毫米波云雷达与地基微波辐射计联合反演云微物理参数

使用地基微波辐射计与毫米波云雷达进行主被动遥感联合探测,从理论上能够更准确地反演云微物理参数。分别以2010年7月于广东阳江和2015年3月于江苏南京进行的联合观测试验所获取的数据为例,对其中的层状云、层积云个例进行联合反演微物理参数的试验和分析,获取云演变过程中微物理参数和其他环境场物理量的变化特征。结果表明:1云雷达-微波辐射计联合反演法反演的云微物理参数与基于雷达反射率因子的经验法有较好的一致性,与实测经验值相比,结果比较可靠;2在层状云发展初始阶段,凝结增长作用在云体中上部位置起主要作用,碰并作用主要导致云滴粒子的显著增长和云层垂直方向的延展;在垂直上升速度较大的层积云中,云滴粒子通过碰并作用增长;3低层湿区高度与大范围层状云系出现的高度较为一致,抬升作用和水汽通量的辐合为云的产生提供了动力和水汽条件。     

浅积云中微物理特性的垂直分布对短波辐射的影响

使用大涡模拟模式获得浅积云的液态水含量和云滴有效半径的垂直廓线, 结果显示云的液态水含量和云滴有效半径均随高度的增加而增加( 垂直非均一) 。使用辐射传输模式 SBDART 研究了浅积云中微物理特性的垂直廓线对短波辐射的影响, 以及液态水路径相同时, 垂直非均一云和垂直均一云的短波辐射强迫的差异, 结果表明微物理特性的垂直廓线对云的辐射特性有很大影响。当垂直均一云的云滴有效半径约为垂直非均一云在云顶处的云滴有效半径的76% ～90% 时, 两种类型云的短波辐射强迫相当。结果说明在气候模式中对云的辐射传输特性进行参数化时, 如果假定云是垂直均一的, 则采用的云滴有效半径应该大致为实际观测的云顶云滴有效半径的76% ～90% 。对于浅积云的研究结果与以往对于层状云的研究结果一致。     

浅积云中微物理特性的垂直分布对短波辐射的影响

使用大涡模拟模式获得浅积云的液态水含量和云滴有效半径的垂直廓线,结果显示云的液态水含量和云滴有效半径均随高度的增加而增加(垂直非均一)。使用辐射传输模式SBDART研究了浅积云中微物理特性的垂直廓线对短波辐射的影响,以及液态水路径相同时,垂直非均一云和垂直均一云的短波辐射强迫的差异,结果表明微物理特性的垂直廓线对云的辐射特性有很大影响。当垂直均一云的云滴有效半径约为垂直非均一云在云顶处的云滴有效半径的76%～90%时,两种类型云的短波辐射强迫相当。结果说明在气候模式中对云的辐射传输特性进行参数化时,如果假定云是垂直均一的,则采用的云滴有效半径应该大致为实际观测的云顶云滴有效半径的76%～90%。对于浅积云的研究结果与以往对于层状云的研究结果一致。     

藏东南墨脱地区云中过冷水的毫米波雷达观测

云中过冷水识别对于人工影响天气及预防飞机积冰具有重要意义，但过冷水的识别一直是气象探测中的难点，毫米波雷达是连续探测云结构和物理特征的有效工具。本文利用布设在藏东南水汽通道入口处墨脱地区的Ka波段毫米波云雷达基数据，结合微波辐射计温度资料，采用基于模糊逻辑法、阈值法进行过冷水识别，识别出的粒子相态包含冰、雪、过冷水及混合态。并利用同址的微波辐射计的液态水路径(LWP)对墨脱云雷达观测的两个层积云过程的过冷水识别效果进行了分析和初步验证。结果表明:模糊逻辑法和阈值法识别的过冷水基本合理，但模糊逻辑法可以识别更多的过冷水，从定量分析来看，模糊逻辑法相对于阈值法识别的LWP更接近于微波辐射计。藏东南墨脱地区层积云中过冷水的微物理参数与其他地区较为一致，有效半径主要位于7~15 μm之间，液态水含量(LWC) 主要分布在0.01~0.3 g·m-3之间，但墨脱地区过冷水的分布比其他地区更为丰富，往往云顶、云底及云中同时存在过冷水。     

