On mechanism of hail cloud modification by seeding

The hail cloud modification is carried out by means of the cloud seeding with AgI particles of submicron sizes, in order to create nuclei competing with the natural ones for supercooled liquid droplet water, or in order to accelerate precipitation generation. However, scientific research confirmed a primary role of giant and supergiant particles in generation of nuclei of convective precipitation and hail. Such particles occur also at high levels (5 km and more). As the cloud and precipitation particles grow mainly through coagulation, one can hardly expect a high success from the cloud seeding with AgI submicron particles for hail suppression. In the hail process modification, the Ag particles settle down not at the nuclei but in the hailstone layers. So, AgI is spent not for additional hail nuclei creation, but for intensification of coagulation growth of natural cloud and precipitation particles. This can represent one of the main mechanisms for cloud modification.     

Advances in cloud physics and weather modification in China

The capabilities of cloud-resolving numerical models, observational instruments and cloud seeding have improved greatly over recent years in China. The subject of this review focuses on the main progresses made in China in the areas of cloud modeling, field observations, aerosol–cloud interactions, the effects of urbanization on cloud and precipitation, and weather modification.Well-equipped aircraft and ground-based advanced Doppler and polarized radars have been rapidly applied in cloudseeding operations. The combined use of modern techniques such as the Global Positioning System, remote sensing, and Geographical Information Systems has greatly decreased the blindness and uncertainties in weather-modification activities.Weather-modification models based on state-of-the-art cloud-resolving models are operationally run at the National Weather Modification Centre in China for guiding weather-modification programs.Despite important progress having been made, many critical issues or challenges remain to be solved, or require stronger scientific evidence and support, such as the chain of physical events involved in the effects induced by cloud seeding. Current important progresses in measurements and seeding techniques provide the opportunity and possibility to reduce these deficiencies. Long-term scientific projects aimed at reducing these key uncertainties are extremely urgent and important for weather-modification activities in China.     

An inadvertent transport of the seeding material as a result of cloud modification

Successful seeding of clouds in weather modification experiments essentially depends on the seeding time and dynamics, amount of seeding material and location of the initial seeding area. In the present study, we focus on the influence of the initial seeding zone location on the transport of seeding agent material into the target cloud. In addition, the inadvertent transport of seeding material is analysed. During weather modification activities, a lot of seeding material can be transferred far from the seeding zone in a downwind direction. The primary motivation for this research was to prove this statement. We use a three-dimensional, mesoscale cloud-resolving model to achieve our goal. We performed sensitivity tests with respect to the distance between the mass centres of the initial seeding area and the cloud. Different seeding scenarios are analysed. Our principal findings are as follows: (1) For distances between the mass centres of the initial seeding area and the cloud below 2.5 km, all seeding agent material would be activated after a short time. For distances above 10 km, most of the seeding agent would remain inactivated, because horizontal transport of the seeding agent becomes more important than transport induced by the main updraft. For these scenarios, the seeding agent is injected in the cold peripheral part of the cloud. (2) Sensitivity tests show that the inactivated seeding agent would remain close to the seeding area if the seeding is performed below cloud base. This effect occurs even for large distances between the seeding area and the target cloud (>20 km) due to low-level convergence. Thus, this seeding method suppresses the inert seeding material from being transferred far from the seeding zone. (3) The complete seeding material stays inactivated if the seeding is performed between the ?8 and ?12°C isotherms in front of the increased reflectivity zone. As a consequence, it would be transferred far from its initial area. The cloud would not be able to capture the seeding agent even during its greatest lateral extent.     

Calculation of cloud modification factors for the horizontal plane eye damaging ultraviolet radiation

This research examines the influence of cloud on the cataract effective UV (UV_Cat) irradiances on a horizontal plane over an extended period of 12months that included the range of cloud conditions, solar zenith angle (SZA) and ozone conditions experienced over that time. The data were collected at five minute intervals. Cloud modification factors were determined from the influence of clouds on the global broadband solar radiation and these were applied to the cloud free cataract effective UV to evaluate the UV_Cat irradiances on a horizontal plane for all cloud conditions. A comparison of the measured and calculated UV_Cat irradiances for the 2004 data set in the range of SZA of 70° or less provided an R² value of 0.85. The data in the first 6months of 2005 for an SZA of 70° or less that were at a different time to that when the technique was developed provided an R² value of 0.83 for the comparison of the measured and calculated UV_Cat irradiances.     

