| Nuclear explosion or nuclear leakage will release a large number of radionuclides,endanger national security,affect human health and pollute the natural environment due to the lack of observation data model and data assimilation technology limitations,the accuracy of radionuclide diffusion prediction is relatively low at present.The main content of this paper is to design two kinds of variational assimilation schemes for nuclides,general assimilation and logarithmic assimilation,based on WRF-Chem model,using the observation data of specific activity concentration of nuclides and taking the radionuclide Cs-137 as the control variable.In addition,a case study of March 15,2011in the Fukushima area of Japan with a wide distribution of nuclear pollution was selected to carry out the 3-Dimensional Variational Assimilation and prediction experiment.NMC method was used to calculate the background error covariance of Cs-137variable.The results show that the horizontal correlation coefficient decreases with the increase of distance,and the horizontal attenuation scale is about 21km.The vertical profile of the standard deviation of background error decays rapidly within the boundary layer and is close to 0 above the 13th layer.Vertical related within the boundary layer,layer and ground floor 5 correlation coefficient can reach above 0.5 ideal field test results show that the direct assimilation of Cs-137 concentration can make the observation information to spread around,but when the site observation data in different order of magnitude difference is bigger,small scale of observation information can’t reflect in the assimilation analysis field Logarithmic transformation assimilation of Cs-137 concentration can overcome this problem,but the concentration of large scale observation sites in the assimilation analysis field is low.By analyzing the data of Fukushima nuclear accident on March 15,2011,it was found that the Cs-137 concentration order of 32 sites ranged from 10-2to 102Bq/m3,showing a skewed distribution.90%of the values were below 50 Bq/m3,and the data of Cs-137 concentration after logarithmic transformation was close to normal distribution.The direct assimilation experiment of Cs-137 concentration showed that the root mean square error（RMSE）of the initial field decreased from 1.38 Bq/m3to 0.08 Bq/m3,the correlation coefficient（CORR）increased from 0.57 to 0.98,and the average deviation（BIAS）decreased from 0.8 Bq/m3to 0.01 Bq/m3and FAC2/5/10 increased from 0.24,0.52 and 0.67 to 1,respectively.From 06:00 to 23:00 on March 15,the average RMSE decreased by 17%,the CORR increased by 16%,the average BIAS decreased by 18%,and FAC2,FAC5,and FAC10 increased by 67%,32%,and 17%,respectively.Especially in the first 10 hours,the assimilation improved the forecast significantly.The improvement effect of Cs-137 concentration logarithmic transformation assimilation on the initial field is slightly lower than that of direct assimilation,but the improvement effect on the follow-up forecast is better.From 06:00to 23:00 on March 15,the mean RMSE decreases by 29%,the CORR increases by 20%,the mean BIAS decreases by 37%,and the FAC2,FAC5 and FAC10 increase by 67%49%and 27%,respectively.The effective duration of assimilation was more than 18hours. |
