| With global warming,the acidification of Loess Plateau has intensified,the area of semi-arid zone has increased significantly,and the problem of water shortage is significant.To alleviate the local water shortage,artificial precipitation enhancement operations have been implemented in various parts of China to increase precipitation.However,whether the ground precipitation enhancement cannot be quantitatively evaluated after seeding,so evaluating the effect of rainfall enhancement has become a key problem that needs to be solved urgently.In the evaluation of precipitation enhancement effect,there is still no clear conclusion on how the macro and micro characteristics of cloud layer change and whether the ground precipitation increases after seeding.Therefore,in order to evaluate the effect of precipitation enhancement and find convincing physical evidence of precipitation enhancement,the macro and micro characteristics of mixed convective-stratiform clouds on the Loess Plateau are detected and analyzed based on the detection data of various foundation equipment,to deepen the understanding of the target cloud of artificial precipitation enhancement.With the progress of detection means,supercooled stratus in front of the mixed convective-stratiform clouds was seeded by aircraft and the physical response characteristics of the cloud cluster after seed was studied.Based on the physical response characteristics obtained by different observation equipment,the conceptual model of the evolution of clouds and precipitation changes is obtained,which is helpful to deeply understand macro and micro evolution characteristics of cloud after seeding.It not only clearly answers the questions of how clouds move,distribute and evolve after artificial seeding,the duration of the Ag I impact and whether the ground precipitation increases,but also promotes the transformation of the precipitation enhancement evidence from statistical evidence to physical evidence and the effect assessment method from statistical assessment to physical assessment.This change has important guiding significance for the development of technology and theory of artificial precipitation enhancement.The main results and conclusions are as follows:(1)A space-time matching principle is proposed for the screening of cloud radar and sounding observation samples to effectively reduce the observation errors caused by the drift of sounding balloons.The analysis of 406 groups of cloud base and cloud top height samples observed by sounding and cloud radar in central Shaanxi for 508days shows that the correlation coefficients of cloud base and cloud top heights improved to 0.98 and 0.97,the root mean square error decreased to 602 m and 708 m,and the mean absolute error was reduced to 14%and 9%,respectively.The cloud height samples screened by the space-time matching principle are very effective in improving the consistency of the observed cloud heights between sounding and cloud radar.Based on the spatial-temporal matching principle,the cloud radar's cloud measurement capability is examined by sounding.It is found that the cloud base and cloud top heights of each cloud layer observed by the cloud radar correspond well to those observed by the sounding.The cloud radar has the ability to detect clouds accurately.In addition,cloud radar can compensate for the missing details of the vertical structure of clouds due to horizontal drift of sounding balloons.(2)Through an in-depth study of the microscopic characteristics of mixed convective-stratiform clouds and precipitation on the Loess Plateau,we provide a basis for natural precipitation characteristics and an empirical formula for radar estimation of precipitation for the evaluation of artificial rainfall enhancement effects in the region.Based on the observations of distrometer and weather radar in central Shaanxi from May to October 2013-2014,the average raindrop spectra,microphysical characteristic quantities and the relationship between radar reflectivity factor(Z)and rain intensity(R)for 43 mixed convective-stratiform cloud precipitations were statistically analyzed.The results show that the mean raindrop spectrum of mixed convective-stratiform cloud precipitation is unimodal,and the Gamma distribution fits the precipitation macroparticles significantly better than the M-P distribution.The raindrops less than 1mm in diameter of mixed convective-stratiform cloud precipitation contribute most to the raindrop number concentration and rain intensity.The Z-R relationship Z=168R1.43was established using the least squares method for mixed convective-stratiform clouds in central Shaanxi.When the echo intensity calculated from raindrop spectrum data is less(greater)than 30 d BZ,the radar overestimates(underestimates)the echo intensity significantly.For this reason,we propose a 5-step correction scheme for radar echoes of mixed convective-stratiform cloud.Using the relational expression(Z=168R1.43)to estimate individual cases of mixed convective-stratiform cloud precipitation,it is found that the rainfall estimated by the newly established Z-R relationship is closer to the measured value,and the relative error of estimated rainfall is reduced from 51.3%to25.4%.The localization of the Z-R relationship for mixed convective-stratiform clouds is necessary to improve the accuracy of radar precipitation estimation in central Shaanxi.