人工影响天气飞机播云作业有效催化范围的模拟评估

针对人工影响天气飞机播云作业效果评估需求,提出一种基于拉格朗日粒子扩散模式FLEXPART-WRF的催化剂催化范围模拟评估方法.以典型催化剂碘化银为例,开发催化剂物理化学特性参数清单模块,结合飞机播云特点和模式源项特点,将飞机不规则线性播撒方式离散化为连续移动点源播撒方式,实现模式对飞机播云的模拟能力.通过对一次飞机播云作业的模拟试验,证实了模拟评估方法的可行性和有效性.     

On the use of a cloud modification factor for solar UV (290–385 nm) spectral range

Summary  Knowledge of ultraviolet radiation is necessary in different applications, in the absence of measurements, this radiometric flux must be estimated from available parameters. To compute this flux under all sky conditions one must consider the influence of clouds. Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth’s surface. This cloud radiative effect is different for the different solar spectral bands. In this work, we analyse the cloud radiative effect over ultraviolet radiation (290–385 nm). This could be done by defining a cloud modification Factor. We have developed such cloud modification Factor considering two different types of clouds. The efficiency of the cloud radiative effect scheme has been tested in combination with a cloudless sky empirical model using independent data sets. The performance of the model has been tested in relation to its predictive capability of global ultraviolet radiation. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almería, a seashore location (36.83° N, 2.41° W, 20 m a.m.s.l.), while the second one is located at Granada (37.18° N, 3.58° W, 660 m a.m.s.l.), an inland location. The database includes hourly values of the relevant variables that cover the years 1993–94 in Almería and 1994–95 in Granada. Cloud cover information provided by the Spanish Meteorological Service has been include to compute the clouds radiative effect. After our study, it appears that the combination of an appropriate cloudless sky model with the cloud modification Factor scheme provides estimates of ultraviolet radiation with mean bias deviation of about 5% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum provides evidence for the spectral dependency of the cloud radiative effect. Received November 15, 1999 Revised September 11, 2000     

Recent Progress in Cloud Physics Research in China

A review of China cloud physics research during 2003-2006 is made in this paper. The studies on cloud field experiments and observation, cloud physics and precipitation, including its theoretical applications in hail suppression and artificial rain enhancement, cloud physics and lightning, and clouds and climate change are included. Due primarily to the demand from weather modification activities, the issue of cloud physics and weather modification has been addressed in China with many field experiments and model studies. While cloud physics and weather modification is still an important research field, the interaction between aerosol, cloud and radiation processes, which is the key issue of current climate change research, has become a new research direction in China over the past four years.     

Environmental aspects of cloud seeding

The estimates are presented of the possible level of environmental pollution with reagents resulting from precipitation modification by cloud seeding. It is demonstrated that the amount of reagents (crystallizing reagents based on silver iodide (AgI), cooling reagents, and powder reagents) injected to the clouds does not exceed the level of inflow of these chemicals to the atmosphere from natural and anthropogenic sources. These data and the data on the pollution level in the areas where AgI is actively used, allow stating the extremely low impact of cloud seeding on the environmental pollution. No change in the precipitation regime was detected in the regions adjoining the areas where weather modification experiments on the meteorological protection of megalopolises were conducted.     

Hail modification in the Republic of Moldova

The weather modification technologies designed to mitigate hail damage for economic and social objects become more and more demanded, despite ambiguous estimates of their results and thus expediency of their practical use [15, 17]. In this connection, results of antihail work in the areas where this activity lasts for many years are of direct scientific and practical interest. The Republic of Moldova is a good example of such an area. Antihail shield, based on the rocket technology of reagent injection into the cloud, has been under operation for more than 40 years. In the paper, the conceptual basis of hail-dangerous Cb cloud modification technique are presented along with some data on modification results during the last 10 years; an optimistic estimate of the antihail shield efficiency in the country is obtained.     

A cold air outbreak near Spitsbergen in springtime — Boundary-layer modification and cloud development

A moderate cold air outbreak from the Arctic ice over the warm West-Spitsbergen current on 15 and 16 May 1988 during the field experiment ARKTIS '88 is analysed using data from four aircraft and one research vessel.The downstream development of cloud coverage appears to depend sensitively on the moisture content above the inversion. The cloud amount determines the energy balance at the sea surface. Under daytime conditions and little cloud cover, energy is added to the ocean in spite of sensible and latent heat losses.The downstream temperature increase in the boundary layer is controlled by sensible heat flux and by longwave radiation cooling. The entrainment sensible heat flux is the dominating term in the region near the ice edge. The downstream moisture increase is controlled by surface evaporation. Condensation processes play no significant role.On 16 May 1988 cloud streets near the ice edge changed to closed cloud meanders in the downstream direction. The aspect ratio increased from 3 to around 10 over a distance of 200 km. In the cloud street region, the dynamical generation of turbulent kinetic energy due to wind shear at the tilted inversion was larger than the thermal generation.Cloud droplet concentration, mean droplet radius and liquid water content increased linearly with height. The maximum liquid water content was only 0.1 g/kg near the top of a 400 m thick closed cloud and clearly below the adiabatic value. The net longwave radiation flux decreased by 50 W/m² at cloud top and increased by 13 W/m² at cloud base.     

