Study On The Influence Range Of Wintertime Operational Cloud-seeding In Jiangxi Province During 2008-2014

Generally speaking,much effort has been devoted to investigations about the cloud-seeding impact on precipitation at target region.But the influence on downwind-areas of the target region drew less concern.Obvious enhancement in precipitation was revealed by foreign researches not only in target regions but also downwind regions.Evaluation on the range of influence by cloud-seeding has great significance for both science research and social economics.To explore the range of influence by long-term cloud-seeding operation,this paper examines the effects in target area and downwind area in the north of Jiangxi province in southwestern China during the operational wintertime(November-February)aircraft cloud-seeding project from 2008 to 2014.A-posteriori analysis method of a simple adaption of the often used historical target/control regression approach was used.Nine target stations and twelve downwind stations were selected as the studied object according to the mean wind direction of 71 cloud-seeding operations in 7 years.A group of 5 control stations that provided the highest correlation of average seasonal precipitation with the selected 21 stations was determined.Linear regression equations between the control group and each downwind station were established.Then for the purpose of verifying the reliability of statistical results,a-posteriori case study for a seeded event on 29 November 2014 was conducted.A comparison between the target and control units was made,with respect to cloud characteristics including echo top,echo volume,maximum reflectivity,vertically integrated liquid water content,and precipitation flux,which were derived from operational China New Generation Weather Radar System(CINRAD)volume-scan radar data in Jiangxi.The key findings obtained from this study are summarized as follows.Average wintertime rainfall increased by 17.30%(p=0.25)and 21.67%(p=0.0013)for target and downwind regions,respectively.After removing the abnormal rainfall data of the year 2011,Average wintertime rainfall increased by 19.59%(p=0.03225)and 22.50%(p=0.0009)for target and downwind regions,respectively.The p values for each downwind station were less than 0.05,indicating a pronounced effect for downwind area.Besides,the positive seeding effect can be detected as far as 126 km downwind of the target region.The five radar-derived physical parameters containing echo top,echo volume,maximum reflectivity,vertically integrated liquid water content,and precipitation flux,were found to deviate positively after comparing with control unit before and after cloud seeding.Lifetime of target unit was extended and the reflectivity was stronger during seeded period.The positive deviation persisted for more than two hours and the seeded cloud moved out the fixed target area for about 150 km.The protracted positive deviation effect may explain why rainfall of downwind-area increased when taking in local wind speed into account.The case studied in this paper may provide the proof of positive “downwind-area” effect for this project.However,this research is a posteriori estimation.Due to the small sample size and lack of specified randomized experiments and multiple observations,there must be deficiency in our conclusion.Scientific and convincing evidence needs lager size and high-quality experiment samples.Stabilizing an experiment area,extending sample size and utilizing more advanced observation apparatus will contribute to the follow-up study.