用最小距离分类法和云基本物理属性对全天空红外图像进行了分类测试。在200个测试样本中, 层状云、积云、高积云、卷云和晴空的识别率分别为100%, 100%, 90%, 100%, 100%, 平均识别率达到98%。基于模糊纹理光谱的云分类算法对单一云空具有很好的分类效果, 可进一步应用于全天空红外图像的云分类识别。     

Calibration of a three‐component angular distribution model of sky radiance

Abstract

A three‐component (isotropic, circumsolar and horizon‐brightening) model of the angular distribution of sky (short‐wave) radiance has been tested and validated against a data base of measured sky radiance. The data base encompasses cloud cover 0.0 to 1.0 and solar zenith angles 30 to 80°. Empirical constants have been derived for the model enabling the prediction of sky radiances for any sky condition.     

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.     

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。     

半透明云风矢量高度算法中云下背景辐射的估计

半透明云风矢量高度指定是卫星风矢量算法的重要部分,需要来自半透明云体的辐射和云下背景辐射两个变量。云下背景辐射发生在云层下面,未被卫星直接观测到,为了在半透明云风矢量高度指定算法中更精确地获得云下背景辐射,使用风矢量附近无云区的红外/水汽散点图,估计云下背景辐射。分析表明:在追踪区域里存在无云区的情况下,追踪区的最高红外亮温可代表红外通道的背景辐射;而水汽通道的背景辐射,却在红外亮温高值区段内水汽亮温相对较低区段。追踪区内找不到无云区时应扩大搜索范围,找到无云区后可估计云下背景辐射。在半透明云风矢量高度指定算法中使用云下背景辐射估计的改进算法前后,计算FY-2气象卫星进行风矢量,并将结果与欧洲中期天气预报中心（ECMWF）分析场进行对比表明,在半透明风矢量高度指定算法中使用云下背景辐射估计,FY-2气象卫星风矢量误差明显降低。     

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     

A practical approach for the assimilation of cloudy infrared radiances and its evaluation using airs simulated observations

Abstract

A variational estimation procedure for the simultaneous retrieval of cloud parameters and thermodynamic profiles from infrared radiances is proposed. The method is based on a cloud emissivity model which accounts for the frequency dependence of cloud absorption and scattering and possible mixed phase situations. An effective cloud top height and emissivity are assumed. Monte Carlo experiments performed in a 1D‐var assimilation context using simulated Atmospheric Infrared Radiance Sounder (AIRS) observations from 100 channels demonstrate the substantial added value, in theory, of cloudy radiance assimilation as opposed to clear‐channel assimilation. Improved temperature and humidity retrievals are obtained for a broad layer above the cloud as well as below cloud level under partial cloud cover conditions. The impact is most pronounced in broken to overcast situations involving mid‐level clouds. In these situations, the effective cloud top height and emissivity are retrieved with estimated rms errors typically lower than 30 hPa and 3%, respectively. Expected relative errors on the retrieved effective particle size are of the order of 30–50%. The methodology is directly applicable to real hyperspectral infrared data upon inclusion, for local estimation, of the cloud parameters in the Canadian 4D‐var assimilation system.     

Multiphase chemistry and acidity of clouds at Kleiner Feldberg

The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH₃ was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO₃. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO₃ from the air feeding into the cloud, or to heterogeneous HNO₃ formation via NO₂ oxidation by O₃ to NO₃ and N₂O₅. Sulfate in cloud droplets mainly originated from aerosol SO ₄ ^2– scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H₂O₂ and low pH of cloud droplets, which made O₃ and metal catalyzed S(IV) oxidation inefficient.     

Effects of cloud types on cloud-radiation interaction over the Asian monsoon region

This paper quantifies the sensitivity of radiation budget quantities to different cloud types over the Asian monsoon region using the International Satellite Cloud Climatology Project. Multiple regression was used to estimate the radiative effects of individual cloud type. It was observed that the regression performed better when the solution was constrained with clear sky fluxes, which is evident by an improvement in R ² statistics. The sensitivity coefficients calculated for the Asian monsoon region reveal that, while the LWCRCF and SWCRF will be most vulnerable to changes in cloud cover of deep convective clouds, NETCRF will be susceptible to changes in the nimbostratus clouds. Although the cloud radiative forcing of individual cloud types are found to be similar in sign to previous global findings, their magnitudes are found to vary. It is seen that cirrus clouds play an important role in governing the radiative behavior of this region.     

