一种新的COSMIC大气折射率资料观测误差估计方法及在GRAPES全球三维变分同化中的应用

观测误差协方差是变分同化系统中决定分析及预报效果的关键参数之一,观测误差的估计精度直接影响变分同化分析和预报效果。分析了新息增量法(H-L法)估计全球定位系统无线电掩星这类观测点不固定资料的观测误差的适用条件,并利用1年的气象、电离层及气候星座观测系统(COSMIC)折射率资料,针对局地观测算子,估计了COSMIC折射率在南、北半球高、中、低6个纬度带四季的观测误差,分析了COSMIC折射率观测误差的纬度、高度和季节变化的特点,并将估计的折射率观测误差应用于GRAPES(Global/Regional Assimilation and Prediction Enhanced System)三维变分同化系统。结果表明,折射率观测误差随高度和纬度有明显变化;在中、高纬度带,折射率观测误差有显著的季节变化:夏季折射率的观测误差约为冬季2倍,春、秋两季折射率误差具有较好的南北对称性,冬、夏两季折射率观测误差南、北半球差异较大。与GRAPES原来使用的全球平均单一的折射率观测误差相比,在GRAPES全球三维变分同化系统中使用本研究估计的较高精度的随纬度变化的COSMIC折射率观测误差能够提高GRAPES全球变分同化系统的预报水平。

A new method for estimating observation error of the COSMIC refractivity data and its impacts on GRAPES-GFS model weather forecasts

The observation error covariance matrix plays a key role in the variational data assimilation. Assumptions for using the innovation vector method to estimate the observation error of the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) GPS (Global Positioning System) RO (radio occultation) data are analyzed and the algorithm for estimating the observation error of refractivity is composed. Using the H-L method, the observation errors of the COSMIC refractivity are estimated over the low-latitudes (0°-30°), mid-latitudes (30°-60°) and high-latitudes (60°-90°) of the north and south hemisphere respectively in the different seasons. The impacts of the refractivity observation error on the GRAPES (Global/Regional Assimilation and Prediction Enhanced System) assimilation and forecast skill are assessed by the assimilation and forecast experiments of GRAPES. The fractional refractivity observation errors vary with the latitude and height significantly. The results illustrated that a single error profile is not a good global representation of the refractivity errors. On the other hand, the fractional refractivity observational errors vary with the season over the mid-latitudes and high-latitudes of the south and north hemisphere, and in those locations the error in the boreal summer is twice of that in the boreal winter. The observation error profiles are symmetric in the south and north hemisphere in both the boreal spring and autumn. Compared with the single fractional refractivity error profile, the new refractivity observation error estimated in this paper results in an improvement in the anomaly correlation scores and a reduction of geopotential height errors, temperature errors and wind errors of the GRAPES forecasts.