星载毫米波云雷达反演液态水云含水量的算法性能分析

为了充分了解毫米波雷达遥感液态水云微物理参数的性能,介绍了一种基于最优估计理论从星载毫米波雷达反演液态水云微物理参数的方法,并通过模拟试验对算法性能做了初步分析.模拟分析表明,假定云中粒子半径的垂直分布不均一,而数密度和谱宽分布均一,当观测不确定度等于1.5 dBZ时,整层廓线几何平均半径、数密度、谱宽以及液态水含量LWC(liquid water content)的相对不确定度分别为:13%、59%、15%和50%.各谱参数之间存在着一定的相关性,在计算LWC不确定性时,考虑这种互相关性可以使廓线的反演不确定度显著减小.介绍了联合雷达反射率和可见光厚度信息反演云微物理参数的方案.对比试验表明,不同的参数假定方案下,联合反演方法对反演结果的影响不同.若假定云中粒子半径的垂直分布不均一,而数密度和谱宽分布均一,则联合反演方法可使LWC反演不确定度从50%减小到27%;而若假定云中3个参数的垂直分布均不均一,联合反演对于结果的改进相对较小,仅从50%减小到了48%.     

基于卫星遥感的全球洋面降水暖云与非降水暖云的云参数差异

利用中分辨成像光谱仪(Aqua/MODIS)和云廓线雷达(CloudSat/CPR)两部星载仪器的准同步探测资料,分析了全球洋面暖云覆盖量和暖云降水频率的空间分布特征.并就暖云占优的多个中低纬海域,对降水暖云和非降水暖云的云微物理性质进行了对比,特别分析了由MODIS云滴有效半径(R16,R21,R37)相对大小所指示的云层顶部云水垂直结构在两类云之间的差异.研究结果表明,降水暖云与非降水暖云的云顶温度、光学厚度、云滴有效半径、云水路径等参数的有效范围接近一致,但均值存在明显差异.降水暖云的云顶高度显著偏高,其光学厚度、云滴有效半径、云水路径显著大于非降水暖云.针对6种云水垂直结构模态的初步统计结果显示,对应最大降水概率的模态为R37R21R16.该观测事实揭示了洋面暖云发展至降水阶段时,云层上部的云滴尺寸和云水含量多呈现为向下增长,该结果也反映了多光谱云参数反演资料在暖云降水识别中的潜在价值.     

河南春季一次冷锋降水过程的云物理结构分析

利用Cheyenne ⅢA探测作业飞机机载PMS探测设备、GPS航迹资料、飞机探测宏观记录、天气预报等资料,对河南春季云粒子浓度、粒子直径、液态水含量及云滴谱进行了分析。结果发现,冷锋前后云中粒子的浓度、液态含水量以及粒子的粒子谱型存在较大的差异,从而揭示出锋面前后云中粒子的不同特征,云粒子谱分析表明：锋前云粒子谱主要为单峰型,锋面过境时,谱型逐渐转为双峰型或多峰型。混合云及更高层的云谱型比层积云更加复杂,粒子直径比积云大很多,谱宽较宽。     

利用云雷达反演层状云空气垂直速度及微物理参数的个例研究

为了研究层状云内空气垂直运动特征,深入了解层状云微物理性质,通过雷达功率谱估算层状云内空气垂直运动速度反演得到了层状云的微物理参数值。利用雷达实测反射率分布与反演的反射率谱分布的对比分析,验证了反 演结果的合理性。研究结果表明：对受湍流影响较小的层状云,利用毫米波雷达的功率谱数据能够比较准确的反演空气的垂直运动及层状云的微物理参数值。     

