CLOUD AND PRECIPITATION PHYSICS AND WEATHER MODIFICATION
QIN Yu 1 and HU Zhijin2
1. Physics College, Peking University, Beijing 100871, China
2. Chinese Academy of Meteorological Sciences, Beijing 100081, China
The recent work and study results of laboratory experiments, numerical simulations and field experiments on cloud and precipitation physics and weather modification are briefly presented in this paper. Researches on cloud and precipitation physics, research on aerosols, cloud-radiation and atmospheric chemistry, research on sound, light and electricity in the atmosphere are involved. Weather modification covers rain enhancing, hail suppression, fog dissipation and artificially triggered lightning.Key words: cloud and precipitation physics, weather modification, clond radiation, aerosol, atmospheric chemistry, numerical simulation
For the last four years, studies on cloud and precipitation physics, research on aerosols, cloud-radiation and atmospheric chemistry, research on sound, light and electricity in the atmosphere in China have been carried out extensively and profoundly, which include laboratory experiments, numerical simulations and field experiments. Weather modification has been carried out in some provinces and cities, involving laboratory experiments and field experiments of rain enhancement, hail suppression, fog dissipation and artificially triggered lightning. Meanwhile, a series of modeling calculations have been made. The seeding methods include increasing number concentration of ice, introducing seeding agent or refrigerant into clouds. Much research work on cloud and precipitation physics and weather modification have been done all over the China, and some significant results have been obtained. The following sections introduce the results and research work in China from 1999 to 2002. They include cloud and precipitation physics, research on aerosols, cloud-radiation and atmospheric chemistry, research on sound, light and electricity in the atmosphere and weather modification.
II. CLOUD AND PRECIPITATION PHYSICS
1. Field Experiments
Between 1997-2001, Torrential Rainfall Experiment over the both Sides of the Taiwan Strait and Adjacent Area—HUAMEX was carried out on heavy rainfall in South China (Zhou, 2000). Between 2001-2002, research on the Formation Mechanism and Prediction Theory of Severe Climatic and Synoptic Disasters was organized on Meiyu rainfall in Yangtze River areas (Tao 2001). In both experiments, a number of instruments were employed, such as radars, satellite, automated stations, GPS stations, lightning locating, wind profilers and soundings, etc. and significant data were obtained. Circulation, dynamic, thermodynamic and microphysical characters about the formation of heavy rainfall in South China had been analyzed. On Meiyu front, it has been basically understood that the formation and mechanism of continual heavy rainfalls in Yangtze River areas, and a clear physical pattern was presented. Besides, the mseoscale structure, circulation background and microphysical characters on Meiyu front have been studied. In Shanghai, a mapper sensor carried by air balloon was used to record the spatial contents, number concentrations and phases of hydrometeors during heavy rainfall processes in 1999 (Yang et al., 2002).
In the field experiments of rain enhancement, a variety of detecting and remote sensing measurements have been employed, such as radar, microwave atmospheric sounding both based on ground and aircraft, densified sounding, airborne particle measurement system. Meanwhile in some provinces and regions, observational studies on raindrop size distribution and ice nuclei have been carried out. By use of the collected data, the distribution of the vapor and liquid water in the atmosphere, the microphysical structure and the particle distribution of the atmosphere, the precipitation mechanism and the distribution of ice nuclei have been studied. Thus, the characters of the cloud physics have been clearly appreciated (Duan and Xu, 1999).
2. Numerical Simulation
There has been made great progress in the aspect of the formation of the mesoscale cloud model of new cloud physics scheme. An explicit cloud scheme has been incorporated in HALFS model and has been used in operational forecast (Liu, 2000). An implicit double-moment microphysical scheme has been presented in these days, in which the description of microphysical processes was more reasonable than ever before. Nowadays, the model has been incorporated into MM5 model. After simulating a series of heavy rainfall cases, the model performed best on microphysical processes, and was more reasonable on precipitation simulation than ever before (Lou, 2002).
Based on the previous numerical models, a stratiform cloud droplet category model has been presented (Guo et al., 1999), by which the typical stratiform clouds in North China have been studied. By means of spectrum function and category methods, the evolution of the diameters of the hydrometeors in cloud has been studied (Xu et al., 1999). A three-dimensional hail category model has been developed, by which the hail formation and the particle distribution have been studied (Guo et al., 2001).
Using a three-dimensional non-hydrostatic meso-β—γmodel, the effects of the hail transfer coefficient acquired in lab experiments on the hail's growth have been studied (Fang et al., 2002). Using a hail model with detailed microphysical processes, the hail's growth and microphysical processes in hailstorm have been studied (Qi et al., 2002; Hong et al., 2002; Zhou et al., 2001; Xu et al., 2001).
