The Role of Observations in ECMWF Tropical Cyclone Initialization and Forecasting

Thirteen observing system experiments (OSEs) were conducted at ECMWF during a 37-day period in August-September 2020, using the same resolution as the operational data assimilation. These OSEs provided a unified framework to evaluate and understand the role of different types of observational data for tropical cyclone (TC) forecasts, keeping in mind that removing or adding individual types is expected to have a limited effect due to resilience in the full observation system.

The strongest impact came from withholding of microwave radiance observations, which on average degraded the TC position for all evaluated lead times and central pressure forecasts out to 3 days. The all-sky assimilation of AMSU-A temperature sounders improved the TC position forecasts. Scatterometer data improved the forecasts of both quantities, with further improvements from reduced spatial thinning. The impact for other satellite data such as GNSS-RO, AMVs and Aeolus were mixed/neutral. While the aircraft-borne data (dropsonde, flight-level data) were mostly beneficial, further investigations on how to best use this data in the TC core are needed. The assimilation of interpolated hourly central pressure and position data estimated by forecasters had an overall positive impact on both position and central pressure.

While this study has illustrated the difficulties in obtaining strong results for observation impact on TCs for an operational data assimilation system, we suggest that the inclusion of new observations and improvements to the use of existing ones are necessary to keep improving the accumulated positive effect from the observation system. Additionally, future work is needed to advance diagnostic tools to judge the impact of individual observation systems on TCs.