基于模糊逻辑法的粒子识别研究

毫米波雷达相比于微波雷达对云的探测具有更高的敏感性,本文利用Chilbolton观测场的94GHz毫米波云雷达的基本探测量,结合探空仪的温度廓线数据,根据Shupe总结得到的云粒子相态识别的阈值,采用模糊逻辑算法对一次实际探测进行了反演研究.该模糊逻辑算法采用的主要隶属函数为不对称的梯形函数,识别粒子的相态类别主要分成雨、毛毛雨、液态水、混合相态、冰、雪等6种,对比经典的阈值算法反演的过冷水区域结果后认为,本文所研究的模糊逻辑算法对云中水凝物粒子的相态识别分类基本合理,其反演的结果对于人工影响天气作业以及飞机安全飞行等方面具有重要的意义.     

云凝结核数浓度观测影响因子的实验室研究

气溶胶的云滴活化能力影响云的微物理特性, 还表征气溶胶吸湿性的强弱。活化气溶胶(即云凝结核, Cloud Condensation Nuclei, CCN)的观测研究中的重要方法是CCN计数器(CCN Counter, CCNC)的应用。针对CCNC观测的影响因子进行了实验室研究。实验结果表明, CCNC所处的气压和设置的流量会对过饱和比产生等比例的影响, 而进气温度可能导致非线性的影响。实验中发现当气压差超过300 hPa时, CCNC气压调节装置会导致气溶胶的损失, 且损失的比例随着压差的增大而增大。在气溶胶浓度超过10000个/cm³、过饱和比低于0.2%时, 会出现对CCN数浓度的低估, 可能是由于水汽消耗导致CCN无法全部活化而不能被CCNC观测。研究结果及给出的建议对CCN的观测有指导意义, 同时有利于CCN数据的质量控制和分析工作。     

The impact of humidity above stratiform clouds on indirect aerosol climate forcing

Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol forcing, is thought to be comparable in magnitude to the forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect climate forcing.     

Deep convective clouds with sustained supercooled liquid water down to -37.5 degrees C

In cirrus and orographic wave clouds, highly supercooled water has been observed in small quantities (less than 0.15 g m(-3)). This high degree of supercooling was attributed to the small droplet size and the lack of ice nuclei at the heights of these clouds. For deep convective clouds, which have much larger droplets near their tops and which take in aerosols from near the ground, no such measurements have hitherto been reported. However, satellite data suggest that highly supercooled water (down to -38 degrees C) frequently occurs in vigorous continental convective storms. Here we report in situ measurements in deep convective clouds from an aircraft, showing that most of the condensed water remains liquid down to -37.5 degrees C. The droplets reach a median volume diameter of 17 microm and amount to 1.8 gm(-3), one order of magnitude more than previously reported. At slightly colder temperatures only ice was found, suggesting homogeneous freezing. Because of the poor knowledge of mixed-phase cloud processes, the simulation of clouds using numerical models is difficult at present. Our observations will help to understand these cloud processes, such as rainfall, hail, and cloud electrification, together with their implications for the climate system.     

Observational evidence of a change in radiative forcing due to the indirect aerosol effect

Anthropogenic aerosols enhance cloud reflectivity by increasing the number concentration of cloud droplets, leading to a cooling effect on climate known as the indirect aerosol effect. Observational support for this effect is based mainly on evidence that aerosol number concentrations are connected with droplet concentrations, but it has been difficult to determine the impact of these indirect effects on radiative forcing. Here we provide observational evidence for a substantial alteration of radiative fluxes due to the indirect aerosol effect. We examine the effect of aerosols on cloud optical properties using measurements of aerosol and cloud properties at two North American sites that span polluted and clean conditions-a continental site in Oklahoma with high aerosol concentrations, and an Arctic site in Alaska with low aerosol concentrations. We determine the cloud optical depth required to fit the observed shortwave downward surface radiation. We then use a cloud parcel model to simulate the cloud optical depth from observed aerosol properties due to the indirect aerosol effect. From the good agreement between the simulated indirect aerosol effect and observed surface radiation, we conclude that the indirect aerosol effect has a significant influence on radiative fluxes.     

