Cloud Base Height and Effective Cloud Emissivity Retrieval with Ground-Based Infrared Interferometer

Based on ground-based Atmospheric Emitted Radiance Interferometer (AERI) observations in Shouxian, Anhui province, China, the authors retrieve the cloud base height (CBH) and effective cloud emissivity by using the minimum root-mean-square difference method. This method was originally developed for satellite remote sensing. The high-temporal-resolution retrieval results can depict the trivial variations of the zenith clouds continu-ously. The retrieval results are evaluated by comparing them with observations by the cloud radar. The comparison shows that the retrieval bias is smaller for the middle and low cloud, especially for the opaque cloud. When two layers of clouds exist, the retrieval results reflect the weighting radiative contribution of the multi-layer cloud. The retrieval accuracy is affected by uncertainties of the AERI radiances and sounding profiles, in which the role of uncertainty in the temperature profile is dominant.     

红外高光谱观测值反演云参数

最小局部比辐射率变化MLEV算法根据在红外长波10～15 μm 区, 云的吸收、发射和散射具有相对有限的局部谱变化的特征, 利用对云敏感的长波红外辐射观测值来同时反演单层云的云顶高度和有效云比辐射率谱。先给定一些假想的云高初始猜测, 最佳的云高和比辐射率谱解使得用这些不同云高计算得到的比辐射率谱的局部变化最小, 该算法适用于高光谱(光谱分辨率从0.25～1 cm-1) 的大气红外探测器。通过用两种不同方案的内部比较及与激光雷达观测和MODIS业务云高产品的对比验证, 说明对卫星红外高光谱观测资料采用MLEV算法同时反演单层云的云顶高度和云有效比辐射率谱是非常有效的, 尤其是对中高云。     

基于卫星辐射率资料的两种三维云反演方法对比研究

基于格点统计插值分析系统（Gridpoint Statistical Interpolation analysis system,简称GSI）,利用粒子滤波（Particle Filter,简称PF）方法对卫星红外辐射率资料进行了云覆盖、云高等三维云图产品的反演研究。选取了具有高时空分辨率的静止卫星GOES（Geostationary Operational Environmental Satellites）-Imager辐射率资料进行了云反演试验,初步评估了PF云反演方法的可行性及其与多元极小残差（Multivariate and Minimum Residual,简称MMR）云反演方法的异同。结果表明:两种方法反演得到的云覆盖和云顶气压与NASA基于CO₂切片法反演得到的GOES云产品一致性较高。PF和MMR方法反演产品的优点是云图信息是三维分布的,相对于NASA提供的GOES云产品能提供更全方位立体的云信息。MMR方法需要利用一维变分逐步拟合观测来反演三维云图产品;PF方法采用不同模式垂直层的云覆盖比例作为不同粒子来近似后验概率分布,计算效率大大提高。进一步提出了一种新的基于“扰动粒子”的粒子滤波云反演方法,结果表明:在滤波过程中采用足够多的粒子样本（样本数量约为250）可以改进后验概率密度函数的估计,有效地避免了粒子发散问题,改善了云反演的结果。     

有云环境下MODIS亮温资料的变分同化Ⅱ——对暴雨预报的影响

卫星资料提供了大量关于云和雨的观测信息,在暴雨预报中可发挥巨大的作用,然而在数值模式资料同化中的应用水平仍然不高,特别是红外辐射资料的应用。由于有云环境下辐射传输过程的模拟难度很大,因此通常只同化晴空环境下的红外辐射资料。基于GRAPES-3DVAR(Global and Regional Assimilation and Prediction Enhanced System,全球/区域同化预报系统),根据RTTOV辐射传输模式(fast radiative transfer model for TOVS,快速辐射传输模式)的特点,增加云水含量、云冰水含量和云量作为同化系统控制变量,在改进辐射传输模式对红外资料模拟的同时,利用红外资料调整初始云参数和大气参数。针对2007年5月26日南海季风爆发后广东地区的一次暴雨过程,选取MODIS(Moderate Resolution Imaging Spectroradiometer,中分辨成像光谱仪)传感器水汽(第27)和云顶观测(第36)通道进行了同化试验,利用WRF(Weather Research and Forecasting Model,天气研究和预报模式)进行了数值模拟,结果表明同化MODIS资料,可以改进初始场水汽和温度分布,间接调整高空风场,调整趋势符合卫星观测,对短时降水预报有正面影响。     

