Effect of Direct Assimilation of Precipitation on Initial Field
降水量的直接同化对初始场的影响

Four-dimensional variational data assimilation(4D-VAR) is a logical and rigorous mathematical method to obtain the "best" estimate of the model initial conditions from observations and model itself.It is one of the most advanced data assimilation methods today. Automation weather station(AWS) precipitation data are assimilated by 4D-VAR directly,and the increment of initial field is analyzed.The effect of precipitation assimilation on the moisture field and temperature field is more obvious than on other element fields,and the effect on low levels is more evident than on high levels.Most of the maximum increments appear between 925 hPa and 850 hPa.Because of precipitation assimilation,the convective instability is enhanced in Beijing area.Analysis of wind field increments shows that some mesoscale information is added to the initial field.The increments of divergence and moisture flux divergence show the tendency that convergence is enhanced in low levels.By assimilating precipitation,the cloud water and rainwater appear in the initial field while there is not any cloud water and rainwater in the original initial field of MM5.But the effect of assimilation on the vertical circulation of the initial field is not evident.In the experiment,the increment of test II in which 6-hour accumulative rainfall is assimilated is more obvious in most situations than that of test I in which 6-hour hourly precipitation is assimilated.This is because the test I contains more information of rain time than test II and the test II has less limitation than test I.By the increments of test II,effect of precipitation assimilation on the initial field is more evident.However,test II also shows that if only precipitation data are assimilated,effect of assimilation is not always good.The rain structure in test I is simulated better than that in test II.Experiment results show that,due to addition of information of AWS precipitation data,the initial fields of test I and test II are enhanced in mesoscale information,which are more consistent with the model in thermo-dynamical mechanism.After assimilating,the simulation of torrential rain is improved.The precipitation during the early period of simulation is increased,and the position of simulated precipitation is more consistent with real situation.The "spin-up" problem of the model is weakened.