94GHz云雷达回波及测云能力分析

重点利用英国的94 GHz Galileo测云雷达,结合35 GHz云雷达、地面雨滴谱仪、雨量计和探空资料等,分析了94 GHz 雷达的回波特征及测云能力。结果表明：（1）94 GHz云雷达能清楚反映出云及弱降水过程的云系结构变化和云内小尺度变化,可以探测到雾,雾的多普勒速度杂乱;（2）94 GHz云雷达区别于厘米波雷达的较显著的回波特征是层状云降水的0℃层亮带下面雷达反射率因子降低不明显或没有降低及0℃层亮带上面存在0℃层暗带,分别是因为雨滴较大及冰晶较大产生非瑞利散射引起的,暗带区域的宽度一般在600 m以下,暗带区域的许多冰晶聚合物最大尺度可超过3 mm,有些暗带区域的许多冰晶聚合物最大尺度可超过6.8 mm,多普勒速度及谱宽显著增大的地方是融化层顶;（3）与35 GHz测云雷达相比,由于衰减和非瑞利散射,降水时的94 GHz雷达反射率因子远小于35 GHz雷达反射率因子,使探测到的高云云顶高度偏低,但94 GHz云雷达抑制地物杂波的能力更高,在晴空低云探测方面具有优势。这些结果为中国正在研制的94 GHz云雷达回波可靠性分析提供了参考。94 GHz云雷达与其他探测手段结合,可揭示各种天气形成的物理机制,对天气预报、云物理的发展、人工影响天气、气候变化的研究均有重要意义。     

Key issues in developing numerical models for artificial weather modification

The scientific foundation of artificial weather modification is meso- and small-scale dynamics and cloud–precipitation microphysics. Artificial weather modification requires the realistic coupling of weather patterns, dynamical processes, and microphysical processes. Now that numerical models with weather dynamical characteristics have been widely applied to artificial weather modification, several key points that should not be neglected when developing numerical models for artificial weather modification are proposed in this paper, including the dynamical equations, model resolution, cloud–precipitation microphysical processes, numerical computation method, and initial and boundary conditions. Based on several examples, approaches are offered to deal with the problems that exist in these areas.     

Kinetic limitations on cloud droplet formation and impact on cloud albedo

Under certain conditions mass transfer limitations on the growth of cloud condensation nuclei (CCN) may have a significant impact on the number of droplets that can form in a cloud. The assumption that particles remain in equilibrium until activated may therefore not always be appropriate for aerosol populations existing in the atmosphere. This work identifies three mechanisms that lead to kinetic limitations, the effect of which on activated cloud droplet number and cloud albedo is assessed using a one‐dimensional cloud parcel model with detailed microphysics for a variety of aerosol size distributions and updraft velocities. In assessing the effect of kinetic limitations, we have assumed as cloud droplets not only those that are strictly activated (as dictated by classical Köhler theory), but also unactivated drops large enough to have an impact on cloud optical properties. Aerosol number concentration is found to be the key parameter that controls the significance of kinetic effects. Simulations indicate that the equilibrium assumption leads to an overprediction of droplet number by less than 10% for marine aerosol; this overprediction can exceed 40% for urban type aerosol. Overall, the effect of kinetic limitations on cloud albedo can be considered important when equilibrium activation theory consistently overpredicts droplet number by more than 10%. The maximum change in cloud albedo as a result of kinetic limitations is less than 0.005 for cases such as marine aerosol; however albedo differences can exceed 0.1 under more polluted conditions. Kinetic limitations are thus not expected to be climatically significant on a global scale, but can regionally have a large impact on cloud albedo.     

Air mass modification over Lake Michigan

The modification of a relatively cold air mass over the warm water of Lake Michigan is studied by using a two-dimensional nonlinear mesoscale model. Considerable amounts of heat and water vapor are supplied from the water surface to the lower atmosphere by turbulent eddies. A convective mixed layer develops and grows toward the downwind region with stratocumulus clouds over the lake.The model simulates the warming and moistening of the mixed layer, the development of a boundary layer, the divergence and convergence of wind near the coastlines, and the turbulent fluxes.The model warming of the mixed layer across the lake was about 2.2 °K and the moistening of the mixed layer was about 0.8 g kg^–1, which are comparable to 2.7 °K and 0.8 g kg^–1 observed by Lenschow (1973). The convective boundary layer, which includes the cloud layer, subcloud layer, and superadiabatic layer near the water surface, is well simulated. The tilt of the inversion which coincides with the cloud top is also well reproduced. When a prescribed cooling rate is applied at the cloud top, stronger turbulence and a deeper cloud layer are generated. Without the cooling, the cloud is shallow and the shape of the cloud base is determined by surface conditions. The rise of the inversion is due to upward vertical motion, and deepening of the convective layer in the downwind region.     