Using a three-dimension terrain-following coordinate cloud model with detailed microphysical processes, the orographic effects on the convective cloud development have been studied (Lou et al., 2001). Using a three-dimensional convective cloud model, the cloud structure and precipitation mechanism of heavy rainfall in Meiyu front in Wuhan City have been studied (Wang et al., 2002). Using high-resolution non-hydrostatic mesoscale model (MM5) with explicit scheme, the cloud physical processes of heavy rainfall in South China have been studied (Wang et al., 2002). After studying the applicability of the mesoscale cumulus parameterization scheme over low latitude plateau areas, it was found that the influence of steep topography of plateau and the moistening coefficient should be considered (Deng et al., 2002).
3. Analysis and Study on the Satellite Cloud Picture
Using satellite data, the product development and its applicable research have been exploited. Using ISCCP data, the spatial characteristics and yearly, seasonally and daily variation properties of total cloud cover have been analyzed (Liu et al., 2002). Using GMS infrared radiation TBB data, the development regularity of cumulus and mesoscale convective system over the Tibetan Plateau and its ambient areas have been studied (Jiang and Fan, 2002). By analyzing the satellite cloud picture and numerical forecast products, the heavy rainfalls during flood season have been studied, furthermore mastering the characteristic structure of convective clouds, forming prediction and alarm system of heavy rainfall (Wei et al., 2001). Using the spectrum characteristic difference between MSU-1from TOVS and HIRS/2-8, the detection on clouds have been done, meanwhile the means of snow detection in clear sky have been studied (Liu and Luo, 1999).
III. RESEARCH ON AEROSOLS, CLOUD-RADIATION AND ATMOSPHERIC CHEMISTRY
The 1980's mean aerosol optical depth (AOD) over China has been analyzed (Luo et al., 2001), and the geographical distribution and temporal variation of the aerosol optical depth (AOD) from 1961 to 1990 over China have been studied (Luo et al., 2002). The aerosol scattering optical depth in the arid region of Northwest China has been analyzed (Li and Ji, 2001). The regularity of atmospheric photochemical process for surface ozone has been preliminarilly studied (Bai and Wang, 2001). The variational regularity and calculating method of surface ozone among visible spectrum have been presented( Bai et al., 2002). The thermodynamics mechanism of the O3 layer formation and extinction has been studied. It was explained why NOx, CO2 or the global rising air temperature can destroy the O3 layer. Using look-up table method, the tropospheric ozone and NOx evolution and their distributi ons have been studied (Zhu et al., 2001). Using atmospheric photochemical model, the main physical or chemical factors of influencing the photochemical transformation rate during daytime in troposphere have been studied (Zhu et al., 2002). Using earth-atmosphere system outgoing long-wave radiation (OLR) and cloud cover data, the seasonal and yearly mean radiative forcing of OLR and greenhouse effects causing by cloud cover have been calculated and studied (Liu and Weng, 2001).
In the aspect of numerical simulations, by evaluating MM5 (the fifth Generation Penn State/NCAR Mesoscale Model), RADM (Regional Acid Deposition Model) and MARS (the Model for an Aerosol Reacting System), a coupled regional atmospheric photochemical model was developed to study the tropospheric photochemical process, and study the transformation process from gas to particle. With this coupled model, the characteristics of spatial and temporal distribution and variation of regional tropospheric photochemical precursors, products in the northern China were studied, such as SO2, NO2, O3, PAN and so on (Ji et al., 2001; Ji et al., 2001). The modeling research on the effects of the atmosphere on the solar ultraviolet radiation incident to the earth surface has been carried out (Guo et al., 2002). Using a regional climate model (RegCM2-1996), the direct radiative forcing and its temperature effect due to tropospheric anthropogenic sulfate over the eastern Asia have been studied (Wang et al., 2002).
IV. RESEARCH ON SOUND, LIGHT AND ELECTRICITY IN THE ATMOSPHERE
Based on the summary of the previous lightning detection from space, some suggestions have been presented about how to develop satellite-borne instruments for lightning detection and mapping in China (Chen and Lu, 2001). Using the data obtained by the lightning location system, satellite and Radar, the variation characters of lightning in various periods of hail cloud formation and evolution have been studied (Feng et al., 2001). By use of the observational lightning data, the anti-polarity discharge and the structure character of the electric charge during a thunderstorm have been studied (Zhang et al., 2002).
Using a two-dimensional axis-symmetric convective model and a ground corona discharge model, the spatial and temporal distribution of spatial charge density and the formation of space charge layer under the thunderstorm have been studied, especially concerning transportation mechanism for all kinds of irons (Sun et al., 2001). Using GCM model, simulating the global cerauno activity, the global lightning frequency and its variation with seasons have been studied.
