Numerical Simulation Study On Seeding Potential Of Stratiform Cloud Systems

In order to mitigate the severity of drought, projects of artificial precipitation enhancement have been conducted extensively in China. The major object of artificial precipitation augmentation is stratiform cloud system, which usually occurs in Spring and Autumn. The question whether cloud systems have seeding potential attracts much attention of the investigators. It is very necessary to explore seeding potential of stratiform cloud systems, in order to strengthen positive seeding effects for precipitation enhancement. Some researchers regard precipitation efficiency as an index of evaluating seeding potential, while others use vapor contents, supercooled liquid water, ice concentrations and the area in which vapor water is supersaturated over ice to assess seeding potential. A question of seeding potential is complicated, so it is restricted and incomplete to judge it by any one factor. Based on previous research, observational data and numerical simulation results of the precipitation process in the stratus cloud system, which affected Henan province on 18-20 October 2002, are studied in this paper. Some new factors of seeding potential are analyzed, and the way of qualitative integrated assessment for seeding potential is suggested. The main conclusions are as follows: 1 Synoptic situation, satellite photograph, radar echo, cloud macroscopic feature, thermodynamics characteristic and raindrop spectrum are analyzed, and it make us know about the simulated cloud system to a degree. The results indicate that the precipitation produced by the stratiform cloud system results from the interaction between high and surface weather settings. Instability areas in low and high levels occur before surface front in the low-trough cold-front cloud system arrives. Inversion layers occur in the frontal zone. The cloud color is relatively white and bright and the texture is relatively even in infrared pictures. Bright band echoes, undulating echo top and generating cells are evident in radar echo display. As far as cloud forms are concerned, Asop and Fn usually exist, but Ns appears once in a while. There are "seeder-feeder" processes in stratus clouds. Small raindrops make a great contribution to number concentration, and big raindrops make a large contribution to rain intensity (water content). Raindrop spectrum expands along with the more intensive rainfall. Raindrop spectrum accords with M-P spectrum on the whole, whereas it is more similar to Γspectrum. 2 The cloud system is successfully simulated using MM5 model. Fields of geopotential height, temperature, cyclonic wind, shear line and 6-hr precipitation are considerably simulated, and the evolution of column vapor and liquid water is also well simulated. 3 The structure of cloud layers and its relationship with the precipitation. There are various cloud vertical structures and "seeder-feeder" processes in cloud systems. Some clouds are jointed continuously from high to low, with stratified microphysical structure. In such jointed clouds, ice crystals and snow particles constitute high level cloud, and snow, graupel and supercooled water constitute medium level cloud, and cloud water and rain water form low level cloud. Other clouds are composed of separated high, middle and low level clouds, intercalated with cloud-free areas between two layers. Cloud vertical structures have an important effect on the precipitation. In jointed "seeder-feeder" clouds, water content of all kinds of particles is sufficient, and snow layer is rather thick, then the precipitation is intensive. In the clouds in which there are dry layers between high and middle layers, total water content and rainfall intensity are low. Microphysical structures and precipitation mechanisms are apparently different when precipitation intensity is high or low. Cold cloud process makes a significant contribution towards rain water when precipitation is intensive. 4 Regional water budget and conversion in the cloud system. ⑴ Water transport zone is made up of warm, moist airflow coming from the west and the south. It is very likely that Cloud water and rain water which flow into the region exert a great influence on precipitation formations inside the region. There is an intimate relationship between inputted vapor contents and total water contents in the clouds. ⑵ There are three conversion ways of water substance in the clouds. The conversion of vapor to liquid water and solid phase water by condensation and deposition. 54.1% of the vapor contents are changed into cloud water, and 25.7% of vapor contents are transformed into ice particles. ② Conversion of cloud water to precipitation particles. 86.7% of cloud water is changed into rainfall particles. Conversion of precipitation particles to surface rainfall. 36.5% of condensation water and 41.3% of deposition water constitute surface precipitation. Ice melting contributes to about one half of rain water content. Deposition growthcontributes to more than 35% of rain water, but accretion growth contributes to about 12% of rain water. Nearly 70% of rainwater produced by melting ice comes from vapor deposition. 5 Production processes of precipitation particles. Growth of ice crystals is mainly achieved by deposition. Snow is formed by auto-conversion of ice crystal, and then grows by deposition. Graupel is produced by conversion of snow, and then grows dominantly through accretion, and deposition growth is less. Besides warm cloud process, rain water is mainly formed by melting of ice particles, which can contribute to uppermost 60% of rain water. Rain water is also produced through the process that melting ice particles collect cloud water in warm cloud sector, which contributes to 5-7% of rain water. Forming processes of particles, precipitation mechanisms, cloud vertical structures and water contents of ice particles are different when precipitation intensity is highest or lowest. When rainfall intensity is highest, snow grows mainly by deposition, and both warm cloud process and cold cloud process make major contributions to the precipitation, and the cloud system is deep, with layered clouds jointed together. However, when rainfall intensity is lowest, snow grows mainly by accretion of cloud water, and only warm cloud process is significant, and there is low water content or non-cloud sector between adjacent cloud layers.Surface precipitation is mainly produced by three approaches. ① Rain water grows by collection of cloud water, which contributes to 49.2% of rain water. ② Ice particles melt and form rain water when they fall into warm cloud section, and the process contributes to 48.5% of rain water. Melting ice particles collect cloud water in warm cloud sector, and then convert to rain water, whereas its contribution to growth of rain water is small. Ice crystals are essential to begin the process of cold cloud precipitation. 