飑线型降水云微波辐射特性的数值研究

本文将三维云的微物理模式的输出作为三维辐射传输模式的输入，对降水云的微波上行辐射率同地球物理参数间的关系进行了初步研究。在侧边界条件不对称的情况下，如果不考虑实际存在的云间相互辐射作用而模拟降水云的上行辐射率时，模拟出的亮温在低频段将低估卫星上测到的亮温值；在高频段则依降水云上层含冰量的大小而异，即在降水云上层含冰量小的区域，将低估星上测到的亮温，且低估的幅度较低频通道的大，而在降水云上层含冰量大的区域，可能高估星上测到的亮温，但高估的幅度较小。     

Development and initial assessment of a new land index for microwave humidity sounder cloud detection

This paper describes a new quality control (QC) scheme for microwave humidity sounder (MHS) data assimilation. It consists of a cloud detection step and an O–B (i.e., differences of brightness temperatures between observations and model simulations) check. Over ocean, cloud detection can be carried out based on two MHS window channels and two Advanced Microwave Sounding Unit-A (AMSU-A) window channels, which can be used for obtaining cloud ice water path (IWP) and liquid water path (LWP), respectively. Over land, cloud detection of microwave data becomes much more challenging due to a much larger emission contribution from land surface than that from cloud. The current MHS cloud detection over land employs an O–B based method, which could fail to identify cloudy radiances when there is mismatch between actual clouds and model clouds. In this study, a new MHS observation based index is developed for identifying MHS cloudy radiances over land. The new land index for cloud detection exploits the large variability of brightness temperature observations among MHS channels over different clouds. It is shown that those MHS cloudy radiances that were otherwise missed by the current O–B based QC method can be successfully identified by the new land index. An O–B check can then be employed to the remaining data after cloud detection to remove additional outliers with model simulations deviated greatly from observations. It is shown that MHS channel correlations are significantly reduced by the newly proposed QC scheme.     

Comparisons of Two Cloud-Detection Schemes for Infrared Radiance Observations

The cloud-detection procedure developed by McNally and Watts （MW03） was added to the Weather Research and Forecasting Data Assimilation System. To provide some guidelines for setting up cloud-detection schemes, this study compares the MW03 scheme to the Multivariate and Minimum Residual （MMR） scheme for both simulated and real Advanced Infrared Sounder （AIRS） radiances. Results show that there is a high level of consistency between the results from simulated and real AIRS data. As expected, both cloud-detection schemes perform well in finding the cloud-contaminated channels based on the channels＇ peak levels. The cloud- detection results from MW03 are sensitive to the prescribed brightness temperature innovation threshold and brightness temperature gradient threshold. When increasing the brightness temperature innovation threshold for MW03 to roughly eight times the default threshold, the two cloud-detection schemes produce consistent data rejection distributions overall for high channels. MMR generally retains more data for long-wave channels. For both cloud-detection schemes, there is a high level of consistency between the cloud-free pixels and the visible/near-IR （Vis/NIR） cloud mask.     

利用MODIS数据反演多层云光学厚度和有效粒子半径

利用卫星资料反演云微物理参数不仅有助于对天气变化的监测和预报,而且对人工影响天气的研究十分有益.目前卫星反演云微物理参数的算法一般是假设视场中只有一层云,但是实际环境中多层云出现很频繁.文中研究了多层云的光学厚度和有效粒子半径微物理参数的反演算法,主要针对薄的冰云覆盖在低层水云的多层云情形.算法利用中分辨率成像光谱仪(MODIS)吸收通道和非吸收通道同时进行反演,在此基础上利用SBDART辐射传输模式模拟冰云覆盖在低层水云上的多层云对云微物理参数反演的影响,模拟表明反演时将多层云作为单层云处理会使反演结果产生较大误差.为此,文中提出了云光学厚度和有效粒子半径反演算法中要考虑多层云的因素,并设计了一套云光学厚度和有效粒子半径反演方案.该方案使用SBDART辐射传输模式建立不同观测几何条件、下垫面类型、大气环境等条件下以光学厚度和有效粒子半径为函数变量的多层云、水云和冰云辐射查找表.经过云检测、云相态识别和多层云检测后,在该查找表的基础上,对MODIS通道1和通道7的数据采用最小方差拟合法反演光学厚度、有效粒子半径.利用该方案对2006年7月12日TERRA卫星MODIS数据进行反演试验,反演结果与NASA发布的MOD06产品中云的光学厚度和有效粒子半径的结果较一致,表明方案具有合理性.     

