Case Study On The Weather Conditions And Physical Evaluation Of Precipitaion Enhancement Operations In Tai’an Region, Shangdong Province

In this paper, some studies are carried out on the practical exploration in physical evaluation of precipitation enhancement combined with the analysis of doppler radar detection data, routine weather data and surface precipitation. There is demand for precipitation enhancement in spring that is more vulnerable to drought by analyzing the surface precipitation in Tai’an recent years. Based on radar echoes, satellite images, surface precipitation and circulation patterns in the upper air and at surface, the operational weather conditions and effect of the two experiments of precipitation enhancement in spring of Tai’an, the first of which is conducted on May 8, 2013 that under the influence of Yangtze-Huaihe Cyclone in southern weather system and the second is performed on April 24, 2012 that under the influence of Yellow River Cyclone and the upper cold vortex in northern weather system, are investigated. We judged whether the weather conditions are conducive to precipitation enhancement operation and a method for estimating the effect and catalytic timing of seeding based on various parameters of radar echoes is discussed through analyzing the case by comparing the target cloud with the contrast cloud, as well as the changes in rainfall. It shows that there are obvious variations in some aspects such as precipitation and various vertical parameters of radar echoes between the target cloud and contrast cloud, which indicates the two seedings all play a positive role on precipitation enhancement in different degrees.A test is made for a complex system of the stratiform and embedded convective clouds in Tai’an on May 8, 2013 that the precipitation is under the influence of Yangtze-Huaihe Cyclone in southern weather system. Under the co-influence of southwest flow in front of trough at 500 hpa, shear line, dry line and cyclone over Yangtze-Huaihe River valleys at 850 hpa, high humidity and strong water vapor transportation at 925 hpa, as well as the high energy gradient of pseudo-equivalent potential temperature and cold cloud moves to Tai’an, the good moisture conditions and dynamic conditions which benefit to the development of strong convection are provided for the precipitation operation owe to the favorable weather situations. The positive effect starts in 12 min after seeding in the target cloud with all the values of radar echo parameters for the target cloud increase, while the enlargement rate of the contrast cloud is much smaller than that of the target cloud. There are positive deviations between the target cloud and contrast cloud such as echo top, echo volume, maximum reflectivity, vertically integrated liquid water content(VIL) and precipitation flux, which are gradually increasing with their increase rates are respectively as follow: 200%, 288.2%, 29.4%, above 65% and 384.8%. Analysis of the optimum seeding time should be earlier than the original time of 12 minutes, it is expected to receive more ainfall effect if start work at that point when the echo parameters of the target cloud are beginning to increased obviously. The ground rainfall shows there is a large increase appears in rainfall of the affected area after seeding with the effect lasts 3 hours, while its hourly rainfall is significantly higher than that of the contrast area, considering the operation plays a good effect.Analyzing the operation of a complex system of the stratiform and embedded convective clouds in Tai’an on April 24, 2012 that the precipitation is under the influence of Yellow River Cyclone and upper cold vortex in northern weather system.There exist wind shear and dry line at levels 700 to 850 hPa with strong cold air moves to southeast that provides dynamic lifting for the development of convective activity. And there are intense moisture sinks and intense upward motion in Tai’an, along with large K index and large pseudo-equivalent potential temperature with its high energy gradient, the enough water vapor and intensified upward motion can easily stimulate the rainfall which conducive to precipitation enhancement. Compared with the echo parameters of contrast cloud decrease or remain unchanged instead of increasing, the seeding operation plays a positive effect in 45 min after seeding in the target cloud with its echo top rises from 4.7km to 5.8km and the increase rates are larger for such parameters as the echo top is 23.4%, echo volume is 169.2%, maximum reflectivity is 10.5%, vertically integrated liquid water content is 137.9% and precipitation flux is 500%. The increase rates of positive deviations between target cloud and the contrast cloud such as echo top, echo volume, maximum reflectivity, VIL are respectively as follow: 100%, 273.3%, 250%, 57.8% with the positive deviations of precipitation flux increases significantly. The results also indicate that the seeding operation indeed extend the life of the target cloud and the time of maintaining maximum echo top. It is appropriate to seeding at that time when the target cloud at the initial stage of development with all the parameters begin to increase in a certain scale of development. The calculation results of this case show that the rainfall increases about 3~4h after seeding operation, and its amount increases by 110%~180%.