Performance validation of the Gueymard sky radiance model

Abstract

The Gueymard model of short‐wave sky radiance has been validated against sky radiance date measured in the Rocky Mountain foothills of Alberta. Except for sky points within the circumsolar region in clear skies and for low sun positions in near overcast skies the model performs as well as other current sky radiance models.     

中尺度强暴雨云团云特征的多种卫星资料综合分析

针对2002年6月23—27日发生于江淮地区的一次中尺度强暴雨过程，利用FY-1D，EOS和NOAA卫星的可见光、红外、微波通道遥感观测、反演资料，从相态、光学厚度、垂直结构等各方面分析云特征，并将分析结果与同时段地面雨量观测进行对比分析，发现云光学厚度大且云顶粒子为大粒子、冰相态是此次降雨过程中云团的主要云特征，地面雨量的大小与云光学厚度密切相关，两者间基本呈正相关关系；稳定少变的大光学厚度云或云光学厚度显著增大均可带来强的地面降水。微波资料可以很好地体现降雨云团的垂直结构。分析结果显示，卫星遥感对揭示中尺度强暴雨云团的云特征，具有很好的指示作用。     

三维辐射传输模式分析非均匀云对天空辐射场影响

为了解非均匀分立云体分布状况下的天空辐射场与无云晴空辐射场的差异, 本文借助一个三维辐射传输数值模式SHDOM模拟了离散云块分布条件下的天空辐射场分布, 重点分析不同云况分布情况下“非云”大气的辐射分布特征, 并将该区域与无云晴空大气辐射场的相同区域进行了比对。工作主要从辐亮度以及450 nm/650 nm的辐射比两参数入手进行讨论。研究发现, 相对无云晴空大气, 云的存在会对周围“非云”大气散射辐射产生影响, 影响程度与云量、云及气溶胶光学厚度等参数相关。数值模拟结果表明, 在云量不太大的条件下, 无云视场空间的大部分辐射值与无云晴空相比变化很小, 集中在±2％之间。这一结果表明, 已有的一维均匀大气辐射传输模式运算结果所得云与非云相元的判据基本适用于非均匀分布的有云大气。另一方面, 模拟结果表明我们也完全可以利用非均匀有云大气中的无云视场观测结果进行大气气溶胶等晴空大气光学特征的探测研究。     

物理-统计反演和云-晴空辐射特征差异检测结合的全天空高云判别能力改进算法与初步试验

在全天空红外-可见光云像观测反演方法基础上,针对已往阈值法判断高云的局限性,提出改进的算法。对可见光图像可利用以太阳-天顶为连线的主平面两边相元特性的对称性来判断云的存在,对红外成像仪可利用同一仰角扫描时所得不同方位角亮温分布的非均匀性来判断云的存在。以北京2011年1 3月观测资料为基础,选择原阈值法未能判别出来的案例进行再处理,全天空可见光成像观测仪高云判别能力最高可达73.9%,全天空扫描式红外成像仪可达70.1%。该工作为天空的高云识别提供新的复合算法。     

Characterization of atmospheric aerosols across Canada from a ground‐based sunphotometer network: AEROCAN

Abstract

Ground‐based sunphotometry measurements acquired under clear sky conditions can be used to investigate atmospheric aerosol optical properties. Such measurements are not only important in their own right as a technique for monitoring generic aerosol dynamics, but also represent a direct means of evaluating the contribution of aerosol induced radiative forcing in the modelling of climate change. In this paper we analyze derived aerosol optical properties using datasets from the Canadian AEROCAN (AERosol CANada) sunphotometer network.

The AEROCAN network currently includes eight sunphotometers distributed across Canada at sites chosen in order to obtain a diverse sampling of continental, maritime and arctic aerosols. Some of these sites have been operational since 1993 as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). These instruments permit standard and automatic multi‐wavelength measurements of solar extinction radiance centred on the solar disk as well as sky radiance scans off the solar disk. These data yields aerosol optical depth, the Ångström exponent, aerosol particle volume size distribution, refractive index, column‐averaged single scattering albedo, and precipitable water vapour content.

Spatial and temporal trends of these parameters as well as observed inter‐correlations are discussed. The results demonstrate the utility and significance of these types of measurements and illustrate the potential applications of networked sunphotometry data.     