基于亮温通道变率的FY-3C微波湿度计陆地云检测新方法

云检测是卫星资料同化的重要前处理步骤,无论是晴空资料同化还是有云资料同化,都需要准确地区分有云和晴空资料。由于陆地地表发射率的多变性和微波能穿透部分云类的特点,微波湿度计资料在陆地上空的云检测研究一直是难点。利用快速辐射传输模式（CRTM）分析了不同云类条件下FY-3C微波湿度计（MWHS-Ⅱ）各通道亮温的通道间变率特征,根据MWHS-Ⅱ亮温通道间变率随云高以及云中液态水含量的增大而减小的特点,提出了一个基于亮温通道变率的MWHS-Ⅱ陆地资料云检测方法。与已有的云产品比较结果表明:新的云检测算法能有效地剔除大部分受云影响的资料,剔除后的晴空资料观测和模拟偏差更好地符合高斯分布。新方法对过冷水云、冰云、重叠云的检测能力较强,正确检测率可以达到80%,对卷云以及高度较低的水云的检测能力相对较弱。新方法能有效利用MWHS-Ⅱ观测资料自身完成云检测,在MWHS-Ⅱ资料同化中有很好的应用前景。      

利用FY-2C卫星数据反演云辐射特性

本文利用FY-2C静止卫星提供的可见光、中红外和热红外观测数据, 开展了水云光学厚度、粒子有效半径和云顶温度的云参数遥感探测理论和反演方法研究。基于FY-2C可见光、中红外(3.75 μm)与热红外(11 μm)通道辐射率对云光学厚度、 云滴有效半径、云顶温度辐射参数的敏感性分析, 提出三通道同时反演云的光学厚度、云滴有效半径及云顶温度的迭代方案; 通过个例分析进行了云参数反演试验, 并将结果与MODIS的云反演产品进行了对比, 最后对反演误差进行了分析。主要结论如下:(1) 个例反演得到的云参数与各通道探测数据有着较好的对应关系, 迭代计算标准偏差在允许的计算精度范围内(<0.89%), 反演结果具有合理性; (2) 通过与MODIS云反演产品的对比可以看到, 两者云光学厚度、云滴有效半径的均值和直方图分布都非常一致, 而MODIS的云顶温度比FY-2C反演值要高, 考虑到FY-2C的 11 μm通道测量的辐射值与MODIS相比偏小, 因此认为我们的反演方法与MODIS方法的精度是相当的。     

Study of TOVS applications in monitoring atmospheric temperature,water vapor,and cloudiness in East Asia

Summary A new Statistical-Physical Retrieval Method (STPRM) has been developed for applications with TIROS Operational Vertical Sounder (TOVS) measurements. The method uses physical modeling combined with statistical regression to develop straightforward retrieval expressions for temperature, moisture content, and tropopause height based on brightness temperature measurements from both the HIRS and MSU instruments used in the TOVS system. The selection of HIRS and MSU channels used in the various retrievals is based on channel sensitivity coefficients which relate to the amplitude of the change of a given channel's brightness temperature to a given change in a retrieval parameter. The temperature retrieval procedure is designed for all-weather situations, and emphasizes the use of MSU microwave channels including linear combinations of these channels to improve vertical resolution. Cloud parameters are retrieved using radiances from HIRS channels to generate clear column radiances for the moisture and tropopause height retrievals, which depend almost exclusively on the HIRS channels. The STPRM scheme is then used to obtain and evaluate distributions of temperature, mixing ratio, and tropopause height. Distributions of ozone content are also retrieved from an independent iterative retrieval procedure using the cloud corrected radiances from the STPRM scheme as input. The retrievals are compared to in situ measurements obtained from radiosonde observations and ground-based Dobson spectrometer measurements situated throughout East Asia with generally good results. The temperature retrievals are also used in a synoptic analysis of a winter storm situation that developed over northern China in January 1989, giving a geopotential height distribution which is confirmed by aerological observations.With 16 Figures     