V. WEATHER MODIFICATION
1. Rain Enhancement
Based on the project of “ Pre-study of the Rain Enhancing Technique Series” from 2000 to 2003 and the project of “ the Research of Rain Enhancing Technology and Field Experiments ” from 2001 to 2005, field experiments are carried out in many provinces in China. The seeding mechanism, seeding condition and seedability of stratiform clouds were analyzed (Wang et al., 2001). For the cold stratiform clouds, the real-time seedability with the help of radar and PMS data was used to aid the rain enhancement operation (Tao et al., 2001). Recently, many advanced equipments of observation, along with the routine detecting equipments were applied in field experiments (Wang et al., 2001). With these data, the conditions of rain enhancement by aircraft, the mechanism by rocket and gunfire were studied (Tang et al., 2001). Rain enhancement operational systems have been developed in many provinces.
Using 3-D convective cloud model, seeding simulations of hail suppression were made on the position, time and amount of releasing AgI and on the seeding effect (He et al., 2001). Another seeding method with increasing ice concentration numbers in the model was also tested (Yu et al., 2001). Furthermore, the seeding method with releasing refrigerant of liquid CO2 showed that it not only increased the amount of rainfall, but also aroused dynamic effect (Guo et al., 2002). A 3-D puff trajectory model was developed to simulate the heterogeneous and unsteady diffusion and transportation of line seeding sources in stratiform clouds (Yu et al., 2002).
The drop-freezing experiments have been carried out for several kinds of precipitation water that were sampled in Beijing from 1995 to 1998 (Feng et al., 2002). The freezing nucleus content calculated form spectra of freezing temperature of drops was given for each kind of precipitation, the differences among them were significant and the reasons were preliminarily discussed.
2. Hail Suppression
The relations between echo parameters and polarization features during different lifetime of hail clouds were studied with the data of Doppler radar and dual polarization radar (Zhang et al., 2001). Through the simulations of hail cloud model, the growth track of hailstones, the formation mechanism and the possible approaches of hail suppression were analyzed, which could guide the field experiments (Hong, 1999). The analysis of the possible dynamic mechanism of explosion with model showed that the explosion caused the strong disturbance for air motion to affect the airflow.
3. Fog Dissipation
Between 1995 and 1999, the fog dissipation experiments were carried out in Beijing Airport. The seeding agent was liquid nitrogen which was sprayed out from a truck and released from air balloons (Song and Wang, 2000). A three-dimensional time-dependent frog model suitable for complicated terrain has been presented, which has been applied to the study of the formation and extinction of frogs over complex terrain, and the study of the microphysical structure and its characteristics (Huang et al., 2000; Huang et al., 2000). The experimental researches of a kind of seeding agent that dissipates warm fog have been carried out (Wang and Lu, 2000).
4. Artificially Triggered Lightning
The artificially triggered lightning technology was developed. The comprehensive observation of lightning and triggered lightning experiments in South and North China discovered the features of lightning and the differences between them. A model was developed to simulate the spatial charge in the lightning clouds and to provide the data for triggered lightning (Zhang et al., 2002). The relations of electric field and radiant electric and magnetic field was analyzed so that the line model of cloud-to-ground lightning was improved (Zhang et al., 2002). With a high-speed photographer, the shining phenomena of triggered lightning were studied, and in the first time, the two-way characteristic was detected and validated when the lightning electric channel developing (Liu et al., 1999).
In this paper the recent work and study results of laboratory experiments, numerical simulations and field experiments on cloud and precipitation physics and weather modification are briefly presented.
1. Cloud and Precipitation Physics
Two mass field experiments are designed and finished in China, some data of many cases with a various detecting and observing equipment, and have been used to analyze the physical and dynamical structure, the mechanisms of these cases. Works on the numerical model includes the development of new microphysical schemes of mesoscale model and the improvement of meso-β—γmodels, besides, numerical simulations have been done with these models. Using satellite data, the product development and its applicable research have been exploited.
2. Research on Aerosols, Cloud-Radiation and Atmospheric Chemistry
Using models and observed data, researches on the aerosol optical depth, O3 , NOx, CO2 , OLR and greenhouse effects have been carried out and abtained some important conclusions, a regional atmospheric photochemical model and a regional climate model are used to study the transformation from gas to particle, effects of the atmosphere to the solar ultraviolet radiation, the anthropogenic sulfate aerosol's direct radiative forcing and its temperature response.
3. Research on Sound, Light and Electricity in the Atmosphere
Lightning and anti-polarity discharge in hail clouds have been studied with observed data. Using a two-dimensional axis-symmetric convective cloud electrization model and GCM model, the features of the spatial electricity layer, the global lightning frequency and its variation with seasons have been studied.
4. Weather Modification
Weather modification has been carried out in some provinces and cities, involving laboratory experiments and field experiments of rain enhancement, hail suppression, fog dissipation and artificially triggered lightning. Meanwhile, a series of modeling calculations have been made with different seeding agents and methods.Acknowledgement: The authors gratefully acknowledge Dr. LOU Xiaofeng, Ms. SHI Yueqin for their support. Thanks are also extended to Dr. LIU Qijun.
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