Impact of Refractivity Profiles from a Proposed GNSS-RO Constellation on Tropical Cyclone Forecasts in a Global Modeling System

A global Observing System Simulation Experiment (OSSE) system was used to assess the potential impact of a proposed Global Navigation Satellite System (GNSS) radio occultation (RO) constellation on tropical cyclone track and maximum wind speed forecasts. The OSSE system was based on the 7-km NASA nature run G5NR and used simulated RO refractivity determined by the horizontal and vertical distribution of observations from the original planned Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2). The RO data was assimilated using NOAA’s GDAS/GFS weather analysis and forecast system. Three experiments were conducted to generate global forecasts over 6 weeks of the Northern Hemisphere’s tropical cyclone season and a tropical cyclone tracker was used to determine track and maximum wind speed forecasts.

Global statistics for 132 tropical cyclone forecasts show adding or excluding explicit errors to RO profiles made very little difference to forecasts. Track forecasts had small, mainly statistically insignificant impacts when comparing an experiment with RO assimilation to one without. Small (1-3kts) statistically significant degradations in the maximum wind speed forecasts were found in the first 60-hr of lead time with neutral impacts thereafter. There was large forecast-to-forecast variability in RO impact, especially for track forecasts. Further investigation through binning forecasts by the geographic location, intensity, and coverage of RO observations near the cyclone core revealed insensitivity of RO impact to these storm attributes. Two individual forecasts were explored in more detail, revealing track improvements and degradations were driven by RO impact on mid-level geopotential heights.