Relationships of along-the-track local variations of GPS RO impact parameter to strong local vertical gradient of wet refractivity in the tropics

Atmospheric refractivity fields are more strongly affected by more water vapor in the tropical lower troposphere than elsewhere.In this study,based on model simulations,we first collocated the radio occultation(RO)data in 2009 from the Constellation Observing System for Meteorology,Ionosphere,and Climate(COSMIC)with CloudSat cloud profiling radar data.We then investigate where and why a RO ray path intersects with other simulated rays of impact heights above and/or below the impact height of the ray.Using the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis as input to a raytracing observation operator,we show that the simulated impact parameters could vary along the ray in the tropical lower troposphere.For brevity,an intersection of a GPS RO ray path with rays above it or below it or both will be called an impact multipath.By overlapping simulated ray trajectories over the vertical and horizontal gradients of refractivity in 2D occultation planes,impact multipath occurrences are vividly illustrated.Statistical results show that the impact multipath is caused mostly by strong local vertical gradients of atmospheric wet refractivity mostly within 300 km horizontal distances from their perigee positions.The impact multipath occurs more frequently below 5-km impact height,with a maximum occurrence around the 3.4 km impact height(about 1.8 km geometric height).It is shown that both the simulated impact multipath phenomena and locally strong vertical gradients of wet refractivity are found in both cloudy and clear-sky conditions,and most frequently in presences of stratocumulus clouds.

The Impact of Assimilating FY-3C GNOS GPS Radio Occultation Observations on GRAPES Forecasts

In the present study, a gross quality control (QC) procedure is proposed for the Global Navigation Satellite System Occultation Sounder (GNOS) Global Positioning System radio occultation (GPS RO) refractivity data to remove abnormal data before they are assimilated. It consists of a climate extreme check removing data outside the range of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) climate maxima and minima over approximately five years, and a vertical gradient check that rejects profiles containing super-refractions. These two QC steps were applied sequentially to identify outliers in GNOS GPS RO refractivity data during boreal winter 2013/2014.All of the abnormal refractivity profiles and the outliers at each level of the GNOS GPS RO observations were effectively removed by the proposed QC procedure. The post-QC GNOS GPS RO refractivity observations were then assimilated in the Global/Regional Analysis and PrEdiction System (GRAPES) using the three-dimensional variational(3D-Var) system. The impacts of the GNOS refractivity observation on GRAPES analysis and forecasting were evaluated and analyzed using an observation system experiment run over one whole winter season of 2013/2014. The experiment results demonstrated a positive impact of GNOS GPS RO data on analysis and forecast quality. The root mean squared error of GRAPES analysis temperature was reduced by 1%in the Southern Hemisphere (SH) extratropics and in the tropics, and the anomaly correlation scores of the forecasted 500-hPa geopotential height over the SH increased significantly during days 1 to 5. Overall, the benefits of using GNOS GPS RO data are significant in the SH and tropics.