水成物对GNOS掩星弯曲角同化的影响评估

The impact of water content on the assimilation of GNOS occultation bending angle

本文在无线电掩星弯曲角射线追踪正演算子中引入水成物的影响,针对台风个例,利用FY-3c GNOS弯曲角资料的同化展开研究.通过分析水成物对掩星弯曲角正演精度的影响,指出当掩星剖面跨越一定厚度的台风区云雨大气时,多相态水成物对GNOS弯曲角正演精度的影响不可忽略.进而提出一种考虑云雨影响的掩星折射率正演算法,将掩星折射率的正演分别在晴空区和云雨区进行,在云雨区正演算子中增加多相态水成物含量对正演掩星折射率的贡献,改进了FY-3c GNOS弯曲角资料在云雨大气环境的同化方案.针对2018年24号台风个例,进行了同化的参照试验、未考虑和考虑水成物影响时GNOS弯曲角的3DVAR同化试验,考量云雨环境下的GNOS弯曲角资料同化对台风模拟的影响差异.试验结果表明,两种同化方案皆能改善台风路径预报,台风中心海平面气压模拟都能接近实际观测,台风最大风速也不同程度增大.而考虑水成物含量的影响后,资料同化能更有效缩小观测空间与背景场空间之间的偏差,同化后观测与分析的偏差更接近高斯分布,台风外围动力场和热力场环境能够得到更优的调整,使得96 h的台风路径模拟平均距离误差较不考虑水成物影响的情形减小了约14%.

In this study,the impact of multi-phase water content is introduced into a RO(radio occultation)bending angle forward operator based on two-dimensional ray tracing algorithm,and then performed to 3DVAR assimilation for FY-3c GNOS bending angle based on WRFDA.The evaluation shows that the impact of multi-phase water content cannot be ignored when the RO plane passes through the thick cloudy region of typhoon.Thus,an improved forward operator is established based on an algorithm that contains the effect of water content,and the bending angle is respectively calculated in clear sky and cloudy region.Then several experiments were performed to GNOS bending angle data assimilation for typhoon No.24 in 2018.The original bending angle forward operator was used for a test,and the new operator was used for another one.It is shown that the simulation of typhoon’s track and sea-level center pressure is improved in both tests,and the sea-level maximum wind speed increase as well.However,the shape and magnitude of the analysis increments for geopotential height,temperature,vertical velocity,and water vapor mixing ratio exhibit significant differences between the use of original and new operators.The test using new operator produces more accurate analyses and more improvements of deviation Gaussian distribution when compared with observations,enhances typhoon warm core structure in middle and low level,increases the asymmetry of typhoon humidity distribution,and optimizes the vertical velocity in typhoon region.Also,the adjustments of environment fields outside typhoon region are more effective,and the mean distance error of 96 h track simulation is reduced by 14% when the new operator was used.As expected,an operator that includes the effect of multi-phase water content is much beneficial for GNOS bending angle data assimilation.