Comparison of China Southwest Vortex Structures from AHI Observations, ERA5 Reanalysis, and NCEP GFS All-Sky Simulations

This study first modifies a conventional method for locating the center of the Southwest Vortex (SWV) and applies it to center identification in the fifth generation ECMWF reanalysis (ERA5) and the Global Forecast System (GFS) analysis from the NCEP of the U.S. for an SWV event on  26-27 June 2020. Significant discrepancies are found between the SWV trajectories in ERA5 and GFS. Secondly, according to observations from the Advanced Himawari Imager (AHI), a row of isolated clouds over the eastern Qinghai-Xizang Plateau exhibits characteristics of mountain gravity waves and generates SWVs as they propagate eastward; the SWV related cloud/rain structure in cloud-top pressure and cloud type varies consistently with the observed lower brightness temperatures. Thirdly, using ERA5 and GFS as inputs to a radiative transfer model, we generate all-sky simulation of AHI channel-13 brightness temperatures. On the synoptic scale, the ERA5 simulations more closely match the observed brightness temperatures, whereas for the small-scale cloud/rainband structure of the SWV, the GFS-simulated brightness temperatures more closely resemble the observations. The ERA5 liquid water clouds extend to altitudes as high as the 140-hPa level, with temperatures as low as -32°C. The AHI liquid water cloud tops are located in the middle and upper troposphere, while the GFS liquid water cloud tops are confined to the lower troposphere. The findings suggest that while global reanalysis/analysis datasets are useful for synoptic-scale understanding of SWVs, caution is needed when using them for fine-scale cloud structure analyses or as training data for deep learning models targeting convective systems.