Analysis of ice water path retrieval errors over tropical ocean

Retrieval of multi-layered cloud properties, especially ice water path (IWP), is one of the most perplexing problems in satellite cloud remote sensing. This paper develops a method for improving the IWP retrievals for ice-over-water overlapped cloud systems using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Visible and Infrared Scanner (VIRS) data. A combined microwave, visible and infrared algorithm is used to identify overlapped clouds and estimate IWP separately from liquid water path. The retrieval error of IWP is then evaluated by comparing the IWP to that retrieved from single-layer ice clouds surrounding the observed overlapping systems. The major IWP retrieval errors of overlapped clouds are primarily controlled by the errors in estimating the visible optical depth. Optical depths are overestimated by about 10–40% due to the influence of the underlying cloud. For the ice-over-warm-water cloud systems (cloud water temperature Tw > 273 K), the globally averaged IWP retrieval error is about 10%. This cloud type accounts for about 15% of all high-cloud overlapping cases. Ice-over-super-cooled water clouds are the predominant overlapped cloud system, accounting for 55% of the cases. Their global averaged error is 17.2%. The largest IWP retrieval error results when ice clouds occur over extremely super-cooled water clouds (Tw 6 255 K). Overall, roughly 33% of the VIRS IWP retrievals are overestimated due to the effects of the liquid water clouds beneath the cirrus clouds. To improve the accuracy of the IWP retrievals, correction models are developed and applied to all three types of overlapped clouds. The preliminary results indicate that the correction models reduce part of the retrieval error.     

南海中尺度大气海浪耦合模式及其对该区一次强台风过程的模拟研究

利用中尺度大气模式MM5(V3)和第3代海浪模式WWATCH建立考虑大气-海浪相互作用的风浪耦合模式.在耦合模式中引入考虑波浪影响的海表粗糙度参数化方案,大气模式分量提供海面10 m风场驱动海浪模式分量运行,并利用海浪模式分量反馈的波龄参数计算海表粗糙度.利用耦合模式模拟南海的一次台风过程,通过3组对比试验,检验耦合模式对台风过程的模拟效果并研究大气-海浪相互作用对台风过程的影响.结果表明:耦合模式能够较好地模拟南海的台风过程,与非耦合大气模式相比,其模拟的台风强度略有增强,路径变化不大;耦合模式对台风过程中海表热通量及降水影响显著,在台风充分发展过程中,耦合模式模拟的海表热通量增强,台风螺旋雨带上尤其是台风路径的右侧,耦合模式模拟的降水强于非耦合模式;耦合模式较好地模拟了台风过程海浪场的分布和演变,与非耦合模式相比,其模拟的海浪场增强,与实际更为接近;考虑了海表粗糙度对波浪的依赖关系后,海浪场同时影响海表的动力过程和热力过程,从本次个例看,在台风发展初期,海浪对海表动力作用影响显著,其反馈作用使台风系统减弱,但在台风充分发展后,耦合系统中海表热通量增加,热力作用显著增强,海浪的反馈作用有利于台风系统的发展和维持.     

On the assimilation of satellite sounder data in cloudy skies in numerical weather prediction models

Satellite measurements are an important source of global observations in support of numerical weather prediction (NWP). The assimilation of satellite radiances under clear skies has greatly improved NWP forecast scores. However, the application of radiances in cloudy skies remains a significant challenge. In order to better assimilate radiances in cloudy skies, it is very important to detect any clear field-of-view (FOV) accurately and assimilate cloudy radiances appropriately. Research progress on both clear FOV detection methodologies and cloudy radiance assimilation techniques are reviewed in this paper. Overview on approaches being implemented in the operational centers and studied by the satellite data assimilation research community is presented. Challenges and future directions for satellite sounder radiance assimilation in cloudy skies in NWP models are also discussed.     

