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.