67.5% of ice contents are converted to snow, and 82.1% of snow contents melt into rain water, and only 7% of snow contents are transformed into graupel, and almost all of graupel contents melt to rain water. Different cloud layer make a different contribution to the precipitation in "seeder-feeder" clouds. Ice cloud, mixed-phase cloud, and liquid water cloud contribute to 24%, 48%, 28% of rainfall amounts, respectively. Seeder cloud contributes to 10-45% of precipitation, and feeder cloud contributes to 55-90% of precipitation. Generally, mixed-phase clouds make a major contribution to precipitation when cold cloud precipitation mechanism prevails in "seeder-feeder" clouds. 6 Factors of seeding potential are put forward. The results are as follows: ⑴ The conditions of conversion of cloud water have a relationship with the distribution of cloud water resources andcloud structures. Therefore, "seeder-feeder" clouds are beneficial to precipitation formation. ⑵ seeding agent mainly composed of silver iodide works by affecting cold cloud process for precipitation enhancement. Therefore target cloud in which cold cloud precipitation mechanism is dominant or important is seedable. ⑶ Mixed-phase clouds in "seeder-feeder" cloud systems contribute to 40-50% of surface rainfall, and supercooled water contents and rainfall intensity in mixed-phase clouds are high. When cold cloud precipitation mechanism makes a large contribution to precipitation, the magnitude of ice growth by deposition remarkably exceeds that by accretion at some portions of cloud system, and the variations of supersaturated vapor contents over ice (obtained by subtracting saturation vapor content with respect to ice from real vapor content) have the same tendency with that of precipitation intensity, which suggests that it is important for vapor contents (especially supersaturated vapor contents over ice) to form precipitation. Melting ice particles continue to grow by collecting cloud water in warm cloud sector. If cloud water content is high in warm cloud sector, and the cloud is thick in feeder clouds, then growth amount of melting particles is large; therefore, precipitation intensity is strengthened. Based on above research and combined with previous results, factors of seeding potential are offered, including cloud vertical structure, rainfall mechanism, supercooled water content, ice concentration, supersaturated vapor content over ice, vapor flux and precipitation efficiency and so on. 7 The qualitative and integrated thought to assess seeding potential. Cloud structure is analyzed. Generally speaking, cloud system that has "seeder-feeder" structure is seedable, and cloud system in which seeder cloud and feeder cloud are connected together has high potential by seeding. ⑵ For "seeder-feeder" cloud system, the contribution of cold cloud process to precipitation need to be investigated. The more contribution cold cloud process makes to rainfall, the higher seeding potential the cloud system has. The rates of vapor condensation and deposition, and precipitation efficiency of condensation and deposition, should be considered. If vapor condensation rate is high and precipitation efficiency of condensation is low, then the cloud has high seeding potential. Supercooled cloud water content, ice concentration in supercooled area, cloud water content and the thickness of warm cloud sector, and the thickness of feeder cloud layer etc ought to be studied. If supercooled water content is high, and ice concentration in supercooled area is low, and cloud water content of warm cloud sector is high and the thickness ofwarm cloud sector is big, and feeder cloud layer is thick, then seeding potential would be large. ⑸ Vapor conditions, including vapor flux and supersaturated vapor content over ice etc, need to be analyzed. If there are strong updraft, strong horizontal convergence of vapor, upward and high vapor vertical flux, and sufficient supersaturated vapor content over ice in thick cloud levels, then seeding potential should be remarkable. It's not necessary that all of these factors of seeding potential need to be analyzed, however, several factors should be chosen and comprehensively studied, when selecting the opportunity and the location for one seeding operation. 8 Seeding factors and evaluating approaches are preliminarily verified by seeding simulation. The analysis of simulation results shows: Non-seeded cloud in seeding regions basically accords with main seeding factors, including jointed "seeder-feeder" cloud, cold cloud process making an important contribution to rainfall, thick cloud water layer, having supercooled water content, low ice concentration, high cloud water content in warm cloud sector, upward vapor vertical flux and so on. ⑵ Seeding only changes the amount of particles and their source production by physical process, but does not change physical process of precipitation formation. In some areas precipitation is increased, while in other areas precipitation is decreased after seeding experiment. It implies that the cloud system which has different structure and condition is seeded by the same way is unreasonable. It is "seeder-feeder" cloud structure which is an great factor of seeding potential, but not all of these clouds are seedable. If only "seeder-feeder" clouds in which cold cloud process makes a great contribution to rainfall are beneficial to precipitation enhancement. For those cloud systems in which warm cloud process is dominative, the seeding way based on cold cloud seeding principle is improper, and will lead to surface precipitation reduce. In summary, water budget and conversion in cloud system, cloud structure and precipitation mechanism are detailedly analyzed in this paper, mainly using simulation results. Moreover the factors of seeding potential are studied and the way by which seeding potential is assessed is advanced. Integrated evaluating seeding potential using many factors is more reasonable than just according to one or two factors. Some new seeding factors are put forward, including cloud structure, precipitation mechanism and supersaturated vapor content over ice etc. Based on the theory of artificial precipitation enhancement principle, the calculating method of precipitation efficiency is improved, in order to match the need for better assessment of seeding factors. Thetheoretical conclusion is significant for the evaluation of seeding potential before seeding project of stratiform cloud system is performed.