A modelling study of aerosol processing by stratocumulus clouds and its impact on general circulation model parameterisations of cloud and aerosol

A model of the aqueous phase processing of an aerosol population undergoing multiple cycling through a stratocumulus (Sc) cloud layer is presented. Results indicate that a significant modification of the aerosol properties is achieved following the first cycle through cloud. In a polluted atmosphere, further modification in subsequent cycles is seen to be hydrogen peroxide limited unless there is a flux of ammonia entering the system through cloud base (CB). The modification of the aerosol population is seen to have little effect on the microphysics (specifically the cloud droplet concentration and effective radius) of the processing cloud. However, it enables processed aerosols to subsequently act as efficient cloud condensation nuclei (CCN) in less vigorous clouds (as a result of reducing the critical supersaturation required to activate them). The effects of variations in the internal mixture of soluble components of aerosols on the microphysics of clouds forming on them are also investigated using the cloud model. A (K2) parameterisation of the effects of variations in internally mixed nitrate loadings on the cloud droplet number concentration is presented. The effects of applying this K2 correction to the droplet number (derived from a parameterisation based on sulphate) for the presence of nitrate in aerosol have been investigated using the HadAM3 version of the Hadley Centre General Circulation Model (GCM). The effect on global annual mean simulations of the indirect forcing and effective radius is small, but more pronounced regionally. Suggestions (based on model results and observations) for parameterising the size distribution and in-cloud growth of aerosols for use in GCMs are presented.     

地形云催化增雨试验研究进展

地形云作为最具有前景和可行的人工影响云系,受到人工影响天气工作者和研究人员的关注。本文分析了国内外地形云催化增雨野外科学试验的历史进程,总结了野外科学试验中取得的成果,梳理了在地形云催化增雨试验中需关注的几个关键科学问题,包括对地形云自然降水过程的分析、地形云系统中过冷水在云内的分布、山地云系的微物理过程演变特征及其与中尺度动力结构的关联,介绍了宁夏开展地形云野外科学试验的实践,提出了加快地形云催化野外科学试验,提高地形云云水资源开发利用的对策及建议。为解决中国西北地区干旱问题,推动黄河流域生态环境保护及高质量发展提供了一种思路。     

一次西风槽过程过冷云水分布特征观测研究

过冷云水生消演变规律是云物理学和人工影响天气的重要研究领域。根据Hobbs 1974年提出的假定,利用飞机、卫星、雷达和雨量计等观测资料,对2012年9月21日河北一次西风槽天气过程进行观测研究,分析其过冷云水分布特征及演变规律。结果表明,槽前云系过冷水区宽厚并且过冷水含量较高,云滴浓度和均立方根直径较大并且均匀,冷云区厚而且没有分层,没有暖云配合;近槽云系中冷云区小粒子浓度降低但云滴直径增大,冷云区夹有干层,云系变厚出现暖云配合,冷暖云液态水含量较高,冷暖云区大粒子和降水粒子浓度和尺度增大,中尺度云团移动较快;槽后云系中云滴浓度最大,但云滴均立方根直径明显减小,过冷水区出现的高度下降、厚度很薄、过冷水含量较低,冷、暖云之间有干层,暖云对应的大粒子浓度和降水粒子浓度非常大,地面降水主要由暖云过程产生;云水(过冷水)含量峰值常出现在云内逆温层的上方;利用云粒子测量系统(PMS)资料分析过冷云水生消演变特征与卫星和雷达资料具有较高的一致性。     

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.     

毫米波雷达测云个例研究

云参数是影响降水和大气辐射过程的重要因子,但对云参数的遥感探测存在许多困难。利用35GHz的毫米波雷达进行云探测,并进行云参数反演研究,反演了云水含量、冰水含量和云滴有效直径的垂直廓线,得到了6类云况的垂直分布。结果表明：1）不同类型的云具有不同的云参数分布;2）在低于-15dBz的非降水云情况下,反演的云水含量及云滴有效直径较可靠;3）雷达探测的线性退偏振比因子,可以用于判别云中的过冷却水和冰晶,有助于更好了解云的宏微观特征。