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.     

Two-moment bulk stratiform cloud microphysics in the grid-point atmospheric model of IAP LASG (GAMIL)

A two-moment bulk stratiform microphysics scheme, including recently developed physically-based droplet activation/ice nucleation parameterizations has been implemented into the Grid-point Atmospheric Model of IAP LASG (GAMIL) as an effort to enhance the model’s capability to simulate aerosol indirect effects. Unlike the previous one-moment cloud microphysics scheme, the new scheme produces a reasonable representation of cloud particle size and number concentration. This scheme captures the observed spatial variations in cloud droplet number concentrations. Simulated ice crystal number concentrations in cirrus clouds qualitatively agree with in situ observations. The longwave and shortwave cloud forcings are in better agreement with observations. Sensitivity tests show that the column cloud droplet number concentrations calculated from two different droplet activation parameterizations are similar. However, ice crystal number concentration in mixed-phased clouds is sensitive to different heterogeneous ice nucleation formulations. The simulation with high ice crystal number concentration in mixed-phase clouds has less liquid water path and weaker cloud forcing. Furthermore, ice crystal number concentration in cirrus clouds is sensitive to different ice nucleation parameterizations. Sensitivity tests also suggest that the impact of pre-existing ice crystals on homogeneous freezing in old clouds should be taken into account.     

有云时大气红外遥感及其反演问题 I. 理论研究

我们已经研究了晴空情况下的大气红外遥感及其反演问题。本文对有云情况下的大气红外遥感及其反演问题进行了研究，首先指出国际上通用的处理有云反演的晴空订正法存在误差放大问题，然后提出了3×3相邻视场同步反演法，在该方法中，假定3×3相邻视场具有共同的大气温度廓线和大气水汽廓线，从而使求解方程数增加到9倍，而反演参数只增加有限的几个，大大提高了有云情况下反演的稳定性。     

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.     

Impact of assimilating IASI radiance observations on forecasts of two tropical cyclones

The impact of assimilating Infrared Atmospheric Sounding Interferometer (IASI) radiance observations on the analyses and forecasts of Hurricane Maria (2011) and Typhoon Megi (2010) is assessed using Weather Research and Forecasting Data Assimilation (WRFDA). A cloud-detection scheme (McNally and Watts 2003) was implemented in WRFDA for cloud contamination detection for radiances measured by high spectral resolution infrared sounders. For both Hurricane Maria and Typhoon Megi, IASI radiances with channels around 15-μm CO₂ band had consistent positive impact on the forecast skills for track, minimum sea level pressure, and maximum wind speed. For Typhoon Megi, the error reduction appeared to be more pronounced for track than for minimum sea level pressure and maximum wind. The sensitivity experiments with 6.7-μm H₂O band were also conducted. The 6.7-μm band also had some positive impact on the track and minimum sea level pressure. The improvement for maximum wind speed forecasts from the 6.7-μm band was evident, especially for the first 42 h. The 15-μm band consistently improved specific humidity forecast and we found improved temperature and horizontal wind forecast on most levels. Generally, assimilating the 6.7-μm band degraded forecasts, likely indicating the inefficiency of the current WRF model and/or data assimilation system for assimilating these channels. IASI radiance assimilation apparently improved depiction of dynamic and thermodynamic vortex structures.     

用红外和水汽两个通道的卫星测值指定云迹风的高度

提出了一种改进的用红外和水汽两个通道的测值指定云迹风高度的算法。利用红外水汽散点图和云迹风本身自动地将有低云的目标区、有半透明卷云的目标区、有密蔽高云的目标区分开。在有低云的目标区和有密蔽高云的目标区用红外一个通道指定云高。在有半透明卷云的目标区用红外和水汽两个通道指定云高。在用两个通道的测值指定云高时，假定卷云高度以上不存在水汽，以红外和水汽两个通道亮温相同这个方程与两个通道辐射测值线性相关的方程联立解出云的高度