Parameterization for narrow band shortwave optical properties of water clouds

Based on the drop size distribution models built from the observational data of microphysical properties of water clouds, the relationships between the optical properties and microphysical characteristics of water clouds have been investigated, and some different parameterization schemes of cloud optical properties have been analyzed. It is found that with the parameterization scheme, in addition to the equivalent radius, also including the mean radius of cloud droplet size distribution, the role of small cloud drops might be able to be considered more properly and better accuracies of parameterization calculation of cloud optical properties can be obtained, compared with that of using only the equivalent radius as parameter.     

基于MODIS数据与数值模式的云垂直结构估计

为了利用MODIS数据与数值模式估测云的垂直结构,通过运行PSU/NCAR MM5非静力数值模式,以模式输出的温湿廓线与MOD06云顶气压产品插值得到云顶高度,并利用WR95探空分析算法判定云底高度;以CloudSat/CPR主动遥感数据为真值进行云垂直结构的对比试验.初步结果表明:结合MODIS云产品与中尺度数值模式...     

A method for distinguishing and linking turbulent entrainment mixing and collision-coalescence in stratocumulus clouds

This paper presents a method to distinguish and link inhomogeneous mixing with subsequent ascent and collision-coalescence. Three stratocumulus clouds analyzed were collected over the U.S. Department of Energy’s Atmospheric Radiation Measurement Southern Great Plains site during the March 2000 cloud Intensive Observation Period. The criteria are presented to distinguish the two processes. Inhomogeneous mixing with subsequent ascent is identified if cloud along an aircraft horizontal leg is non-drizzling and the relationship between cloud volume-mean radius and liquid water content is negative; in contrast, drizzling and positive relationship between the above two properties are the criteria for collision-coalescence. To link the two processes, threshold function, the possibility of occurrence of collision-coalescence, is employed; the big droplets generated during the inhomogeneous mixing with subsequent ascent increase the threshold function, initiates collision-coalescence and produces drizzle drops. To the authors’ knowledge, this is the first study on distinguishing and linking inhomogeneous mixing with subsequent ascent and collision-coalescence based on observational data.     

基于模糊逻辑的CloudSat卫星资料云分类算法

为了提高星载毫米波雷达资料云分类的准确性,从基于云角色的分类思想出发,利用源于CloudSat/CPR(云廓线雷达)和CALIPSO/Lidar(激光雷达)的云几何廓线数据产品2B-GEOPROF-LIDAR以及相关资料,通过对云的特征参数进行提取,采用模糊逻辑技术对特征参数进行处理并完成对云的分类,将分类结果与Clo...     

Dusty cloud radiative forcing derived from satellite data for middle latitude regions of East Asia

The dusty cloud radiative forcing over the middle latitude regions of East Asia was estimated by using the 2-year (July 2002?June 2004) data of collocated clouds and the Earth’s radiant energy system (CERES) scanner and moderate resolution imaging spectroradiometer (MODIS) from Aqua Edition 1B SSF (single scanner footprint). The dusty cloud is defined as the cloud in dust storm environment or dust contaminated clouds. For clouds growing in the presence of dust, the instantaneous short-wave (SW) forcing at the top of the atmosphere (TOA) is about -275.7 W/m² for cloud over dust (COD) region. The clouds developing in no-dust cloud (CLD) regions yield the most negative short-wave (SW) forcing (-311.0 W/m²), which is about 12.8% stronger than those in COD regions. For long-wave (LW) radiative forcing, the no-dust cloud (CLD) is around 102.8 W/m², which is 20% less than the LW forcing from COD regions. The instantaneous TOA net radiative forcing for the CLD region is about -208.2 W/m², which is 42.1% larger than the values of COD regions. The existence of dust aerosols under clouds significantly reduces the cooling effect of clouds.     

A satellite view of aerosols in the climate system

Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.