How well is outer tropical cyclone size represented in the ERA5 reanalysis dataset?

The outer tropical cyclone (TC) size was extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation global atmospheric reanalysis (ERA5), the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and the ECMWF Interim reanalysis (ERA-I), as well as a hybrid reanalysis-observation dataset, the Cross Calibrated Multi-Platform (CCMP) ocean surface wind vector product. Results are compared with the QuikSCAT Tropical Cyclone Radial Structure Dataset (QSCAT-R). Several size metrics were applied including the radii of 6-15 m s(-1) winds. The capability of ERA5 in describing TC outer wind fields was evaluated, and improvements in the representation of outer size assessed. The ERA5 reanalysis can resolve stronger TC winds, which leads to more valid outer size samples, especially for the radii of 12 and 15 m s(-1) winds, and for the cases with outer sizes of <300 km. This makes the distribution characteristics of the outer size in the ERA5 reanalysis closer to the observation. These improvements of ERA5 are more obvious in the eastern North Pacific basin where TCs are generally of smaller size. However, ERA5 still underestimates the outer size, similarly to its predecessor ERA-I. The size error in ERA5 becomes more negative with increasing observed outer size, with this tendency also occurring in previous reanalysis datasets. Nevertheless, among the size metrics examined, the radius of 9 m s(-1) winds is regarded as the optimal size metric for examining the extent of outer winds, thus providing an effective reference for constructing an objective, multi-decadal dataset of outer size using the ERA5 reanalysis.

Automated summary

Multiple reanalysis datasets and observation data were used to analyze the outer size of tropical cyclones, with ERA5 showing improvements in describing TC outer wind fields and size, especially in the eastern North Pacific basin. However, ERA5 still tends to underestimate the outer size, with the radius of 9 m/s winds considered the optimal metric for studying the extent of outer winds.