降水量的直接同化对初始场的影响

四维变分法用严格的数学方法、合乎逻辑的物理过程根据观测资料和模式本身来获得一个最优的初始场,是当前最先进的四维资料同化方法之一.作者用四维变分法对自动站降水量作直接同化,对初始场增量进行了分析.结果表明降水量的同化对初始场中热力性质的量影响相对较大,对低层的影响大于高层,大部分增量在925 hPa至850 hPa层间达最大.通过同化,北京地区对流不稳定性明显增加.对风场的增量分析表明,同化增加了一些中尺度信息.对于散度场和水汽通量散度场,同化有使低层辐合加强的趋势.通过同化,在MM5初始场上增加了原先没有

Effect of Direct Assimilation of Precipitation on Initial Field

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.