基于晴空阈值法的全天空红外图像云量计算

为提高新疆戈壁地区云的自动化观测水平,基于全天空红外成像仪(WSIRCMS)获取的红外辐射图像,利用辐射传输模式SBDART分析了仪器测量波段对有云无云状况的敏感性并构建了拟合方程,同时利用典型季节的晴空辐射样本拟合了晴空曲线并统计形成了晴空阈值,最后利用统计晴空阈值对全天空红外辐射图像进行云像素识别和总云量计算。将不同季节总云量计算结果同人工观测结果对比验证表明：观测时段算法计算总云量和人工观测总云量差值在±2成以下的概率均在80%以上,说明该方法具有较高的准确度和较强的实用性,在观测业务中具有较好的应用前景。     

Cloud measurements on day 245 of gate

Abstract

On day 245 of GATE (2 September 1974) two lines of convection ‐ north (N) and south (S) ‐ were studied by aircraft stacked vertically, patrolling a north‐south line at longitude 22.85° W of length about 200 km. These lines were part of a complex of convection related to an easterly wave. Photogrammetry shows that the northern line consisted at first of a dense aggregate of small clouds, of width roughly 50 m at cloud base. There were a few tall clouds. No line organization was apparent from the aircraft at 1208 GMT. Fifteen minutes later there was line organization, readily apparent from the DC‐6 aircraft. From study of Electra and DC‐6 records, it appears that a vigorous cold northeasterly surface current initiated line N, and that this was a downdraft originating at altitude about 2.5 km in a mesoscale cloud feature to the north. It appears further that penetrative convection to 14 km followed after the first rain, which moistened air near the surface, and thus lowered cloud bases from about 500 to 350 m.

In overhangs of cloud, anvils to the north of line S, active cloud towers only about 2 km wide were found. The anvils were roughly 2 km thick. Patterns of clouds corresponded to a profile of A/B‐scale divergence; active towers at low levels corresponded to convergence up to the 60 kPa level, and stratus coincided with A/B‐scale divergence aloft at about 50 and 26 kPa.

Statistical analyses from the aircraft films indicated that the area covered by clouds of dark base ‐ signifying concentrated updrafts ‐ was ～5%, much less than that covered by rain at cloud base, ～ 18%. Cloud cover at altitude 4 km was ～ 10%.     

On the detection of clouds

Abstract

Short‐ and long‐wave sky radiances measured with an all‐sky automatic recording radiometer are used to infer the sky cloud‐cover conditions at 10‐min intervals (night) and 20‐min intervals (day). During the day a simple net short‐wave detector provides supplementary data and can be employed as a clear‐sky indicator. This system of cloud detection is controlled by a PET microcomputer and provides a basis for the automatic computer programmed estimation of cloud cover.     

Comparison of model predicted cloud parameters and surface radiative fluxes with observations on the 100 km scale

Summary Cloud parameters and surface radiative fluxes predicted by regional atmospheric models are directly compared with observations for a 10-day period in late summer 1995 characterized by predominantly large-scale synoptic conditions. Observations of total cloud cover and vertical cloud structure are inferred from measurements with a ground-based network of Lidar ceilometers and IR-radiometers and from satellite observations on a 100 kilometer scale. Ground-based observations show that at altitudes below 3 km, implying liquid water clouds, there is a considerable portion of optically non-opaque clouds. Vertical distributions of cloud temperatures simultaneously inferred from the ground-based infrared radiometer network and from satellite can only be reconciled if the occurrence of optically thin cloud structures at mid- and high tropospheric levels is assumed to be frequent. Results of three regional atmospheric models, i.e. the GKSS-REMO, SMHI-HIRLAM, and KNMI-RACMO, are quantitatively compared with the observations. The main finding is that all models predict too much cloud amount at low altitude below 900 hPa, which is then compensated by an underestimation of cloud amount around 800 hPa. This is likely to be related with the finding that all models tend to underestimate the planetary boundary layer height. All models overpredict the high-level cloud amount albeit it is difficult to quantify to what extent due to the frequent presence of optically thin clouds. Whereas reasonably alike in cloud parameters, the models differ considerably in radiative fluxes. One model links a well matching incoming solar radiation to a radiatively transparent atmosphere over a too cool surface, another model underpredicts incoming solar radiation at the surface due to a too strong cloud feedback to radiation, the last model represents all surface radiative fluxes quite well on average, but underestimates the sensitivity of atmospheric transmissivity to cloud amount. Received August 31, 2000 Revised March 15, 2001