A perspective on the fundamental quality of GPS radio occultation data

[摘要] Radio occultation (RO) is a promising source of observation for weather andclimate applications. However, the uncertainties arising from imperfectretrieval algorithms may weaken the overall confidence in the data anddiscourage their use. As an alternative approach of assessing the quality ofRO data while avoiding the nuisance of retrieval errors, this study proposesto use minimally processed data (measurement) instead of derived RO data.This study compares measured phase paths with their model counterparts,simulated with an effective ray tracer for which the refractive indicesalong the complete ray path linking the transmitter and the receiver arerealistically specified. The comparison of phase measurements with theEuropean Centre for Medium-Range Weather Forecasts (ECMWF) data made in theobservation space shows that the RO measurements are of sufficient accuracyto uncover regional-scale systematic errors in ECMWF's operational analysisand the 45-year reanalysis (ERA40), and to clearly depict the error growthof short-term ERA40 forecasts. In the southern hemispheric stratosphere, inparticular, the RO measurements served as a robust reference against whichboth of the two analyses were significantly biased in opposite directionseven though they were produced by the same center using virtually the sameset of data. The measurement and ECMWF analyses showed a close agreement inthe standard deviation except for the regions and heights that the qualityof the ECMWF data is controversial. This confirms the high precision of ROmeasurements and also indicates that the main problem of the ECMWF analyseslies in their systematic error.

[学科分类] 几何与拓扑