Investigation Into the Potential Value of Stratospheric Balloon Winds Assimilated in NOAA's Finite‐Volume Cubed‐Sphere Global Forecast System (FV3GFS)

Near‐space balloon networks have the potential to improve numerical weather prediction (NWP) through data assimilation (DA) by providing in situ observations in an otherwise data‐sparse stratosphere. This study investigates the prospective value of stratospheric balloon winds to NOAA NWP by examining Loon data quality and conducting a 2‐month observing system experiment (OSE) using NOAA's Finite‐Volume Cubed‐Sphere Global Forecast System. During the study period (December 2018–January 2019), Loon winds show good correspondence to collocated rawinsonde (RAOBS) winds, while a considerable difference in tropical stratospheric winds is observed between NOAA and ECMWF operational analyses. The OSE, one run with Loon winds assimilated and another without, suggests that additional stratospheric wind observations can improve NWP. Loon wind assimilation acts to decelerate the strong tropical easterly stratospheric jet west of where Loon winds are observed while accelerating less intense subtropical easterly motions. Differences in observation‐minus‐background statistics between the two runs reveal that the backgrounds for Loon winds and RAOBS winds and temperatures improve when Loon winds are assimilated. This positively impacts short‐term forecasts of the lower stratospheric circulation during the study period. Additionally, Loon wind assimilation improves the tropical stratospheric response to the 2019 New Year sudden stratospheric warming (SSW) event by significantly reducing the short‐term forecast error following the SSW. The findings emphasize the value of stratospheric winds toward improving global NWP, and imply the importance of including near‐space observing systems in the Earth‐observing architecture. Plain Language Summary Networks of large superpressure balloons have the potential to address a known data gap in the global observing system by providing in situ observations of the lower stratosphere. This study investigates the potential value of stratospheric winds observed by Loon balloons to NOAA by conducting two experiments, one run with Loon winds added and another without, using NOAA's Finite‐Volume Cubed‐Sphere Global Forecast System over a 2‐month period (December 2018–January 2019). Comparisons between the two runs reveal the impact of adding Loon winds to the system. We find that Loon winds act to weaken the strong tropical easterly stratospheric jet in the model near where Loon winds are observed. Differences between the observed and model ba