(3)A radar echo enhancement area which appeared parallel to the route of the aircraft spreading Ag I in north-central Shaanxi on March 19,2017 may become the physical evidence of precipitation enhancement.At 08:00,the seeded area was in front of upper-troposphere trough,the surface temperature in the area was greater than 0?.The seeded cloud was a single cloud layer with an inverse temperature layer at the top of the cloud.The average wind speed was 12.3 m s-1 and the wind direction was 239°at the seeding layer.The Ag I was dispersed from 10:41 to 11:14,lasting 34 min.The total length of the seeding line was 125 km,the seeding height was 3200-3975 m,and the temperature was from-10.5 to-7.5?.Under the background cloud system with the same altitude and temperature,the aircraft seeding was proved to be effective by dynamically tracking the radar echo change of the seeded cloud cluster.By calculating the velocity of the seeded cloud echo,it is found that the moving speed of the seeded cloud echo(8.55 m s-1)is not equal to the wind speed at the seeding height.Application of the seeded cloud echo velocity in the diffusion transport model can accurately simulate the movement of the seeded cloud echoes.Smaller diffusion coefficients simulate narrower linear seeded cloud echoes better,while larger diffusion coefficients simulate the expansion of wider seeded cloud echoes better.No matter how large the diffusion coefficient is,it is not possible to simulate the splitting of the seeded cloud echo from one radar reflectivity peak into two peaks.(4)Combining space-based and ground-based detection data to analyze the physical response characteristics after seeding on March 19,2017,helps to understand the whole process of how the cloud changes after cloud seeding,and provides the theoretical basis for artificial precipitation enhancement operation.The effect of cloud seeding was observed by radar 18 min after seeding,and last 234 min.The wide of cloud seeding echoes with reflectivity>15 d BZ were up to 19 km.The seeding cloud echoes not only move along the wind direction,but also gradually broaden,with the broadening rate first fast and then slow(the expansion rates before and after 12:25 is approach 1.4 m s-1and 0.3 m s-1,respectively).After 11:44,the seeding line gradually splits into an echo band with two reflectivity peaks during the broadening process.It takes 21 min for the glaciation particles to appear and land on the ground,and the velocity of the ice crystals is 1.7 m s-1.Both TERRA and FY-3C satellites captured a glaciation cloud trench formed by the collapse of glaciation particles at the cloud top.The satellite observation of the cloud seeding signal appears no later than 15 min after seeding.Ag I seeding occurs in supercooled water with cloud top temperatures below-15°C and the effective radius of particles in the cloud trench from 20 to 30?m.Precipitation from the seeded cloud was detected by distrometer at Chunhua station in40 min after seeding.The seeded cloud echoes with reflectivity>15 d BZ took 11 min and 40 min to pass through Chunhua and Tongchuan stations,respectively.The raindrop concentration,water content and effective radius in Seed period at Chunhua station increased rapidly,much larger than those in Pre-seed.The raindrop spectra obtained from Chunhua and Tongchuan stations completely depict the changes of raindrop spectra of surface precipitation in Pre-seed,Seed and Post-seed.Cloud seeding has a positive impact on the surface precipitation at Chunhua and Tongchuan stations.A conceptual model of cloud and precipitation evolution after seeding is proposed based on the physical response characteristics,which not only explains that the newly generated small-particle water clouds within the glaciation cloud trench on the satellite are caused by the release of latent heat of freezing,but also provides a theoretical basis for the change of reflectivity from single to double peaks after the broadening of the seeded cloud echo.The proposed model contributes to an in-depth understanding of the physical evidence of the effectiveness of artificial precipitation enhancement,providing a scientific basis and theoretical foundation for the physical assessment of artificial precipitation enhancement. |
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