Extended-Range Prediction of the Tropical Cyclone Environment in Coupled Global Ensembles

We examine the predictability of large-scale environmental conditions relevant to tropical cyclones (TCs) in several coupled global ensemble prediction systems, including the Navy Earth System Prediction Capability (Navy ESPC) and ECMWF, UKMO, and NCEP CFSv2 from the Subseasonal-to-Seasonal (S2S) database. An operational 16-member version of the Navy ESPC ensemble has been run operationally once a week out to 45 days since August 2020, allowing us to examine the TC environment in the 2021 and 2022 northern hemisphere summers. Forecasts are verified against ERA5 reanalyses and NOAA CDR OLR.

We examine the actual and potential predictability of several fields related to TC genesis including shear, low-level vorticity, and convectively coupled equatorial wave (CCEW) -filtered OLR using 20 years of ECMWF ensemble reforecasts. In examining the extended-range predictability of the TC environment we examine the sensitivity of predictive skill and potential predictability to averaging forecasts in time (1, 2, 4, 7, and 14 day windows are used) and spatial averaging (500 and 1000 km radial averages are used). Spatial averaging of the forecasts and observations increases skill especially for window lengths less than 7 days. In addition we analyze predictions of the TC environment in the 2021 and 2022 northern hemisphere summers in the Navy ESPC ensemble and operational ensembles from the S2S database.